ILLIN I S UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN PRODUCTION NOTE University of Illinois at Urbana-Champaign Library Large-scale Digitization Project, 2007. UNIVERSITY OF ILLINOIS BULLETIN IBSUED WEEKL Vol. XXIV November 16, 1926 No. 11 [Entered as second-cla matter December 1 191, 1912 at the pot offie at Urbana, mino, ud the Act of August 4, 1912. Acceptance for mailing at the specil rate of potage provided for in section 1108, At of October 3, 1917, authorised July 31, 1918.] AN INVESTIGATION OF TWIST DRILLS PART II BY BRUCE W. BENEDICT AND ALBERT E. HERSHEY BULLETIN NO. 159 ENGINEERING EXPERIMENT STATION PUBLISHED BY THE UNIVERSITY OF ILLINOIS, URBANA PaBIC: FOeTr CNTrs jT HE Engineering Experiment Station was established by act of the Board of Trustees of the University of Illinois on December 8, 1903. It is the purpose of the Station to conduct investigations and make studies of importance to the engineering, manufacturing, railway, mining, and other industrial interests of the State. The management of the Engineering Experiment Station is vested in an Executive Staff composed of the Director and his Assistant, the Heads of the several Departments in the College of Engineering, and the Professor of Industrial Chemistry. This Staff is responsible for the establishment of general policies gov- erning the work of the Station, including the approval of material for publication. All members of the teaching staff of the College are encouraged to engage in scientific research, either directly or in co8peration with the Research Corps composed of full-time research assistants, research graduate assistants, and special investigators. To render the results of its scientific investigations available to the public, the Engineering Experiment Station publishes and distributes a series of bulletins. Occasionally it publishes circu- lars of timely interest, presenting information of importance, compiled from various sources which may not readily be acces- sible to the clientele of the Station. The volume and number at the top of the front cover page are merely arbitrary numbers and refer to the general publica- tions of the University. Either above the title or below the seal is given the number of the Engineering Experiment Station bul- letin or circular which should be used in referring to these pub- lications. For copies of bulletins or circulars or for other information address THE ENGINEEIN NG EXPEIMENT STATION, UNiVMIvsnr or ILLINOIS, UrAUNA, ILLINOIS UNIVERSITY OF ILLINOIS ENGINEERING EXPERIMENT STATION BULLETIN NO. 159 NOVEMBER, 1926 AN INVESTIGATION OF TWIST DRILLS PART II BY BRUCE W. BENEDICT MANAGER OF THE SHOP LABORATORIES AND ALBERT E. HERSHEY RESEARCH GRADUATE ASSISTANT IN MECHANICAL ENGINEERING ENGINEERING EXPERIMENT STATION PUBLISHED BY THE UNIVERSITY OF ILLINOIS, URBANA CONTENTS I. INTRODUCTION . . . . . . 1. Introductory Statement 2. Object of Investigation 3. Acknowledgments . . . II. DESCRIPTION OF APPARATUS . 4. Drilling Machine . . . 5. Motor Drive . . . . 6. Dynamometer . . . 7. Calibration of Dynamometer 8. Test Blocks . . . .. 9. Test Drills . . . III. DESCRIPTION AND RESULTS OF TESTS . . . 10. Dimensions and Properties of Twist Drills . 11. Helix Angle Test in Gray Cast Iron . 12. Endurance Test in Gray Cast Iron . 13. Summary of Results of Tests in Gray Cast Iron 14. Helix Angle Test in Steel . . . . . 15. Endurance Test in Steel . . . . 16. Machineability of Metals . . . . .. 17. Summary of Results of Tests in Steel . IV. CONCLUSIONS . . . . . 18. Summary of Conclusions . . . . . . V. BIBLIOGRAPHY ... PAGE . . 7 7 . . 8 S . . 8 . . 8 S . . 8 . . . 9 . . . 13 . . . 17 . . . 19 . . . 22 . . 24 . . 24 . . 27 S . 37 S . 40 S . 40 . . 48 . 50 57 S57 S57 LIST OF FIGURES NO. PAGE 1. Drilling Machine and Dynamometers, View from the Rear ..... . 10 2. Driving Motor and Dynamometer . . . . . . . . . . . . 11 3. Details of Motor Mounting and Dynamometer . .. . . . . . . 12 4. Drill Dynamometers and Recording Apparatus . . . . . . . . 13 5. Drill Dynamometers . . . . . . . . .. . . . . . . 14 6. Detail of Thrust Recording Indicator . . . .. . . . . . . 15 7. Typical Dynamometer Record . . . . . . . . . . . . . 16 8. Apparatus for Calibrating Drill Dynamometers . . . . . . . . 17 9. Calibration Curves of Drill Dynamometers . . . ... . . . . 18 10. Test Drills ................... 22 11. Torque of Drills with Various Helix Angles and Feeds, in Gray Cast Iron . 28 12. Thrust of Drills with Various Helix Angles and Feeds, in Gray Cast Iron .. 29 13. Chips from Drills with 26- and 35-degree Helix Angles, in Gray Cast Iron . 31 14. Comparison of Drills of Different Design . . . . . ... . . .32 15. Power Consumption at the Drill Point of Drills with Various Helix Angles, in Gray Cast Iron ................ . .33 16. Torque of Drills with Various Helix Angles in Endurance Test, in Gray Cast Iron ...... . .. .. ....... ..35 17. Thrust of Drills with Various Helix Angles in Endurance Test, in Gray Cast Iron . . . . . . . . . . . . . . . . . . . . 36 18. Condition of Test Drills at Close of Endurance Test in Gray Cast Iron . 39 19. Section of Gray Cast Iron Test Block Showing Run-in Hole .. . . . 39 20. Torque of Drills with Various Helix Angles, in Seven Grades of Steel . . . 42 21. Thrust of Drills with Various Helix Angles, in Seven Grades of Steel . . 43 22. Variation of Torque and Thrust with Feed, for Drills with Various Helix Angles, in Two Grades of Steel . . . . .. . . . . . .. . .45 23. Power Consumption at the Drill Point of Drills with Various Helix Angles, in Two Grades of Steel .. . . . . .. . . . . . . . 49 24. Torque of Drills with Various Helix Angles in Endurance Test, in Medium- carbon Steel . ............ .. . .. 51 25. Thrust of Drills with Various Helix Angles in Endurance Test, Medium- carbon Steel . .. . . . . . . . . . . . . . . 52 26. Condition of Test Drills at Close of Endurance Test in Medium-carbon Steel 53 27. Comparison of Machineability and Hardness of Seven Grades of Steel . 54 28. Micrographs of Test Steels at Bottom of Holes under Drill Point . . . . 56 LIST OF TABLES NO. PAGE 1. Properties of Gray Cast Iron Test Blocks . . . . . . . . .. 20 2. Properties of Steel Test Bars ...... . . . . . . . . 21 3. Dimensions and Properties of Test Drills . . . . . . . ... . 23 4. Dimensions and Properties of Stock Twist Drills . . . . . . . . 25 5. Variations in Dimensions and Properties of Stock Twist Drills . . . 26 6. Torque of Drills with Various Helix Angles . . . . . . . . . . 27 7. Thrust of Drills with Various Helix Angles . . . . .. . . . . 27 8. Power Consumption of Drills with Various Helix Angles . . . . .. 34 9. Relative Endurance of Drills with Various Helix Angles . . . . . . 37 10. Relative Performance of 26- and 45-Degree Helix Angle Drills . . .. 38 11. Torque and Thrust of Drills with Various Helix Angles .. . . . . 41 12. Power Consumption of Drills with Various Helix Angles . . .. . . 48 13. Relative Endurance of Drills with Various Helix Angles .. . . . . 50 14. Physical Properties of Steels Used in Test . . . .. . . . . .54 AN INVESTIGATION OF TWIST DRILLS PART II I. INTRODUCTION 1. Introductory Statement.-This investigation of twist drills is the second of a series which has been planned to determine the factors re- lating to the design and performance of this widely used production tool. Results of the initial investigation were published in Bulletin No. 103 of the Engineering Experiment Station in 1917. As stated in the preface of that bulletin, the first part of the investigation was essenti- ally preliminary in scope with the object of determining the general characteristics of the twist drill as a cutting tool, and of opening the way for further and more extensive investigations in the phenomena of drilling. The second part, with which the present bulletin is concerned, deals specifically with the relation between helix angle, and torque, thrust, and endurance of the drill when drilling in gray cast iron and steel. During the late European war the original program of tests was temporarily abandoned, but in 1920 it was revived, and with it plans for an improved testing equipment. A heavy duty drilling machine was installed and equipped with a dynamometer capable of measuring forces and recording data beyond the capacity of the original apparatus. As the present dynamometer is of novel design it is described in some detail in the following pages. A period of two years has been employed by the authors in obtaining the results herewith presented. Since the publication of Bulletin No. 103 investigators in both the United States and Europe have added to the store of knowledge on the subject of drills and drilling, but as yet no work has been done in this field comparable with Taylor's classic experiments on lathe cutting tools. As a factor in the production of metal goods the process of drilling ranks in importance with turning or milling, and is, therefore, worthy of a thorough and systematic study with the aid of precision apparatus. An investigation of the magnitude which such a study demands can be accomplished within a reasonable period of time only through co6per- ative effort on the part of drill and drilling machine manufacturers, and laboratories equipped for scientific testing. Such an investigation would accomplish in a reasonable time what would require years of effort by individual investigators working independently, and would furnish information in the immediate future which should remove the twist drill from the realm of the rule-of-thumb. 7 ILLINOIS ENGINEERING EXPERIMENT STATION 2. Object of Investigation.-In the initial investigation of twist drills reported in Bulletin No. 103 the fact was disclosed, and, so far as the authors are aware, for the first time, that power consumption at the drill point is influenced more by changes in helix angle than by any other one factor of design. It was found that at heavy drilling rates the power required at the drill point in gray cast iron was reduced more than 20 per cent by increasing the helix angle from 26 degrees to 35 degrees. As these tests were conducted on gray cast iron only, the results were not considered generally applicable until similar tests had been made on the carbon and alloy steels commonly used in industry. The question of drill endurance, especially in drilling steel, was likewise an unde- termined matter. It was, therefore, decided to make a comprehensive study of the relation of the helix angle of twist drills to the power con- sumption and endurance, in both gray cast iron and steel, with the ob- ject of determining what helix angle best satisfies the inseparable re- quirements of (a) economy in the use of power and (b) resistance of the cutting edge to wear and destruction. 3. Acknowledgments.-The testing apparatus was constructed by the mechanical staff of the Machine Laboratory; the test blocks of gray cast iron were made in the Foundry Laboratory; the metallographic work and heat treating of steel test blocks was done in the Forge Labor- atory. The CENTRAL STEEL COMPANY, Massilon, Ohio, donated all of the alloy steels used in the test. THE FULTON COMPANY, Nashville, Tennessee, cobperated by furnishing "Sylphons" for the dynamo- meters. Drills of special dimensions were supplied by the DETROIT TWIST DRILL COMPANY and by the CLEVELAND TWIST DRILL COMPANY. THE NEW DEPARTURE MANUFACTURING COMPANY donated one large ball bearing for the drill dynamometer. MR. KLASS KNIBBE, Holland- American Foundation student, Delft, Holland, assisted in the conduct of the tests for a period of three months. II. DESCRIPTION OF APPARATUS 4. Drilling Machine.-The drilling machine used in this series of tests was made by the Minster Machine Company. It is of the heavy duty type, with 12 speeds and 12 feeds arranged particularly for testing purposes. The speed range is from 35 to 650 r.p.m., in geometric pro- gression with a ratio of approximately 1.30. The feed range is from 0.0084 to 0.1000 in. per rev., likewise in geometric progression with a ratio of approximately 1.25. AN INVESTIGATION OF TWIST DRILLS Slippage between the driving motor and the machine is avoided by employing a 5-in. Link Belt drive between the two. In the machine itself, the power for driving and feeding the drill is transmitted entirely by means of gears. Ball bearings are provided throughout the trans- mission gear box and these, together with provision for ample lubrica- tion of all gears and bearings, insure a minimum power loss. Another noteworthy feature is the main spindle drive. A large cylindrical sleeve is driven directly by gears, and this sleeve in turn drives the main spindle through the medium of two wide and deep keyseats extending the full length of the sleeve and engaging two hardened driving keys mounted in the driving head. The driving head is thus supported and guided in the driving sleeve while free to move up and down without binding even when the machine is operating under a heavy load. 5. Motor Drive.-The power for driving the drilling machine is supplied by a two-phase 25-h.p. induction motor operating at 1200 r.p.m. The method of mounting this motor so that it may serve as a dynamom- eter for measuring the power input to the drilling machine is shown in Figs. 1, 2, and 3. As will be seen from Fig. 3, the frame of the motor is bolted to two brackets R which are in turn mounted on ball bearings at S and T so that the whole is free to oscillate. One of these trunnion bearings is hollow, permitting the motor drive shaft to pass through it. This extension of the drive shaft carries the driving gear and is supported at the end by the outboard bearing U. An arm V screwed into the field casting has at its outer end a hardened steel button which bears against a similar button mounted in the end plate of the sylphon X. By this arrangement the torque reaction of the motor frame is transferred to the hydraulic system of the dynamometer. The pressure in the dynamometer system, which is proportional to the motor torque, is indicated by two diaphram gages and recorded by a Bristol recording gage. The motor-torque dynamometer is calibrated in place by hanging standard weights from an arm projecting from the motor casting on the side opposite to that on which the dynamometer is located. The length of this arm being the same as that of the arm which applies the load to the dynamometer sylphon, the pressure in the system corresponding to a given dead weight will be the same as that which exists when an equal force is applied to the sylphon due to torque reaction. The gages may, therefore, be calibrated to indicate the motor torque in ft. lb. The torque and the motor speed being known, the latter indicated by a tachometer (M in Fig. 4) driven directly from the shaft of the motor, ILLINOIS ENGINEERING EXPERIMENT STATION I I FIG. 1. DRILLING MACHINE AND DYNAMOMETERS, VIEW FROM THE REAR AN INVESTIGATION OF TWIST DRILLS FIG. 2. DRIVING MOTOR AND DYNAMOMETER the power required to drive the drilling machine, independent of the motor losses, may readily be calculated. The sylphon, which forms the load absorbing unit of the motor torque dynamometer, is a flexible, seamless copper bellows such as is employed frequently in thermostats. The ends of the sylphon are closed by brass plates soldered in place, one plate containing a pipe connection for the gages and the other the hardened button through which the load is applied. By this construction it is possible to obtain all the advantages of a closed hydraulic system and to have at the same time a system capable of changes in volume of about the same order as would be obtainable with a cylinder and piston. ILLINOIS ENGINEERING EXPERIMENT STATION AN INVESTIGATION OF TWIST DRILLS FIG. 4. DRILL DYNAMOMETERS AND RECORDING APPARATUS 6. Dynamometer.-The dynamometers* used in measuring the torque and thrust at the drill point are essentially the same as those used throughout the initial investigation.t Considerable modification was necessary, however, before the tests in steel could be completed. Some of the external details of the dynamometers are shown in Fig. 4, while construction details are shown in Fig. 5. Both of these dynamo- meters are of the hydraulic type, each consisting of a load supporting element and a recording element. The thrust load of the drill is trans- mitted to the fluid through the medium of the platen A, Fig. 5, and a heavy rubber diaphragm. The plate B, mounted on a ball thrust bear- ing on top of the platen, is free to rotate under the action of the drill torque. Such rotation applies a direct load to the sylphon C by means of the arm E bolted to plate B and the plunger F which slides on balls in the guide G. The fluid used in the torque dynamometer is ordinary *Designed by T. H. Schance, B.S. in M.E., University of Illinois, 1912 t"An Investigation of Twist Drills," Univ. of Ill. Eng. Exp. Sta. Bul. No. 103, p. 16. ILLINOIS ENGINEERING EXPERIMENT STATION 'y/e Area)" 59'. i7 ePn~oo FIG. 5. DRILL DYNAMOMETERS - - - - I I - AN INVESTIGATION OF TWIST DRILLS FIG. 6. DETAIL OF THRUST RECORDING INDICATOR machine oil, but in the thrust dynamometer it was necessary to use castor oil because of the rapid deterioration of rubber diaphragms under the action of mineral oil. Additional support for the rotating table B is secured by means of a cage bolted to the table. A large radial ball bear- ing H is mounted at the top of this cage and bears on the sleeve I, which is in turn bolted to the frame of the drill press. This sleeve, indicated by the dotted lines in Fig. 5, takes the place of an inner race for the bear- ing H, and being ground to an accurate fit in the bearing without a ball groove, allows the table B to rotate and move vertically but not hori- zontally. The recording element of each dynamometer consists of a modified Crosby steam engine indicator. As may be seen in Fig. 4, these indi- cators are mounted on a table at the side of the machine and are con- nected to the load supporting elements of the dynamometers by brass tubing. Details of the construction of the thrust indicator are shown in Fig. 6. Instead of the usual piston and cylinder, the pressure element ILLINOIS ENGINEERING EXPERIMENT STATION IG. - - Y-IAL iNMc -- --ETE - RC . nEnueuraec, T7e/ /11 Cer / roe/7 zf/o/ S, m9e H/yt Aig/mu Bo/ou/ Nat e-ari iofSpe te indcatorm. siia r to those sd in te otor comparatively large volume changes in a closed hydraulic system, also allows easy and change of the measuring element, the indicator spring, so that records with maximum ordinates can be obtained under widely varying load conditions. The attempt was made in designing the drill dynamometer to obtain a permanent and automatic record of all the factors entering into twist drill performance. With this in view all the recording instruments were Torq.ur arranged on a table at one side of the machine so that simultaneous records of drill torque, thrust, and speed were made on a moving strip of paper. This strip of paper was driven by a cord attached to a drum on FIG. 7. TYPICAL DYNAMOMETER RECORD othe indicator consshaft of the drill feed mechanism so that the traverse of the motor paper was equl to the vdynamomerticals. The piston of the indicator is replaced by a btorque indicators traced theird which bears directly on thie end plate of the sylphon, and the speed record was obtained by means any movement of the latter due to pressure changes in the system is communne of whicated to the pencil though thspindle revolutions, and the Thise type iof construction, in addition to possess. A typical record is shown ing the advantages of comparatively large volume changes in a closed hydraulic system, also allows easy and rapid change of the measuring element, the indicator spring, so that records with maximum ordinates can be obtained under widely varying load conditions. The attempt was made in designing the drill dynamometer to obtain a permanent and automatic record of all the factors entering into twist drill performance. With this in view all the recording instruments were arranged on a table at one side of the machine so that simultaneous records of drill torque, thrust, and speed were made on a moving strip of paper. This strip of paper was driven by a cord attached to a drum on the pilot shaft of the drill feed mechanism so that the traverse of the paper was equal to the vertical movement of the drill. The thrust and torque indicators traced their diagrams on this same strip of paper, while the speed record was obtained by means of two electrically controlled pencils, one of which indicated total drill spindle revolutions, and the other, time in seconds. A typical record is shown in Fig. 7. AN INVESTIGATION OF TWIST DRILLS FIG. 8. APPARATUS FOR CALIBRATING DRILL DYNAMOMETERS 7. Calibration of Dynamometer.-Due to the size and arrangement of the dynamometers it was necessary to calibrate them in place. The method of doing this is shown in Fig 8. The torque dynamometer was calibrated by means of standardized weights. The weights were sup- ported by a sling, and the vertical force was converted into a horizontal load and applied to the dynamometer sylphon through the medium of a chain passing over a sprocket wheel* and a second sling. In cali- brating a given indicator spring it was simply necessary to apply the dead weight in uniform increments and determine the corresponding indicator pencil travel. The procedure in the case of the thrust dyna- mometer was slightly more involved. Known loads were applied to the table by compressing a spring which had previously been calibrated in a universal testing machine. This spring had a load-deflection rate of approximately 800 lb. per inch and a very uniform calibration curve to over 8000 lb. compressive load. A dead weight of 250 lb. was first placed on the table of the thrust dynamometer and a corresponding base line obtained at the thrust indicator. The dead weight was then replaced *Since the sprocket was mounted on ball bearings and the movement of the chain was slight, the frictional resistance was considered negligible. 18 ILLINOIS ENGINEERING EXPERIMENT STATION p,0o7 7 ,-Ao.gZ SU) o) S N i Ir I CQ o a a !x AN INVESTIGATION OF TWIST DRILLS by the calibrating spring, this being compressed by feeding down the drill spindle by hand until the pencil of the indicator returned to the base line obtained with the dead weight load. In applying a load to the dynamometer by this method the calibrating spring A, Fig. 8, was placed between two plates B-B which served as reference planes for the measurement of the spring deformation. The actual measurement of the spring compression was made with an Ames dial indicator at three equally spaced intervals around the spring. In order to allow the top plate to tilt slightly and distribute the load over the first coil of the spring, a ball and socket bearing was interposed between the drill spindle and this top plate. The loads were applied to give equal incre- ments of spring compression starting with that corresponding to the 250 lb. load as an initial reading. For each point the pencil travel of the indicator was recorded and the corresponding load obtained from the calibration curve of the loading spring. In Fig. 9 are plotted curves of indicator pencil travel and load for both dynamometers with several different indicator springs. These curves are straight lines for all springs, and calibrations at frequent intervals during the tests showed practically no variation. From the indicator records made in calibrating the dynamometers, scales were prepared for measuring the test records. 8. Test Blocks.-Gray iron test blocks, approximately 2 inches by 6 inches by 12 inches in size, were cast in the University foundry. Uni- formity in structure was secured by pouring the blocks used in single tests at one heat, and generally from one large ladle. Blocks were cast in pairs and later broken apart. About 18 in. was planed off the bottom of each block to give a drilling surface. It was possible to drill 10 holes, 5 in. deep, in each block. Physical properties and chemical composition of blocks used in the endurance tests, typical of all gray cast iron test blocks used, are shown in Table 1. In determining the influence of helix angle on torque and thrust, care was taken to drill all holes of each series in a single block. To accomplish this result blocks were cut in two lengthwise parallel with the bottom surface, giving three finished surfaces for drilling in each block. In the endurance tests one hole was drilled with each of the seven test drills in each block. The steel test bars were 11Y in. square and 10 to 12 in. long. Seven different kinds of steel were used in the tests, two carbon steels and five alloy steels. These were selected as being typical steels used in industry. Table 2 shows the physical characteristics and chemical ILLINOIS ENGINEERING EXPERIMENT STATION 0 C .0 %.0 C0) 0)0 *0E IC 0 ,0) 0)0 00 .0 0 0) 0)0 0) 0 0: 0; 0 0 Q .0 0 0) 0) 0 0 . 00 s 0 s » fri H I E- 00 0 0- 0 a o I OD Pk E o 0) < 0) J 9 £ 0; 00 0 "0 0 00 0 00 00 0 00 00 oia N CO MN 00 00 00 00 ~0000 Si0 0 0 0000 0 N 0 0 0 0 0 00 00 00l 00 N^ 1 0 *^ 00 d N d N N IN b- 00 0 00 s : - 00 00 0 00 - - 00 00 00 00 -1 00 . - 0 N 3 00 00 00l 00 N- 0 QO S3 d o dn O1 ~o o* t: x2 t COiic 0 0 3- ·- 3- 0 d d d CO 1N CC CO (0 CO CO. CO M CO CO 0 0 0 0 0 0 0 o o 0 o o 0 0 0 0 0 0 0 0 0 C o 0 0 0 0 0 0 - CC CO C <0 0 o do 0 o 0 CsO N 0 ^ CO 0 00 0 0 *- 0 0 0 0 o d 0 0 0 0 0 CO N- 0 -^ 0 0 CO o 0 0 0 0 0 0 CO O 0 0 0 c0 0- 0 0 CO 0- 0 CC N o 0 0i 00 0 0S 0 o- a- t0 t0 t 0 00 00000000~l 0 CC Co C-0 C CO C CC 0 C N CC C C N O C)I~ -a a ILLINOIS ENGINEERING EXPERIMENT STATION Seven drills were used throughout the tests (see Fig. 10). They were similar in dimensions except as to helix angles, which varied as follows: 15 degrees, 26 degrees, 28 degrees, 32 degrees, 35 degrees, 40 degrees, 45 degrees. The thickness of the web in all drills was made 0.10 in., and was constant from point to shank. The principal dimen- sions of the test drills are given in Table 3. The drills were ground by machine as follows: point angle, 119 degrees, edge angle, 125 degrees, clearance angle, 7 degrees (see sketch accompanying Table 3). Hereafter in the discussion the point angle will be designated as A, the edge angle as B, and the clearance angle as C. With a point angle of 119 degrees the cutting edges of all the drills were straight. The service obtained from the test drills was re- markably uniform and consistent. III. DESCRIPTION AND RESULTS OF TESTS 10. Dimensions and Properties of Twist Drills.-Milled twist drills are furnished to the trade by a score of independent manufacturers located in industrial centers of New England, New York, Pennsylvania, Ohio, and Michigan. With the exception of a small number of new plants brought into existence by the war, twist drills are produced by old established companies specializing in tool manufacture. Individual initiative has governed the development of designs and production methods on lines similar to those followed by the makers of machine tools, although it would appear that the latter had been the first to per- ceive the advantages of cooperative effort. Notwithstanding the wide distribution of production facilities and the independent character of the industry, twist drills as furnished to the trade are remarkably uniform in appearance and in productive capacity. Without the usual trade marks it would be difficult, in a cursory examination, to identify the drills of different manufacturers and to distinguish one brand from another; and it is doubtful if the usual shop test would show marked differences in their performance. This similarity is but superficial, however, as a careful examination of the commercial brands of drills shows many and pronounced variations in design and details of con- struction. For purposes of comparison, 17 1-in. milled high speed twist drills from as many manufacturers were accurately measured, and their dimensions and properties are recorded in Table 4. To assist the reader in a critical study of these data, maximum variations in these particulars between drills of various manufacturers are given in Table 5 Although made to rather close limits, the twist drill is not classed as a precision tool. In the shop it is generally employed on roughing AN INVESTIGATION OF TWIST DRILLS g,3E *I f . C~ C) ea~ C3 C c %IlL.5 m ^3 003 a) c -C) CC 0 a -i -c) CLO IC rEi .aa ^£ VI ,E1 :r::r-C CL CC O C C C C C C 0 N O C C ~ C C CO ~ c CO C O C O C O C O C O C C C CC C . C C CC CC CC CC CC CC C C C C CC C C - - C - C C - CC C - C CC C 0 CC CC - CC 0 C C C CC C CC C - CC C CC- - CC C C 0 CC CO C CC N C CC CCCC - -C0 00 C '00000000 CC) C - CC~ CC C C - N C CO C~mC C C-C CC CC 7C CCn ~ a~" CC CC C CC C C C C C C C C C C C) C*a) cc o; d C -i C) 0 S CC C) Cd VI ~li *at %C C.CC ~ S. . . . . . . . .o . ILLINOIS ENGINEERING EXPERIMENT STATION TABLE 5 VARIATIONS IN DIMENSIONS AND PROPERTIES OF STOCK TWIST DRILLS 1-in. High Speed Steel Drills 17 Brands Maximum Minimum Total Item Value Value Variation Helix angle, at point, deg.............................. 36. 23. 13. Drill length, overall, in ................................ 11.48 11.20 0.28 Drill length, point to shank, in ......................... 7.05 6.65 0.40 Drill diameter, at point, in............................. . 1.0010 0.9990 0.0020 Drill diameter, at shank, in............................ 0.9969 0.9902 0.0047 Drill diameter, taper from shank to point, in............. 0.0101 0.0025 0.0076 Web thickness, at point, in........................... 0.138 0.110 0.028 Web thickness, at shank, in ............................ 0.265 0.151 0.124 Web thickness, taper from point to shank, in............. 0.140 0.000 0.140 Web thickness, at periphery, in ........................ 0.665 0.452 0.213 Land, width, in...................................... . 0.10 0.05 0.05 Land, depth, in...................................... 0.05 0.02 0.03 Clearance angle, deg .................................. 28. 5. 23. Point angle, deg ...................................... 123. 117. 6. Edge angle, deg ..................................... 130. 115. 15. Hardness, by scleroscope. ............................. 70. 57. 13. Drill weight, lb....................................... 1.62 1.42 0.20 Drill price, dollars ................................... 3.35 2.50 0.85 Drills with constant helix angle-10, or 59 per cent of total. Drills with variable helix angle-7, or 41 per cent of total. Drills with constant web thickness-1, or 5.9 per cent of total. Drills with variable web thickness-16, or 94.1 per cent of total. work, as accuracy in hole size is obtained by reaming. Even if made to very accurate limits the almost universal practice of hand grinding would destroy any possible accuracy which could be secured from a drill of exact size. Unquestionably the observed variation in diameter of 0.002 in. between drills of the lot recorded in Table 5 is of trifling importance. Drills made within the recorded limits will not cause trouble from in- correct hole sizes, although some difficulty might arise on certain jobs with drills having the maximum taper (0.01 in.) after they had been in use for some time. Generally, helix angles have been increased by makers of twist drills during the period between the initial and the present investigation, but drills are still made with helix angles of 23, 24, and 25 degrees. Hand grinding continues to be the prevailing method employed by drill manu- facturers for pointing drills, but this practice is no longer universal. To a large extent hardness and durability have been standardized by the employment of high grade steels and scientific methods of heat treat- AN INVESTIGATION OF TWIST DRILLS TABLE 6 TORQUE OF DRILLS WITH VARIOUS HELIX ANGLES Drilling Gray Cast Iron Torque, ft-lb. Helix Angle Feed, in. per rev. deg. 0.0165 0.0256 0.0399 0.0629 15 32.0 43.8 67.7 102.3 26 30.5 40.2 58.1 90.3 32 29.1 36.8 55.6 84.1 35 28.6 35.5 53.0 80.2 40 27.1 33.2 53.4 79.1 45 26.0 32.4 51.0 76.2 TABLE 7 THRUST OF DRILLS WITH VARIOUS HELIX ANGLES Drilling Gray Cast Iron Thrust, lb. Helix Angle, Feed, in. per rev. deg. 0.0165 0.0256 0.0399 0.0629 15 1135 2070 3761 5225 26 1167 1682 2939 4750 32 1029 1596 2855 4510 35 1005 1420 2645 4205 40 919 1261 2355 3828 45 824 1158 1831 3278 ment. A variation of over 30 per cent in the price of drills in the open market raises questions relating to production and distribution which cannot be considered here, although they are of economic importance to both manufacturers and users of drills. 11. Helix Angle Test in Gray Cast Iron.-In this test a series of holes was drilled in gray cast iron blocks with the object of determining the relative torque and thrust of drills having helix angles of 15, 26, 28, 32, 35, 40 and 45 degrees respectively. An equal number of holes was drilled in the same set of test blocks with each of the drills men- tioned, at speeds of 100, 230 and 375 r.p.m., and at feeds of 0.0165, 0.0256, 0.0399, and 0.0629 in. per rev. The readings at a given feed were averaged and this average used as a typical value of torque and thrust for that feed. The results appear in Tables 6 and 7, and are shown graph- ically in Figs. 11 and 12. ILLINOIS ENGINEERING EXPERIMENT STATION I U t N A, t Samt N1 N ' o 0 0 z Is, ^ ? 1 » N N N '' C t< 3 e a S S; 0i S S: 0* -4 -4 N N \I-"n- - - Y t] 14 Lz Qs <^ z 4 G C a 1ý ^ ,i s-iL 3 t , s '-/» nTO^-JioOJ ~I AN INVESTIGATION OF TWIST DRILLS Sp/n ! n//70 7 ILLINOIS ENGINEERING EXPERIMENT STATION Referring to Fig. 11, it is apparent that there is a direct relation between the helix angle of the drill and the torque required to remove the chip. At all the feeds employed the torque decreased as the helix angle of the drill increased, the decrease being more pronounced at the highest drilling rate. As the clearance angle was maintained at 7 degrees on all drills used in the test, changes in helix angle result in equal changes in lip angle at the periphery of the drill, as follows: Helix Angle, Lip Angle, Degrees Degrees 15....................... ........... 68 26.................................. 57 28.................................. 55 32.............................. ... . 51 35.................................. 48 40 ................................. 43 45....................... ........... 38 Thus an increase in helix angle from 15 to 45 degrees, a total of 30 degrees, is accompanied by a decrease of 30 degrees in the lip angle at the periphery of the drill. Larger helix angles mean smaller lip angles and correspondingly sharper cutting edges. This explains why the work of removing the chip decreases as the helix angle is increased. At a feed of 0.0629 in. per rev. the torque decreases from 102.3 ft. lb. for a helix angle of 15 degrees, to 76.2 ft. lb. for a helix angle of 45 degrees, a decrease of 25.5 per cent. Drills with the traditional helix angle of 26 degrees require 18.5 per cent greater torque than those with a 45-degree helix angle at this drilling rate in gray cast iron. A comparison of the torque of test drills at various feeds is shown in Fig. 11. If power consumption were the only factor to be considered there would be no reason for making drills with helix angles less than 45 degrees, even for use in gray cast iron. Lip angles of cutting tools for gray cast iron usually are much greater than those found in similar tools for cutting steel, since a blunter form is necessary to prevent crumbling of the cutting edge. Gray cast iron is of a brittle structure and in cutting it breaks off in small particles, throwing the pressure of the chip close to the cutting edge. This condition, coupled with the higher cutting speeds used, makes it necessary to provide tools with blunter lip angles for gray cast iron than for steel. Tradition, therefore, is against small lip angles for cutting gray cast iron, and doubtless this is largely responsible for the practice of making high speed twist drills with large lip angles, although such lip angles are a relic of the age of carbon steel. Taylor's recommendation to use "the keenest cutting angle which is free from danger of spalling" for lathe tools unquestionably AN INVESTIGATION OF TWIST DRILLS FIG. 13. CHIPS FROM DRILLS WITH 26- AND 35-DEGREE HELIX ANGLES, IN GRAY CAST IRON is applicable to twist drills also. That the action of the two tools is dissimilar should not cause confusion, as the ultimate functions of both are identical, i.e., reduction of mass by removing a chip of definite dimensions at a predetermined rate In practice, the chips from drills of different helix angles tell much about the action of the cutting lip, and indirectly show why less torque is necessary to remove chips with drills of larger helix angles. A typical collection of gray cast iron chips from drills with 26-degree and 35-degree helix angles, respectively, operating in the same block and at the same drilling rate, is shown in Fig. 13. From the 26-degree angle drill the chips are spl:nters of metal broken or pushed off the block by the blunt cutting edge. In sharp contrast are the shavings of iron cut from the block by the 35-degree angle drill with its more acute lip angle. This practical evidence of the superior cutting qualities of drills with lesser lip angles serves to confirm the results of the tests with the dyna- mometer which already have been discussed. The effect of flute contours on torque has not yet been investigated but there is some evidence at hand to support the belief that it is meas- urable, and is a subject worthy of further attention. In the initial in- 35~ e2'~/M e60 Dr/// 35£ ril/ 26" Ori/ll ILLINOIS ENGINEERING EXPERIMENT STATION FIG. 14. COMPARISON OF DRILLS OF DIFFERENT DESIGN vestigation* the lowest values of torque in drilling gray cast iron were found with the 35-degree helix angle drill, slightly higher readings being registered by drills of 40- and 45-degree helix angles. These results were not duplicated in the present test using drills of the same helix angles but with considerably larger flutes and thinner chisel edges. At all drilling rates in both gray cast iron and steel the torque readings of the 40- and 45-degree helix angle drills invariably were lower than those of the 35-degree helix angle drill. Contrasts in the design and dimensions of the particular drills mentioned are shown clearly in Fig. 14. Drills of the present investigation are marked A and A', those of the initial investigation are designated by the letters B and B'. The conclusion seems reasonable, although not yet definitely supported by test results, that the larger flutes serve to reduce torque by improving the cutting action of the drill. Thrust on the end of the drill is influenced by a number of factors including helix angle, width of chisel edge (web thickness), point angle, and clearance angle. As the chisel edge operates with a scraping and not a cutting action, it virtually crushes a cylinder of metal having a diameter equal to its width and a length corresponding to the depth of the hole. Approximately from 60 to 70 per cent of the thrust on the drill of usual design is set up by the chisel edge.t If this edge is of unnecessary width, *See "An Investigation of Twist Drills," Univ. of Ill. Eng. Exp. Sta. Bul. 103. tSee "An Investigation of Twist Drills," Univ. of Ill. Eng. Exp. Sta. Bul. 103, p. 58. AN INVESTIGATION OF TWIST DRILLS /-e// z r'4g./e /7 D ,egr-ees FIG. 15. POWER CONSUMPTION AT THE DRILL POINT OF DRILLS WITH VARIOUS HELIX ANGLES, IN GRAY CAST IRON excessive thrust results, and an abnormal load is thrown upon the drill and the drilling machine. Helix angle has a marked effect on thrust in drilling gray cast iron, as indicated by the test results shown in Table 7, and in Fig. 12. Referring to the latter, it is observed that the thrust decreases progressively as the helix angle is increased. Obviously, the decrease is most pronounced at the heaviest drilling rate. At a feed of 0.0629 in. per rev. there is a decrease in thrust from 5225 pounds with the 15-degree helix angle drill to 3278 pounds with the 45-degree helix angle drill, a total of 1947 pounds, or 37.2 per cent. Reduction in thrust with the 35-degree helix angle drill over that with the 26-degree helix angle drill is approximately 12 per cent. By a logical combination of helix angle and web thickness it is possible to reduce thrust to a mini- mum, and thus improve the cutting qualities of the drill. ILLINOIS ENGINEERING EXPERIMENT STATION TABLE 8 POWER CONSUMPTION OF DRILLS WITH VARIOUS HELIX ANGLES Drilling Gray Cast Iron Total H. P. at Drill Point Helix Angle, deg. Feed, in. per rev. 0.0165 0.0256 0.0399 0.0629 15 2.40 3.26 5.10 7.79 26 2.25 2.98 4.35 6.89 32 2.15 2.73 4.17 6.41 35 2.10 2.64 4.01 6.12 40 1.99 2.46 3.99 6.01 45 1.91 2.40 3.80 5.77 H.P. Consumption due to Torque 15 2.38 3.21 4.95 7.47 26 2.23 2.94 4.24 6.60 32 2.13 2.69 4.06 6.14 35 2.09 2.60 3.91 5.86 40 1.98 2.43 3.90 5.78 45 1.90 2.37 3.73 5.57 H.P. Consumption due to Thrust 15 0.018 0.051 0.145 0.320 26 0.019 0.042 0.113 0.290 32 0.016 0.040 0.110 0.270 35 0.016 0.035 0.102 0.260 40 0.015 0.031 0.091 0.230 45 0.013 0.029 0.071 0.200 For practical purposes, the power consumption at the drill point when drilling gray cast iron or steel may be computed from the torque alone, since the power required to feed the drill is but a small portion of the total power. At a feed of 0.0629 in. per rev., 3.4 per cent of the total power used is due to thrust with the 45-degree helix angle drill, while at a feed of 0.0165 in. per rev. with the same drill less than 0.7 per cent of the power used was accounted for by thrust. For the 26-degree helix angle drill the power loss due to thrust at these drilling rates is 4.2 per cent and 0.84 per cent respectively. The relative power consumption of different drills at various feeds, in gray cast iron, is shown graphically in Fig. 15, and the corresponding figures are presented in Table 8. At all the drilling rates employed in the test on gray cast iron, power con- sumption at the drill point decreased as the helix angle was increased from 15 to 45 degrees, confirming the conclusion that twist drills with smaller lip angles require less power to remove the chip, as is the case with other metal cutting tools also. AN INVESTIGATION OF TWIST DRILLS FIG. 16. TORQUE OF DRILLS WITH VARIOUS HELIX ANGLES IN ENDURANCE TEST, IN GRAY CAST IRON f ILLINOIS ENGINEERING EXPERIMENT STATION Tota/ Deptf Dr///ed in /nches FIG. 17. THRUST OF DRILLS WITH VARIOUS HELIX ANGLES IN ENDURANCE TEST, IN GRAY CAST IRON AN INVESTIGATION OF TWIST DRILLS TABLE 9 RELATIVE ENDURANCE OF DRILLS WITH VARIOUS HELIX ANGLES Drilling Gray Cast Iron Speed, 500 r.p.m. Feed 0.0623 in. per rev. Per cent Increase at Depth Helix Angle Total Depth Indicated deg. Drilled, in. In Torque In Thrust 15............... 135 15.0 18.5 26............... 123 18.3 7.5 28................ 135 21.2 14.3 32............... 135 19.2 9.4 35............... 137 40.0 21.3 40............... 21 3.9 1.0 45............... 138 15.4 6.4 12. Endurance Test in Gray Cast Iron.-The relative endurance of drills with helix angles of 15, 26, 28, 32, 35, 40, and 45 degrees, respective- ly, was determined by driving each of the drills to destruction under identical operating conditions. All drills were ground by machine ac- cording to the specifications in Table 3. A drilling rate in excess of that used in customary practice was used to accentuate, if possible, differences in the various drills. This rate was: spindle speed, 500 r.p.m., or 131 ft. per min. at the drill periphery; feed, 0.0623 in. per rev., or a penetration of 31 in. per min. The outside corners of the drills were not rounded to secure maximum endurance, as the test was planned to reveal the rela- tive stamina of drills having widely diverging lip angles, and it was be- lieved that this aim would be furthered by allowing the most vulnerable point in the cutting edge to remain unchanged. Although the test was continued to absolute destruction, the factor of endurance in drilling gray cast iron was measured in terms of depth drilled before sudden in- creases in torque and thrust indicated the beginning of the final break- down in the cutting edge. The results of this test are shown graphically in Figs. 16 and 17. It is apparent from the curves in these figures that the cutting edge of a high speed drill is subject to a small but progressive deterioration for a certain period of service, and that eventually a criti- cal condition develops which leads to rapid and complete failure. This critical point for each of the drills is shown clearly by the breaks in the curves of torque and thrust in Figs. 16 and 17. Neglecting the eccentric behavior of the 40-degree helix angle drill, due probably to uncontrolled test conditions, the results show that the endurance of drills in gray cast iron is not influenced materially by the factor of helix angle. The ILLINOIS ENGINEERING EXPERIMENT STATION TABLE 10 RELATIVE PERFORMANCE OF 26- AND 45-DEGREE HELIX ANGLE DRILLS Drilling Gray Cast Iron Speed 500 r.p.m. Feed 0.0623 in. per rev. (1) (2) Difference Factor Helix Angle Helix Angle between 26-deg. 45-deg. (2) and (1) Lip angle, deg.................................. 57.0 38.0 -19.0 Inches drilled.................................. 123.0 138.0 + 15.0 Torque, at start, ft-lb.................... ........ 82.0 65.0 -17.0 Torque, at close, ft-lb........: .................. . 97.0 75.0 -22.0 Increase in torque, per cent .................. .. 18.3 15.4 - 2.9 Thrust, at start, lb. ....... .................... 4000.0 3100.0 -900.0 Thrust, at close, lb............................. 4300.0 3300.0 -1000.0 Increase in thrust, per cent....................... 7.5 6.4 - 1.1 number of inches drilled by each test drill before the critical period of the cutting developed is given in Table 9. An examination of these figures reveals the interesting fact that the endurance of the 45-degree helix angle drill with a lip angle of 38 degrees was appreciably greater than that of the 26-degree helix angle drill with a lip angle of 57 degrees. As the latter drill represents traditional practice in drill manufacture, the large lip angle being considered necessary to insure endurance, it is instructive to compare in some detail its performance with that of a drill having a more acute lip angle, as shown in Table 10. The conclusion is obvious from the results therein presented that in high speed twist drills lip angle is not the only factor affecting endurance when drilling gray cast iron. Reduction in the lip angle reduces the torque and thrust required for drilling, indicating freer cutting action and less heat generated in removal of the chip. Apparently the heat reduction from this cause is considerable as otherwise the endurance of the high-angle twist drills is not explainable. The appearance of the test drills at the close of the endurance test in gray cast iron is shown in Fig. 18. It will be observed that while the cutting edges of all the drills are intact, although dull, the sharp corners. at the periphery are badly burned. After the critical condition at the sharp corner is reached the high-angle twist drills break down more rapidly at a heavy drilling rate than those with smaller helix angles. At the critical point the former are no longer cutting freely at the peri- phery, and they break down rapidly at the corner from overheating. Since the sharp corner is the weakest portion of the cutting edge, en- durance may be increased by rounding it on the grinder to a definite curve. AN INVESTIGATION OF TWIST DRILLS FIG. 18. CONDITION OF TEST DRILLS AT CLOSE OF ENDURANCE TEST IN GRAY CAST IRON The excellence of the modern high speed twist drill was demon- strated during the endurance test in gray cast iron. Through mis- adjustment of the test block when drilling with the 45-degree helix angle drill, a "run-in" hole was drilled as shown in Fig. 19. The drill broke close to the shank at a depth of 1% in., the point being 716 in. out of line with the center of the spindle. As a twist drill is not designed to resist large stresses resulting from cross bending, this minor occurrence in the investigation illustrates in a striking manner the unusual strength of high speed steel when heat treated to secure toughness and hardness. FIG. 19. SECTION OF GRAY CAST IRON TEST BLOCK SHOWING RUN-IN HOLE ILLINOIS ENGINEERING EXPERIMENT STATION 13. Summary of Results of Tests in Gray Cast Iron.-The following observations were made during the tests: (1) Torque and thrust decreased progressively as the helix angle was increased from 15 degrees to 45 degrees; both were lowest when the helix angle was 45 degrees. (2) At the highest rate of feed used, 0.0629 in. per rev., increasing the helix angle from 15 to 45 degrees resulted in reducing the torque by 25.5 per cent, and the thrust by 37.2 per cent. (3) Thrust was influenced most by helix angle, and by the thick- ness of the web. Test drills with web thickness of 0.1 in., constant from point to shank, gave entire satisfaction at all speeds and feeds. (4) Chip removal when drilling holes 5 in. deep was effective with drills having constant helix angles from point to shank. (5) Savings in power consumption at the drill point by increasing the helix angle from 15 to 45 degrees ranged from 2 to 26 per cent, at drilling rates of 0.0165 and 0.0629 in. per rev. respectively. (6) From 95 to 99 per cent of the power consumption at the drill point was due to torque, the remainder resulting from thrust. (7) Drill endurance was not influenced by changes in helix angle from 26 to 45 degrees, inclusive, at normal drilling rates. 14. Helix Angle Test in Steel.-This test was planned to determine the relative torque and thrust of drills having helix angles of 15, 26, 28, 32, 35, 40, and 45 degrees respectively, when drilling at rates ordinarily used in practice, in typical carbon and alloy steels used in industry. The test procedure was the same as employed in the corresponding test in gray cast iron except that the drills were cooled by a heavy stream of water. A series of five holes was drilled in each of the steels listed in Table 2, at a speed of 300 r.p.m. and at feeds of 0.0131, 0.0206, and 0.0320 in. per rev. Each set of readings at a given feed was averaged and the resulting figure used as a typical value of torque and thrust at that feed. The results are presented in Table 11, and are shown graphically in Figs. 20 and 21. From a comparison of the torque curves for gray cast iron (Fig. 11) and for steel (Fig. 20) it is obvious that the effect of helix angle on torque is, in general, the same when drilling either of these metals. Increasing helix angles from 15 to 45 degrees serves to reduce the torque required for drilling either metal at all the drilling rates employed in the test. An equivalent reduction in torque is observed for gray cast iron and chromium nickel steel (S.A.E. 3130), although the actual torque values AN INVESTIGATION OF TWIST DRILLS TABLE 11 TORQUE AND THRUST OF DRILLS WITH VARIOUS HELIX ANGLES Drilling Various Steels Grades of Steel Helix Feed, Angle in. per deg. rev. S.A.E. S.A.E. S.A.E. S.A.E. S.A.E. S.A.E. U.M.A. 1025 1045 2315 2345 3130 6145 5 Torque in Foot-Pounds 15 0.0131 66.2 54.8 63.6 62.2 53.4 60.4 62.8 0.0206 99.0 81.2 110.2 99.4 78.8 96.8 99.6 0.0320 110.0 125.0 . .. .... .. ... .. 26 0.0131 51.2 48.6 52.4 53.0 48.4 54.8 54.4 0 0206 78.4 73.2 77.0 86.2 72.8 75.6 80.6 0.0320 112.4 103.0 139.8 141.6 103.6 120.6 140.0 28 0.0131 52.0 50.6 51.0 54.8 48.6 54.8 57.8 0.0206 75.6 75.0 77.6 83.2 73.2 74.6 80.0 0.0320 106.8 99.8 126.6 137.0 105.6 111.0 136.5 32 0.0131 48.4 45.6 47.4 51.4 46.4 49.6 51.4 0.0206 69.2 68.8 68.6 78.4 68.8 73.4 75.0 0.0320 98.4 97.4 107.4 129.2 99.8 104.4 122.4 35 0.0131 44.6 43.6 46.0 48.2 43.8 48.8 49.0 0.0206 64.8 69.4 67.4 74.0 67.4 70.8 76.2 0.0320 98.0 97.0 103.2 122.0 98.6 104.6 117.6 40 0.0131 41.8 42.6 43.4 47.6 42.4 48.0 49.2 0.0206 60.8 67.4 67.0 72.8 62.4 70.2 73.8 0.0320 93.8 94.4 96.2 109.6 95.4 101.4 111.2 45 0.0131 41.0 45.8 43.2 50.8 44.8 48.8 48.2 0.0206 56.6 65.8 67.0 73.4 61.6 68.0 71.4 0.0320 89.2 94.4 95.6 109.4 92.4 102.4 106.0 Thrust in Pounds 15 0.0131 1950 1738 1908 1974 1740 1918 1964 0.0206 2590 2430 3300 5875 3100 5510 5890 0.0320 5475 4420 .... .... .. .... .... 26 0.0131 1628 1778 1716 1910 1834 2038 2084 0.0206 2460 2600 2640 3970 2480 2910 3020 0.0320 3620 3770 5080 6480 3500 4320 5500 28 0.0131 1638 1776 1744 1888 1758 1930 2030 0.0206 2390 2600 2500 3920 2570 2730 3000 0.0320 3370 3520 4540 5990 3400 4400 5600 32 0.0131 1598 1676 1656 1914 1696 1896 1956 0.0206 2110 2370 2270 3230 2240 2590 2760 0.0320 3040 3530 3860 5690 3290 4060 4880 35 0.0131 1486 1610 1518 1794 1518 1794 1894 0.0206 1950 2280 2130 3220 2180 2520 2610 0.0320 3220 3250 3590 4950 3150 3980 4560 40 0.0131 1354 1490 1404 1684 1496 1660 1704 0.0206 1790 2210 1980 2810 1980 2340 2440 0.0320 2560 3050 3080 4480 2920 3550 4030 45 0.0131 1180 1350 1290 1686 1336 1556 1590 0.0206 1640 1840 1650 2330 1700 2060 2110 0.0320 2190 2710 2700 3800 2760 3310 3510 42 ILLINOIS ENGINEERING EXPERIMENT STATION r12 0 0 0 0 H a aT 8 rn o c a ffi 6 0 E 5 Q- 6, 0 w AN INVESTIGATION OF TWIST DRILLS k _ r Lj_ spunc7 uq/ 41nU/t/l rpunod ue/ll n,9/-/I ILLINOIS ENGINEERING EXPERIMENT STATION are not the same. For the remaining steels the reduction in torque due to increasing the helix angle through the range indicated is much greater than in the case of gray cast iron or chromium nickel steel. It was assumed at the beginning of the test, in the absence of exact information on the subject, that increasing the helix angle would result in reductions in torque for the steels used, approximately in proportion to their hardness. The correctness of this assumption was not confirmed by the test. On the contrary, it was found that the effect on torque of increasing the helix angle was not uniform for steels of different chemical composition, and, further, that there appeared to be no definite relation between hardness and torque. Proof of the first state- ment is seen in the widely divergent torque curves in Fig. 20. For the purpose of a quantitative comparison, the per cent reduction in torque resulting from an increase in the helix angle from 26 to 45 degrees for each of the test steels is tabulated below (feed 0.0320 in. per rev., speed 300 r.p.m.): Steel, grade Per cent Reduction Low-carbon.................. S.A.E. 1025........... 20.7 Medium-carbon ................. S.A.E. 1045........... 8.3 Low-carbon Nickel............. S.A.E. 2315........... 31.6 Medium-carbon Nickel........... S.A.E. 2345............ 22.8 Chromium Nickel .............. S.A.E. 3130........... 10.8 Chromium Vanadium .......... S.A.E. 6145............ 15.1 Chromium ................... U.M .A. 5........... 24.3 It is obvious from these values that increasing the helix angle of the drill lowers the torque of drilling in all of the steels employed in the test, but the amount of the reduction is influenced by the factor of machine- ability.* Thus the medium carbon steel with which the minimum re- duction in torque (8.3 per cent) from an increase in helix angle from 26 to 45 degrees occurs, has a machineability corresponding to a drill torque of 103.0 ft. lb., while the low-carbon nickel steel with which the maxi- mum reduction in torque (31.6 per cent) occurs has a machineability corresponding to a torque of 139.8 ft. lb. At a corresponding drilling rate the reduction in torque from a change in helix angle from 26 to 45 degrees is 15.3 per cent in drilling gray cast iron, the machineability of which corresponds to a torque of 49.0 ft. lb. As lip angles invariably are less in tools for cutting steel than in those for cutting gray cast iron it is interesting to observe that increasing the helix angle produces a smaller reduction in the torque required for drilling steels of low machine- ability than in that required for drilling medium gray cast iron. In steels *See defnition of machineability in Section 16, p. 50. AN INVESTIGATION OF TWIST DRILLS Feed /i~ /nches per l 'eva/le//on FIG. 22. VARIATION OF TORQUE AND THRUST WITH FEED, FOR DRILLS WITH VARIOUS HELIX ANGLES, IN TWO GRADES OF STEEL ILLINOIS ENGINEERING EXPERIMENT STATION of high machineability the effect on torque of increasing the helix angle is shown by the tabulation above. While these results are definitely established by the tests, no explanation of them can be advanced in the absence of exact information relating to the action of cutting tools on the structure of metals. Considering the universal practice of making certain cutting tools with lesser lip angles for steels than for gray cast iron it may be logical to assume that the twist drill should be so constructed. The tests do not support this assumption; instead, they go to prove that twist drills with helix angles of 32 degrees or more are equally effective in both types of metal. Smaller helix angles than this produce unnecessarily large torque in drilling. A comparison of the torque of the test drills at various feeds in medium-carbon steel and in low-carbon nickel steel is presented in Fig. 22. Thrust on the end of the drill is a factor which has little effect on power consumption, and apparently, from the data available, it exer- cises small influence on drill endurance. It can not be stated, however, that thrust is not responsible, under some conditions, for break down of the cutting edge and also for unnecessary drill breakage. At heavy feeds the total thrust carried by the drilling machine may result in mis- alignment of shafts, overheating of bearings, and breakage of parts. It is only within recent years that the drilling machine has been made stiff enough to resist thrust without distortion of the column and suffi- ciently powerful to drive high speed drills to capacity. With large drills and heavy feeds the thrust developed by the drill may be over 10 000 lb.; in the tests herein reported the largest thrust recorded was 8750 lb. for the 26-degree helix angle drill at a feed of 0.0320 in. per rev., and a speed of 480 r.p.m., in medium-carbon steel. Helix angle also exercises a pronounced effect on thrust in drilling steel as indicated by the values given in Table 11, and shown graphically in Fig. 21. Referring to the latter it is observed that the thrust de- creases progressively as the helix angle is increased, neglecting a number of unexplained values obtained from the 15-degree helix angle drill at lower feeds. At the highest feed (0.0320 in. per rev.) the effect on thrust of increasing the helix angle is greatest, but it varies considerably in the steels tested as is shown by the thrust curves in Fig. 22. The com- parative effect of helix angle on thrust in drilling various steels is revealed by the following tabulation of the per cent reduction in thrust resulting from an increase in the helix angle from 26 to 45 degrees (feed 0.0320 in. per rev., speed 300 r.p.m.): AN INVESTIGATION OF TWIST DRILLS Per Cent Steel, grade Reduction Low-carbon....................S.A.E. 1025.......... 39.5 Medium-carbon.................S.A.E. 1045........... 28.1 Low-carbon Nickel............ S.A.E. 2315............ 46.8 Medium-carbon Nickel......... .S.A.E. 2345............ 41.3 Chromium Nickel ..............S.A.E. 3130............ 21.1 Chromium Vanadium ...........S.A.E. 6145........... 23.4 Chromium .....................U.M .A. 5........... 36.2 It will be observed that increasing the helix angle from 26 to 45 degrees reduces thrust in drilling for all of the test steels by large but varying amounts. For the low-carbon nickel steel (Brinell hardness 134) the reduction is from 5080 lb. to 2700 lb. or 46.8 per cent. The minimum reduction (21.2 per cent) occurs with the chromium nickel steel having a Brinell hardness of 163. The chromium steel (U. M. A. 5) with a Brinell hardness of 202 shows a reduction of 36.2 per cent. At a corresponding drilling rate the reduction in thrust in medium gray cast iron (Brinell hardness 146) is 30.3 per cent. There is no apparent relation between the total thrust load induced by the drill and the hardness of the metal (Brinell number), and no recorded data to indicate that the factor of hardness affects the amount of reduction in thrust accomplished by in- creasing the helix angle of the drill. A comparison of the thrust at various feeds on the end of the test drills in two commonly used steels is shown in Fig. 22. Power consumption at the drill point in drilling steel, as in drilling gray cast iron, is influenced more by helix angle than by any other factor. As explained in Section 11, torque is an approximate measure of the power consumed by the drill, hence the torque values for drills of different helix angles indicate the relative power required to drive them. The actual power consumed at the drill point of the test drills at various feeds, in medium-carbon steel and in low-carbon nickel steel is shown in Table 12 and graphically in Fig. 23. These steels represent the ex- tremes in machineability (of those tested) and the power values given may be considered as typical for driving a 1-in. drill into steel at various drilling rates. At the three feeds employed in the test, power con- sumption at the drill point decreased as the helix angle was increased from 26 to 45 degrees in proportion to the amount of power required to drive the drill. Thus in the low-carbon nickel steel, which has a high ma- chineability factor,* the saving in power consumption by increasing the helix angle from 26 to 45 degrees was 31.8 per cent, while in the medium-carbon steel it was but 8.5 per cent. The latter steel has a low *See Section 16, p. 50. ILLINOIS ENGINEERING EXPERIMENT STATION TABLE 12 POWER CONSUMPTION OF DRILLS WITH VARIOUS HELIX ANGLES Drilling Two Different Steels Total H.P. at Drill Point Medium-carbon Steel Low-carbon Nickel Steel Helix Angle, S.A.E. No. 1045 S.A.E. No. 2315 deg. Feed, in. per rev. Feed, in. per rev. 0.0131 0.0206 0.0320 0.0131 0.0206 0.0320 15 3.16 4.67 7.25 3.65 6.34 26 2.90 4.21 5.97 3.01 4.44 8.10 28 2.91 4.32 5.78 2.93 4.47 7.34 32 2.62 3.97 5.63 2.73 3.96 6.23 35 2.51 4.00 5.62 2.65 3.88 5.99 40 2.44 3.88 5.46 2.49 3.85 5.58 45 2.63 3.79 5.46 2.47 3.85 5.52 machineability factor and requires 35.7 per cent less power (using the 26-degree angle drill) to drill at a feed of 0.0320 in. per rev. than the low-carbon nickel steel. It is obvious that increasing the helix angle from 26 to 45 degrees materially reduces the power consumption of the drill, but that the amount of the reduction varies with the machineability factor of the metal drilled. 15. Endurance Test in Steel.-This test was conducted in medium- carbon steel, S.A.E. 1045, to determine the relative endurance of drills with helix angles of 15, 26, 28, 32, 35, 40, and 45 degrees respectively. The test procedure was similar to that employed in the corresponding test with gray cast iron. All drills were ground alike by machine to the specifications in Table 3. The outside corners of drills were not rounded. The drilling rate was: speed 480 r.p.m., feed 0.0320 in. per rev. A heavy stream of water was directed into the drill hole from above. Drills were run to absolute destruction of the cutting edge under identical operating conditions. The total depth drilled and the values of torque and thrust at the beginning and end of the test are shown in Table 13. These results are presented graphically in Figs. 24 and 25. From a study of the graph it is apparent that in drilling medium-carbon steel at a cutting speed of 124 ft. per min. helix angle is not the deciding factor in drill endurance. It appears from the results of this particular drilling experiment that drills with helix angles above 35 degrees and below 28 degrees possess less endurance than those with helix angles between these limits. The conventional drill with a helix angle of 26 degrees and a correspondingly AN INVESTIGATION OF TWIST DRILLS /