1. Twist Drills
Unit 40

2. Objectives Identify the parts of a twist drill
Identify four systems of drill sizes and know where each is used
Grind the proper angles and clearances on a twist drill

3. Twist Drills End-cutting tools
Used to produce holes in most types of materials
Two helical grooves, or flutes, are cut lengthwise around body of drill
Provide cutting edges and space for cuttings to escape during drilling process

4. Twist Drill Parts Most made of high-speed steel Carbide-tipped drills
Replaced carbon-steel drills for two reasons
Can be operated at double the cutting speed
Cutting edge lasts longer
Stamped with letters H.S or H.S.S.
Carbide-tipped drills
Speeds for production have increased up to 300% over high-speed drills

5. Three Main Parts of a Drill
Point
Three Main Parts of a Drill
Body
Shank
A drill (Fig. 40-1 on p. 312) may be divided into three main parts: shank, body, and point.
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6. Shank Straight-shank drills Tapered-shank drills Held in drill chuck
Up to ½ in. in diameter
Tapered-shank drills
Fit into internal taper of drill press spindle
Tang provided on end to prevent drill from slipping
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7. Body Portion of drill between shank and point
Consists of number of parts for cutting
Flutes
Two or more helical grooves cut around body of drill
Form cutting edges, admit cutting fluid, allow chips to escape hole
Body Clearance
Undercut portion of body between margin and flutes
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8. Body, cont. Margin Web Narrow, raised section on body of drill
Next to flutes and extends entire length of flutes
Provides full size to drill body and cutting edges
Web
Thin partition in center of drill, extends full length of flutes
Forms chisel edge at cutting end of drill
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9. Point Consists of chisel edge, lips, lip clearance, and heel
Chisel-shaped portion of drill point
Lips
Cutting edges formed by intersection of flutes
Must be of equal length and have same angle
So drill run true and not cut hole larger than drill

10. Point
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11. Lip Clearance
Is the relief ground on point of drill extending from cutting lips back to the heel
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12. Drill Point Characteristics
The use of various point angles and lip clearances, in conjunction with thinning of the drill web, will allow:
Control size, quality and straightness of drilled hole
Control size, shape and formation of chip
Control chip flow up flutes

13. Increase strength of drill's cutting edges
Reduce rate of wear at cutting edges
Reduce amount of drilling pressure required
Control amount of burr produced
Reduce amount of heat generated
Permit use of various speeds and feeds for more efficient drilling

14. Conventional Point (118º)
Most commonly used drill point
Gives satisfactory results for most general-purpose drilling
Lip clearance of 8º to 12º for best results
Too much weakens cutting edge and causes drill to chip
Too little results in use of heavy drilling pressure
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15. Long Angle Point (60º to 90º)
Used on low helix drills for drilling of nonferrous metals, soft cast irons, plastics, fibers, and wood
Lip clearance generally from 12º to 15º
Flat may be ground on face of lips to prevent drill from drawing itself into the soft material
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16. Flat Angle Point (135º to 150º)
Used to drill hard and tough materials
Lip clearance on flat angle point drills only 6º to 8º to provide as much support as possible for cutting edges
Shorter cutting edge tends to reduce friction and heat during drilling
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17. Four Systems of Drill Sizes
Fractional
Range from 1/64 to 4 in. (steps of 1/64th )
Number
Range from #1 (.228 in.) to #97 (.0059 in.)
Letter
Range from A to Z (A = .234 in., Z = .413 in.)
Millimeter (Metric)
Miniature (0.04 to 0.09 mm, steps of 0.01 mm)
Straight-shank standard (0.5 to 20 mm)
Taper-shank (8 up to 80 mm)

18. Types of Drills
Wide variety manufactured to suit specific drilling operations and materials
Design of drills vary
Number and width of flutes
Amount of helix or rake angle of flutes
Shape of land or margin
Shape of flute: straight or helical
Whether helix is right-hand or left-hand

19. Twist Drills Manufactured from three main materials
Carbon-steel drills
Used in hobby shops not for machine shop work
Cutting edges wear down quickly
High-speed steel drills
Used in machine shop work
Cutting edges withstand more heat and wear
Cemented-carbide drills
Operated at high speeds, withstand higher heat, and can drill hard materials

20. General-Purpose Drill
Has two Helical flutes
Designed to perform well on wide variety of materials, equipment and job conditions
Can be made to suit different conditions and materials by varying point angle, speeds and feeds
Straight-shank drills called general-purpose jobbers length drills
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21. Low-Helix Drill Developed primarily to drill brass and thin materials
Used to drill shallow holes in some aluminum and magnesium alloys
Can remove large volume of chips formed by high rates of penetration
Used on turret lathes and screw machines

22. High-Helix Drills
Designed for drilling deep holes in aluminum, copper, die-cast material, and other metals
Material where chips have tendency to jam hole
High helix angle (35º to 45º)
Wider flutes assist in clearing chips from hole

23. Core Drill Three or four flutes
Used to enlarge cored, drilled, or punched holes
May be used instead of reamer for hole finishing
Produced in sizes from ¼ to 3 in. in diameter

24. Oil Hole Drills
Have one or two oil holes running from shank to cutting point
Compressed air, oil, or cutting fluid can be forced through when deep holes being drilled
Used on turret lathes and screw machines
Cutting fluid cools drill's cutting edges and flushes chips out of hole
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25. Straight-Fluted Drills
Recommended for drilling operations on soft materials such as brass, bronze, copper and various types of plastic
Straight flute prevents drill from drawing itself into material while cutting

26. Deep Hole (Gun) Drills
Used for producing holes from approximately 3/8 to 3 in. in diameter and as deep as 20 feet
Consists of round, tubular stem, on end is fastened flat, two-fluted drilling insert
Cutting fluid forced through center of stem to flush chips from hole
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27. Spade Drills Similar to gun drills Usually clamped in holder
Cutting end is flat blade with two cutting lips
Usually clamped in holder
Easily replaced or sharpened
Available in wide range of sizes
Micro to 12 in. dia.
Some have replaceable carbide inserts
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28. Hard-Steel Drill Used for drilling hardened steel
Made from heat-resistant alloy
As brought into contact with workpiece, fluted, triangular point softens metal by friction and then removes softened metal ahead in chip form
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29. Step Drills
Used to drill and countersink or drill and counterbore different sizes of holes in one operation
May have two or more diameters ground
Each size or step separated by square or angular shoulder
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30. Saw-Type Hole Cutter
Cylindrical-diameter cutter with twist drill in center to provide guide for cutting teeth on hole cutter
Made in various diameters
Used for drilling holes in thin materials
Little burr produced
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31. Drilling Facts and Problems
Excessive speed
Excessive clearance
Excessive feed
Insufficient clearance
Cutting lips with unequal angles
Cutting lips with unequal in length
Loading and galling
Examples of each on following slides

32. Excessive speed will cause wear at outer corners of drill
Excessive speed will cause wear at outer corners of drill. This permits fewer regrinds of drill due to amount of stock to be removed in reconditioning. Discoloration is warning sign of excess speed.
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33. Excessive clearance results in lack of support behind cutting edge with quick dulling and poor tool life. Despite initial free cutting action. Clearance angle behind cutting lip for general purposes is 8º to 12º.
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34. Excessive feed sets up abnormal end thrust, which causes breakdown of chisel point and cutting lips. Failure induced by this cause will be broken or split drill.
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35. Insufficient clearance causes the drill to rub behind the cutting edge
Insufficient clearance causes the drill to rub behind the cutting edge. It will make the drill work hard, generate heat, and increase end thrust. This results in poor holes and drill breakage.
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36. The web is the tapered central portion of the body that joins the lands.
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37. Cutting lips with unequal angles will cause one cutting edge to work harder than the other. This causes torsion strain, bellmouth holes, rapid dulling, and poor tool life.
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38. Cutting lips unequal in length cause chisel point to be off center axis and will drill holes oversize by approximately twice the amount of eccentricity.
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39. Loading and galling is caused by poor chip removal with insufficient dissipation of heat so that material anneals itself to the cutting edge and flute. This condition frequently results from using wrong drills for the job or inadequate cutting fluid application.
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40. Characteristics of a Properly Ground Drill
Length of both cutting lips equal
Angle of both cutting lips be the same
Lips should be free from nicks or wear
No sign of wear on margin
Note: Resharpen drill if it does not meet all of these requirements.

41. Conditions That Indicate Drill Be Examined and Reground
Color and shape of chips change
More drilling pressure required
Drill turns blue because of excessive heat
Top of hole out of round
Poor finish produced in hole
Drill chatters when it contacts metal
Drill squeals and may jam in hole
Excessive burr left around drilled hole

42. Factors Causing Premature Dulling of Drill
Drill speed may be too high for hardness of material being cut
Feed may be too heavy and overload cutting lips
Feed may be too light and cause lips to scrape rather than cut
May be hard spots or scale on work surface

43. More Factors
Work or drill may not be supported properly, resulting in springing and chatter
Drill point may be incorrect for material being drilled
Finish on lips may be poor

44. Procedure to Grind a Drill
Wear approved safety glasses
Check grinding wheel and dress it to sharpen and/or straighten wheel face
Adjust grinder tool rest so it is within in. of wheel face
Examine drill point and margins for wear

45. Hold drill near point with one hand, other hand hold shank of drill slightly lower than point
Move drill so it is approximately 59º to face of grinding wheel
Hold lip or cutting edge of drill parallel to grinder toolrest
Bring lip of drill against grinding wheel and slowly lower drill shank

46. Check angle of drill point and length of lips with drill point gage
Remove drill from wheel without moving position of body or hands, rotate drill one-half turn, and grind the other cutting edge
Check angle of drill point and length of lips with drill point gage
Repeat operations 6-10 until cutting edges are sharp and lands are free from wear nicks
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47. Web Thinning
Webs gradually increase in thickness toward shank to give drill strength
As drill shortens, web becomes thicker and more pressure required to cut
Resultant heat reduced by thinning web
Webs thinned using
special web-thinning grinder, tool and cutter grinder, or freehand on conventional grinder
Grind equal amounts off each edge