Drilling Machines

38-1

Drilling Machines

• Probably first mechanical device developed • Principle of rotating tool to make hole • One of most common and useful machines in industry • Come in several types and sizes – From hand-fed to computer-controlled 38-2

Drilling Press

• Drilling machine – Spindle • Turns drill to advance into work (hand or automatically) – Work table • Holds workpiece rigidly in place as hole drilled – Used primarily to produce holes in metal – Other operations: tapping, reaming, boring, counterboring, countersinking, spot-facing 38-3

Standard Operations

• Drilling – Operation of producing hole by removing metal from solid mass using twist drill • Countersinking – Operation of producing tapered or cone-shaped enlargement to end of hole

Standard Operations

• Reaming – Operation of sizing and producing smooth, round hole from previously drilled or bored hole

• Boring • Truing and enlarging hole by means of single-point cutting tool

Standard Operations

• Tapping – Cutting internal threads in hole with cutting tool called tap • Counterboring – Enlarging top of previously drilled hole to given depth to provide square shoulder for head of bolt or capscrew

Standard Operations

• Spot Facing • Smoothing and squaring surface around hole to provide seat for head of cap screw or nut – Boring bar fitted with double-edged cutting tool • Pilot section on end to fit into existing hole

• Wide variety of drill presses • Size of drill press may be designated in different ways by different companies – Some state size as distance from center of spindle to column of machine – Others state size by diameter of largest circular piece that can be drilled in center 38-8 Sensitive Drill Press Parts • Only hand feed mechanism – Control downfeed pressure • Manufactured in bench and floor model • Four main parts – Base, column, table and drilling head

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 40-11

Twist Drill Parts

• Most made of high-speed steel – 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 40-12 Three Main Parts of a Drill

Point

Body

Shank

• Straight-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

• 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

• Margin – 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

Point

• Is the relief ground on point of drill extending from cutting lips back to the heel

• 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) 40-19

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 40-20

1.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

40-21 2.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

3.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

4.Step Drills

• Used to drill and countersink or drill and counter bore different sizes of holes in one operation • May have two or more diameters ground • Each size or step separated by square or angular shoulder

5.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

Drilling Facts and Problems

• Excessive speed • Excessive clearance Examples of each • Excessive feed on following slides • Insufficient clearance • Cutting lips with unequal angles • Cutting lips with unequal in length • Loading and galling 40-26

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.

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.

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.

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.

The web is the tapered central portion of the body that joins the lands.

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.

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.

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.

Cutting Speeds and Feeds

• Two important factors – Diameter and material of cutting tool – Type of material being cut • Speed of twist drill referred to as cutting speed, surface speed or peripheral speed – Distance point on circumference of drill will travel in 1 min

41-35

Recommended Cutting Speeds

For every job, choose the drill speed that will result in the best production rates!

Steel Casting Tool Steel Drill Size Cutting Speeds in Feet per Min or Meters per Min in mm 40 ft/min 12 m/min 60 ft/min 18 m/min … 1/16 2 2445 1910 3665 2865 1/8 3 1220 1275 1835 1910 3/16 4 815 955 1220 1430

Portion of Table 41.1 from text 41-36 Economical Drilling Speed Variables

• Type and hardness of material Most important! • Diameter and material of drill • Depth of hole • Type and condition of drill press • Efficiency of cutting fluid employed • Accuracy and quality of hole required • Rigidity of work setup 41-37

Revolutions per Minute

• Compute correct number of r/min of drill press spindle for given size drill – Type of material to be drilled – Recommended cutting speed of material – Type of material from which drill is made 41-38 Formula (Inch)

CS(feet per minute)x12 r / min   D(drill circumference in inches)

where CS = recommended cutting speed in feet per minute for the material being drilled D = diameter of drill being used

Revolution per minute = number of revolutions of the drill necessary to attain proper cutting speed for metal being machined. 41-39

Simplified Formula

• Since not all machines can be set to exact calculated speed, pi (µ) divided into 12 to simplify formula CS x 4 r / min  D Example: Calculate r/min required to drill a ½ in hole in cast iron (CS 80) with a high-speed steel drill. 80 x 4 320 r / min    640 1/ 2 1/ 2 41-40

Feed

• Distance drill advances into work for each revolution • May be expressed in decimals, fractions of an inch, or millimeters • Three factors govern rate of feed – Diameter of drill – Material of workpiece – Condition of drilling machine 41-41 Drill Feeds

General –purpose Work Drill Size Feed per Revolution in. mm in. mm 1⁄8 and smaller3 and smaller.001 to .002 0.02 to 0.05 1⁄8 to ¼ 3 to 6 .002 to .004 0.05 to 0.1 ¼ to ½ 6 to 13 .004 to .007 0.1 to 0.18 ½ to 1 13 to 25 .007 to .015 0.18 to 0.38 1 to 1 ½ 25 to 38 .015 to .025 0.38 to 0.63 Table 41.2 Drill feeds 41-42

Drill Feeds

• General rule: feed rate increases as drill size increases – Too coarse – chip cutting edges – Too light – chattering noise, dulls cutting edge • Hard steels or alloys use slower feed • Softer metals drilled with faster feed • Blue steel chips indicate too much heat at cutting edge – Dull cutting edge or too high speed 41-43 Cutting Fluids

• Provide both cooling and lubrication • Properties of an effective liquid in dissipating heat – Able to absorb heat rapidly – Have good resistance to evaporation – Have high thermal conductivity

Oil: good lubricant, poor coolant Water: best coolant, no lubricating value (promotes rust) 42-44

Lathe Center Holes

• Use a combination drill and countersink – Commonly called center drill • Must be drilled to correct size and depth – Too shallow or deep, poor support for work

• Chisel end on drill wider than center-punch mark on work – Spot center-punch mark with center drill

Small point on center drill will accurately follow center-punch mark and provide guide for larger drill

Spotting Hole

Drilling to an Accurate Layout

1. Clean and coat surface with layout dye 2. Locate position of hole from two machined edges of workpiece and scribe lines 3. Lightly prick-punch where two lines intersect 4. Check accuracy of punch mark

5. Scribe circle to indicate diameter of hole 6. Scribe test circle .060 in. smaller than hole 7. Punch four witness marks on circles up to .750 in. in diameter and eight witness marks on larger circles 8. Deepen center of hole location with center punch to provide larger indentation for drill to follow

9. Center drill work to just beyond depth of drill point 10. Mount proper size drill in machine and drill hole to depth equal to one-half to two- thirds drill diameter 11. Examine drill indentation; should be concentric with inner proof circle

12. If spotting off center, cut shallow V-grooves with cape or diamond-point chisel on side toward which drill must be moved 13. Start drill in spotted and grooved hole • Drill will be drawn toward direction of grooves 14. Continue cutting grooves into spotted hole until drill point drawn to center 15. Continue to drill hole to desired depth

Drilling Large Holes

• Drills increase in size; thickness of web also increases to give drill strength – Thicker web, thicker point of drill – Thick web not follow center-punch mark easily • Two methods to overcome poor cutting action of thick web on large drills – Web is thinned – Lead, or pilot, hole is drilled 43-52

Reamers

• Rotary cutting tool with several straight or helical cutting edges along body • Used to accurately size and finish hole previously formed by drilling • Two classifications – Hand – Machine 43-53 Reamer Parts

Three main parts 1. Shank 2. Body 3. Angle of chamfer

• Finishing tools • Holes bored to .003-.005 in. • Square on shank for wrench • Teeth on end tapered so can enter hole easily • Never turn backwards • Taper hand reamers – Remove frequently to clean

Machine Reamers

• Used in any machine tool for both roughing and finishing hole • Called chucking reamers for holding method • Wide variety of types and styles – Rose reamers – Fluted reamers – Carbide-tipped reamers 43-56

Fluted Reamers

• Have more teeth than rose reamers for comparable diameter • Lands relieved for entire length • Fluted reamers cut along side as well as at chamfer on end • Considered finishing tools and used to bring hole to size

Carbide-tipped Reamers

• Similar to rose or fluted reamers, except carbide tips been brazed to cutting edges – Resist abrasion and maintain sharp cutting edges even in high temperatures • Outlast high-speed steel reamers • Can run at higher speeds and still maintain their size

Shell Reamers

• Reamer heads mounted on driving arbor • Shank of driving arbor may be straight or tapered • Two slots in end of reamer fit into lugs on driving arbor • Sometimes locking screw in arbor

• Adjustable reamers – Have inserted blades that can be adjusted approximately at .015 in over or under nominal reamer size • Adjusting nuts on either end • Emergency reamers – Drills whose corners have been slightly rounded and honed if reamer of particular size not available

Another Reamer

• Expansion reamers – Amount expanded limited – Body slotted and tapered, threaded plug fitted into end – Turning this plug will allow 1 in. reamer to expand up to .005 in.

Counterboring

• Operation of enlarging end of hole that has been drilled previously – Depth slightly greater than head of bolt, cap screw or pin it is to accommodate • Supplied in variety of styles – Each have pilot in end to keep tool in line with hole • Some interchangeable pilots to suit variety of holes 44-63

Countersinking

• Process of enlarging top end of hole to shape of cone to accommodate conical- shaped heads of fasteners – Head will be flush with or below surface • Countersinks available with various included angles – 60º, 82º, 90º, 100º, 110º, and 120º 44-64

Countersinking

• 82º countersink used to enlarge top of hole so it will accommodate a flat-head machine screw • Holes to be threaded countersunk slightly larger than tap diameter • Speed is approximately 1/4th of drilling speed

Tapping

• Performed by hand or under power with tapping attachment • Done immediately after drilling operation • Hand taps – In sets containing taper, plug, bottoming tap • Machine taps – Designed to withstand torque required to thread hole and clear chips 44-66 Set of Hand Taps

Taper

Plug

Bottoming

Types of Machine Taps

Gun

Stub-flute

Spiral-flute

Fluteless Tap

• Actually a forming tool used to produce internal threads in ductile material – Copper, brass, aluminum, and leaded steels

Fluteless tap Lobes of the tap