DRILLING OPERATIONS Chucking the Drill

Transcription

1 DRILLING OPERATIONS Chucking the Drill Drilling and Countersinking 83 WARNING Before installing or removing drill bits, countersinks, or other devices in an air motor, be sure that the air line to the motor is disconnected. Failure to observe this precaution can cause serious injury. Install proper drill in the motor and tighten with proper size chuck key. Be sure to center the drill in the chuck. Do not allow flutes to enter the chuck. Connect the air hose to the motor inlet fitting. 3. Start the drill motor and check the drill for wobble. The drill must run true, or an oversize hole will be made. Replace bent drills. Drilling Holes 1. Hold the motor firmly. Hold the drill at 90 angle to the surface, as shown in Fig Fig Square drill with work. 2. Start the hole by placing the point of the drill on the marked centerline. With the fingers, turn the chuck until an indentation is made. (Omit this step when drilling through a drill bushing or when a pilot hole exists.)

2 84 Standard Aircraft Handbook Position thumb and forefinger to prevent the drill from going too far through the work, which can cause damage to items on the other side or result in an oversized hole. Drill the hole by starting the drill motor and exerting pressure on the centerline of the drill. Exert just enough pressure to start the drill cutting a fairly large size of chip and maintain this pressure until the drill starts to come through the work. Decrease the pressure and cushion the breakthrough with the fingers when the drill comes through. Do not let the drill go any farther through the hole than is necessary to make a good, clean hole. Do not let the drill spin in the hole any longer than necessary. Withdraw the drill from the hole in a straight line perpendicular to the work. Keep motor running while withdrawing drill. To ensure proper centering and a correct, final-sized hole, rivet holes are usually pilot drilled with a drill bit that is smaller than the one used to finish the hole. Selected larger-diameter holes must be predrilled after pilot drilling and before finalsized drilling to ensure a round, accurate hole for the rivet. This procedure is sometimes referred to as step drilling. Note: When drilling thin sheet-metal parts, support the part from the rear with a wooden block or other suitable material to prevent bending. For enlarging holes in thin sheet metal use: Plastic-type drills for hole diameter 'A" and under. Hole saws for holes over 3/8" diameter (Fig. 4-8). Do not use counterbores or spotfacers. Fig For cutting clean, large-diameter holes in thin sheet metal, hole saws are commonly used.

3 Using an Extension Drill Drilling and Countersinking 85 Special drills can be used with the air-drill motor. The long drill (sometimes called a flexible drill) comes in common drill sizes and in 6-inch, 8-inch, 10-inch, or longer lengths. Do not use a longer drill than necessary. See Fig CAUTION Before starting the motor, hold the extension near the flute end with one hand as shown in Fig Don't touch the flutes and don't forget to wear safety glasses or a face shield. Drill through the part. Do not let go of the drill shank. Keep the motor running as the drill is removed. Jig Fixture Assembly 1/4" RIGHT Fig Select a drill of the correct length and size. WRONG Fig Hold the extension drill near the flute end with one hand. An unsupported drill might whip around and cause injury.

4 86 Standard Aircraft Handbook Drilling Aluminum and Aluminum Alloys Drilling these materials has become quite commonplace and few difficulties are experienced. Some of the newer aluminum alloys of high silicon content and some of the cast alloys still present several problems. General-purpose drills can be used for all sheet material. High rates of penetration can be used when drilling aluminum; hence, disposal of chips or cuttings is very important. To permit these high-penetration rates and still dispose of the chips, drills have to be free cutting to reduce the heat generated and have large flute areas for the passage of chips. Although the mechanic has no direct indication of drill motor speed, a relatively high rpm can be used. Drilling Titanium and Titanium Alloys Titanium and its alloys have low-volume specific heat and low thermal conductivity, causing them to heat readily at the point of cutting, and making them difficult to cool because the heat does not dissipate readily. Thermal problems can best be overcome by reducing either the speed or the feed. Fortunately, titanium alloys do not workharden appreciably, thus lighter feed pressures can be used. When using super-high-speed drills containing high carbon, vanadium, and cobalt to resist abrasion and high drilling heats, a speed (rpm) considerably slower than for aluminum must be used. See Chapter 3 for further information regarding the drilling of titanium. Drilling Stainless Steel Stainless steel is more difficult to drill than aluminum alloys and straight carbon steel because of the work-hardening properties. Because of work hardening, it is most important to cut continuously with a uniform speed and feed. If the tool is per-

5 Drilling and Countersinking 87 mitted to rub or idle on the work, the surface will become work hardened to a point where it is difficult to restart the cut. For best results in cutting stainless steel, the following should be adhered to: Use sharp drills, point angle 135. Use moderate speeds. Use adequate and uniform feeds. Use an adequate amount of sulfurized mineral oil or soluble oil as a coolant, if possible. Use drill motor speeds the same as for titanium. Hint When drilling through dissimilar materials, drill through the harder material first to prevent making an egg-shaped hole in the softer material. Deburring Drilling operations cause burrs to form on each side of the sheet and between sheets. Removal of these burrs, called debunking or burring, must be performed if the burrs tend to cause a separation between the parts being riveted. Burrs under either head of a rivet do not, in general, result in unacceptable riveting. The burrs do not have to be removed if the material is to be used immediately; however, sharp burrs must be removed, if the material is to be stored or stacked, to prevent scratching of adjacent parts or injury to personnel. Care must be taken to limit the amount of metal removed when burrs are removed. Removal of any appreciable amount of material from the edge of the rivet hole will result in a riveted joint of lowered strength. Deburring shall not be performed on predrilled holes that are to be subsequently form countersunk. Remove drill chips and dirt prior to riveting to prevent separation of the sheets being riveted. Burrs and chips can be minimized by clamping the sheets securely during drilling and

6 88 Standard Aircraft Handbook backing up the work if the rear member is not sufficiently rigid. A "chip chaser" (Fig. 4-11) can be used when necessary to remove loose chips between the material. BURRS ON HOLES AND CHIPS BETWEEN SHEETS CHIP CHASER Fig A chip chaser can be used to remove chips between material. COUNTERSINKING Flush head rivets (100 countersunk) require a countersunk hole prepared for the manufactured rivet head to nest in. This is accomplished by one of two methods: machine countersinking or form countersinking (dimpling), as shown in Fig SURFACE COUNTERSUNK y SURFACE DIMPLED SUB-SURFACE COUNTERSUNK Fig Countersinking and dimpling. SURFACE DIMPLED SUB-SURFACE DIMPLED

7 Drilling and Countersinking 89 Types of Countersinking Cutters The straight shank cutter is shown in Fig The cutting angle is marked on the body. Cutting angles commonly used are 100 and 110. The diameter of the body varies from %" to 1 'A". A countersink of 'A" diameter is most commonly used. Shank Body Flutes Cutting Angle / oe Threaded Shank Body Cutting Angle Fig Straight shank and rosebud countersinking cutters. A countersink cutter (rose bud) for angle drills, also shown in Fig. 4-13, is used if no other countersink will do the job. The stop countersink (Fig. 4-14) consists of the cutter and a cage. The cutter has a threaded shank to fit the cage and an integral pilot. The cutting angle is marked on the body. The cage consists of a foot piece, locking sleeve, locknut, and spindle. Spindle NCUTTER Z98(3-,11\ Cutting Angle Threaded Shank Body Pilot Locking Sleeve Foot- Piece CAGE Fig The stop countersink.