DRILLING AND HOLE PREPARATION

 

HOLE PREPARATION

· One of the first operations when preparing to install a rivet is to locate and drill an accurate hole with a hand held drill. Theoretically, drilling is the simple matter of pushing the rotating twist drill through material at the desired point. However, the successful joining of critical and highly stressed parts requires top quality holes. Poor quality holes waste time and money. The salvage value of scrapped materials and parts is very low. More important, mislocated holes can weaken a structure directly or set up stress points with the resulting danger of fatigue failure.

 

TYPES OF HAND DRILL MOTORS

 

Electric drill motors

·110 Volt or plug in electric drills are not normally used in aviation settings, for a number of reasons. Although they are cheaper to purchase, they are more bulky than air drill motors, which means they are more awkward to use and over time creates greater operator fatigue. They also create sparks at the commutator, which presents a fire hazard around aviation fuels, and other aviation maintenance related products.

 

· Lower voltage battery operated drill motors turn too slowly for efficient drilling, however they are often used as screw drivers around aircraft for removing panels and the like. They should never be used around fuel openings.

 

Pneumatic tools (Air tools)

·The tools of choice in the aviation industry are pneumatic tools (more commonly known as air tools). These tools require compressed air to run. The compressor takes ambient air and compresses it from approximately 14 psi (pounds per square inch) to around 100 psi. for most shop tool operation. This high-pressure air is carried via piping and hoses to the air tools. Generally air tools are smaller and lighter than comparable electrically driven tools and they do not overheat no matter how hard they are worked. The air compressor must be of adequate size for the volume of air required and should have water traps in the lines to remove water that condenses inside the system. Air tools should be oiled regularly to provide lubrication and protection against corrosion

 

Hand drills

· The standard pneumatic hand drill is a turbine type tool, that is, the chuck is turned by a turbine reacting with the compressed air. Like most air tools the pneumatic drill motor is smaller and lighter than an electrically driven tool. Air drills have good speed and are available in a number of shapes and sizes. The standard drill motor usually has a 3 jaw universal chuck with a 1/4" opening.

 

 

 

 

 

Angle Drills and/or angle drill attachments

Drilling in restricted areas sometimes presents problems. These problems can often be solved by the use of drills or drill attachments configured to access tight areas. These drill motors or attachments may be of the 45º, 90º or 360º adjustable type. They may have three jaw chucks, internally threaded spindles to take threaded twist drills and other tool bits or use very small collets for each drill size. For obvious reasons angle drills and attachments are sometimes more difficult to control than standard air drills and care must be taken to achieve accurate drilling when using them.

 

30 degree angle drill

90 degree angle drill

 

90 degree drill attachment

 

 

 

 

 

 

 

 

360 degree angle drill attachment with extension

 

                               

Off-set drill attachment

 

Straight drill

 

DRILL BIT STYLES

 

Split Point Drills (sometimes called notched point)

· Split point drills are specially designed for use in the aircraft sheet metal industry. They are designed for easy and accurate starting. The heel is back ground to reduce the size of the dead centre. This makes the very centre of the drill a cutting surface and improves the starting accuracy of the drill when free-hand drilling. With care, a sharp split point drill can be started on flat sheet without a starting indent from a centre punch.

· There are drawbacks to split point drills. Because the heel has been partially ground away there is a tendency for the drill bit to bite into piloted holes. There is also the tendency of diameters larger than 3/16" to produce triangle shaped holes in thinner sheet.

· When opening out piloted holes to sizes larger than 3/16" you may want to use a drill bit that is ground to a standard configuration. This will reduce the likelihood of triangle shaped holes.

· Although not as pronounced, even regular ground drills in diameters over 1/4"will tend to produce triangle shaped holes in sheet metal. Any holes over 1/4 " should be produced with specially ground spur bits, hole cutters, or Unibits (one bit with a number of sizes in steps).

                                                      

Regular twist drill	Split point drill
Parts of the standard twist drill. The split point drill has cutting lips on the dead center.

 

                       

 

 

1/4 28 threaded twist drill for use in angle drills

 

 

 

DRILL SIZES

· Rivet hole size. The effectiveness of structural fasteners is dependent on the elimination of relative movement between parts being joined by the fasteners. Some fasteners achieve a tight fit installation of the fastener in the hole by using a close tolerance fit or an interference fit. However, because rivet shank diameters vary slightly within their designed tolerances, solid-shank rivets are designed to be installed in a slightly oversize hole. The mechanical swelling or hole filling action of the rivet during installation provides a tight fit in the hole.

 

A rivet will not fit in a hole with the same diameter as the rivet shank; it must be installed in an oversize hole.

A 1/8" rivet will have trouble fitting in a 1/8" hole	A 1/8" rivet needs a #30 hole which is 3 thou larger than a 1/8" rivet
The swelling action of the installation process causes the .003” oversize hole to be filled by the rivet

 

 · It is important that the correct size holes be prepared for the particular diameter rivet to be installed. A hole diameter equal to the nominal rivet diameter will not accept rivet diameters that have been manufactured closer to their plus tolerance. On the other hand, if too large a hole is made it will be difficult to hold the rivet straight while riveting, “dumped” rivets (clinching) or an off center rivet tail may result. If the rivet is driven enough to fill an oversize hole it may crack from excessive cold working or have insufficient material to make a good formed shop head  (rivet tail). In order to make a correct size rivet tail a longer rivet than normally called for would be required.

 

 

 

 

 

	REJECT

 

Oversize holes can cause rivets to "dump"

 

 

In aircraft construction, different companies will specify the drill type and sizes used to drill holes for the fasteners used on their aircraft. For example the de Havilland Production Process Standard PPS 2.01 specifies types of drills, pre-drill and final drill sizes for various nominal sizes and types of rivets used on de Havilland aircraft. De Havilland also provides a production process standard (PPS 1.09) for drilling and reaming. However, as a common rule, rivet holes should be of the smallest size that permits easy insertion of the rivet. This is usually about .003" larger than the nominal shank diameter. 

 

The common aviation rivet sizes and recommended clearance drills are listed below in Table 1.

TABLE 1 - Solid Rivet and Hole Sizes
Rivet Size (Nom)	Shank Dia. (Actual)	Clearance Hole Size	Drill Size
3/32	.093 - .097	.0980	#40
1/8	.124 - .128	.1285	#30
5/32	.155 - .159	.1590/.1610	#20 or #21
3/16	.186-.190	.1910/.1935	#10 or #11
1/4	.249-.253	.2570	F

 

After drilling, all holes must be carefully de-burred to eliminate stress raisers and to allow correct seating of rivet heads.

 

 

 

 

 

LAYOUT

 

· Measure and layout location carefully. Holes should be indicated by intersecting lines. Use a non-corrosive fine tipped felt pen to layout intersecting lines for hole location on sheet stock. Marks made with a scriber, which remain on the finished part, are not acceptable.

 

SETUP

 

· Make preparations before drilling. Time spent in setting up work for drilling is usually a good investment.

·Work should be located so that it is accessible and firmly held, normally flat on a drill block on the workbench. DO NOT DRILL INTO THE WORKBENCH TOP.  Some companies allow technicians to drill into the workbench tops; some don't, at BCIT we don't. Use a drill block or a piece of wood to allow the drill to penetrate the part without damaging the workbench top.  

· Work held in a vise should be clamped squarely in such a manner that it is not damaged by the vise, and so that it will not bend while drilling.

· Use only sharp and straight twist drills. They are necessary to produce true holes. Examine the cutting lips of the drill. Twist drill bits will begin to dull at the outer edges of the cutting lips, this is also the area where they tend to break. Using a dull drill bit will require more feed pressure and will tend to work harden the material. The drill bit will tend to wander and create enlarged holes. A sharp drill will minimize glazing and work hardening of the metal.

 

DRILLING SAFETY

 

·Safety glasses are recommended for most drilling operations. However, when drilling at chest height or above, safety glasses are mandatory to protect your eyes against swarf from the drilling operation.There is also the possibility that a drill might break and fly back. Never put yourself in a position where eye injury could occur.

·Protect fingers too. Be sure your fingers or other body parts are not in behind where the drill will come through.

·Make sure work is secure so that it will not slip while drilling. Never attempt to drill small pieces unless they are securely held.

·Be aware of the safety hazards associated with compressed air tools.

STARTING THE HOLE

 

· When drilling steel, heavy plate and castings etc., using a center punch prior to drilling is often recommended. However when using split point drills in aluminum aircraft skins center punching is not necessary and NOT RECOMMENDED. If a punch mark is too small, the cutting lips of the drill may bridge it and "walk off" the exact location before starting. If the punch mark is too heavy, it may deform the metal and/or result in a local strain hardening at the point where the drill is to start cutting.

· With good trigger control, it is possible to set a split point drill on the layout mark and carefully start drilling without any movement of the hole location.

· A safer way to start a split point drill bit is to accurately locate the drill on layout marks made with a fine tipped felt pen and turn the chuck one rotation by hand on the mark. This will make a slight indentation that can then be checked for location. The indentation will help control the drill movement on start up.

· Another method is to start the drill on the mark with a very short controlledburst then inspect the start indentation for correct location. You must stop drilling before the full diameter of the drill has engaged the sheet and check to see if the drill is starting on the mark.

· If any drill start is not on centre it is possible to move the start indentation by carefully angling the drill in the direction that the hole should be moved. With a careful and controlled pull on the trigger you may move the start indentation and then stop and check the centring.

· Be careful not to have the drill run away across the material surface. It is best to practice this technique on scrap until it is mastered.

· Once the start indentation is made on the layout lines, check to make sure the drill is perpendicular to the surface and apply drilling pressure in line with the centreline of the twist drill.

	REJECT

 

Sheet damage from a run-away drill

 

· For holes 3/16 and over in sheet metal, a pilot drill of a size equal to the width of final drill web thickness will often help ensure an accurate location. Avoid using a pilot drill larger than the final size drill web thickness as this tends to cause the cutting lips of the second drill to chatter or climb out of the hole, creating a hole that is off the start mark. It is advisable to start carefully when opening out a piloted hole. Make sure that the drill stays centered in the pilot hole.

 

Ideally pilot drill diameters should not be larger than the drill bit web thickness

 

DRILLING STRAIGHT HOLES

 

· Holdthe drill properly, in such a manner that the drilling pressure is applied as nearly as possible to the center line of the twist drill. This will improve your ability to produce straight true holes. With practice it will become natural to use the first second or third finger on the trigger. As far as possible you should be in a balanced and unstrained position while drilling. Some technicians like to hold their forefinger down the side of the drill motor, pointing at the hole. Some don't.

 

 

Drill alignment

 

· Keep the drill aligned perpendicular to the surface being drilled. You should always sight the drill from two positions 90º apart until skilled in straight drilling even when you feel that you have straight drilling mastered it doesn't hurt to check once in while.

· Ask your bench partner to check your drill positioning from their side of the bench, they will have a better view of your drilling angle. This will help you develop the feel of holding the drill straight. An experienced technician will use the reflection of the drill on the metal surface as a visual aid to keeping the drill perpendicular.

· A common error is to tip the drill sideways when drilling and/or when withdrawing the drill. DO NOT DO THIS as it results in slanted misplaced and elongated holes. Rivets driven into elongated or sloping holes will have the tendency to fall over and create a dumped rivet tail. The ability to drill straight holes can be acquired by practice.

· When drilling a curved section, drill perpendicular to an imaginary line tangent to the curved surface.

 

Drill alignment, holding drill

 

DRILLING

· Drill with low RPM and medium pressure. High RPM tends to make the drill wander from its' starting point, or worse, spin uncontrolled across the surface of the work, leaving serious sheet damage. 

· A steady and uniform pressure should be used when drilling. It should be firm enough so that the drill will not spin without cutting. Spinning in the hole results in hardening the material and glazing the drill. In materials harder than aluminum, allowing the drill point to spin in the work will often burn the cutting lips. In aluminum it will simply shorten the sharp life of the drill bit.

· Excessive pressure will result in breaking of the drill or elongation of the hole. Smaller drills will actually bend under too much pressure, causing off center drilling and possible breakage.

· Because of the ease of penetrating aluminum alloys, feeds up to three times those for steel may be used (.005” to .035” per revolution). A well-formed curl of swarf will be produced if the feed and speed are correct and the drill is sharp and properly ground.

· Pressure on a drill should be relieved as it starts to breakthrough. If this is not done it may cause the drill to jam in the material and to slip in the chuck, which can cause a burr to form on the drill shank. At the very least a larger burr will be left around the hole on the back side of the material.

· If breakthrough is a problem, drilling can be done with both hands holding the drill motor. The chuck should never contact the surface of the metal. A support can be formed with the hands to keep the drill under control at all times and especially at break through. If a standard length twist drill is installed in the drill motor, it may be held as shown in the illustration below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Using one hand to steady the drill and hold it back will help prevent the drill from going too far past the back  of the sheet.

 

· Ultimately the drill must be kept under control. Your attention should be on the tip of the drill and it's progress. Control the depth of drilling. Never allow the drill to break through so far that the chuck will mark the surface of the metal. In fact, the drill should not go through the sheet more than 1/4 of an inch, since stringers, flanges or items of equipment on the opposite side may be damaged. The tendency of the drill to pull into the hole as it breaks through the far side of the material should be controlled by proper holding of the drill motor or by using a drill stop on the drill bit.

· Always use a drill block, don’t drill into the bench top.

· A spring drill stop or a drill stop made from a short length of fiber rod or hard rubber is useful to prevent drilling into areas behind the work where items could be damaged by the drill. It is also useful to prevent the drill chuck from marring the surface of the metal.

   

Spring drill stop	Drill stop made from hard rubber

 

                            

· Make sure the drill chuck is not rubbing on or damaging any other areas of the part.

· Make sure that no chips or swarf become lodged between the sheets or in the holes.

 

 

 

 

 

 

 

 

 

 

 

 

Swarf between the sheets and undeburred holes will create rivet/sheet fit problems

 

 

·Stabilize nearby holes with clecos as you drill to prevent material movement and hole misalignment.

 

Put Clecos in nearby holes to stabilize the sheets.

 

 

DRILL LUBRICATION

 

· Drilling thin sections of aluminum does not require lubrication. However, lubricant should be provided for all deeper drilling. It will serve to assist chip removal, prolong drill life, prevent overheating, ensure good finish and dimensional accuracy of the hole.

· The use of a lubricant is always a good practice when drilling castings, forgings, heavy gauge stock or hard materials such as 4130 steel, corrosion resistant (stainless) steels and titanium. A drill lubricant for aluminum must be thin. A mixture of kerosene and cutting oil is satisfactory. For steel a sulfurized mineral cutting oil is best.

· In the case of deep drilling, the drill should be withdrawn at intervals to relieve chip packing, to ensure that the lubricant reaches the point and to ensure that swarf does not jam and break the drill in the hole.

 

REMOVING DRILL HOLE BURRS

After drilling all holes must be carefully deburred to eliminate stress raisers and to allow correct seating of rivet loads.

·         Burrs around holes will prevent proper rivet fit and installation

· Remove any burrs caused by drilling. Sharp, properly ground drills and relief of pressure as the drill point breaks through will produce a minimum of burrs.

· Deburring tools can simply be a 1/4" drill bit or can be made by attaching a file handle to a countersink or to a twist drill 1/4inch or more in diameter that has been ground to a flatter point than the standard 118° included angle. There are deburring tools specially made for this purpose.

· A drill motor that is capable of turning very slowly can be used with skill. It is of the utmost importance that the hole is not over deburred or countersunk.

 

 

Deburr tool

 

· When using deburring tools you should avoid countersinking the metal while deburring it. This is especially objectionable with thin material since it reduces the bearing surface of the metal and weakensthe joint.

 

Over deburring can damage the part

 

· One safe method, wherever it can be applied, is to scrape off the burrs with a thin piece of hard plastic material or a strip of phenolic sheet.

· If the material has been painted prior to drilling, the surface has a protective coating against cosmetic scratches, a small piece of single cut file may be used as a deburring tool. To make this deburring tool a file needs to be scored and broken into approximately 2-inch pieces. All the sharp edges must be ground away and the cutting surface lightly touched to a belt sander (with the file teeth held backwards to the direction of the belt) to slightly flatten the cutting surface and take some of the aggressiveness out of the cutting action of the file. This small piece of file can be glued to a small block of aluminum or wood to serve as a handle or left as is. This tool can be used where a regular deburring tool won't fit, or in normal situations provided the surface has an epoxy primer or other tough primer on the surface.

· Care must be taken to avoid scratching the material surface. Even light cosmetic scratches may be inappropriate on some surfaces.

Deburring tool made from a small piece of file

 

· When drilling several sheets together that cannot be separated conveniently for cleaning, a tool called a “Chip Chaser” is useful for chip and burr removal. It should have all edges rounded and smoothed in order to avoid scratching the work.

Chip chaser, used for removing swarf trapped between metal surfaces.

 

· If universal head rivets are to be installed then hole preparation is complete once the holes have been drilled and de-burred. If bare, these components may now be sent for finishing (primer) prior to fastener installation. If countersunk rivets are to be installed some deburring steps will obviously be eliminated or left until after the counter sinking operation.

 

PUNCHING HOLES

Holes may be punchedwith hand tools or machines but must be drilled or reamed afterwards due to the tendency of punched holes to form minute cracks around the perimeter.

Whitney hand punch

SPECIAL DRILL BITS

 

Extension Drills

· Extension drills are widely used for drilling holes in locations where it is necessary to reach through small openings or past projections.

· These drills, which come in 6” and 12” lengths, are high speed drills with spring tempered shanks.

· Extension drills are often ground to a split point, in order to reduce end thrust, which would increase the bending loads on the drill bit.

 

Using an extension drill bit with a small diameter pipe as a guide

 

When using extension drills always:

 

I. Select the shortest drill that will do the job. They are easier to control.

 

2. Check for bent drills that might easily go out of control.

 

3. Control the drill by guiding the shaft of the drill lightly with your free hand as long as it is turning. Wear a stiff leather glove, do not wear light duty rubber gloves while guiding the drill shaft as the gloves may get caught and drawn into the drill shaft. Never run the drill motor without supporting the drill since the end could whip around and cause serious injury. A drill guard, made of a short length of tubing, is the best way to support the drill.

 

4. As with any drilling operation avoid letting the drill penetrate the work far enough to damage anything behind, be it aircraft structure, people or simply the workbench. Always investigate to see who or what the drill point could hit when it goes through the material.

TOOLS TO AID DRILLING

 

Drill bushings

Drill bushings are used when transferring holes from one sheet to another sheet when the holes are large enough to make it difficult to find the centre accurately. The drill bushing has an outside diameter that matches the hole that is to be transferred. The drill bushing has a smaller hole on centre that will allow a smaller diameter drill to pass through and drill into the underlying sheet.

 

 

 

 

 

 

 

 

 

Drilling guides

Drilling guides contain hardened steel bushings that are installed in a block at 90 ° to the blocks surface. The drill guide will automatically hold the drill at 90° to the surface being drilled.

Two styles of drilling guides

 

Transfer punch

Transfer punches are used to transfer the exact center of holes to an underlying sheet. The outside diameter of the punch must fit exactly inside the hole to be transferred. When the correct size punch is fitted in the hole a light tap on the punch will leave a small mark on the undrilled sheet underneath at the center of the hole. The technician must be careful not to hit the punch too hard and distort the sheet underneath. This type of hole transfer does not work well on thin sheet because the punch will not fit far enough into the hole.

Transfer punch

 

Hole Finders

A useful tool that permits the transferring of holes from drilled members to a sheet of metal placed over them is called a hole finder or “duplicator.” It is especially handy when fitting skin where holes in the new piece must align with original holes in the inner structure. A pilot in one of the straps of the tool is set into the existing hole and a punch in the other strap marks the exact center for the hole in the top piece of metal. On one type, a drill bushing is put in place of the centrepunch. Holes to be matched are drilled through the bushing while the pilot is in place. A third type has the pilot inverted to allow hole duplication where the sheets cannot be pried apart.

One style of hole duplicator or hole finder

 

 

TEMPORARY FASTENERS

 

It is essential that members to be joined are closely and securely clamped together while drilling and especially while rivets are being installed. This will ensure that all fastener holes remain in alignment and that rivets do not swell out between the members as they are installed (see Figure 1)

Figure 1 - Swelling Between Sheets

 

In the early days of aviation, sheet metal to be drilled or riveted during construction or repair was temporarily held together with bolts and nuts while the work was being done.

Many types of threaded fasteners are still used as temporary fasteners while riveting, especially when the sheets are large or when a set of formed pieces must be "nested" together. However, for most sheet metal assembly jobs the most commonly used temporary holding devices are a sheet fasteners made by the Cleco company commonly called "Clecos"(Figure 2, pin type Cleco). Pin type, or standard Clecos are spring loaded clamps that are inserted and removed from fastener holes using Cleco pliers as shown. Pin type Clecos are colour coded for the diameter of rivet hole for which they are intended, the most common sizes are:  

 

 

                            

Rivet Size	Cleco colour
3/32 1/8 5/32 3/16	Silver Copper Black Gold

 

Table 1 Cleco Colour Codes

Cleco also makes a spring loaded side grip sheet clamp that will hold a sheet by the edge (Figure 2 Cleco clamp) and threaded temporary fasteners that can be tightened by a hex nut or a wingnut. The wing nut style is also shown in Figure 2.

 

Figure 2 Cleco Devices

 

 

 

 

 

 

 

 

 

CLECO COLOR CODE       SILVER     – 3/32” COPPER   – 1/8” BLACK     – 5/32” BRASS     – 3/16 ZINC         – 7/32” COPPER   – 1/4” BLACK     – 5/16” BRASS     -- 3/8     Pushing the plunger, extends the steel wires with hooked ends over the centre pin. In this position the wires converge at the tip, making insertion or extraction an easy operation.

 

Figure 3 Cleco Fasteners

 

 

 

 

 

 

 

Plain Cleco
Wing Cleco

 

Figure 3  The innards of  "Cleco" Type Sheet Holders

 

 

The Avdel company also makes a screw type temporary sheet fastener. There is also a fastener known as a Nomar which is threaded and has a plastic cover on the end to help protect the sheet from scratches. With spring type Clecos it is important to make sure the bottom of the Cleco is clean and free of burrs. If necessary touch up the base with a file. If a spring type Cleco continues to leave cosmetic scratches around the hole it is used in, it is possible to use a plastic plug cap with a hole drilled in it slipped over the end of the Cleco. Rivet tape on the end of the Cleco would also work but masking type is not recommended as the tape tends to pick up debris. The tape itself eventually deteriorates into a dried on sticky mess.

 

 

DRILL FEEDS AND SPEEDS

· Calculating drill speeds is not an issue with hand held air drills in aluminum as the average air drill speeds are well within the cutting speed range for aluminum. However, it is useful to understand the theory behind drill speeds when drilling steels and titanium. For efficient drilling the speed and feed ratio is important. If the speed is too slow unnecessary time is consumed. Also, there is an increased tendency to break smaller drills, even with normal pressure. If the speed is too high the drill will overheat and become dull. A burning of the outer edges of the cutting lips of the drill often indicates excessive speed in hard material. As a general rule, “The larger the drill the slower the speed and the smaller the drill the higher the speed”. Often the equipment available does not adjust to the most desirable speed. In any case the best adjustment provided should be used. Choose a portable drill that has a speed suitable for the twist drill and material to be drilled.

· Technically, the speed of a drill means its speed at the circumference, in surface feet per minute. The recommended speed for drilling aluminum alloy is from 200 to 300 surface feet per minute. That for mild steel is from 60 to 70 sfm and for corrosion resistant steel 30 to 50 sfm. In practice, this must be converted into revolutions per minute for each size drill. Drill rpm charts are included in machinists’ handbooks and many mechanics’ handbooks. Drill rpm may also be computed by use of the formula.

 

RPM	= 4CS
	D
		where:	CS = the recommended cutting speed in sfm
			D = the diameter of the drill

 

Example: At what rpm should a 1/8” drill turn to drill aluminum at 300 surface feet per minute?

 

RPM	= 4CS	= 4 x 300	= 1200	=    9624 rpm  (Most hand air drills operate                              between 2000 and 3000)
	D	.125	.125

                                   

 

DRILLING TITANIUM OR STAINLESS STEEL

· As has already been stated, with practice and a sharpsplit point drill an accurate hole should be possible in aluminum with no prepunching at all. This is not true for stainless steel and titanium. Starting a split point drill accurately in harder materials is not always successful. It is also more difficult to move a hole that is not started on center. When starting drills in stainless steel or titanium use an automatic center punch made for light metals. This eliminates the need for a prick punch and the center punch.

 

Automatic center punch

 

· When center punching thin sheets make sure the sheet is laying flat and well supported from behind, preferably on a smooth thick piece of metal as a back support

 

Larger holes in aluminum sheet are best made by punching. Conventional twist drills larger than 1/4" will tend to distort the metal around the hole and will create triangular shaped hole. If a large drill must be used be sure the work is securely clamped with a backup piece of hardwood or thick aluminum. A hole cutter or Unibit will work better and is recommended. Holesaws are reserved for larger holes. Special sheet metal “spur point” drills may be purchased or hand ground. These drills act as miniature hole cutters, eliminating most of the distortion. To grind your own, hollow the heel surfaces of a twist drill point on a grinder to the contour shown.

 

 

 

 

 

 

 

Spur bit

 

· As we have already discussed, in general, drilling speed should be suited to the size of twist drill as nearly as your equipment permits. When in doubt keep the drill speed slow and sufficient feed pressure to continue to cut with no slippage. If drill speed is too high the excess peripheral speed will cause burning of the outer corners of the drill. Also, with a high speed it is difficult to maintain sufficient feed pressure to keep the drill cutting. This would be especially so in drilling corrosion resistant steels. The required drill feed pressure for corrosion resistant steel is greater than for aluminum alloy. For example, at least 35 pounds thrust is recommended for use of a #30 drill in stainless steel and 56 pounds for a #10 drill. The drill feed pressure should be applied steadily all the way through the metal so that the drill will continue to cut and not spin in the hole. Spinning will work-harden the metal at the bottom of the hole, dull the drill, and cause the familiar trouble of burned drills. This is also true when drilling titanium.

 

 

Cobalt Drill bits

The components of the exhaust and reverser system of a jet engine encounter high heat and also require high strength. These parts are fabricated of 17-7 PH and 19-9 DL stainless steel alloys. Rivets and high shear blind bolts of A-286 alloy join them. When disassembly is required these fasteners are removed by drilling. The A-286 material is so tough and has such inherent work-hardening properties that even the NAS 907 high-speed drills will not stand up when used on it. Because of this, special cobalt alloy drills are used. These drills have properties between the high speed and the tungsten carbide types. In general, tungsten carbide drills would be too brittle for hand drilling. The cobalt alloy will give a desirable combination of hardness, abrasion resistance, and heat transfer. Cobalt drills are made in standard sizes. The shank is made longer than standard in order to reduce the likelihood of breaking.

 

These drill bits are expensive and should be given more than normal care in use. As with any drill, they must be kept sharp. Using a dull drill is a safety hazard and also will cause it to be completely ruined. Dipping the point in a lubricant such as Kool Tap or Bo Lube before use helps. Drilling speed should not exceed 1000 rpm for drills less than 3/16 drills or 750 rpm for larger ones. With proper use cobalt drills will do the tough job expected of them as satisfactorily as any other type available.

 

· Another type of drill used for aircraft work is the NAS-907. This is a high quality twist drill specially designed for hard or tough metals like aircraft corrosion resistant steels. Since corrosion resistant steel is tough and also will work harden very rapidly it is very hard on ordinary High-Speed Steel twist drills. The NAS-907 is a split point (notched point), nitrided drill. It has a heavier web than a standard drill and a point angle of 135 degrees, compared to 118 degrees for a standard drill. Also the lip clearance angle is from 10 to 14 degrees compared to 12 to 18 for a standard drill. They are made with both regular and short shank lengths.

 

The notch is made necessary by the thicker web and blunter point of this drill. With it, the drill will start more easily and require less feed pressure during drilling. The nitriding process hardens the surface of the drill. Nitriding consists of holding a special alloy of steel at a temperature of 1000⁰F in an atmosphere of ammonia gas for several hours. During this period the aluminum and iron in this steel alloy combine with nitrogen in the ammonia to produce iron nitrides. This causes the formation of a coating that approaches diamond hardness.

 

An important feature is that the hardness of a surface so treated will be unaffected by subsequent heating unless the temperatures are extreme. This coating penetrates to a depth of about .015 leaving the core soft enough to retain some flexibility. Resharpening these drills is not considered practical unless they are also to be re-nitrided.

 

Twist drills of standard design are satisfactory for drilling aluminum alloys and the notched point nitrided drill is sometimes stocked for general use on all metals except for some extremely hard types where special drills are required. Actually, the point angle of 135 degrees is preferred for thinner material since it permits the drill to cut almost the full depth before the point breaks through. Also it reduces the tendency for chips to pack and for burrs to form. This blunter point requires that greater care be taken to start the drill accurately. Drill points must, of course, be sharp to do satisfactory work. All surfaces over which chips must pass must be smooth to minimize friction.

 

In all deeper drilling (1/2 inch or more) there is a great tendency for chips to pack in the flutes of the drill. This causes rough, oversize holes and often breaks the drill. Removing the drill frequently to let the chips clear can prevent it. Clean well-polished flutes and the use of a lubricant will also help. Lubricant helps keep the drill bit cool, over heating can cause the drill bit to seize up in the hole.

  

 

 

 

 

STUFF

If this deburring is not scrupulously done for each hole, the rivets will not pull up flush and gaps may develop in the joint.

 

This means that, after drilling all the holes in an assembly, you MUST take it apart and deburr every hole. Every hole!

 

The traditional way of doing this is with a sharp, larger diameter (about 3/16”-l/4”) drill bit held between your fingers and twirled lightly on the drilled rivet hole. The idea is to remove the burrs—not to countersink the hole.

 

After you have painstakingly deburred a hundred holes or SO, you will develop a great interest in saving your fingers from further wear and tear. It would help if you were to slip a small file’s wood handle on the end of the drill bit to protect your hand from the sharp flutes.