Nuts & Bolts
Building Basics DRILLING ACCURATE ROUND holes in aluminum and corrosion-resistant steel is the beginning of many sheet metal projects. Although the basics are not , difficult, the techniques used differ from drilling holes in wood or building materials. This month we'll discuss drilling equipment, and next month we'll delve into drilling techniques. Drilling is a simple matter of pushing a rotating twist drill through material at the desired point. And learning good drilling habits from the start is easier (and better) than unlearning bad habits later.
aviation, metalwork makes use of just four of them. The high-speed steel (HSS) drill is the Part I: Drilling equipment & techniques standard for making holes in aluminum and RONALD STERKENBURG mild steel. Available in both short shank and standard (jobber) length, HSS drill bits have a 135-degree split point to eliminate "drill walking," and they are generally black. Vanadium cobalt (CoVan) drill bits are designed to drill corrosionresistant steel or titanium. Extremely hard (and expensive), they are great for drilling out bolts and screws. You can recognize CoVan drills by their longer shank, thicker webs, and the tapered end piece on the shank. A gun barrel drill is an HSS drill designed for back-drilling through When shopping for a drill, pay an existing hole, and it has a very close attention to its trigger. It blunt point, which eliminates the should be sensitive so you can con- elongation of existing holes during trol the drill rpm. You can get a back-drilling. Finally, a step drill or pneumatic drill at the hardware store core drill is designed to enlarge existfor around $30, but it's not suitable ing holes or drill larger holes in two for quality aircraft sheet metal work steps, and it has a smaller pilot drill because the trigger has two posi- at the drill point. tions—no speed and full speed. An aviation-grade pneumatic drill uses a sensitive trigger to control its variable speed.
A pneumatic (air-driven) drill is the tool of choice, and a small lightweight pistol-grip drill is perfect for drilling aluminum and corrosion-resistant steel.
Drills A pneumatic (air-driven) drill is the tool of choice, and a small lightweight pistol-grip drill is perfect for drilling aluminum and corrosion-resistant steel. Good pneumatic drills cost between $100 and $400, and when investing in tools, buy the best one you can afford. If properly maintained, the drill will last a lifetime. (The only maintenance the drill will require is regular oiling: Put a drop of air tool oil in the male air fitting, connect the air hose, and squeeze the trigger.) 9O
Although the basics are not difficult, the techniques used differ from drilling holes in wood or building materials.
Drill Bits Many different types of drill bits make life easier for craftsmen, but in
Aviation metalwork uses four different types of drill bits (from top): the standard high-speed steel drill, the vanadium cobalt drill, the gun barrel drill, and the step (or core) drill.
Vanadium cobalt drills are for boring holes in corrosion-resistant steel and titanium, and they have a longer shank with a tapered end and thicker webs. : , i
Core (or step) drills enlarge existing holes or drill larger holes in two steps. Note the smaller pilot drill on the point.
Drill bits are measured by their diameters and grouped by three standards: number, letter, and fractional. Table 1 gives the decimal equivalents of general standard drills between 3/32- and 1/4-inch. When building, repairing, or restoring a sheet metal airplane, most drilling is for rivet A pancake drill attachment gets to holes. Rivet holes should be approxi- hard-to-reach areas. mately 0.003-inch larger than the rivet size. Table 2 gives the recommended clearance drills for the com- very useful for drilling out hard-toreach rivets in tight spaces. A more mon rivet diameters. economical solution could be a 90-
Drills are not the only way to create a hole, especially when you need to enlarge a hole and finish it smooth to a required size. A reamer will give the desired result. There are many different types of reamers, but a piloted reamer (see photo) works well for drilling close tolerance holes.
Ninety-degree threaded drills are useful tools, especially when you need to drill out hard-to-reach rivets in tight spots.
degree drill attachment, which fits your pistol-grip drill. There are a number of tools that make drilling straight holes easier or
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On most straight reamers the grooves do not create cutting edges, they provide a way for chips to escape and a channel for lubricant to reach the cutting edge. The reamer's tip does all the cutting, and it's normally ground to a 45-degree bevel. Note: Attempting to withdraw a reamer by turning it in a reverse direction is an unacceptable practice because it can force chips into the surface, scarring the hole.
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Attachments & Aids
There are some situations where you can't use your pistol-grip drill, but there are alternative solutions. A pancake drill attachment (see photo) fits your standard drill and is great for those hard-to-reach areas. For those really tight spaces you can get a 90-degree drill that uses threaded drill bits. Because the bits screw into the drill, the tool doesn't need a bulky chuck. Although rather expensive, these are great tools and
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Table 1: Drill Sizes Drill size Decimal inches .0937 3/32
.0960 .0980 .0995 .1015 .1040 .1065 .1093 .1100 .1110 .1130 .1160 .1200 .1250
41 40 39 38 37 36 7/64 35 34 33 32 31 1/8
Drill size Decimal inches Drill size Decimal inches Drill size Decimal inches .1285 18 .1695 13/64 .2031 30 .2040 .1360 1 1 / 6 4 .1718 j 6 29 .2055 .1405 17 .1730 5 28 .2090 9/64 .1406 16 .1770 4 .1440 15 .1800 3 .2130 27 .2187 .1470 14 .1820 7 / 3 2 26 .2210 .1495 13 .1850 2 25 .1875 1 .2280 24 .1520 3 / 1 6 .2340 .1540 12 .1890 A 23 .2343 .1562 11 .1910 1 5 / 6 4 5/32 .2380 .1570 10 .1935 B 22 .2420 .1590 9 21 .1960 C .2460 .1610 8 .1990 D 20 .2500 .1660 ! 7 .2010 1/4 19
Table 3: Cleco Color Code
Table 2: Rivet Hole Sizes
Rivet 3/32 1/8 5/32
Drill #40 #30 #21
Rivet Drill 3/16 #11 7/32 #2 1/4 F
3/32 1/8 5/32 3/16
Cleco color code Zinc/silver Copper Black Brass
from drilling into an underlying structure that doesn't need a hole. Drill stops also prevent the drill chuck from marring the surface. They come in different sizes, and
prevent unwanted damage to the structure. Drill stops are inexpensive devices that can save you a lot of money because they limit the depth of the hole, which prevents you
A drill stop is an inexpensive device that limits a hole's depth.
their only disadvantage is the small setscrew has the tendency to loosen,
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A drill bushing holder (or eggcup) ensures perfectly perpendicular holes, even on curved surfaces.
fall out, and get lost. If you are drilling just a few holes, you can make your own drill stops from small hard rubber tubing or by putting tape around the drill bit to indicate the correct depth hole. If you're an absolute perfectionist, get a drill bushing holder (also called an eggcup). It will help you drill perfectly perpendicular holes, even on curved surfaces. Many of them are available with interchangeable bushings.
Use sheet holders to assure hole alignment when drilling through two or more sheets of material. If "sheet holder" is an unfamiliar term, try Cleco, the brand name most people use for sheet holders. By whatever name, you'll need special pliers to use them.
Clips work well to hold sheets together temporarily, and the rubber ends won't damage the material.
Standard spring Clecos come in standard rivet hole sizes: 3/32, 1/8, 3/16, 7/32, and 1/4 inch. Wing-type Clecos are harder to use, but they clamp the sheets together more tightly and do not allow any space, great to prevent burns between the
Cleco fasteners ensure hole alignment when drilling through two or more sheets, come in several styles, and require special pliers.
sheets. To tell one size from the other, use the color code given in Table 3.
To align sheets of metal before drilling use side Clecos. Clips work well to temporarily hold two sheets together, and the rubber end pieces do not damage the material. Next month we'll drill into (sorry) the techniques for boring holes in different materials.
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