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Drills and Drilling
By Tony Bingelis (originally published in EAA Sport Aviation, April 1994)
I guess drilling holes is no big deal. Just about anyone can do that . . . and drill them where they want them to be. Well, most of the time.
All you need is an electric drill (nobody uses those old hand crank jobs anymore) and the right size drill bit. Jab the point of the drill against the place you want the hole to be, pull the trigger and, voila!, a hole.
Not surprisingly, drilled holes don’t always turn out to be nice and round, or located precisely where they were intended to be . . . after all, a certain amount of skill is involved. Nevertheless, you do have a hole.
Building an aircraft, be it wood, metal or composite, entails drilling countless holes in different kinds of materials. Not only that, sometimes access to where you have to drill a few of the holes seems impossible. That ordinary electric drill and standard twist drill just can’t do the job without elongating or otherwise botching the holes . . . no matter how hard you try.
That is when you first realize there must be more than one way to drill a good hole where you want it with a high degree of accuracy. After all, other builders seem to be able to do it consistently.
It is all in being able to think your way through each drilling problem as it arises . . . before you reach for the drill. Of course, it pays to have some good tools and a few drilling tricks at your disposal.
With that in mind, why not consider this as an opportunity to review your drilling know-how and, perhaps, pick up a few tips that may help augment your drilling skills and speed up your work.
The Right Equipment Helps
You are not expected to have a special tool for every type of job that confronts you during the construction of your homebuilt - sure would be nice though.
However, most builders, in time, do acquire a few special tools. These can save the builder a lot of building time and make it much easier to do good work.
If you already have an electric drill or two, good! Use them. On the other hand, if you have to buy one, opt for one with variable speed capability and a 3/8" capacity.
Buy the best drill you can afford. One with bearings instead of bushings will see you through your project and then some.
Better than the familiar plug-in electric drill most folks have is a powerful variable speed cordless drill. The good ones are not cheap but, again I say, buy the best one you can afford. (I like my powerful Makita with its extra battery. Anytime the battery runs down, I slip in the spare, plug the run-down battery into its charger and go on drilling without losing valuable building time.)
Imagine the convenience of a drill that doesn’t have to have an extension cord dragging around behind it. I think it is an essential luxury every builder is entitled to.
A pneumatic drill is another valuable tool to have. If you do have one, I’m sure you like using it much more than your plug-in electric hand drill.
On the other hand, if you don’t have one but are planning to buy an air drill, be sure it has a "teasing" throttle. A cheapie pneumatic drill will probably have a throttle button that can only turn the drill on full bore and its speed cannot be controlled. That makes drilling a hole in an exact spot a rather exciting and unpredictable experience.
Another valuable tool worth having is an angle drill gun. The best, by far, are the compact self-contained units that take threaded drill bits.
A less effective substitute would be a small 90 degree device that can be attached to your handheld electric, or pneumatic, drill. It is adequate for the occasional hard-to-get-at hole you have to drill . . . besides, it is much cheaper.
Fortunate indeed is the well-equipped builder who owns several power drills. Collectively, they are a great convenience, especially when building a metal airplane.
It seems that you are constantly changing the drill bit size. Having several power drills can save a lot of time. For example, you can put a #30 bit in one drill, a #40 in another, and a countersink bit in still another. Even so, you will still have plenty of practice changing to other sizes and functions.
Starting the Drill
I don’t always do it but a careful craftsman will.
That is, first make a punch mark at the exact point where you intend to drill a hole.
This is most important, especially when the hole is to be in some external surface that you don’t want marred by a drill bit that skids away from its intended mark.
Anyhow, don’t say I didn’t warn you. Make the punch mark first.
As your project progresses, you will become bolder and bolder and you will frequently skip making that punch mark and start drilling with the odds stacked against you.
You will get away with it often, but sometimes the drill bit will slip slightly and the hole will not be exactly where you had intended it to be. The worst consequence is drilling the hole too close to the edge because the drill had walked away from its mark and you didn’t notice it in time.
And where does this sort of thing happen? Always on top where the whole world can see it.
Anyway, if locating a hole precisely is important, always make a punch mark to help the drill get started.
Here is another way to ensure starting a hole exactly where you want it. Press the point of the drill bit on the mark and turn the chuck by hand two or three turns before squeezing the trigger. A short slow burp will immediately let you know if you did it right.
This method is quite effective but only with a small drill size - say a #40 or a 1/16" bit.
If you have to drill a 1/4" or larger size hole, always drill a pilot hole (about 1/8" diam.) first. The operation will go faster.
Needless to say, your deeper holes can be more accurately drilled with the work clamped on a drill press.
Breaking Through Unexpectedly
The moment a drill unexpectedly breaks through the material you are drilling (particularly thin metal), you will risk:
1. Jamming the bit in the thin jagged edges of the partially completed hole.
2. Losing control of a small part as the drill grabs and tries to screw itself into the metal. If the breakthrough is violent and the drill is quite powerful you could lose a finger or get your wrist slashed.
3. Breaking the bit because the sudden jamming may cause the drill to be suddenly and unexpectedly deflected.
You can avoid these potential drilling hazards when drilling holes in thin metal by:
1. Reducing the applied pressure on the drill as soon as you sense that the bit is about to break through. After a little practice this is easily detected by a change in the drilling sound.
2. Wedging your thumb or fingers between the material being drilled and the drill to serve as a manual stop. This will allow you to resist the tendency of the bit to grab and pull itself into the partially completed hole.
3. In drilling small parts, whenever possible, securely clamp to something, preferably the drill press.
4. Using a piece of scrap metal or a wood block as a solid back-up to minimize the breakthrough effect.
It’s A Matter Of Technique
You will be drilling in a lot of awkward locations and positions. This means you may not know what is close behind the part you are drilling, if you don’t take the precaution of checking before you start the hole.
Sometimes the structure behind may be an angle or tube that will deflect the drill bit and cause it to break . . . so, be careful.
Drill bits are expensive and it is bad enough that they get dull but much worse if you keep breaking them.
Your instinct might tell you that the best way to enlarge a hole is by progressively using a larger and larger drill. This is wrong. It is much safer to start with a pilot hole and then complete the hole with the correct finish size drill bit.
Trying to enlarge a drilled hole by running a slightly larger drill bit into the undersized hole is asking for trouble. This is because the pressure of the cutting action will be concentrated on the outside edges of the drill’s lips and flutes. The resultant wedging action causes the drill bit to bind and ruins its cutting edges.
If a hole is to be enlarged slightly, the proper technique is to use a reamer, or progressively larger reamers as needed.
Often you will be overwhelmed with the temptation to apply side pressure to a drill in an attempt to move a small pilot hole over . . . or to try to align the holes through two pieces that don’t quite align.
Resist the temptation, or suffer the consequences of a badly elongated hole, or a broken drill bit.
Your best bet for correcting this sort of misalignment is with a taper reamer or a small round file worked through the holes to obtain the alignment. Of course, the assumption here is that the hole will need to be enlarged to some larger diameter.
It is possible to make this so complicated a matter that you will spend most of your time studying charts and graphs rather than working on your project.
Most of the time the exact drill speed isn’t critical. Simply remember this:
1. Small drill bits up to 1/8" diameter or so work best at rather high speeds . . . full throttle in your air drill or handheld electric drill.
2. The larger drill bits, 1/4" diameter and larger, must be turned at lower (slow) speeds (especially in hard steels) with sufficient pressure applied to ensure that the drill is constantly cutting. This is extremely important when drilling in tempered steel.
If you allow the drill to turn fast without cutting, the metal will get extremely hot and possibly harden to the point where the drill point turns blue from the heat, dulls, and will no longer cut.
Should this happen, switch to a small bit and try to cut through the hardened metal before you switch back to a larger bit.
In addition to the rather slow speed that must be used with larger drill sizes is the need to keep the bit from getting too hot. A dab of cutting oil, kerosene or turpentine will help prevent burning and the risk of breaking the bit.
One way you can tell that your drill is turning too fast for the size is by the chatter you will experience as the drill tries futilely to cut into the metal . . . at times you may even see smoke.
Holes In Stainless Steel
Stainless steel firewalls are nice and definitely fireproof. The stainless steel, however, is mean stuff to work with. It is hard to cut and even harder to drill decent holes into.
No use attempting to drill a hole in the stainless unless you first make a good center punch dimple in it. Then, very definitely, start with a small #40 drill before you even think of drilling the hole with anything larger.
Stainless is very malleable (ductile?) and when a drill breaks through the backside it leaves jagged edges. Don’t brush your fingers over the hole . . . you’ll get blood on that nice shiny firewall.
Smooth the hole from the backside with a countersink bit, a smooth cut file or a round rotary file.
What about larger holes? The best way to enlarge a hole (up to about 3/4" diam.) is with a "Unibit" step drill. This step drill is ideal for making large holes in thin metal to accommodate rubber grommets.
But what about larger holes in the firewall. Say you need to open a 2" hole for the cabin heat valve, or to accommodate ducting for cockpit ventilation. What then?
If you have easy access to the firewall area, you can use a good sharp pair of left, or right, tin snips to rough out a larger opening. Or you could enlarge the step drilled hole with a nibbler.
A reasonably good 2" hole can be finished with a rotary file chucked in your pneumatic drill. Be careful, it will have a tendency to bounce around and chatter . . . use a light pressure and vary the drill’s speed. Finish the hole with a file using your best filing technique.
Other Drilling Problems
We seldom have to drill deep holes in building an airplane but should that problem arise, keep the following pointers in mind.
Soft aluminum, like 6061T6, tends to gum up the drill bit flutes. Clamp the work. Drill slowly, removing the bit frequently to free it of chips that could ultimately jam the drill tight in the hole. Holes in soft aluminum always need to be deburred because the backside where the drill pops through will be quite jagged.
Drilling deep holes in steel are best made in a drill press. Use slow speed, plenty of lubricant and keep pressure on the drill to keep it cutting.
If you have to drill a deep hole in steel (a landing gear leg, for instance) with a handheld drill, maintaining alignment of the drill is critical to avoid wallowing out the hole. It is advisable to clamp a small square next to where the hole is to be drilled to give you a visual alignment reference for the drill. As previously mentioned, drill a small hole first and then complete the hole with the correct diameter drill bit.
To drill a 1/4" hole in wood, a 1/4" drill will result in an exceedingly tight hole. The same 1/4" drill in metal will result in a sloppy hole. If a metal hole for a 1/4" bolt must have a close tolerance (wing spar, landing gear, etc.) it should be drilled slightly undersized and reamed for a close fit.
Holes in wood are not normally reamed. A sharp drill bit results in as good a hole as you can get in wood.
And, finally, a small select assortment of fractional size drills, numbered drills, and letter sizes, will give you sufficient options for obtaining whatever hole tolerance you may need. No need to buy whole drill sets because you will most likely need less than a dozen different drill sizes for your project.