For most robotics work, we are drilling in metal, mostly aluminum. We also drill plastics and wood on occasion. Different materials act differently when being drilled, so it is important to know what kind of drill is best for each material, and how fast to spin and feed the drill. There are lots of fancy and expensive drills out there for various materials, but we will concentrate here on things we are likely to find in our shops. To start, take look over this diagram of twist drill nomenclature...
Most twist drills we are likely to see in our shops have either a 118 degree or 135 degree tip. The 118 degree tip has a longer cutting edge, and follows a pilot hole more easily. The 135 degree tip has a shorter cutting edge and gets past the tip earlier in the drilling process, but does not follow a pilot hole as easily. A common variation is the "split point" design, which is often found paired with a 135 degree tip angle. This drill type has a "multifaceted" tip with a more aggressive cutting angle and a greatly shortened flank behind the cutting edge. The extra facet that shortens the flank also cuts into the chisel edge, drastically shortening it so that it walks much less when drilling without a punched or drilled pilot.
Because these drills take a deeper bite, they are generally not recommended for aluminum, and though the tip is better at self centering, it does not follow a pilot hole as well as a conventionally ground drill.
Fast and Slow Spiral Drills
Aluminum is considered a "stringy" metal because it forms long chips when it is cut. In fact, some cutting tools will form a single chip several feet long when drilling a hole in aluminum. A big challenge is providing an escape route for the aluminum chip. Drills with a higher "rake angle" provide a better path for getting aluminum chips out of the cutting area, particularly for deep holes. These are called, among other things, fast spiral drills. Slow spiral drills are stronger and better for more brittle materials with smaller chips. Most drills found in our shops will have a medium spiral, which is generally fine for general use, including aluminum. Having fast spiral drills in a few sizes for deep holes in aluminum is an advantage, but not critical. Drills with all three rake angles are shown here.
Drill materials and finishes
For most robotics work with aluminum, high speed steel (HSS) drills are a good choice. For drilling in harder and tougher materials, cobalt alloy steels are commonly used because they hold their edge longer and handle heat better; these are certainly a better choice for quality and long life, but are considerably more expensive. Cheaper drills are made of carbon steel or high carbon steel. These are not appropriate for robotics work, because they get dull very quickly.
Drills come either uncoated (bright,) black oxide coated, or titanium nitride coated (gold.) There are also much more expensive titanium-based coatings, but these are not generally appropriate for our work. Black oxide bits are marginally more durable than bright bits, and provide better natural lubrication when drilling steel. Titanium nitride is a very hard ceramic coating, and can extend the life of a bit used on hard materials quite a bit assuming the basic bit is high quality. But if these bits are sharpened in the shop, the coating is lost on the cutting edge.
Many "titanium" drill bits sold in retail stores are not much better than their uncoated low quality counterparts. It is generally a better bet to buy high speed steel or cobalt steel bits from machine tool suppliers or a good hardware store.
Note that drill coatings are the most useful for drilling in steel and stainless steel. They are not helpful for aluminum, and in some cases, perform worse in aluminum, which usually does better when cut with uncoated bright polished bits. Note that some uncoated high speed steel bits have a very slightly yellow color (much less yellow than the titanium nitride coating) due to heat treatment processes.
For precision drilling, it is important drills be sharp, straight, strong, round, and balanced. These all seem obvious, but a lot of cheaper drills fail on one or more of these points.
Dull drills fail to feed, and the points overheat. If a drill seems to be slow to feed and starts smoking, it is probably dull.
Besides dull drills, the most common flaw is that the point of the drill is not symmetrically ground, resulting in an off-center point. Using this drill on a piece of wood may result in an adequate hole, but using it for precision drilling in aluminum or steel results in a wandering drill and a hole bigger than the diameter of the drill. When you get a messy hole that seems too big, check the point of the drill, and if it is not perfectly symmetric, throw it away or have an expert re-grind it.
A bent drill is pretty obvious because the point does not stay in one place when the drill is spinning. Straightening a bent drill is almost impossible, so don't waste your time.