Drill Basics

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Revision as of 22:06, 8 September 2011 by Eburdick (talk | contribs)

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.

Tip designs

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, forming a point that walks much less when drilling without a punched or drilled pilot.

Split point drill tip.jpg

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 as well as a conventional 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.

Fast spiral drill.jpg Normal spiral drill.jpg Slow spiral drill.jpg

Drill quality

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 several of these points. 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 to big, check the point of the drill, and if it is not perfectly symmetric, throw it away.