Difference between revisions of "How to Drill Holes where You Want Them"

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===Feed fast on too-large pilot holes===
 
===Feed fast on too-large pilot holes===
There are times when a too large pilot hole is unavoidable, such as when you are drilling out a tap sized hole to body size, or otherwise slightly increasing the size of a hole. In this case, the pilot hole is only about 20% smaller than the final hole, and chattering can easily happen.  But because so little material is being removed, it is possible to feed the drill much more quickly than when drilling a full hole, and unless the drill is not ground symmetrically, this fast feed will eliminate the chattering problem.
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There are times when a too large pilot hole is unavoidable, such as when you are drilling out a tap sized hole to body size, or otherwise slightly increasing the size of a hole. In this case, the pilot hole is only about 20% smaller than the final hole, and chattering can easily happen.  But because so little material is being removed, it is possible to feed the drill much more quickly than when drilling a full hole, and unless the drill is not ground symmetrically, this fast feed will eliminate the chattering problem by maintaining strong pressure throughout the cut, which provides good centering forces.  If you cut too slowly, the drill loses center when there is no longer material to cut and it is just turning in an already cut hole.  A fast feed keeps it cutting all the time.
  
 
[[Drill Basics|'''Previous Tutorial: Drill Basics''']]
 
[[Drill Basics|'''Previous Tutorial: Drill Basics''']]
 
<div style="text-align: right;">[[Keeping Bolt Holes Aligned when Drilling and Tapping|'''Next Tutorial: Keeping Bolt Holes Aligned when Drilling and Tapping''']]
 
<div style="text-align: right;">[[Keeping Bolt Holes Aligned when Drilling and Tapping|'''Next Tutorial: Keeping Bolt Holes Aligned when Drilling and Tapping''']]
 
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Revision as of 21:23, 10 October 2011

Previous Tutorial: Drill Basics

Introduction

Usually when you drill holes as part of robot construction, it usually matters where they are, and this is often a significant challenge, even when you are using a drill press. Besides just taking the time to measure and mark accurately, the most commont issue that makes it difficult is "walking" of the drill as the initial cut starts. This is just because the drill is turning, and the tip is not a single point. Even on the drill press, smaller drills flex, and a hole can start off center. To start, let us look at the tip of a typical twist drill.

Twist drill nomenclature.jpg

The Walking Drill

Note that the tip of the drill is not a single point, but a straight chisel edge that runs across the end of the drill's web. If you put this chisel edge against a piece of metal (the work) and turn it, then it will want to walk along the surface, pivoting on the chisel edge corners. If the drill is held rigidly in place, the chisel edge will scrape away a circle of material as the beginning of the drilling process, and as it goes deeper, the cutting edges start cutting a cone-shaped depression that matches the shape of the drill point. This depression keeps the drill on center for the rest of the drilling operation, but even then, if the tip of the drill is not perfectly symmetric, or if the drill is not perpendicular to the surface, or if the material has uneven hardness, the drill can cut more on one side than another and drift until the entire point is below the surface. There are drills that are ground to have a smaller chisel point, to to this, other compromises need to be made that makes the drill less desirable for the kind of materials we use, specifically aluminum.

The Flexing Drill

Smaller drills will walk even when they are held rigidly in the drill press. This is because they are flexible, and have very little side to side rigidity. If they were not flexible, then they would break under side pressure, so this flexibility is not all bad. There are also ways in which we can take advantage of the tendency of a drill to flex, as described below.

Center Punching

The most common approach to keeping a drill from walking is to use a center punch to create an initial starting cone. If this cone is significantly bigger than the chisel point of the drill, then we have centering forces from the beginning, but as soon as the center punched part is cut away, the drill can still drift because of uneven cutting. The best situation is if the punched cone is bigger than the entire diameter of the drill, which is clearly not going to happen with even moderately large drills. The flexing of a small drill is actually an advantage if it fully fits in the punched cone, because the drill will follow the cone even if the drilling machine is not on center.

Center drilling

Another way to form an initial guide for the drill tip is with a center drill. This is a rigid drill with a small spur point that allows you to make a small hole without the drill flexing away from where you want the hole.

Center drill.jpg

The key advantage of center drilling over center punching is that it can usually be done more accurately when used on scribed layout. A center drill is normally used in the drill press as the first (pilot) step of a sequence of drilling steps. The center drill should go deep enough to form a small cone outside of the diameter of the spur.

Pilot drilling

For small holes, where center punching or drilling creates a big enough starting cone, it is fine to just drill the final hole in one step. For larger holes, whether this is adequate depends on the rigidity and accuracy of the drilling setup. For drilling with a hand held machine, or in some situations on the drill press, it is useful to drill a smaller pilot hole to guide the larger drill through the work. Referring to diagram above, the diameter of the pilot hole should be about the same as the length of the chisel point or the thickness of the web of the final drill. This eliminates walking because the chisel point is confined in the pilot hole, and never touches the work. If the pilot hole is too big, though, centering depends on the balance of the cutting edges on either side of the drill, which often causes the drill to vibrate (chatter) from side in the pilot hole, which results in a hole bigger than the drill, and not quite round.

Feed fast on too-large pilot holes

There are times when a too large pilot hole is unavoidable, such as when you are drilling out a tap sized hole to body size, or otherwise slightly increasing the size of a hole. In this case, the pilot hole is only about 20% smaller than the final hole, and chattering can easily happen. But because so little material is being removed, it is possible to feed the drill much more quickly than when drilling a full hole, and unless the drill is not ground symmetrically, this fast feed will eliminate the chattering problem by maintaining strong pressure throughout the cut, which provides good centering forces. If you cut too slowly, the drill loses center when there is no longer material to cut and it is just turning in an already cut hole. A fast feed keeps it cutting all the time.

Previous Tutorial: Drill Basics