Keys to Small Hole Drilling

Applications/programming engineers get to hear all kinds of machining problems on a daily basis. The one that seems to stand out as the most baffling really is not that difficult to solve once you analyze exactly what is happening.

Drilling small diameter holes presents numerous problems. Small diameter holes refers to holes of 0.125″ diameter and smaller. What seems to go wrong when drilling these holes? Have you ever had bell mouthed holes? Tapered holes? How about crooked holes? Everyone has dealt with these at some time or another.

There are a few issues that can serve as possible solutions to these conditions. Let’s start with the drill. The most important aspect to consider is the drill point geometry. The smaller the drill size, the more critical it becomes that the drill point geometry is perfectly symmetrical. Bad geometry can cause a drill to “walk” off, giving you a crooked hole, a triangular hole or even a broken drill.

The length of the drill protruding from the collet will greatly affect the drill’s ability to track straight. If the drill length is five times the diameter or more, you can expect some problems. But that doesn’t mean that lengths less than five times the diameter are immune from problems, either. Always keep all tooling as short as possible for maximum stability. Also watch out for any “stamped” lettering on the shank. This lettering can cause bumps on the shank and can cause the drill to orbit – causing bell mouthing, walking off and other difficulties.

The most obvious error normally encountered is the feeds and speeds used. To avoid this, try using The Machinist Handbook for recommended starting parameters for a given alloy. You can make appropriate adjustments after you get rolling.
Most of the above solutions everybody knows about, but sometimes they just don’t think about it. When you have problems, always go back to the basics to find the error

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