Shop Doc – Off-Center

Today’s Machining World Archive: October 2006 Vol.2, Issue 10

Dear Shop Doc,

In our shop, we have a debate as to what type of tool is the most effective for centering a drill. Is it a spot drill or center drill? Since both of the choices are available in several different angles, what influence does the angle have on centering the drill or anything else, such as tool life? We have had mixed results from a variety of these tools. Please help us choose the tool that will give us the best centering and life of a drill.

Signed, Off-Center

Dear Off-Center,

This is a very common issue and is easily explained. A center drill doesn’t do anything to center a drill. It’s made to leave a centering feature in a part so a live center or dead center can be located into its corresponding angle and be used in some sort of supporting application like a tail stock in a lathe.

A spot drill has a better chance of centering your drill, but only if the included angle of the spot drill is greater (blunter) than the drill’s included point angle. A great example of this is a 120° spot drill and a 118° drill point. The spot drill is 1° blunter on each side, allowing the drill’s point to reach the spot drill’s point before anything else comes into contact, such as the corners of the drill or the lip of the drill.

The problem with this is that many drills on the market today have drill points of 135°, 140° or even greater. So in order for this theory to work properly, a 145° spot drill needs to be used so the drill point makes contact fi rst, thus centering the drill in the most effective manner.

A center drill actually has two incorrect angles to deal with. One angle is from the pilot, and the other angle is what the center drill is measured at or known by, such as a 90° center drill, which eventually leaves a 45° chamfer per side once the hole is drilled. Tool life suffers greatly when using a 90° center drill or a 90° spot drill with an incorrect angle. If you want a chamfered hole, chamfer it with a chamfer tool after the proper spot drill and drill are fi nished. By following these simple steps you will see drills lasting longer, and the holes will be more accurately located, straighter and rounder.

Jim Rowe
Application Specialists / Medical Accounts
Mahar Tool Supply, Warsaw, IN


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