Making a Countersink

There are advantages to making your own countersink cutters apart from a big saving in cash (HSS countersink cutters are expensive for some reason). You can make them any angle that suits, and you can make them any diameter within reason. Base material is silver steel (drill rod) which, when hardened, will last a very long time and stay sharp provided they don't get overheated in use (unlikely for a countersink). Anyway, they can be easily re-sharpened with a slip stone.

Believe me, they are a lot quicker and easier to make than it is to describe how to make them!

60 degree countersink.

The example in the pictures is a 9/16" diameter 60-degree countersink, handy for holes up to and including 1/2" dia. First job is to set the top-slide over to 30 degrees and use it to turn a taper on the end of a suitable length of silver steel rod, fine finish is not essential at this point. At the same setting, use the cross-slide to turn the shank behind the cutting head down to a diameter to fit your drill chucks (I selected 5/16" which will fit any machine chuck I'm likely to use - including the hand drills). Part off and remove to the milling machine (or lathe vertical slide). Next, cut the 2 flutes 180-degrees apart using and end mill about 3/8" diameter. For this machining setup the countersink blank should be positioned so that the final cutting edges are cut by the *end* of the endmill, whereas the trailing edges are cut by the the *side* of the endmill. Makes sense if you just look at it!

Machining setup for making countersink.

The picture shows that I used a chuck mounted on a rotary table for this job. If you don't have either dividing head or rotary table you can make a holder from a short length of square bar (though hex might be even better as it's easier to drill a hole through the middle) - drill a hole through the center the same diameter as the cutter shank and use a grubscrew (headless setscrew) to hold the cutter in place. You can now grip the square section in the machine vice to cut the two flutes.

As it stands the embryo cutter is useless as the cutting edges have no back-relief (ie., it won't cut anything). Easy to get around this - just mount in the lathe chuck eccentrically so that the trailing edge of the cutter surface is furthest from the centre line, this is done twice (once for each cutting edge). I first set the chuck in position with a square off the lathe bed resting against the side of 2 of the 3 jaws. The remaining jaw on the far side will of course be horizontal in this position. Now, place a piece of shimstock (mine was 6 thou thick, the thicker the shim the more aggresively the countersink will cut - 6 thou will produce a fine-cutting tool, 30 thou an aggresive one) between the horizontal jaw and the cutter shank, and push the cutter into the chuck as far as it will go until the cutter head (which is larger diameter than the shank) rests against the ends of the jaws. Lightly close the jaws. Next, turn the cutter in the chuck until the 2 cutting edges are vertical (that is, the trailing edge is opposite the jaw with the shim) and nip up the chuck jaws. A quick check turning the chuck by hand should show that the trailing edge of the cutting edge is more eccentric than the leading edge, which is what we want. Start the lathe and take a cut of a depth that just reaches the leading edge. It helps to smear the surface with engineer's blue so you can see just how much is being cut. You want the finest finish possible at this stage. Leave the cross-slide feed in that position. Stop the lathe and turn the cutter in the chuck 180 degrees. Take a cut across the second trailing edge with the top-slide at the same setting as before. That completes the lathe work.

You can stone the inside edges to remove any burrs and sharpen the edges of the cutter. Next job is to harden and temper which is easy - just heat to cherry red and quench, polish so you can see the tempering colours develop and heat to light-straw, immediately dunk into cold water.

The first picture shows the countersink I made plus an example workpiece that was countersunk to full width of the cutter very easily. Just remember to drop the lathe speed to about half turning speed and you'll get no chatter. Use a neat cutting oil for best results.

Download Drawing - This small drawing might help to visualise what the countersink looks like.

(c) Chris Heapy 1996.

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