MORTISING MACHINE DRILLING
&
SHARPENING HOLLOW MORTISE CHISELS

This article is the follow up to the first of the month issue on our Special Drill Press Fence for Mortising. If you have built our fence assembly for your drill press, you will enjoy the special hold downs when you mortise.  The heart of your system is the two part bit that is used.  On the outside is a square sided hollow chisel that comes down to a point at each corner.   These points are what squares up the hole.   There is a relief slot in one face of the chisel, which allows the chips to be ejected from the bit. This slot should always face to the front.  Inside the chisel is an auger style drill bit.  This bit removes the waste from the mortise.  This bit has a flat cutting edge and a spur which will develop a flat bottomed hole.

Setting Bit Adjustment

The setting for the bit is important to the set’s performance.  If it bit leads the chisel too far it can move off the mark and in some cases break the bit.  Manufactures recommend from 1/16” to 3/16” which we have found to be too far.  We have found that the closer the better.  In some cases you will find that the bit can be longer than the chisel with your drill press.  In that case a selection of hardened steel washers that fit the milled end of the chisel will act as spacers to bring them in alignment. One of the things that we use for

spacing in our shop is a deck of playing cards.  Take three cards and slide them under the top of the mounting lip of the chisel and tighten the chisel in place.  Move the bit up to the bottom of the chisel and tighten.  As with any drill press always tighten the chuck at each of the three holes.  This assures that it will not slip.  Loosen the chisel and move up and tighten. You now have the perfect distance for the spacing. 

Drilling a Mortise

You are now ready to drill the mortises.  After laying out the mortise on the work piece, drill the first hole at one end of the mortise.  Then drill the second hole at the opposite end.  On the first pass, we always drill out several holes along the mortise, leaving a little stock between each hole.  On the last pass, go back and remove the remaining stock between the holes.   If necessary, clean up any ridges on the walls with a chisel.  We have found that one of the best investments is a 3/8” corner chisel and we prefer the Sorby Chisel.

Sharpening

There are easy ways and hard ways to sharpen the chisel and Lee Valley has a set of stones that make the job very easy.   You clamp the chisel in your vise and using the first stone, which is set at 51 degrees use your drill, and lightly grind.  Remove and install the 50 degree stone and finish with a slow grind.  You will need to lap the outside of the chisel as there will be a slight burr.  The auger bit is sharpened with a fine file on the cutting edge and the spur.  Be careful not to reduce the outside diameter as that will effect the hole size.

Brian Murphy
American Furniture Design Co