I engraved a circuit board today and before starting I decided I needed a better engraving tool for the job, so I made one. This is a very easy tool to make and it works very well. It's designed to take 1/4" bits, in this case a HSS router engraving bit. There are no adjustments and changing the tool bit is a one second process. It is spring loaded and very repeatable.
It has only one machined part, the main barrel. It is drilled out on the lathe to a few thousandths of an inch less than 1/4" and then reamed to 1/4". This is the critical part in this type of tool holder as the bit must not have any free play from side to side or it will distort the engraving as the lash is taken up each time it changes direction. The barrel must be of a non magnetic material such as brass or aluminum. The upper part of the barrel is turned to fit a convenient collet size, in this case 3/4" but it could be anything down to 1/2 or even slightly smaller. The top .5" of the bore is tapped 5/16 x 18 NC to fit a set screw. A spring is needed and almost anything that will fit freely inside a .25 hole will do, even a ball point pen spring.
The key items in this design are a couple of small super magnets, in this case .312" in diameter. A .25" through hole is drilled in the lower barrel and it is then counterbored for a press fit of the magnets. The magnets should seat on a rim at the bottom of the counterbore so they cannot pass into the central bore. If a good solid press fit cannot be obtained then the magnets should be retained with locktite or similar. If there is any doubt as to the security of the magnets then an outside collar of brass or aluminum should be fitted to ensure the magnets do not fly loose in operation.
Another feature of this tool holder is that there is no need for a drive pin or dog of any sort. The magnets are emplaced so that one presents a south pole face and the other a north pole face. In other words they should be attracting, not repelling. The tool bit is wiped across one of the magnets before loading and this is sufficient to magnetize the tool so that the magnets not only retain the tool but drive it as well.