Small hole slotting tool.
When I was making the speed reducer for the lathe I needed to cut 3 mm keyways in pulleys that had a bore of 8 mm. My toolpost cutting tool was just too large to tackle this job. The first pulley I filed out to size. This was very time consuming and the resulting keyway was not very accurate. I had to cut keyways in several pulleys. The tool described here was devised to do the job. It is a kind of adjustable broach
As can be seen in the photo the tool consists of a length of 8 mm round steel with a HSS steel cutter protruding at a narrow angle. The protrusion of the cutter can be adjusted by a small grub screw. In use the cutter protrusion is initially set at zero and the tool is then forced through the pulley with a hammer. The tool is then withdrawn and the protrusion increased by a small amount (0.05 mm - 0.25 mm) and the tool forced through the pulley again. This operation is repeated, increasing the protrusion each time, until the required depth of the slot is achieved. The slot formed in this way is accurate and square sided.
As can be seen in the photo the tool consists of a length of 8 mm round steel with a HSS steel cutter protruding at a narrow angle. The protrusion of the cutter can be adjusted by a small grub screw. In use the cutter protrusion is initially set at zero and the tool is then forced through the pulley with a hammer. The tool is then withdrawn and the protrusion increased by a small amount (0.05 mm - 0.25 mm) and the tool forced through the pulley again. This operation is repeated, increasing the protrusion each time, until the required depth of the slot is achieved. The slot formed in this way is accurate and square sided.
This shows the slotting tool without the cutter in place. It is a piece of 8 mm round bar that is an easy sliding fit in the pulley. A 3 mm wide slot has been milled out for a length of about 60 mm. The slot is slightly over 3 mm deep. On the right hand side, in the bottom of the slot, can be seen the tip of a M3 grub screw.
In this photo the tool has been rotated 180 degrees to show the grub screw. This screw is only 4 mm long so it is completely below the surface of the tool.
This photo shows the HSS cutter in the tool in the fully retracted position.
The tip of the HSS cutter is ground to an angle of about 10 degrees. This angle worked well for aluminium. I have not experimented with other angles to try to find an optimum.
This tool is certainly easier, quicker and more accurate than using a file to cut keyways. The pitch of the M3 grub screw is 0.5 mm so about 1/4 of a turn between each pass of the tool is about right in aluminium. One problem with the tool is that the cutter sometimes gets stuck in the slot on withdrawal of the tool. This could probably be overcome by grinding some relief on the sides of the tool behind the cutting tip.