Required Material: - Rod of 6 mm (or 0.25 in.) diameter, about 75 mm in length of silver steel or steel that will hold a cutting edge. Photographs 19 and 20 show the cutter. The rod is held in the lathe chuck with about 15 to 20 mm exposed, the top slide is set to 45º to determine the grooving tool shape, and the lathe tool is adjusted so that only its cutting tip touches the rod during the entry of the turning operation. Make the entry cut by adjustment of the top slide only to a depth of about 0.5 mm. Make a small traverse towards the lathe chuck if the lathe rigidity can cope with this depth of cut in one pass do so for about 10 mm, else use the cross slide to reduce the depth of cut and do it in several passes. Trim the end so that the grooving tool comes to a very fine edge at the junction of the 45º face and the end.
Next cut the two flutes 180º apart using an end-mill of about 10 mm (or 3/8”) diameter i.e., only one third of the diameter of the end-mill is used to cut the radius of a 6 mm diameter rod. The machining set up of the grooving blank is positioned so that the end of the end-mill cuts the final cutting edges and the trailing edges are cut by the side of the end-mill. I used my lathe chuck mounted on my simple DIY dividing head to cut the two flutes. To make it an effective cutter the cutting edges must have back-relief, to do this we mount the cutter 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). 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 near side will then be horizontal in this position. Now, place a piece of 10 thou shim stock between the horizontal jaw and the cutter shank, and push the cutter into the chuck until only the 20 mm cutter shank extends beyond 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.
It helps to smear the cutting edge surface with marker pen or engineer's blue so you can see just how much is being cut. Start the lathe and take a cut of a depth that just reaches the leading edge. 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 180º. 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. For cutting Acetal or Delrin I did not harden and temper my cutter. Should you wish to harden and temper your cutter: just heat it to cherry red and quench, polish so you can see the tempering colours develop and heat to light-straw, immediately plunge it into cold water.
