Parting a working piece on the lathe is a very common operation. I use the back tool post sold by Taig with a home-brew parting tool made from a wood carbide sawblade (see Instructable). This is very effective. However, I would also like to be able to use the common 2mm HSS stock to say cut necks at the end of threads.
I have two options: first, make a custom made QCTP with a special 2mm thin tool slot, or second make an adaptor which slips into the standard tool post slot. I may make the former a little time later, but I want to try the adaptor first.
Step 1: Design
This adaptor is made from a short length of mild steel rod, 16mm in diameter and 35~40mm in length. This rod is milled along its length to produce a cross section shown in the diagram above. The milled piece is 8mm in height and has an 8mm tongue which fits into the 8x8mm tool slot of the tool holder. There is a 6mm hole drilled through its length, which is slit open to the side. The open side is milled with a 2x8mm recess, which forms a jaw for grapping the tool piece. As the locking screws on the tool holder are tightened against the adaptor, the jaw closes onto the tool to secure it tightly. Three slots are slit at two places spaced roughtly 12mm apart on the top half of the adaptor to allow the jaw to close more easily and provide a stronger grip.
Step 2: Construction
First I cut a length of mild steel rod, 35mm long. The rod is faced square on both ends. Using the milling adaptor I milled 8mm flats on opposite sides of its length. The top and bottom parts of the tongue are then milled away. With the squared section of the tongue held tightly in the milling adaptor vise, I then carefully milled a 2x8mm recess on the other side. This recess should be just slightly wider than 8mm which allows the 8mm tool to fit snugly without any play. The depth may be slightly deeper than 2mm with no effect on the use of the adaptor.
A 6mm hole is then drilled from one end face to the other. This is started with a 3mm drill followed with a 6mm drill. A slot is then milled from the tool holding recess into this hole along the length of the rod. I originally tried to mill this 2mm wide. Unfortunately I advanced the end-mill a little to fast and the thin tool broke. I replaced it with a 2.5mm bit and slowly milled the slot. This width is not very important, and I would guess anything from 2 to 4mm will do. I then used a slitting saw to cut two 1mm slots at two locations, approximately 12mm apart on the top half of the adaptor. The purpose of drilling the hole and opening one side is to allow the milled rod to behave as a set of jaws which bite onto the tool piece to secure it tightly on the tool locking screws are tightened on the tool post.
Step 3: Conclusion and Test
Finally I placed a 2x8mm HSS tool bit into the adaptor and used it to part a rod on the lathe. Parting brass and aluminium posed little difficulty, but with steel it was a little tricky to use the tool from the front. In any case I will be using this to cut mostly shallow necks and shoulders and this would not be a big problem. The good thing is, it is easy and cheap to get 2mm HSS stock which can be ground even thinner if I want to.