I often have small work items (e.g. valve blanking caps) that I want to hold on the rotary table in order to machine a square or hex head on the top. These often have a thread on the other end and so easily screw into a threaded mandrel that I can grip in a collet chuck on the rotary table. They are short and offer little other surface for gripping.

The problem is that if the mandrel is held vertically, and you touch the work with a conventional (clockwise) rotating tool, the work item will unscrew from the mandrel. Ahh, I thought, I need a left hand spiral milling cutter to be running anticlockwise. But there aren't any, why not? There are LH taps and drills. The only LH spiral milling cutters are designed to turn clockwise and the spiral forces the swarf and hence work item down on the table.

So far I have got round this by setting the rotary table vertically, the mandrel thus horizontally, and now a clockwise cutter drives the work into the mandrel. But the bulk of the rotary table gets in the way of the milling head.

The thread on the work is cut first on the lathe, then the item is parted off. I cannot see that there is another work sequence to let me cut the hex first on a longer piece of stock.

Any thoughts people? and why are there no LH cutters I can find? I shall have to make a simple one.

Pleased I have caused a little bit of enjoyable thought Some interesting feedback, thank you.

Responses in reverse order.

-touch a clockwise rotating cutter on the edge of a round object and that object will try and rotate counter-clockwise, i.e unscrew upward.

– Fly cutters by definition have a moderate radius and this means a thumping wallop each rotation.

– Putting a small piece of work in a Stevenson hex ER block, and rotating that in a chuck manually, produces errors of at least 0.010". Been there, tried it, a mess. I am looking for 0.0001" in the vertical otherwise that small hex I show in my photo has all manner of irregularities in the horizontal land around it.

-The above is why working with the rotary table horizontally is better by keeping the cutting tool at a fixed depth of cut as it runs in the X or Y past each of the six faces in turn. Hence a perfect face around the hex sides.

– A small, 6mm, three flute carbide cutter is the ideal cutting tool for the brass of this size. I just wish the Chinese made left hand cutting ones!

– I like Neil's suggestion and it is what I thought I would do. Make a 6mm dia cutter from silver steel, mill cut a pair of teeth with rake and relief, harden to a straw temper, then diamond hone the tips

They do exist, found these from America, bit too expensive to import, so might keep looking. 2AAAOSwNhpa55Zd”>link

Edited By norm norton on 09/09/2020 22:08:06