Hi experts.
I need an advice on machining an 86 inch (2200 mm) long 3½ inch (89.9 mm) acme thread, with ½ inch (12.7 mm) pitch.
We have machined long acme threads before, but never this long. And in the past we also had the possibility of using a steady rest, as our Daewoo Puma CNC lathe was long enough for this job. Now that we are to machine longer threads, we have purchased a second handed Geminis GHT4x4000 lathe, suitable for this kind of work. The problem is just that this is a flat bed lathe, and there was no “mobile” or following steady rest, following this machine. It has a self centering fixed steady rest, but this means that the slide cannot pass the steady rest.
So my question is, if it will be possible to machine this 86 inch (2200 mm) long thread without a steady rest, still avoiding shattering? Or will it be necessary for me to buy or make a following steady rest? The material is stainless steel.
I think with a screw that long you'll *need* a steady rest, almost certainly a travelling / follow rest, as the part (even at 3"+ diameter) is going to deflect enough to give less than full depth of thread in the middle. As it's an Acme, the easiest would be a pair of roller-bearings close to opposite the cutting tool, as the outer diameter's going to be pretty much constant during the cut (raised burrs excepted!), is there any way you could attach it to the toolpost (even if it means adjusting it for each pass), or into a T-slot in the top of the lathe saddle?
Long full acme thread machining
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