The Ultimate Trick: Cutting off with the Mini Lathe

Hi to all home machinist that like me, have a lathe that has diminished rigidity. Surprisingly, some folks are still not convinced that inverting the parting tool and running in reverse has no real benifitt. Well perhaps I can clarify this phenomenon. Think about your tool post and your parting tool that protrudes out on one side towards your work piece.
Now as the tool enters the work piece there is an immediate down force on the tool bit. Now because your tool bit overhangs on one side and is now being pushed downwards, the tool post bends towards the work piece and this forward movement increased the depth of cut, then the downward forces on the tool bit also increases, so the the tool post bends even more towards the work piece and you get this "run away" effect and it happens is flash, extreme chattering or a sheared off parting tool. Now let's invert the parting tool and run in reverse. As the tool begins to cut, there is an upward force now on the tool. This upward force bends the tool post away from the work piece and this effect reduces the depth of cut and this in turn reduces the upward force, and so on. So what we now have is a level of "self regulation". So without a doubt, on small lathes that don't have the rigidity of bigger machines, running in reverse with an inverted parting tool is a big plus. Please be careful as this procedure in NOT RECOMENDED for screw on chucks.
Regards Basil.
 
Congrats to the person who made an animated simulation to show this. This has confirmed the way I was trying to visualize this problem, after David Fenner wrote about this issue in his book for home machinists. From what I remember, when rear mounting tool posts came into fashion for larger lathes, this was a revelation. Especially for tasks of parting off. I believe he strenuously advises not to part off on a mini-lathe in the regular fashion, and this is specifically why.

It's one of many problems we will have to face with the mini-lathes... even a simple task of knurling has to be done with a specialized tool, as the standard practice would cause too much a load coming directly from the tool post.
 
I find it much easier to part, and form threads in reverse.
 
You must log in or register to reply here.
Back
Top