The Shop > Tools
Rear parting toolpost
awemawson:
It's all about negative feedback (rear tool) and positive feedback (front tool). The rear tool lifts above the centre line disengaging the cut.
The other way of achieving negative mechanical feedback is to use a sprung tool holder, where the parting tool deflects below the centre line if there is a tendency to dig in
Muzzerboy:
It's far better to part off with the machine in reverse if you need to use a rear parting tool. Obviously that requires a spindle nose that isn't simply threaded, for fear of the chuck coming loose. I'd suggest that if you DO need to part off with an upside down tool, you should set up the saddle gibs very carefully. How many people can honestly say they have adjusted the rear gibs recently - or ever? The tool forces act to lift the saddle, so any backlash will result in the tool lifting ie diving under centre and even a well adjusted gib leaves some backlash.
I know people talk about the tool springing or moving as if it's a benefit but the best situation is one where the tool has a well controlled, consistent position and not prone to moving when the tool force exceeds the weight of the saddle. With the spindle turning in reverse, there is no need to worry about backlash, as the saddle is pressed against the flats of the bed.
vtsteam:
Hi appletree. I think in your mental picture of the front tool, the tool bit is the only thing that is bending. In that case, yes, a tool set exactly on center height will bend away from the point of contact. And if you have an absolutely rigid toolholder, toolpost, carriage and lathe ways with no slop, well then no problem with a front toolpost.
But if you have a smaller hobbyist lathe, then the above conditions are unlikely to be perfect. So you will get dig ins, even when the tool is exactly on center.
Why? you are imagining a case where the center of the radius of flex is horizontal and parallel with the tool bit. But with the rest of the system flexible, that radius will effectively be much lower. In that case the tool will tend to dig in.
When it digs in the tool may break, or in a less severe case, you get chatter. The max amplitude of chatter, a harmonic vibration, is much greater then the normal tool cutting force, so once it starts It is hard to stop, and it has the power to flex many of the lathe components, including the carriage attachment to the ways.
A rear toolpost also has flexible connections to the ways, BUT, with the center of that bend radius low and the direction of rotation now upwards instead of downwards, the tendency is to push the tool away from the work.
And this set of behaviors is all borne out by the fact that big solid lathes tend to part material easier than small hobbyist lathes. They are simply more rigid. And they act as you have been imagining. That's why a rear tool bit is more often recommended to cure parting problems with small lathes. But there is one other advantage for either -- a rear tool holder can carry one tool while the front toolholder can carry another. Thus two different operations can carried out without changing tools.
BillTodd:
--- Quote from: appletree on January 16, 2022, 03:46:51 PM ---Sorry to be unable to see this, as far as I can see the work pushing down on the tool which is on centre height will move the tool tip down and away from the work (reduce the depth of cut) this I understand is how the sprung tool works, the rear tool post is the same in my opinion except the word down is replaced with up. Really want to understand this. Thanks for the replies Phil
--- End quote ---
No the down force at the tip is resisted from way behind the centre line forcsng the tip into the work.
Spring tool are purposely more flexible than the lathe toolpost so it flexes above the centre line which does reduce tool contact.
vtsteam:
Below the centerline.
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