Backside parting tools

mickri

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I have been reading a lot about parting off and the problems associated with parting. To be quite frank I am not very good at parting off. After getting less than stellar results whenever I try to part off I usually end up resorting to my trusty hacksaw. Not pretty but it gets the job done and facing off the rough cut is no big deal.

A common solution echoed on this forum and others is to part off from the backside. I have searched high and low without much success looking for ways to mount a parting tool to cut from the backside. All I find are lathes that come with an extended cross slide with slots on the backside to mount a parting tool. But I have found nothing for your typical lathe like my craftsman 12x36. Not even DIY/homemade solutions.

The only thing that I can think of is to make a part that would fit on the dovetails of the cross slide to hold the parting tool. But how would you go about attaching this to the cross slide? Would it need gibs? Has anybody done this?
 
Greetings, try parting with the lathe in back gear. I have an Atlas 618 and have no trouble parting everything from alum. to drill rod.
 
I have one of the notorious Mk2 Clausing lathes, its a 6" by 18 or so. I removed the compound, fabricated a piece of 3/4 by 4 by 8 or nine" 1018 steel to fit on the boss left behind on the cross slide. On top of it I mounted a four way tool post of my own design, and cut T slots in the back for rear mounted tooling. If you do this, study the clamping of the original compound on the boss, and duplicate it. I'''ve since found, after replaciang the plastic compound gibs with steel ones, that my setup is solid enough for parting from the front. I use the rear mounted tools for chamfering.
 
Mickri - parting can be frustrating, and requires some skill to do well. on large lathes like you see on the popular youtube channels, parting from the backside and mounting a tool makes a lot of sense. On a small late like the 12X36, even if you mounted a backside tool, chances are that your results won't be much better...

This is because the cross slide is fairly light weight in relation to the forces involved, and the backside parting puts upward pressure on the dovetails instead of the flatways. I'm sure you'd get more vibration, not less.

so... what to do? - well there's a lot of things to try and check - but the first places to look are:

1) is the tool exactly on, or a tiny bit below the centre line? if you have an adjustable QCTP, this is one place to experiment with.
2) Are you using a very sharp HSS parting tool? On a small lathe like a 12", using carbide puts more pressure on the machine, sometimes causing problems.
3) are you parting at the right speed? for a 1" piece of mild steel, anything over 200RPM can cause chatter, even on a bigger lathe. I'd recommend 100RPM to start.
4) always ensure your carriage is locked and that your gibbs are tight. that helps a lot as well.
 
I have tried both front and back gear. Just about everything I do is in the back gear and at the slowest speed. I typically make a mark on whatever I am trying to part off and try to advance the cross slide no more than .001 for each revolution of the mark. Nothing I try seems work very well.
 
.001 is too slow. You gotta push the tool. Keep putting more and more pressure on the tool, all of a sudden it will stop chattering and turn out a neat coil of steel. RPM slow, plenty of lubrication.
 
I used to have a tough time with parting. pretty much only work in Aluminum and 360 Brass. I would break parting blades and nothing seemed to work. Then I broke down and bought a indexable parting blade that holds an insert similar to this one from Aloris https://www.mscdirect.com/product/details/09061888
It was very expensive however all of my parting problems ended the minute I started using it. Parting is no longer any thing that I worry about so while it was expensive, it was money well spent.
I do realize that parting seems to be one of the areas that what works for one person seems to not always work for others. Good luck in finding what works for you.
 
I have one of the notorious Mk2 Clausing lathes, its a 6" by 18 or so. I removed the compound, fabricated a piece of 3/4 by 4 by 8 or nine" 1018 steel to fit on the boss left behind on the cross slide. On top of it I mounted a four way tool post of my own design, and cut T slots in the back for rear mounted tooling. If you do this, study the clamping of the original compound on the boss, and duplicate it. I'''ve since found, after replaciang the plastic compound gibs with steel ones, that my setup is solid enough for parting from the front. I use the rear mounted tools for chamfering.

I would explore Tom's idea here. Without getting into a protracted discussion about how forces are distributed when parting from the rear, my experience suggests it is the best way to part on a small lathe that doesn't allow you to run in reverse (chuck limitations, etc). I have parted 2" OD steel on my little Sherline at 1200 rpm with a rear mounted parting tool and a little P1-N blade so little lathes definitely do benefit from a set up like this and I would look into it.
 
.001 is too slow. You gotta push the tool. Keep putting more and more pressure on the tool, all of a sudden it will stop chattering and turn out a neat coil of steel. RPM slow, plenty of lubrication.

This.

Chatter is caused by the tool deflecting downward and springing back. Feed it more, it goes down and stays there. It's harder to do when feeding manually.
 
What you are missing here is the powered cross feed. Makes parting a breeze. I too invested in an indexable parting blade (Shars .087 GTN-2 1/2" height ~$35) and I have been very pleased with it. It is a little more limited in the max diameter to cut off compared to HSS blades, however I rarely need to part a 5" bar :).

I agree with the constant pressure and deeper feed suggestion above. Also make sure to add enough oil to make the chip not bind in the slot. Alignment of the blade is critical. Run it against an indicator to make sure it is straight.
 
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