[How do I?] Parting with backlash

ericc

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I am having trouble parting with my new to me South bend 9 lathe. It never achieved a continuous chip and the tip dug in with a bang noise. I broke two parting blades on one piece of 3/4" mild steel. This is really a hassle since the parting blades are time consuming to make. These blades are cut from a defunct carbide wood circular saw blade.

All the standard set up precautions were taken including setting center height, aligning perpendicular to the axis, plenty of oil, etc. These parting tools have been very successful on the smaller Craftsman AA 109 lathe. They have routinely parted 1.25" HR round and even worked on a 2" piece (difficult to hold in the small chuck). They also work great on the Techshop Jet 14-40 lathe. They cut a nice ribbon like chip. They are used in a solid steel block holder, which is very stable.

The main difference with the South bend lathe is that there is a lot of cross slide backlash. The screw is worn and the backlash is about 0.048". But, there are a lot of web posts saying that backlash doesn't matter at all. Here's my theory. It does matter if the tool has a lot of rake like these circular saw blade sourced parting blades. The blade skips and chatters until it is fed sufficiently to dig in, lock and snap the carbide. Does this sound correct? Is there a simple solution that doesn't involve a new cross slide screw and nut (since the Internet posts may be correct and it will be a waste)? I kind of thought that parting was a skill that only had to be mastered once. I'd rather not just do the parting on the little Craftsman 109 lathe, since the heavier South bend should have the advantage. In nearly every other way the South bend is the superior lathe, except for this one.
 
Hi ericc,
The backlash can create problems unless the feed is constant.
You may also wish to inspect and adjust the gibs as well

As far as reducing the backlash,
You can remove the brass nut and slit the nut transverse to the screw axis near one of the ends ,about 1/4" from the end and about 1/4-1/2 of the depth of the nut
Once you have the slit you can manually close the newly formed slit by a few thousandth's (or more in your case) wit a hammer or vise. Then reinstall and retest the backlash.
if the baclash is acceptable you are done, if not repeat the above.

The only real problem with this method is that your feedscrew is not likely worn to the same degree across the entire length and if the slit is closed too tightly it may bind in the less worn sections.
 
I have never run a lathe that didn't have backlash in the cross feed, keep looking
 
backlash is one symptom of wear, but the nut aint the only location it happens. probably have a section of the cross slide that’s tight, another that’s loose. And probably loosey in the section that parting takes place.

Maybe play with speed, tool height, and try tightening up the gibs while parting, both cross slide and compound. Sharp tool, all the usual suspects.
 
Eric, I'm going to guess that the front of the cross slide is lifting as the parting tool encounters the work. This allows the tip of the tool to be pushed below center and dig in. How to fix it on your lathe? No idea but you can test it by somehow pushing down on the cross slide and see if it makes a difference.
 
Backlash is unlikely to be your problem. Normally there is plenty of pressure against the parting tool to keep the backlash from
being a factor. Some materials, like brass, have a tendancy to cause similar problems.

Making a parting tool from a carbide tipped saw blade is a neat idea.

The blade should be horizontal and on centerline. I mention this as the carbide insert could have a bit of rake angle
that causes it to bite or dig in.
Also, if the carbide isn't very sharp too much pressure will be needed and it can reach a point where it grabs and
all that force makes it take a really big bite.

Mike makes a good point as many lathes have play that will cause real problems.
It would best be checked with a indicator but when I used just my hand to bump the inbord end of my
crosslide I could hear the carridge clack as there was a 0.007 gap under it.

My little cheapo lathe wouldn't part ether until I scraped the compound, cross slide, and carridge.
Rigidity was so improved that it will part stuff I never before thought a 13 inch lathe could do.
 
Like others have mentioned, backlash is found on all machines. pictures of the setup go a long way but best guess is your setup is not rigid enough. How is your compound setup? Does it lock? Do you lock your carriage down? Pictures worth a ......you know the rest.
Some machines will never part without drama .
 
Hi Eric,
you mentioned that your parting tip has a lot of rake - if it's not that sharp at the edge it could be rubbing until you put enough pressure on, then self-feeding due to the over-the-top rake and taking up the backlash, putting on a 50-thou cut which is likely to snap the tool tip off...

Just my ha'pennorth
Dave H. (the other one)
 
I keep a wrench handy for snugging down the gibs on the cross slide when parting. Not tight, just snug so it isn't free to dance around.
 
I part on my old C Man 12x36 all the time......plenty of worn points all over. You have to find where yours is moving & make it rigid

A sharp tool is a must.....Buy a real parting blade. I bet that you don't have the proper abrasive to properly sharpen & hone the carbide. I sharpen my blade for every cut in steel. Not so much for aluminum

Works for me
 
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