Parting off problems... advice needed...

[ttrikalin]

Here's my thoughts.. I'd value any feedback...


I get a lot of chatter when I part off on my Sherline. Gibs are tight.

1. I often use a 1/8 inch cutoff tool (from taig) that I have mounted on the forward tool post. The bit cuts exactly at the center. I get lot of chatter with larger diameter stock (above 0.500"). cutting closer to the center (lower forces?) is much better.
2. Today I ground a 1/16 toolbit, again mounted on the center. It does better compared to the 1/8 tool - but I do get a lot of chatter on a 3/4 steel part...


Questions...

1. Why do I get so much chatter? Am I pushing the limits of the machine trying to part off 3/4" steel or aluminum?

2. Can I get rid of the chatter by using a thinner tool? Perhaps make one by grinding a piece of a hacksaw blade (.032")?

3. Does a rear parting tool help?

4. If I clamp down the cross slide on the bed, would that help? Any ideas how to do that (if it's helpful that is...)...



thanks in advance for your thoughts...

tom


PS. I'm posting the same e-mail to the other lists... I'll compose a list of answers as I get them and I'll feed back to this list also...

 

[BAH101]

I had the same problem with my Taig lathe. I built a rear cut-off tool holder and have no problems now. It could be that there is just not enough mass in these small machines to work as well as it should.
Bryan.

 

[Tin Falcon]

Tom the answer to your questions is likely yes. except maybe diameter of the part.
parting can be tricky but remember basics because parting is a little less forgiving than other operations.

speed general slower than turning . you do not want the part resonating.
feed you need to take a nice chip but do not get greedy . steady as she goes.
tool height center line or slightly low never high. (except for reverse parting with the tool behind)
use plenty of the popper lube/ cutting fluid sulpherized oil for steel w-D -40 kero or A-9 for aluminum
make the compound as rigid as possible tightening the ways can reduce chatter.
make sure the tool is dead sharp hone the tool.
use a real parting tool that is much higher than it is wide with clearance. insert tools are nice because they have clearance behind the cutting edge as well as on the sides.
Minimum stick out of the part and of the tool.
Tin

 

[BenPeake]

Hi Tom

I don't have a taig, but I had the same problem. No matter what I did, parting was such a messy affair that I invariably reached for the hacksaw. Since then I built a rear mount parting tool and now I can part just about anything at any speed with a 1/8" hss parting blade. So I have to agree with Bryan on this one. Rear mount parting tools are the way to go.

Ben

 

[GWRdriver]

I'd value any feedback . . . I get a lot of chatter when I part off on my Sherline. Gibs are tight.

Hi Tom,
The world's best industrial machine tools are massive things. But why in the world does a 10" lathe need to weight 3500lbs?? The answer is . . . for rigidity which is a requisite for accuracy. The reason the Sherline and many other small lathes chatter is because there s not enough rigidity and mass in the machine to oppose and damp the tendency of the tool tip to run away from the cut, which is what chatter is. Tightening the gibs helps, as does narrowing the cutting face as you have discovered, but there is still going to be flexure in a small and light machine which won't go away and yes you are pushing the limits of the machine.

The answer to your second question is maybe, but in any case I can tell you that a ground hacksaw blade (in a holder) makes for a great cutoff tool. The problem I have, and you may discover, is that a hacksaw blade is flexible (duh!) and if too much blade is left hanging out of the holder without lateral restraint the blade will drift left or right in the cut, depending upon which way the tip grind slants. The result is that I am only able to expose maybe 1/4" of hacksaw blade without having a problem with drift so I limit my cutoffs with those blades to maybe 3/8" stock, 1/2" at most.

The rear parting toolpost can be an improvement, so I hear from our British cousins where those are popular, due to the dynamics of the lathe and the cutting forces. This may be in part because so many folks in England have Myford lathes and apparently a rear toolpost works better in that particular machine. I've read the technical explanations (or claims) but I haven't tried one. However the newest thing for the UK has for some time been a carbide insert cutoff tool sold by Greenwood Tools (IIRC) which practically eliminates chatter and zips through almost everything, but it's for larger machines 7" swing (3.5" in the UK) and above.

 

[ttrikalin]

Thanks to all for your feedback...

On the side, what is the correct height for a part-off tool (forward mounted)?

1. I always thought that right on center is best. This is also what an experienced machinist told me (I buy stuff from him at the MIT swapfest during the summer)...

2. The Sherline book says a bit lower than center for larger stock (a bit), on center for small diameters.

3. I have received advice to have the tool up to 10 thou per inch HIGHER than the center.

Any firm answer?
tom

 

[Tin Falcon]

always set up tool to defect away from part not dig in .
For normal parting if you set the part on center or slightly below the tool will cut a bit less. If the tool is high there is a risk of rubbing and not cutting or if the tool deflects down in it will take a deeper cut.
again I say set up the tool to defect away from the part not into it.
Tin

 

[peatoluser]

I use a tiag lathe and i found that the 2 big improvements to parting off where a rear toolpost and plenty of cutting oil. I keep a small bottle filled with cuuting oil and use a small paint brush to apply it. I soak the brush and just rest it on the job. Tin's dead right about taking a nice chip and steady feed. if you're too tentative the tool will start rubbing go blunt and chatter will ensue.
Something not mentioned is choice of material for parting off. If you're not going to harden, solder or braze the part afterwards then it's worth investing in the freecutting materials available. they really do make a difference. I have no problem parting 1 inch dia. bar in the tiag with freecutting MS.
I've also parted the heads off some 16mm by 150mm (that's about 5/8 by 6 inches in the understanable scale ) allen bolts in, I believe, high tensile steel. what are they made off? all i know is they blunt my hacksaw blades ruddy quick.
if I've posted the link right this should be a photo of it.

blade is off hand ground 3/32 thick about 1 degree rake . I reckon it was the cutting oil that made it possible.

 

[ttrikalin]

Thank you peatoluser!

I think I'll go for rear mounted tools and revisit setup and cutting oil, as suggested here.

BTW, any clues on why rear cutting seems to work better than forward cutting? The only difference see is the direction of the forces on the knife.

downward and proximal (forward cutting)
upward and distally (backward cutting)

but other things being equal, why would that make a difference?

tom in MA

 

[BenPeake]

BTW, any clues on why rear cutting seems to work better than forward cutting? The only difference see is the direction of the forces on the knife.

downward and proximal (forward cutting)
upward and distally (backward cutting)

but other things being equal, why would that make a difference?

tom in MA

Hi Tom

I think the main reason it works so much better is because you are mounting the tool directly into the toolpost which is mounted directly on the cross slide. This severely limits any flex in the setup:

If you mount it conventionally, the tool is in a toolholder, which is held in the toolpost, which sits on the compound slide, which sits on the cross slide leaving plenty of opportunity for a little flex.

I have a Myford brochure which advertises a parting tool/slide which mounts directly to the bed of the lathe, which I suspect would perform best of all.

I'm no authority on this I just know it works and this is my guess as to the reason. Hope it helps,
Ben

 

[peatoluser]

I'll try and answer that as best as I can. But I can only go on what I've been told and observed personally. Whether to use front or rear for parting off can often degenerate into one of those never ending debates like we have on this side of the pond about the virtues of metric v imperial.
For large machinery with taper headstock bearings, ball leadscrews etc, I honestly think it doesn't make any difference, except in one respect, which I'll mention at the end. (well, so I've been told), but, from what I've observed at our size of lathe it can.
The main problem a rear mounted tool solved for me was dig-in. Given that nothing is truly rigid and that all moving parts have some clearance, as we plough in from the front the work is always trying to climb onto the tool. this is not such a problem until the diameter reduces and, as we are hand feeding, the pressure we apply varies so the tool can stop cutting momentarily . the work piece will deflect minutely and try to climb onto tool. Because we are feeding from the front, the cross slide nut is pushed back onto the cross slide screw and there is clearance in front of it (the backlash we have to deal with when we mill on the lathe) All these clearances, deflections forces etc. can, when the tool tries to start cutting again, cause it to take too big a cut and be pulled into the work, stalling the machine
(I'm sure we've all seen something similar to this when we've inadvertently set the parting tool too low and just when we expect the job to drop off it rides over the tip pulling it forward, or is that just my sloppy workmanship ;D)
now ,from what i understand, when the tool is inverted at the back of the lathe, the forces are trying to push the tool up and away from the cut and the saddle back against the cross slide nut, so when it starts cutting again there's less chance of it being pulled into the job.
Now, the problem with all this theory, is that when I've mentioned to some machinists as to why the work doesn't try to climb under the job and pull the tool forward i never get a clear answer.
like i said , it can be a can of worms starting this debate, I just know i have less problems parting from the rear than the front.
but i did mention one difference at the beginning and that is chip clearance. with a rear mounted tool the swarf just drops onto the bed rather than sit on the tool, and given the slap dash method i use for putting on the cutting oil, it may be that rear mounted the tip is lubricated better than forward facing so cuts better.
incidentally there is also one more reason for using a rear tool post in our size, and that is, if you're using the standard tool blocks like me, you don't have to keep changing them to part off, thereby loosing any datums you are working too. Although It can get crowed on that small cross slide!.

 

[Lakc]

Some of the wisest and most helpful words on parting off by teenut
http://yarchive.net/metal/parting_off.html

That really helped my parting off. Rereading it every so often helps quite a bit as well.

 

[Tin Falcon]

The other option is turn the tool upside down in the front tool post and reverse the spindle direction. But will not work on a threaded spindle nose like a Sherline or south bend. It will work well on a mini lathe as the chuck is bolted on.
Tin

 

[compressor man]

I too have come to dread whenever I have to part steel. I am intrigued by many here saying that a rear mounted cutter works much better on small lathes (I have a mini lathe). Would someone mind elaborating a little on this, I have never before heard of this term.

Chris

 

[Tin Falcon]

CM some lathes have a place or the capability or room to place a second tool holder behind the work . Typical these hold a tool that would be upside down in reference to a tool held in a front tool holder.
If you have a mini lathe try the above mentioned method.
Tin

 

[vedoula]

thanks all for the advice.

Still I do not understand the front vs rear debate.

The setup/arrangement is symmetric and should not matter IF THINGS ARE RIGID... I am sure I'm missing something...

If the setup is not rigid, then the tendency of the rear mounted tool to deflect upwards ("extension") vs that of the front mounted tool to deflect downwards ("compression") is the only difference I can see... And this would only matter if they are not symmetric...

And this is where I lose the ball...

tom in MA.

 

[Lakc]

The setup/arrangement is symmetric and should not matter IF THINGS ARE RIGID... I am sure I'm missing something...

With all things equal, rigidity, tool height, etc. the rear mounted toolpost gets a 9.8m/second head start evacuating the chips thanks to gravity. Sometimes that is a big help.

 

[BAH101]

Not really sure why it works so well, but it does. Before I made my rear holder, I dreaded using the cut-off tool, like Ben, I would use a hack-saw. Now, I can cut pretty much anything, and if you get the feed right, it is like going through butter with a hot knife. I usually just drip cutting fluid onto the cut-off bit and let it run down into the cut.
Bryan.

 

[Ken I]

As Tin mentioned your blade needs to be below centre (not by much) so that the tool always gives away from the work rather than into it.

There should be no difference between front and rear - BUT THERE IS - the rear post is generally mounted directly on the cross slide so you are eliminating the give / clearance etc. in your compound slide / swiveling toolpost etc.
And yes gravity helps in clearing the chips.
The upward forces tend to pull the dovetail slide tighter - downward there is always going to be some clearance or "wiggle room".

I have run a fairly large NC / Auto lathe shop and part to centre is normal.

One thing I have noticed is that chatter is less prevalent in a collet chuck than in a three or four jaw (less so with contoured "soft" jaws) - I have come to the conclusion that internal flexing within the material is also a factor - you have probably broken a part off blade and you will notice the turned diameter has three lobes when turned in a three jaw chuck.
This might just be unjustified superstitious speculation on my part however but I think it significant.

See my post on the laws of physics being out to get you with chatter.

Ken

 

[Ned Ludd]

Hi Guys,
A couple of years ago Cherry Hill's hubby, sorry I can't remember his name, was showing some sprung tooling for parting tools at Harrogate(?). Although not a new idea they are, I am told, very good at preventing chatter.
Ned