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pconnect92352
07-24-2009, 01:51 PM
I am a beginner to lathe work and am having problems with parting off. I seem to have the same problem every time. First is chatter. I get past the chatter by pushing into the work a litter harder which results in a small ribbon of material being cut. All goes well, or I think it does until I get most of the way thru when I no longer get a ribbon of material just small shavings and I am not cutting the material. I have re sharpened the toolbit, checked to make sure I am on center, added oil, but still the cutting action has stopped. The other day I attempted again. Same results, so I pushed the toolbit in a little harder to continue the cutting. The result was a disaster, the toolbit pulled under the work and suddenly a sharp crack sound then the tool post itself pulled under the work. I quickly stopped checked everything and found that the compound slide/toolpost slide had broken in half. Beside replacing the slide, which is turning out to be a difficult task, what the heck am I doing wrong?

Rusty Marlin
07-24-2009, 02:02 PM
your tool is above center. Set it .001-.003 below center.

winchman
07-24-2009, 02:15 PM
I agree.

The radius is larger at the start of the cut, so it works OK. Once the radius gets down to a certain point, the tip of the tool isn't touching anymore. The contact point has shifted to the flat surface just below the tip. More pressure created enough friction to drag the tool tip down just enough to dig in with disastrous results.

That's a real shame about busting your lathe. I hope it's one with replacement parts available for a reasonable cost.

Roger

Jack F
07-24-2009, 02:19 PM
The above post are correct . I would add, you don't give any indication of the size of your lathe or the material diameter and type you are trying to part. I,m going to guess it is a rather small light weight unit. It sounds like your parting blade gets dull before the end of the part. If that is the case you need to re- sharpen the blade. When you sharpen the blade it needs to be as sharp as you can get it. As you have found out never force a dull cutter of any sort into the part. Also make sure you have front clearance and side clearance on the cutter.

Jack.

SGW
07-24-2009, 02:20 PM
You may take some comfort in knowing that parting off is a problem for a lot of people, it's not just you.

Rusty's suggestion is certainly an excellent guess based on the symptoms you describe.

If adjusting tool height doesn't solve your problem, try a narrower parting off blade if you can. That might help. Cutting oil may help. And play around with the speed, noting that the surface speed diminishes dramatically as you get closer and closer to the center of the work. A reasonable rpm at 2" diameter is going to be way slow at 1/4" diameter.

pconnect92352
07-24-2009, 02:47 PM
Thanks for all your input and expert advice. The lathe I have is an Atlas 9x48. It was my Grandfathers. The piece I was parting is approx. 3/4" and I was using a slow speed. I was also using the quick change toolpost with a parting blade in its holder not the rocket post. I ground the toolbit to approx 7 degree and I thought it was sharp, but... so I should use less pressure to cut? when I do I get chatter and no cutting.

Ian B
07-24-2009, 02:54 PM
As SGW says, parting is a common problem (I've had my fair share of parting tools go 'bang') but I've never heard of a compound slide breaking in half!

Was the compound slide wound all the way forwards, and was the section of it holding the toolpost not supported by the mating slide surfaces beneath? Whatever the lathe and however the tool's set, as much overhang as possible should be eliminated.

Is yours a new lathe, or is it old, with backlash in the cross slide feed screw and loose slides? Even if a parting tool's set to just below centre height, if it has top rake and there's wear in the cross slide feed screw, the tool can dig in and suddenly self-feed and break something. Generally, that "something" is the tool or the work.

Ian

Liger Zero
07-24-2009, 03:08 PM
I suspect the slide was damaged at one point and repaired and the repair came undone.

I too have had problems parting off, best advice I can give is to find a Real Machinist nearby and have him show you a few things. Everyone has a sightly different method/technique and way to grind the tool.

Advice on the forum is great but sometimes you need to see it done with the Expert Touch before it makes sense. :)

MTNGUN
07-24-2009, 03:37 PM
I'll join the cast of characters trying to make you feel better by admitting that I, too, have had problems parting. Lots of swearing, things got knocked out of alignment, the work was trashed, but fortunately my lathe has survived intact -- so far.

Good advice has been given.

Like you, I have had more trouble as the cut approaches the center of the work. If it is possible to drill a hole in the center of the work before parting, that eliminates one trouble spot.

I've had better luck with the skinny carbide insert parting tools. This is one case where carbide is better even on small home lathes. Change the insert frequently. Don't wait until the insert fails, because failure at parting is painful, as you have learned.

For a while, I was parting 18-8 stainless steel on a production basis, turning out thousands of parts. I could get it to work 99 out of 100 times, but I could never make it work 100% of the time no matter how carefully I dialed in the process. Call me a wimp, but I gave up and moved the parting operation to the bandsaw, even though it took a little longer and the finish was not as good.

I still part occasionally, when there is no other practical way to do a job, but I try to use the bandsaw more and the parting tool less. I came to the conclusion that my lathe is not rigid enough for 100% reliable parting.

DaHui
07-24-2009, 03:45 PM
Parting can certainly be a pain. It's been said but "no cutting action" is probably a tool that is above center. If you are using HSS try grinding a bit of rake into the top. I'm also a huge fan of insert parting tools. You shouldn't really need much "pressure" at all in either case.

Two things to note, chatter can occur if your parting tool is extended too far from the tool block. The other is if the area you are trying to part is too far from the chuck. Make both as short as possible. Chatter can also be caused by too little in-feed but that doesn't sound like the problem.

I don't know if finish is important but sometimes I'll part it most of the way and then use a hack saw to do the rest. You can run the lathe and just hold the blade in place...but I'm sure someone will want to talk about how that is a really bad idea.

http://www.the-alchemist.com/Images/Shoptask/Parting%20Tool/ground.jpg

Frank Ford
07-24-2009, 03:54 PM
Parting is not sweet sorrow. It's more like bitter misery if you don't have a heavy lathe.

My first lathe was an 11" Taiwanese one, and parting was always a bit scary.

Now I have what I'd consider a fairly stout 11" lathe - it's a Hardinge clone and weighs in at over a ton. I try to avoid parting off anything over a couple of inches.

Got a nice surprise when I first used a pal's 15" Clausing Colchester. It parted off 3" steel without a bit of difficulty - it felt like I was taking a facing cut. (Well, almost.)

PaulT
07-24-2009, 04:27 PM
If you're only going to have one carbide tool in your shop, make it your cutoff tool. HSS cutoff blades can work, but in my experience the window of correct setup and usage is much narrrower than with a carbide cutoff tool.

I'd recommend one of the cutoff tools that uses either the GTN 2 or GTN 3 inserts, depending on how big your workpieces are.

Paul T.

Carld
07-24-2009, 05:18 PM
I'll second that Paul and since I got one my parting trouble have for the most part went away. Since he has a QC tool post that won't be a problem. At least after he repairs his lathe.

It's extremely important to be at center line or just a couple of thousandths low and no more. NEVER above the center line and have the tool as square to the work as possible and watch the speed and feed.

Bguns
07-24-2009, 05:30 PM
Pretty light lathe. Have many hours operating 6, 9, 10, and 12 inchers....

Parting is tough on them (the lathe itself)... Narrow tools, perfectly sharp cutoff tools, still going to have problems.... Too much spring in tiny dovetails, Zamak thrust plates...

I have done it, but it is exciting not fun..

Rather have fun and not break things...

Just avoid parting as much as you can, and live with the limitations of a lightweight Home Shop Lathe.

They can do good work, just in smaller bites...

Have not tried an upside down rear mount cutoff tool on an Atlas, would probably help. You need a custom crossslide to accomodate it.

http://www.statecollegecentral.com/metallathe/A-11.html


Edit: I have time on bigger things, like a 20in Monarch, and have access to a big Pacemaker :) Parting is a joy.. But if something hangs up, big chunks go flying....

My 12 x 36 does OK, and even my 9 in SB is fair parting... The Atlas's are really pushed hard parting..

Oldbrock
07-24-2009, 07:41 PM
I grind the top of my parting tool the same as Da Hui with a dremel so that the grind lines are in line with the path of the chip as it exits the parent material. I have had the same parting tool for years with no problems. Don't have a heavy duty lathe, just a 10K. I always make sure the compound slide is flush, no overhang. I have seen too many broken just as you did. Center height is important or just a few thou low and a touch up with a diamond hone once in a while. I have another flat top cut off tool for brass and bronze. The one with positive top rake will not work for these materials. When parting steel I always use soluble oil and water, never dry. Hope these few comments help and sorry for the broken compound. Should be able to find one on ebay with luck. Peter

Rusty Marlin
07-24-2009, 08:13 PM
Pretty light lathe. Have many hours operating 6, 9, 10, and 12 inchers....

Parting is tough on them (the lathe itself)... Narrow tools, perfectly sharp cutoff tools, still going to have problems.... Too much spring in tiny dovetails, Zamak thrust plates...

I have done it, but it is exciting not fun..

Rather have fun and not break things...

Just avoid parting as much as you can, and live with the limitations of a lightweight Home Shop Lathe.

They can do good work, just in smaller bites...

Have not tried an upside down rear mount cutoff tool on an Atlas, would probably help. You need a custom crossslide to accomodate it.

http://www.statecollegecentral.com/metallathe/A-11.html


Must be I'm spoiled, I read about all these problems with parting and I must confess that the only time I've expiranced chatter was when I was learning to thread rifle barrels and the chamber end was supported on the tailstock center. I cut the the thread releif with my parting blade. Now I toss a spider on the barrel just infront of the cut and use the steady rest to soak up the extra vibration, if I can't hold the barrel directly in the four jaw that is.
I have a 13x40 gear-head and it handles parting with no problem, my parting tool is one of the most used and value added tools in shop. It's a plain old standard HSS parting blade with the side relief ground in. I use the P-2 held in a KDK holder
cutoff blade (http://www.use-enco.com/CGI/INPDFF?PMPAGE=167&PMITEM=317-5027)