Parting Practice Chatter

[RVJimD]

Guys,

I spent a few hours yesterday practicing parting off on the lathe. First let me say I'm sort of talking myself thru this because I think I have read up on the procedure and watched countless videos on the subject. Yesterday I thought I was doing pretty good on the 6061. Today I did a few more with the aluminum and didn't have any problems. So, I pop a 1" piece of 1018 in the chuck and the first attempt goes pretty good. Then the next one starts to chatter like crazy and I have been out there for an hour with no luck finding out what changed.

I'm using a Grizzly G0752 using an import AXA QCTP. I have rigged up a drip oiler because I want to be able to keep both hands on the hand wheel of the cross slide for better control and more even feed.

When it is going well, I part off a narrow piece, loosen the jaws and pull out a bit more for another go. This way I don't have to loosen the lock on the apron.

i have re sharpened the cutter, tried a 1/8" cutter (I was using 5/64") I tried choking up the cutter in the holder to the point that there is no way I would make it thru to the center. I tried slower and faster rpm. I had been using about 350rpm. I re chucked the part. I even have two holders and they both behave the same. I went back to 6061 and although I was able to get it done, I could tell it wasn't working as easily as before. I checked the tool square to the stock and on center. And, I'm not even getting far enough into the work to create and side bind on the tool.

Hum... Frustrating part is I was just about to think I was figuring this out, now I feel like I need sharper teeth so I can try chewing it off!

Here are a few pics of the setup.

 

[Terrywerm]

Jim, the only thing that I can see is that your speed might be a little too fast yet at 350 RPM, depending on the size of the stock, and there may be a problem with the way your cutoff tool is sharpened. In the first photo it appears to be sharpened with a slight concave to it. The curved chatter marks on the work support that as well. When ground that way, the cutting line is longer than a straight grind, but even more significant is the fact that only one spot on the cutter is actually at the correct height on the work. There may be other issues that are not apparent in the photos, so make absolutely certain that there are no chips trapped under the tool holder, or under the toolpost itself. Also try locking down your carriage and make sure that the gibs for the cross slide and compound are not too loose. What method are you using to make sure that you have the work exactly at center height might I ask??

 

[Randy_m]

When I am parting and start to get chatter I increase my feed to get through it. It took me a while to learn too be more aggressive.

 

[RVJimD]

Terry,

thanks for the reply.

the concave is from my 6" wheel. I tried a steeper angle when I thought maybe it was too vertical and possible bumping into the stock before the cutting edge.

to align the tool for center I position it on the end of the part and compare the tip of the cutter to the (in this case) small nub left from the last part off. I have checked a few tools in the past with the dead center in the tailstock but I haven't done that here. I might try putting a small round in the chuck and do the "hold the ruler vertical method"

I did remove the post and holder and clean and inspect for chips.

Jim

 

[RVJimD]

Randy,

i didn't see your post earlier.

i have tried that and it get so bad I'm afraid of snapping the tool.

Jim

 

[Terrywerm]

When you sharpen your cutoff tool, are you holding the tool on it's side? It should be held with the cutting tool vertical, cutting edge at the top, grind only on the face of the tool, never on the top. It appears that you are using what is called a T shaped cutoff blade, they should never be ground on the side or top.
If you are sharpening vertically on the face end of the blade, and your wheel has that much curvature across the face, you might want to dress the wheel to get it flat again.
I also stone my cutting tools with a good Arkansas or India stone after sharpening on the grinder. Makes the cutting edge so much more keen and really improves the character of the cut.

Some guys will sharpen their cutoff blades with a noticeable concave on the front so that the blade completes the cutoff more cleanly on both halves of the cut, usually eliminating the need for a facing cut if surface finish is important. Usually this is done on production lathes that are much more stout than the type of lathes that we have. Of course, most people working in production environments no longer use HSS for parting off, either.

I am not intimately familiar with your lathe model, what size is it, ie: what swing does it have?

 

[Terrywerm]

Scratch on that last question about the size of your lathe, I just looked it up and see that it is a bench model, the same size as my Logan 210, which is also a bench model.

Make sure that your lathe is firmly fastened to the bench on which it sits, and if the bench is a little on the light side, it might be a good idea to fasten it to the floor.

I had a similar problem at times when parting off on my Logan, though not as pronounced as what you are seeing. I found that my lathe was not fastened firmly enough to the bench, getting it tightened down solidly made a noticeable difference.

I also found that a narrower cutoff blade works MUCH better. You will find that a 3/32" wide blade will make life so much easier than a 1/8" blade. You could drop down to 1/16" but that is almost too narrow for doing deeper cuts.

 

[Andre]

Looks like you have a lot of help already from Terry, but I thought I'd throw this in case it helps. No expert my any means, so take this with a grain of salt )

Never had a problem with parting at high speeds, even at 980 RPM or so I can buzz through most metals without chatter with an aggressive feed rate, so I'm not sure but I don't think speed is your problem. I can't get hot rolled to part off without chatter, so it could be a materials issue also, just keep that in mind. Lots of hard spots in HR that lead to vibration.

Chatter is either vibration, tool digging in repeatedly, or a mix of both. Look at your setup, make sure the blade is in the holder tight, toolpost is locked down, gibs are tight, lathe is on a solid (yet ever so slightly flexible, like rubber feet) base, and even make sure the headstock bolts are tight. Parting takes a lot of force, so any weakness in your setup will show.

If in doubt, try holding your parting tool upside-down in the holder and try it. If your holder is like my Aloris 7 holder, it has back rake added to it so you will have to part from the back to get the positive rake angle.

Another member recently posted something brilliant about parting and using form tools on his 7x lathe, and it worked like a dream. The trick is to use your tools upside-down, so instead of the edge catching and being forced down, into the work. It gets pushed up-and-out of the work, reducing chatter.

And of course make sure your parting tool is sharp, or it will give you lots of grief.

 

[Dranreb]

When I am parting and start to get chatter I increase my feed to get through it. It took me a while to learn too be more aggressive.

It took me a while to learn this too, I think it works because of the diameter change affecting the surface speed and depth of cut as the workpiece gets smaller, when you know your tool and everything else is spot on but you still get chattering it comes down to feel...

Hard to explain but over analyzing it made me loose confidence whereas pushing through just works most of the time....

Bernard

 

[Bill Gruby]

There is good reason that it takes a while to get used to greater feed when chatter starts occurring. Natural instinct is to back off not push thru. But that is exactly what you need to do, inceease the feed and push thru till the chatter stops or the piece is parted off.

"Billy G"