Parting off problems... advice needed...

[jpeter]

Parting tools mounted in front tends to lift the work out of the bearings. Any play in the bearings, which all lathes to some degree have, provide the opportunity for chatter. Tools mounted behind the work push the work, chuck, spindle, down into the spindle bearings like the weight of all the parts do. This setup provides less opportunity for the work to move away from the cutter.

Back is better but my lathe doesn't provide for that. If you're using a springy setup like an Armstrong tool holder use a piece of scrap as a prop under the parting tool close to the work to stiffen it up to prevent chatter. You'll be shocked at how it improves your parting jobs. Keep your tool as short as you can.

Regarding tool height, sketch out a profile view of tool and work and you'll see there's no possible way to part with the parting tool above center. Any amount above center and the tool face just rubs on the work surface. Getting exactly on center is tedious so slightly low seems like a good compromise. Parting off low gives you the small tit in the center. The closer to center you are the smaller the tit will be.

 

[rake60]

When I was training people how to part off on a lathe I'd recommend they
put in ear plugs.

Parting is usually noisy. If you can't hear the popping,snapping and grinding
it's easier to do. Seriously!

Nobody looks forward to parting off. It's one of the most difficult operations
to do on a lathe. I don't know of anyone who claims to be a master of it.

I remember having a 9 foot rock crusher adjusting nut on a vertical boring mill
at work. It had a 1/4" thick dust guard on it that needed to cut off so weld
repairs could be made to it. I parted it off. When the steel started to make that
aluminum foil in the wind sound I slapped the E-Stop and RAN! LOL

No big deal. The 200 pound dust guard slid gracefully off the front of the machine.
I picked it up with the overhead crane and hauled it to the scrap hopper.

SO... Parting on a lathe really isn't that tough.

Rick

 

[kcmillin]

I recently started work at a prototype shop. I was given the task of parting off a part from the CNC Lathe, a 4 inch diameter steel motor coupler with an interrupted cut caused by 8 bolt holes going through it.

I can't say exactly what type of insert tool was in the holder, but I was able to part it off using the powerfeed at about 400 RPM, and all this on a "Cheap" 14 inch Chinese lathe.

I never would have though I could so this just a month ago. It goes to show what actually is possible with these tools.

Kel

 

[Blogwitch]

This is a perennial problem that crops up every few months and always seems to bring the same old answers with it. Maybe we should have a special parting heading where people can be directed to, and when someone has a new idea about tooling or technique, they could add to it.

All these people showing different types of tooling must show you that there is not one perfect tool that will work with every parting situation.

Once people realise that there isn't such a thing as a perfect parting tool, then they will start to make progress.

I must have at least a dozen different tools to carry out parting with, and each one has a particular job to do.

Of course, I do have my favourites that I use most of the time, but I also realise that they won't do every job that comes along.

For any job over about 1", I try to part off under power, it is when you hesitate half way through a cut when problems start to occur. Also, don't forget, you might have to stop half way through the cut to increase the spindle speed. If I am cutting all the way through in one go, I try to start off running too fast from what is normal, so that the further in the tool penetrates the better it should cut as it reaches it's perfect cutting speed.

Going too slow when parting is a job and tool killer.


[ame]http://www.youtube.com/watch?v=BG4qEw3eMcQ[/ame]

BTW, the above video was done a while ago before fitting my home made spraymist system, and shows me cutting brass, I can do exactly the same with stainless steel now, if I use a little cutting oil or spraymist to keep things cool and lubed.


John

 

[user 4756]

Hi guys,

When I started out I read all that was said one way or the other.

I use lathes that are getting on for their centenary so the argument about the direction of load on the headstock bearings makes a lot of sense.

I tried both ways.
My problem with front parting was twofold: setting the parting tool exactly square and chips filling the cut, jamming the tool.

I got a cheap rear parting toolpost. One of the main inclusions is a bar on the base that lines up with the T slots so that it will always be mounted square.
This, to me, is the biggest assistance as I can mount it when I want and not worry about anything except the cutting.

As Bogs says, intermittent cuts and too slow a feed does make life difficult!
I just get on with it, wind the saddle across, and it turns out really well.
One thing I read, either part off dry or wet but be consistent; my parting off does not like a little of each, that is, a quick squirt at the start and then no more.

Also, support the end being cut off, but not heavily!

Hope that helps

Andy