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My parting was sucky sweet sorrow - how to fix this?

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PStechPaul:
I'm copying this from my thread in HSM about my second semester of Machine Tool classes. This is a piston for an air/steam wobbler, and apparently it wobbled loose from the chuck:  :( :o
I thought it would be good to cut off the ends of the piston in the school shop, where they have some good parting tools and more powerful lathes. Maybe not the best idea:



 Up close and ugly  :'( :



 The first toolholder with parting blade needed some adjustment to reduce the overhang, but the socket head clamp screw was buggered up, and jammed by the threads having been forced against their limit. The assistant instructor tried a number of things to fix it, but after probably more than a half hour later, I had to get another set out of the drawer. Looking closely at the cutting tip, I saw that it was gouged on the top edge, so I cleaned it up with a diamond hone until it looked and felt OK. I left the shaft end inside the chuck and set the tool perpendicular and on center, although I gauged it at the long end of the 5/8" stock that was extended probably 5" out of the chuck. The actual cut was about 1/2" from the chuck jaws, which was about as close as I could position the tool without hitting the jaws with the toolholder.

 I started parting using manual feed, and it seemed to be cutting nice curls, so I switched to power feed at about 0.003" per revolution, and I was running about 200 RPM. All was going well, but suddenly the part twisted loose from the chuck and caused the damage shown. The instructor noticed that the tool was not really properly ground, and it was also slightly below center. So he resharpened it and showed me how he could part the material using manual feed (where he could feel how it was cutting), and at 600 RPM. But he also said that the three-jaw chuck I was using was old and worn, and I should have used a better one. So this is the piece he cut off:



 I asked if I could cut the damaged piston off of the shaft, and use this piston by using a press fit, and he said it would be OK. So I may bore a hole slightly undersize to fit the shaft, which reads 0.374". I also thought about turning a slight taper on the end of the shaft and a matching tapered hole in the piston, and another idea is to thread the mating parts. In any of these cases, it may be best to use some Loc-tite.

 Not trying to make excuses, but I think a significant reason for this problem was the fact that I was (and still am) in considerable pain due to my multi-level spinal stenosis, and my left knee (the one with no cartilage) has been hurting a lot. Standing at the machines is hard on me, although I often get involved in the work and don't think much about the pain until I need to move around and walk.  :old:
Here is the drawing for this part:

I think I have a thread here already on this project, but I thought this screw-up belonged here. I'd like some ideas on how to fix this, especially if someone can suggest a better way than I have proposed. If I did it over again using one piece, I think I'd just use the bandsaw to get it close, and then face to size.  8)

philjoe5:

--- Quote ---I think I'd just use the bandsaw to get it close
--- End quote ---

That's a perfectly good approach to avoid parting.  However, despite all the bad press, parting is not all that difficult, but it is unforgiving of a few essential techniques.  From your photos it appears that you're tearing the metal.  The large "pip" on the work indicates to me that the parting blade was not centered on the spindle or the feed rate was too fast or both of these.  Lastly, there should be as little overhang as is possible since you don't want to work against a lever.  That will cause chatter or tearing instead of cutting.  I'm assuming you have a rigid setup, using cutting oil and your rpm is as low as the machine permits, definitely less than 100 rpm.  If I read your plans correctly, you're parting a 5/8" rod which should take a few minutes to complete especially on a light machine.

Keep at it and you'll eventually succeed

Cheers,
Phil

ShopShoe:
PSTechPaul,

I think you are learning well and are finding out what works and what doesn't work with parting just like many of the rest of us have done. It will get better.

I am only a hobbiest and I have a small lathe, but here are some things I think apply to your attempt.

Five Inches of a 5/8-inch rod being parted is probably too long: Gravity will get you as you get near the end of the cut. Don't even think about using the tailstock to support the piece while parting: A worse crash will result.

Good idea to cut as near the chuck as possible and you did. Also it's good to have as little of the tool sticking out of the toolholder as possible.

I agree with your instructor that you need the manual feed and need to feel the cut. In my experience, steady feed is best, taking into account that things change as cut nears the center. If you get chatter, feed less, but don't stop feeding. (See below about work hardening.)

Properly sharp cutter and proper setup of tool is more important in parting than in some other lathe operations. If you don't have it right you will force the cut into chatter or bend the piece or put too much of a load on the cutting edge.

You didn't say what material you were cutting: Different ones take different approaches and you will learn in time. Some have a harder surface and need an aggressive start to the cut. Some "grab" more than others. Some will work harden, particularly if your tool is dull or not set right. Some will harden if you take off too little.

Many have suggested that you part down to a certain point, then finish the cut with a hacksaw. I would certainly agree if you are cutting off a long piece. Then face the part.

You didn't say if you used a cutting fluid. That might help.

These are starting observations. I'm sure others can weigh in.

Good luck to you. Thank you for posting and for asking here.

--ShopShoe

PS,

philjoe5 just posted while I was writing.

I have some of the same things, but my whole post is above

Roger B:
I would say much the same as the others. Rigidity, minimum overhang, slow speed, sharp tool on centre height, lubrication. I have used the tailstock centre to support long work when parting off. As long as you don't use too much pressure and release it just before the end it works ok. When parting large diameters you either have to start off too fast or stop and increase the speed as you go deeper.

I recently parted off a piece of 16mm silver steel (drill rod) at 250 rpm with a bit of chatter at the start which went away as the cutting diameter decreased.

Jasonb:
To me it was being held by the wrong end, rather than hold the short bit of 5/8" that you wanted you should have held it by the remaining 5/8" stock and just parted off the bit you wanted. That way you have the full length of the chuck jaws in contact with the work not less than 1/4"

5/8" I would be parting at 5-600rpm and using power feed and a little coolant, but a syou are starting out handfeed will give you teh feel of what can and can't be done but use two hands on te hrim of the handwheel so you can get a constant feed.

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