Turning Question

Martin W

Martin W

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I am making a post for my vice. I have a 8" diameter pipe 48" long. I thought I would clean it up on the lathe to get rid of the rust and scale. I welded a piece of flat stock on one end for the live centre. (was only out 45 thousands with the dial indicator). I made three passes to clean it up and it measured 7.875 on the headstock end and 7.879 on the tail stock end. The problem is its bigger in the middle. My last pass the bit didn't even cut in the middle, only on the ends. What is happening here? I could see if it was cutting a taper, but I don't understand this.
Thanks
Martin


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your pipe has an hourglass figure,
 
What kind of pipe? How much bigger in the middle is it?
 
I'm guessing that the pipe wall is being pushed in under pressure from the tool tip. I'd suggest using high speed steel cutting tool ground with very generous clearance and left very sharp. Use a low RPM (something under 100 if possible) and the slowest feed that you can.

Sandro Di Filippo
 
I am guessing that you do not need .0001" accuracy on the pipe to mount your vise on. Spin it in the lathe and use abrasive cloth roll strips to polish out your tube. Use paint to cover and fill what the polishing doesn't clean up.
 
Using high speed steel, 64 rpm, not worried about tolerances for vice mount. Pipe is just from local salvage yard. It's about 15 thou bigger in the middle. I have never turned anything this big before and was just wondering what was going on here? Just basically trying to get some experience. This is a hobby for me and I really enjoy it. I should have paid more attention to my dad when I was younger but can't do that now.
Thanks
Martin


Sent from my iPad using Tapatalk
 
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It must be smaller in the middle if it is not cutting there, unless flexing is causing issues. How is it held at the head and tail stocks? It sounds like a plate welded on the pipe at the tail stock end with a center recess cut into it for a live center. Is it held in the chuck jaws at the headstock end? Did it clean up completely at the head and tail stock ends, just not making contact at all in the center?
 
It's either smaller in the middle or the ways on your lathe are worn badly where you don't get contact... If the ways are that bad you should be able to see that crater! Take a set of spring calipers and see if it is really wider than the ends.

Take a long straight edge and look at your ways... the bed of that lathe would have to be worn out or bent... It would be the only way you could turn a curve like that. I might believe worn, never seen one bent.
 
This doesn't seem to me the standard for way wear. If you had enough wear for the carriage to fall a significant ammount to cause a drastic difference in DOC on such a large diameter it would be fairly noticeable.

Take micrometer readings every few inches on the pipe and map out it's shape. Then use dial indicators on the side of the pipe and the top to map out what your lathe is doing. I've never heard of such a thing happening (cutting in an oval pattern) so I'm interested in nearing your solution. Is this the first time you've noticed it while turning something so long? Do you have a steady/follow rest you could use?

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If the first two passes cut for the full length but the third one didn't touch in the middle, and the middle is ,015'' larger in diameter than the ends, then there might be wear in the ways or carriage. How old is your lathe? When turning shorter work did you get tapers?

Turning 8'' diameter might be taxing your lathe more than usual depending on its size and ridgidity. Could be some lathe flex as well as wear to consider.

There would be some deflection of the work even with an 8'' pipe but if the tool remained sharp and your set up was good then the third pass should have cut in the middle. If the first passes were not deep enough then the rust and surface grit could dull the tool very quickly and increase deflection during subsequent passes. Mount a dial indicator or dti on the carriage and run it full length along the front of the pipe. A larger reading in the middle compared to ends would show deflection.

If both lathe wear and work deflection are at play, then it would easily explain a difference of .007'' over 24'' (.015'' on diameter).
 
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