Lathe question

I have a Smithy 1220 lathe/mill and was turning a 3/16" threaded coupler down yesterday and I noticed that there is apparently a taper in it. The end away from the chuck is about .006 wider than the end near the chuck. There is no adjustment on the headstock as far as I can tell. I'm going to call Smithy Tuesday but I was hoping some of you knowledgible folks could come up with a solution. It's really wierd. What I'm doing is turning the couplings down to be pressed/glued into the center of a couple of cocobolo shift knobs I made for my Jeep. I need to have them pretty much straight. thanks.

Jim

Reply to
Jim Chandler
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Most likely, you're dealing with part deflection. It bends away from the cutter, making it larger the further from the chuck,

Very finely sharpenned and honed tooling helps a lot with this issue. if you've got it, super high RPM and a light finishing cut help too.

Karl

Reply to
Karl Townsend

What was the l/d (length to diameter) ratio of the part your were machining? Can you rig a test indicator to your carrage and indicate the side of chuck? Might give you an idea if your headstock is mis aligned with ways.

Worn jaws can do it also. Wooped ways also.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

Reply to
Wes

Thanks, Karl. I'll give that a try. I hadn't thought about deflection because I thought everything was solid.

Jim

Reply to
Jim Chandler

The diameter of the part started at about 1/2" and the length is about

2". I an turning it with a 3/16 grade 5 bolt run all the way through it and am chucked (three jaw) on the bolt. I suspect Karl was correct and I'm getting deflection that I hadn't considered. The machine is just over a year old so I don't think that there is excessive wear in any of the parts, especially since I've only used it a few times (not as many as I'd like). Thanks for the reply. Every bit of information helps.

Jim

Reply to
Jim Chandler

Zero deflection, eh.

It's amazing just how elusive 'zero' can be when you start looking hard enough.

Reply to
Tim Wescott

.006" is great compared to my lathe. A few strokes with a file should take care of that.

Reply to
Dan H

This is not correct. A file is completely inappropriate for this problem. The only reason anyone should use a file on a lathe is to take an edge off (chamfer or radius) at a low RPM while paying a lot of attention as to where the file, and his/her apendages are located.

I've never seen an application where a file yields a better result than a correctly formed cutting tool, cutting under appropriate conditions (RPM, feedrate, tool and workpiece overhang, cutting fluid, etc). I'd be very impressed to see anyone file a taper out of a workpiece faster than they can adjust their cutting parameters and actually perform the operation correctly(!)

Not to mention using a file on a lathe is really dangerous (see pics from thread several months ago - see if you can find the file among the twisted flesh and blood).

As we've now found out, the length to diameter ratio on the OP's setup is the cause of this issue. Karl's suggestion is right on the mark.

Regards,

Robin

Reply to
robinstoddart

That's what happens when you're not a real machinist and are judging things by feel. It is amazing how weak things really are. The first time I tried turning this thing I had a 3/16" bolt threaded all the way through it with a nut on the end cinched down tight. I chucked it up and started turning. About the third pass, I got a little overly enthusiastic I guess, and took too big of a cut. The bit dug in and the bolt bent nicely right behind the nut. I was amazed. you don't think of those things. At least I don't.

Jim

Reply to
Jim Chandler

ya know, Wes - there are two types of engineers (actually this probably applies to any group of folks) - those who think they know more than eveyone else and are out to prove it, and those who end up actually knowing what everyone else knows because they listen and ask questions - I haven't worked production line stuff for decades, but I found that the Techs and even the assembly folks could generally teach me something if I would just shut up and listen to them - and at least in my experience, everyone I asked for help was willing to help - maybe it had to do with the asking and listening parts.

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Reply to
William Noble

I listen to the people who do the work, but have learned to separate their observations of what happens, which is valuable, from their interpretation of why it happens which is often incorrect if they don't know the science behind it.

Jim Wilkins

Reply to
Jim Wilkins

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