Unidentified Deflection Source

T

tmenyc

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Haven't posted in awhile but read all the time and appreciate all I've learned here.
I run a Logan 820 for my practice of vintage fountain pen restoration, generally in fabricating replacement parts in delrin (if inside and unseen) or ebonite (if visible; ebonite is processed organic hard rubber, an old material); sometimes acrylic resins and celluloid. Recently, I'm unable to cut without pretty significant error, what I assume is deflection. The error is the same with delrin, ebonite, acrylics, and even cold roll, for me OD is always <.500, usually in .250-.350 range.
Facts;
Lathe is aligned with a test bar between collet chucks at tailstock and in the spindle, <.001" error in 4-5" of length, and since almost all of my turning is <2" long this is acceptable.
Cutting either with an HSS tool I've ground for ebonite or VNMG, sometimes CCGT. Cuts are max .010 while I'm having this problem, but seem to be about the same error at any DOC. QCTP is AXA. Tool stickout is usually .300-.500", actually as litle as I can work with, so I can get to the work; real estate with the QCTP and slim live center is pretty tight. I guess I'm hoping that's not the problem. I always have a tradeoff betwen using a very thin rod as dead center or adding stickout.
Work is in ER32 collet chuck centered with live center in tailstock. With the collet chuck I generally work very close to the chuck, using power feed to less than 1/8". Clearly too close for comfort with a 3J but works fine with collet chuck.
At both 620 rpm and 180 rpm, ebonite cuts very cleanly with feed at .0062.
Cross-slide, compound tightened down; gibs are as tight as I can set them and still turn the y handle.
Why is it cutting .007"-.010" deeper right at the chuck than 1" away? The error is the same with both/either HSS and carbide tools.
I'm at a loss, would appreciate any ideas.

Tim
tim@timsvintagepens.com
 
Sounds like it could be bed wear that is causing the tool to change height and causing the change in cutting depth. I’m not familiar with that lathe, is there a flat way for the tailstock you could run an indicator on while moving the carriage to see if it is raising or lowering as it moves towards the chuck? You want to use a flat way that the carriage doesn’t use since that should be wear free near the chuck and will let you know if the ways the carriage runs on are badly worn.
 
A picture of your setup might give us more to work with
Logans are pretty rigid
 
Mark, here are two photos. Sorry they're large; I can shrink them down if it's a problem.
I'll check the bed level, but the problem is new and my use if the lathe is pretty light. 20240122_172827.jpg20240122_172832.jpg
 
I had another thought while you posted your update. Since it just seemed to recently happen, it’s probably not wear. Check to make sure the live center does not have excessive runout, and that it is solidly in the tailstock. If there is a chip or dirt on the taper, or damage inside the taper of the tailstock spindle, the live center could be rocking back and forth enough to cause the tool pressure to push the work away from the tool causing it to turn larger at that end and then smaller when it gets closer to the chuck.
 
I'll check it. That would be a great solution. Thanks!
 
Recently, I'm unable to cut without pretty significant error, what I assume is deflection. The error is the same with delrin, ebonite, acrylics, and even cold roll, for me OD is always <.500, usually in .250-.350 range.
Why is it cutting .007"-.010" deeper right at the chuck than 1" away? The error is the same with both/either HSS and carbide tools.
I'm at a loss, would appreciate any ideas.
Click to expand...
Luckily, the error is repeatable. This could be a tail stock alignment problem. The alignment of the tail stock may be different depending on how far the tail stock is extended.
You can adjust the tail stock until you don't cut a taper any more. Set a dial on the base of the tail stock to measure the adjustment. Beware that 0.001"adjustment = 0.002" change in diameter.

This is how I check my tail stock:
 
Well, there is a bit of progress. A good friend who's an engineer with a shop, happens to be in NYC for a bit, so came over last night to help, as he has many times. The tailstock is aligned very closely, no doubt about that. We did inspect the inside of the tailstock spindle with a strong light, and found a small burr. He worked at that awhile with a small file and mineral oil and some of the error was removed. I ordered a MT2 finishing reamer, and it should be here in a few days; we hope that will help more.
Why, or indeed, if that burr is the cause of that particular deflection is still a mystery, but we'll get there. Meantime I'm also going to hone my HSS ebonite tool and work with very light cuts; I'll stay away from the carbide for now, thinking that maybe it's pushing the work more than cutting furthest away from the spindle.
Thanks for all the help -- further advice will be appreciated!
Tim
 
I think I have a final report...I purchased an MT2 reamer for the tailstock, and an MT3 dead center for the headstock. After we (well, he...taught me how) reamed out a couple of burrs in the tailstock, there was still a bit of error so I dismantled the tailstock completely, cleaned off every surface, and shimmed it .015, which brought the error down to .007, better but too high. Then I shimmed it .013 and now the error is <.002" over 6". I turned a piece of 6061 and it cut .0015 deeper at the headstock than a few inches away. Since most of my work is short distances very close to the HS, that's getting there! Going to try changing the shim down to .012 and .011 from .013. If the thou or two helps the time will be worth it. UPDATE: the final change to .012 shim brought the error to .0003. Job finished.
thanks for the help and advice; I learned a tremendous amount about the tailstock, a chunk of the old 820 that was unknown to me, the intricacies of what appears to be a crude device but really isn't at all.

Further, I realized that all of my toolholders were too high, since I was measuring them against a center in the TS, which certainly didn't help matters! So I've now carefully adjusted all of them. Making a more objective, or at least independent, tool height gauge that I can place securely on the .64" wide flat way is my next project.

Many thanks again!
Tim
 
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