logan 820 runout

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Franklyn

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I am becoming more familiar with my logan 820 and wanted to ask if the differences in run out that I'm getting is acceptable for this lathe.

I completely disassembled the lathe, cleaned it and reassembled it. There are no broken gears and I've leveled it but not with a machinists level.

I put a piece of approximately 1" round ten inches in length in the chuck and I miked the rod at each one inch mark and got a consistent 9.706 + - .002. for the 10 inches

I also used a live center in the tail stock so the rod was supported at both ends.
I turned .020 and then a finish cut of .003 then measured the diameter of the rod in 1 inch increments from headstock to tailstock with the micrometer.
Results were .930,.929 .928, .927, .926, .924, .922, .924, .923 .923.
So, over 10 inches the taper is .007 or .0007 per inch. Is this acceptable to hobby machining for a lathe made in 1947? Is this flex of the work?
I have also aligned the headstock to tailstock with a rotary dial indicator to + - .0005.
Can this run out be improved by leveling the lathe with a machinists level?
Thanks!
 
if you aligned the tailstock with a dial indicator, i'm assuming that the tailstock was close to the headstock, where all the wear is. If the wear there was .002", then i can see why you got some taper. I don't know what kids are doing these days to check for tailstock alignment, but i'll put in a long bar between centers, make a cut at the headstock, then make a cut at the tailstock, and adjust the tailstock until both sides mic the same. Gets rid of the taper, but any way wear near the headstock will remain. make sense?

and yes, i'd recommend a proper level.

also a point of note, your first test was with the chuck, and your second between centers.
 
Thomas Paine said:
also a point of note, your first test was with the chuck, and your second between centers.
Click to expand...
Indeed;

If you used a chuck for this, you kind of wasted your time by entering a variable, and almost certainly some amount of error. A drive plate with dog and center is the only real way to get the utmost accuracy, and using a fairly long piece of stock will get you past the high wear areas of the bed as TP suggests. Then you can do your adjusting. Keep in mind that the discrepancy may not be in the tailstock. It is not unheard of for the headstock to be at fault, with shimming required to correct it.
 
Thanks for the replies.
Will turn between centers with a long rod between centers as TP stated.
Adjusting the tail stock makes perfect sense.
I'm new to this, so please correct me.
The tailstock on the Logan can be adjusted horizontally with two adjustment screws on the base. Loosen one tighten the other.
A shim can be placed under the gib of the tailstock(not on the way side) moving the tail stock vertically.
Should I start on the x or y axis ? Do you buy or make the shims? Al or steel?
Frank
 
Not to worry, Frank;

I'm new too. I've never even done this stuff, but in the course of my restoration and parallel time spent on forums, I've read about it quite a lot. Firstly, there can be taper that stems from a machine that is not level, more directly from one that has some twist in the bed. You need to look to that painstaking process as a possible solution to this, and the machine should generally be leveled - as a matter of course - before any real work is attempted.

You have the tailstock scheme correct. Since the most common misalignment occurs in the side-to-side, I would start there. Test, repeat. You should be able to get rid of the bulk of any discrepancy in this fashion. If you still have some variation at this point that you consider unacceptable, you might look into the vertical shimming of the tailstock as a source. Most suppliers sell a packet of varying thicknesses of shim stock for just this sort of purpose, and brass is generally used due to its high conformability. Failing all of this, I know someone who had to shim his headstock, having very eventually found a .0015 deviation in the line bore of the spindle bearings. Not the first place I would look, but something to keep in mind.

Frankly, I am not sure how you can differentiate between these various points of error. As far as I understand, it is down to a simple process of elimination, choosing the most likely first and working from there. I will be venturing into this realm sometime in the near future with my own project machine. Hopefully all of my study sees me able to walk-the-walk when I get there!
 
i agree i would suggest #1 leveling the lathe correctly. If your bed is twisted, you'll just be chasing your tail.....check it often if you're not bolted down and/or doing unsymmetrical parts, and/or your son hits it with the tractor or whatever.
 
No real machining experience here, but it's my understanding that tailstock lateral alignment (the setscrews) is most important. I believe the change in turned part diameter isn't nearly as sensitive to tailstock elevation variance as the other adjustment.

That being said, if you end up adding some shims between the plates on your tailstock, please be sure to take some pics. I believe my "worn in" tailstock has to come up about 0.010 inch, so I'm curious as to the correct way to lay in and secure some shims.

After you get a bar cut/aligned, I'm sure we'd all be interested in your dimension variation along the bar. As noted previously, a longer bar may also help.
 
Excellent advice on where to start in diagnosing the problem. I've ordered a Starrett 98-6 level . Since the feet of the lathe are on shims, I'm going to make four adjustable feet. A friend used modified hockey pucks as the hard rubber feet that touched the floor. They are attached to the lathe legs with 9/16 dia. bolts. It's a very sturdy, adjustable setup.
Also, agree with Tom about lateral alignment being more causative of a taper than vertical alignment of the tail stock. If I shim the vertical I'll be sure to post some pictures.
 
This may be slightly off-topic, but here is a trick I have found usefull; look for an 80 0/0 worn
out single cut 8 or 10 inch file of good quality( Nicholson?) in other words, perfectly flat. To check.
ways and ought-to-be-flat surfaces, flloat the file gently over the surface and note"catches" or shiny spots. Use the file , gently again, with common sense, to correct. Stones rub everywhere,
the file will touch only the high spots. Do this before you measure anything on the ways, especially....BLJHB
 
Sorry this has been so long to post. Here's the update. I made four leveling "feet" out of hockey pucks and 5/8 inch bolts. They really worked quite well and along with a machinist's level (Starrett 98 6) I had the lathe leveled in a few minutes. I also aligned the tailstock with the headstock. I obtained an 18 inch piece of .75 cold rolled steel rod and turned between centers with the chuck holding the dead center and dog. The run out was improved but was not consistent, meaning that frequently I would see wavy grooves cut into the rod while turning. I also noticed that during the first pass, the rod would only contact the tool bit on one side of the work. This meant to me that the rod was not completely round in cross section(I think I was wrong about this). I checked the concentricity of the chuck with a dial indicator and the run out was .002-.003. I also found if I was turning a rod and then removed and re-chucked it, I would get some portions of the rod looking like an Acme screw thread. I removed the chuck and checked the spindle which is cut for a 3 MT by placing a 3MT drill chuck(with the chuck removed) and checking the run out. Surprisingly, the run out was .0005...the indicator barely moved. So now I'm thinking it's the jaws that are worn. I put back the original 5" chuck and swapped out the external jaws for internal jaws, being sure to insert them in the correct order. I repeated the turning of the cold rolled steel with no significant change in results. I picked up some .75 drill rod which was 36" long and placed it in the head stock at the 18" mark and ran the lathe. It was obvious that there is a visible wobble in the rod which could be viewed from both sides of the head stock. i.e that it is not running true. If an 18" .75 drill rod is set up between the headstock and tailstock (which I just aligned) a dial indicator attached to the saddle can be run down the rod with .003 variance for the 16.5 inches (the lathe is off). If you turn the lathe on, and place the indicator 10 inches down the rod, the run out is .010. At the chuck its .004 and also at the tailstock center. If you remove the support of the tailstock and turn on the lathe the rod will noticeably wobble probably .02-.03. So at this point here's what I think the problem is: the jaws or the scrolling mechanism of the chuck is not concentric with the chuck or spindle. I'll have a dead 3 MT center due here by Wednesday and will turn a .75 rod between centers(no chuck). I expect to get a good result with no wobble, and if I do I will replace the current chuck with a 3 jaw Bison.

Hope I'm right!

Frank
 
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