Logan Lathe does not run true

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danpatch

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I have 1957 Logan lathe turning a piece of 3/4 DOM tubing 1 1/2 long .After turning the piece is .004 smaller at the very end were I stop than the end were i started.It turns a tapper.Using a collet.I have tightened the ways.
 
Yes, need more info. How does the lathe perform with solid material? Tubing tends to deflect when thin.
How long is the workpiece- inches or feet? What type of cutting tool and how deep of cut are you taking?
A picture of your setup would be helpful
 
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3/4 .188 wall Dom tubing 1 1/2 long .Carbide cutting tip.I think the carriage is worn I can see the tool post move when it starts to cut.Also when you turn the dial in .010 it might cut .005 or .015 never contestant.
P1090213.JPGp .
 
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Just going to commit on one part of this I am competent to speak on. Logan Lathes commonly have cross slide back lash for two reasons.
  1. The cross slide screw, nut, or both are worn. There are a couple of shop "fixes" based on other lathe desigs to address this if replacement isn't an option.
  2. The micrometer dial design on a Logan requires some backlash or else the screw & dial will lock up.
To address #2 with factory components, loosen the two nuts on either side of the handle. While spinning the dial, slowly screw the inner nut tightening down on the micrometer dial until the dial just stops. Then back off the inner nut one flat at a time until the dial spins freely. Back it off two more flats then turn it in two flats to remove the nut/screw backlash. Hold the inner nut in place and tighten down the outer nut.

You have now removed all the backlash you can that is not associated with the cross slide screw and the nut. What is left is the backlash in the screw & nut.

Many folks, including some gifted YouTube machinist's, learn to work around this by backing the tool away from the work past the known backlash and only starting a cut after the backlash has been removed.
 
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VicHobbyGuy said:
The screw holding the cross slide leadscrew block/nut to the cross slide seems to work loose easily on my Logan. So that could be #3 on your list.
Click to expand...
Good point. Fortunately that's "usually" easy to fix with some Locktite.
 
Tool stick out is too much. Need to choke up on it. Long stick out will increase the lever and cause more deflection. The other guys know more about Logans. But excess stick out should be avoided, unless absolutely necessary. Only have the tool stick out a very small amount. We think steel is rigid, (it's really not) but the longer the stick out, the more it will deflect.
 
If it is smaller near the chuck the problem is likely excessive tool pressure causing the tubing and/or collet system to deflect more at the far end of the tube.
Sharper tool, as Martin says is probably the most likely.
Smaller feed rate, power feed not manual
less depth of cut
try small changes to tool height
Try taking a spring cut where you make a second cut at the same dial setting
Backlash is always removed by only turning the dial in before starting a cut.

Let us know what works
It is possible there are other issues like worn bed or head alignment, in which case a special file called a lathe file is your friend.
0.004" over 1.5" is a lot of wear so I suspect it is technique. or tool.
 
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