lathe threading problem

trying to get some help with a treading issue. have a shop fox M1099
haven't used it in a while - had motor go out, took forever to get new one. lead screw bearing went out, replaced. trying to cut 31 tpi thread, first couple passes look good, then starts taking the cut threads off. have checked and rechecked gears, compound set 29.5 turning @ 150rpm, .002 cut each pass. really need some help.

golfish1 said:

trying to get some help with a treading issue. have a shop fox M1099
haven't used it in a while - had motor go out, took forever to get new one. lead screw bearing went out, replaced. trying to cut 31 tpi thread, first couple passes look good, then starts taking the cut threads off. have checked and rechecked gears, compound set 29.5 turning @ 150rpm, .002 cut each pass. really need some help.

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Posting rules prohibit discussion of inexpensive import machines. That being said the usual problem is slipping in the leadscrew or a gear in the gearbox - make sure that all pins are solid (some are used as 'break-away' single use clutches and that gears have keys or other solid retention to their shafts (other than idlers, of course). There's also the possibility of the threading dial (you are using it?) being loose in a similar way, ot that you're not starting on the same number. You can eliminate that possibility by leaving the halfnuts engaged and using spindle reverse to get back to the start (just back the tool out, reverse to the start, engage tool and start spindle forward) - all that is also the process you need to do a pitch that your leadscrew can't divide into.

This is more a discussion of procedure, applicable to any machine. First of all, does this lathe have an inch pitch leadscrew or metric? Are you using the half-nut correctly for whichever case? It can't be disengaged if you're cutting an inch pitch thread with a metric leadscrew - nor vice-versa.

If you've got that covered, check your 29½° setting. Are you turned ½° in the correct direction? The right way is as thus: set your compound at 30° then nudge the rear end of the compound to the left by ½° - lots of guys do this backwards and go to the right on accident, (30½°) then wonder why their threads are disappearing. You can also usually just cut straight in without too much trouble.

From there, check your tool for correct clearance, as well as center height. Report back.

Also, make sure that your using the right 29-1/2°. Some lathes have the zero parallel with the axis of the lathe, others perpendicular.

You want your compound slide perpendicular to the axis of the lathe and then you want to rotate it 29 degrees away from that position, regardless of the actual numbers marked.

fciron said:

Also, make sure that your using the right 29-1/2°. Some lathes have the zero parallel with the axis of the lathe, others perpendicular.

You want your compound slide perpendicular to the axis of the lathe and then you want to rotate it 29 degrees away from that position, regardless of the actual numbers marked.

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Quick look, and that appears to be the problem, leading in at 60 instead of 30.. You can see it
in pics.. Zero is with the compound running parallel to the spindle centerline.