[How do I?] Thread Crest Override - ???

Bamban

Bamban

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Yesterday I worked on the little 10x24 lathe. I mounted the scales for the DRO, afterwards I checked the spindle to bed ways alignment, on a stout 3 inch aluminum, unsupported over a 7 inch section, the taper from end to end I negligible, within measuring error. I then turned to threading on a barrel stub, 16 TPI. I used the keep-the-half nuts-engaged method and just pull out the cutter and reversed the carriage past work piece about an inch, set the cross slide back to zero, advance the top slider, and back to threading again. On the 3rd or 4th advancement of the compound the next pass, the threading bit wiped out the crest of the thread. I did the threading exercise twice and got the same result. To eliminate the possibility of the top slider from slipping I even locked it down after each advancement before making the cut.

Obviously, something is moving, just could not put my finger on it

I have not tried the engage/disengage/engage half nut method, my thoughts it should not make any difference.


Here is what I know, not in any particular order

The half nuts are in good shape, and they move freely up and down and they clamp on the lead screw fully. I checked them when I repaired the mechanical interlock for the threading and carriage feeds after the hold down screws for the interlock lever came loose

The backlash of the cross slide is 0.006 and compound slider is 0.012 range.

The QCTP is rigid

The tool holder is rigid

The work piece is rigid

I was threading at lowest VFD speed the motor can sustain without bogging down

I was using a new Thin Bit tool holder and their 60 degree tool bit and the bit is very sharp

The threading bit is on center

The belts are not slipping


Any thoughts, as I am puzzled right now? Anything obvious I am missing?
 
If you are leaving the half nuts engaged, you can rule out engaging the wrong lead screw thread. Since you are reversing the lathe to go back for the second pass, you could be experiencing lash in the lead screw or threading gears. Normally go past the thread to to right and advancing to the left with the lead screw should take up that lash. However, certain circumstances (a loose key/keyway that slips slightly, a partially tightened carriage lock, etc.) could prevent that from happening completely.

Whatever is causing the misregistration is happening between the spindle and the half nuts. This includes the threading gears, the quick change gears, the lead screw, and any couplings between. It could also be a bearing seizing on the lead screw shaft or a sheared key.

When you are ready to start the next past turn the manual crank clockwise to manually take up any lash.
Edit:
As you start the next past turn, the manual crank clockwise to manually take up any lash. The lead screw has to be rotating while applying the bias.
 
Last edited:
Is it possible that you accidentally moved the top-slide instead of the cross-slide during one of your passes?
Perhaps your top-slide may have been 1 whole turn out from where you wanted it.

Bamban said:
I have not tried the engage/disengage/engage half nut method, my thoughts it should not make any difference.
Click to expand...
Do you have a threading dial? If so, you might try that.
I'd be tempted to use some scrap plastic or even hardwood for practicing 'til you figure this out.

-brino
 
After I crashed my lathe I had a similar issue. The roll pin driving the acme screw was damaged and sometimes would find a different spot changing the timing of the lead screw just enough to mess up threads. Also end play of the lead screw could cause that.
I often thread in that same style and there is alot of backlash in the drivetrain so any that is not consistent will do the same thing.
I turn the chuck by hand when finding my starting point when threading from a shoulder and it amazes me how many turns of the chuck it takes to make the carriage move when watching the DRO to get it to the zero (starting point). 12x36 gear head lathe not sure how that compares to yours.
 
I am puzzled as well. You did not mention the aligment of the cutting tool. I am using a lantern post tool holder and I find that a heavy cut will alter the position of the bit unless I take precautions in tightening down and positioning tool holder to avoid turning in the lantern tool post holder. Is it possible that the entire QCTP is rotating? Not sure what size thread you are cutting but is it possible that the cutting tool is being overloaded? Is the cutter alignment after the problem occurs correct? Are the clearances under the bit correct?
Obviously not much help here but these are my first thoughts about your problem.
Have a good day
Ray
 
Thank you for the responses.

The threading bit was aligned properly to the work piece. The cutter is a Thin Bit 60 degree threading bit. The quick QCTP is rock steady.

When I reversed the carriage I went about an inch past the work piece, my guess the back lash has been taken care off when going back to threading with the carriage going back an inch before the cutter engages the work piece.

I will check the lead screw play and any loose or broken parts around the quick change gear box
 
Hi try adjusting the nut for no end play on the tail end of leadscrew
 
54hawken said:
Hi try adjusting the nut for no end play on the tail end of leadscrew
Click to expand...

Checked the end play, grabbed a piece of leather and clamped it over the lead screw with a large vise grip and tried turning the screw against the bushing trying to find out whether the pin is broke, not it. I grabbed the vise grip and tried to move the screw L and R, no movement.
 
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