Where Do I Begin?

Bamban

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Jul 13, 2014
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Folks,

I need some help as to where to zoom in to fix this problem on my lathe, The video attached is with the half nut engaged. You can see how the carriage crank behaves, There is a definite spot that movement sort of stops and then catches up.

Thank you for your help


View My Video
 
Is the leadscrew turning at a constant rate? If so, I'd check the leadscrew and halfnuts for stripped teeth.

It could also just be the common problem of an off balance handwheel moving from gear slop, put an indicator on the carriage Z axis and see if the movement is constant. I think that's your problem, happens on my lathe too around the same spot of handwheel rotation.
 
Odd! does the carriage pause as well, or is it just the handwheel? If it is just the handwheel, and the carriage movement is constant, then the problem lies in the spur gear or rack that drives the carriage along when the half nut is out. If you think about it some more, as it is repeating every revolution, it is probably the spur gear on the handwheel that is damaged. What make/model is the lathe?
Phil
UK
 
I agree with Andre. It is probably caused by backlash in the rack and pinion that drive the carriage. It happens on my lathe too.
 
Thank you for the responses.

The lathe is a Taiwanese made ACER 1236, 70s vintage.

Have to give full disclosure. I wanted to try internal threading so I got a couple of short schedule 80 PVC pieces for material. All was well, the first pass went well, then I got interrupted and had to stop. Upon resumption of work I dialed the cross slide in back to zero, and engaged the half nuts on the number. Since I retract full rev when threading, this time senior moment took over, I forgot that I already cranked in to zero before I got interrupted. As usual I dialed the compound and so when I cranked the cross slide back zero I put in 0.1 additional.

I was lazy to change to 6J before I started, so when the cutter dug in it flipped the material out of the 3J and stalled the motor. Hit the eStop and released the locked up mess. The half nut lever was real hard to disengage. Immediately I chucked in a 1 inch barrel stub and checked the threading, external this time. After setting the system I made the initial pass, and without advancing the compound I made another pass and another, all 3 did not superimpose.

To forget about the misfortune, the rest of the day I played with dialing a barrel blank using 2 spiders.

Before I went to bed I chucked in a barrel stub, cut the tenon and threaded it 16 TPI. I took my time and meticulously went through the process hoping for a better outcome from the test I did after the mishap. The thread came out atrocious. I took a picture of the finished threading job, if you expand the picture you can see the faint lines made by the cutter.

33mml1z.jpg


For comparison, here is one I took sometime back. I know 2 different materials, but as you can see there are no multiple lines on the threads.

303jr5y.jpg
 
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Have you checked the tool alignment, and tightness< I cant believe you have damaged the lathe with plastic!
Phil
 
Did not check the tool alignment since the incident, but I did the previous day when I threaded a couple of barrels for flash hider. I will check it again. I did checked the cutter tightness though.
 
Given that you are having problems cutting threads now, it appears that you are losing your timing between the spindle and the lead screw. This should put the problem somewhere between the spindle and the lead screw. Have you checked your gear set? You may have sheared a tooth on of the gears. It is also possible that a key has been sheared.
 
I checked the tool alignment, looks good, checked the back gears all are meshing with proper backlash and no missing tooth. I made a test cut with just 2 passes with the same setting. This time I did NOT disengage the half nut, just turned off the motor reversed the carriage direction and turned the motor on again and the second pass started. Stopped the motor just in time before the cutter got to the end of the first pass. The second pass lagged the first one as you will see.

Here is the picture.

11wgeq8.jpg
 
Were you cutting in the reverse direction?

If so the Backlash in the gears would account for the mis-alignment.

Cutting Threads is a One-Way trip.
Cut a pass got back to the beginning
Cut a pass ... that way the backlash is always removed and kept under control.
 
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