# Where Do I Begin?



## Bamban (Mar 7, 2015)

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


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## Andre (Mar 7, 2015)

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.


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## hermetic (Mar 7, 2015)

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


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## mzayd3 (Mar 7, 2015)

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.


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## Bamban (Mar 7, 2015)

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.





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.


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## hermetic (Mar 7, 2015)

Have you checked the tool alignment, and tightness< I cant believe you have damaged the lathe with plastic!
Phil


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## Bamban (Mar 7, 2015)

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.


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## RJSakowski (Mar 7, 2015)

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.


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## Bamban (Mar 7, 2015)

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.


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## yendor (Mar 7, 2015)

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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## hermetic (Mar 7, 2015)

I think it is time to check the half nut/nuts to make sure they are engaging fully, and that the half nut shaft is not bent, or the threads damaged (which I think unlikely).  It could be that a key or shear pin has half sheared, allowing disparity between the carriage position and the  leadscrew, thus messing up the thread by cutting in a slightly different position on the second pass. If the half nuts are still difficult to engage and disengage, that is where I would start. If you have a manual, check to see if there is a shear pin or key.
Phil


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## Andre (Mar 7, 2015)

I wonder if a gear on the leadscrew or spindle is slipping. Also make sure your half nuts are engaging solid, not backing out a little from the detent. If the detent backs them out slightly, that and a bent leadscrew would lead me to believe that might be your problem.


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## Bamban (Mar 7, 2015)

Thank you.

Yendor,

I am not cutting threads in both directions. The last picture I posted was done cutting thread in one direction - towards the spindle. I just kept the half nut engaged, turned off the motor, back out the cutter, reversed the motor to move the carriage back to the right, then made the 2nd pass.

Phil,

Unfortunately the only documents I have is just a few pages on the controls and starting up the lathe. I have a TravAdial blocking any view of the  half nuts. It looks like I need to remove the bracket holding the lead screw and feed rod,  and TravAdial. and slide the carriage all the way to the right to even inspect the half nuts. I don't know if I were to replace the half nuts that I have to completely remove the carriage out of the lathe. I've never been this road before.

Andre,

I do not know and could not tell whether the half nut lever backs out or not, I'll have to make some runs and observe. One thing I never noticed before and was not looking for it before either, but the lead screw bends down every time I push the half nut lever down to engage.

Do you guys know if the half nuts on these 70s era Taiwanese lathes are brazed on, or  soldered on to their housing? Next question, if I were to replace the half nuts and no commercial ones are available, do y'all know who could machine or rebuild mine?

I dread the thought of tearing the carriage apart.


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## Bamban (Mar 11, 2015)

Found the problem, the dowel pin activating the lower half nut sheared off.  It is 8mm x 16mm, a catalog item with Grizzly, but they are out of stock and their best guess for next shipment is 21 April. Local places do not have metric pins. Though 5/16 is close, it may be shy too small for a press fit.  Are these hardened pins designed to shear?

Looking at the piece that came out,  surface does not entirely show fresh break, crashing the cutter into the schedule 80 PVC and unlodging the workpiece off the 3J stalled the motor just finished it off.


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## hermetic (Mar 11, 2015)

Glad you found the problem, if this pin is designed to shear, make one out of aluminium, However it sounds like you mean the pin that slides in the semi circular slot to close the half nuts (assuming that is the way your lathe works) You have a lathe, make one! Good to know it hasn't done any serious damage, big sighs of relief all round;-)
Phil


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## Bamban (Mar 11, 2015)

Phil,

Thank you.

Yes, it is the pin that closes the bottom half nut. I do not know if these are designed to shear or not. I could turn one, but I have the carriage all torn apart right now. I sure hate to put it back together to turn a dowel and had to take it apart again.


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## hermetic (Mar 12, 2015)

Local machine shop? Buddy with a lathe?, local model engineers club? Yes I was being a bit daft, I should have realised you would have the apron stripped


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## DSaul (Mar 12, 2015)

McMaster-Carr sells 8mm dowel pins in multiple lengths.


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## Bamban (Mar 12, 2015)

Thank you, gents

Got some coming from Fastenal, at McMaster-Carr their largest diameter is 6mm. BTW, are these pins in this application designed in as shear pins? Looking at McM-C online, they have 3 different materials,  for dowel pins. The steel alloy is hardened to minimum hardness of 52, which I would guess will shear in 2 pieces, the other 2 are SS.


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## hermetic (Mar 13, 2015)

I Don't think that these pins are designed to shear, usually in the screwcutting chain there is a shearpin in the drive to the leadscrew. On my Colchester it is in 
Philone of the shafts that drive through the banjo gears, so if the leadscrew is overloaded the pin pops and disconnects the drive to the SC gearbox, thus it also protects the half nuts. This pin on yours has sheared because the loading has forced the half nuts apart. It is possible that there is a key that holds one of the gears on to the drive shaft between the lathe and the screw cutting gearbox. You could take this key out and replace it with an aluminium one( which you would have to make) in order to stop this happening again. Have a look through your manual to see if this is possible,


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## Bamban (Mar 13, 2015)

Phil,

Thank you for the input, I appreciate it that you take your time to help out.

I will just have to open the screw cutting box and take a look, the manual I have is just installation and start up, just a couple of pages.


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## hermetic (Mar 15, 2015)

No Problem, Thats what we are here to do!
Phil


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## Bamban (Mar 18, 2015)

No such luck, the parts that came in from Grizzly are not even close to the originals in my lathe I was hoping their 1236 Gunsmith G0750G would be close enough, not so.  Their gib is thinner. Their cam is slotted while mine has holes and the pins are pressed. Looks like I will be forced to make my own gib and the cam. Luckily the cam is set screwed in to the shaft. I hope my mini lathe that I primarily use to turn necks on my long range competition brass is will be up to the task to machine steel, never have tried cutting steel on it, just the cartridge brass and occasional aluminum.

Questions:

What material should I use to make the gib from?
Should it be hardened?
What material should I make the cam, the original appears to be cast?
Should I slot it or drill/bore a hole a press the 8mm pins just like the original.

Thank you.


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## bfd (Oct 8, 2016)

go online and find a piece of metric drill rod right on size. cut and dress the ends and you have a new pin. you can harden it yourself with a torch and quenching bath. depends on oil or water hardening drill rod.  wont be perfect but will work. along those lines can you find a supplier online of metric dowel pins? there are ways to tighten up the hole for 5/16 pin. Loctite would work. peening the top of the hole. knurling a pin . spring pin might also work. unthreaded portion of a high quality metric bolt. there are so many choices that will work. bill


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