I can't cut a straight thread, on my lathe  

 

James Perotti
(@jpsaxnc)
Joined:6 months  ago
Posts: 47
February 1, 2018 13:41  

The only thing that's out of level. is the bench the lathe is on, it's leaning back. It looks like raising the back 1/8" might get it level. I get a tapper on a thread only 1/2" long. Any ideas? TIA


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eeler1
(@eeler1)
Joined:6 months  ago
Posts: 3
February 1, 2018 17:22  

Cut tapered threads?

Really, not enough info, lathe, thread, how much taper, etc


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Tvanpool
(@tvanpool)
Joined:6 months  ago
Posts: 26
February 2, 2018 10:40  

The only thing that makes since to me is that you have to much stick out from the chuck and are getting a lot of tool push off. What size material are you threading, what material is it, what type threading tool are you using? If it is something that has to stick out beyond a 4 to 1 ratio then support it with a center in the tailstock.


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Chris Oldfield
(@kyairgunner)
Joined:6 months  ago
Posts: 5
February 2, 2018 18:42  

is your compound set to 29.5 degrees?  How big a cut are you taking? Have you tried to support end with tail stock?

Chris Oldfield
Independence, Kentucky 41051
859-750-3510
kyairgunner@hotmail.com


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robnewyork
(@robnewyork)
Joined:6 months  ago
Posts: 31
February 3, 2018 03:31  

sounds like the compound is loose and being pushed off the work.. Heres a test .. Do a radial thread , with the compound parallel to the work .. So stop the modified flank cutting ( 29 degrees) and just go straight.. lock the compound  because its now eliminated from the process. Lower the tool height just below perfect , and I mean by .010 or so , nothing dramatic..keep in mind infeed will now be adjusted at the crosslide in this configuration


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robnewyork
(@robnewyork)
Joined:6 months  ago
Posts: 31
February 3, 2018 04:06  

james, this also takes the compound backlash out of the equation completely so you can monitor the crosslide backlash and tool deflection


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James Perotti
(@jpsaxnc)
Joined:6 months  ago
Posts: 47
February 3, 2018 07:56  

No stick out, I cut right up at the collet or jaws.


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James Perotti
(@jpsaxnc)
Joined:6 months  ago
Posts: 47
February 3, 2018 08:02  

I don't the 29 degrees, I always cut straight in.  The carriage, cross slide, compound, and tool post are tight. The lathe is tipped back a bit, I'm going to level it up, and see if that makes a difference, I would be surprised if it did.


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robnewyork
(@robnewyork)
Joined:6 months  ago
Posts: 31
February 3, 2018 08:36  

well I think you said this machine has always been a problem for threading..


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James Perotti
(@jpsaxnc)
Joined:6 months  ago
Posts: 47
February 5, 2018 06:11  

I think the problem is free play in the head stock spindle bearing. I can push and pull on the chuck, and get a couple thousandths, movement each way. I'll look at adjusting the bearing.


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hddriver1
(@hddriver1)
Joined:5 months  ago
Posts: 8
February 10, 2018 20:43  

Are you using the tailstock to hold you work in center? It could be the the tailstock is not centered. There are aluminum bars that are certified for checking tail stock center for under $100.00 and they are worth every penny, and will save you a lot of headaches down the the road. A tapered thread or any type of taper cut usually means tailstock is out of alignment, that would be the first place I would look. You can try this  https://www.amazon.com/BLAKE-Blake-Co-ax-Indicator-Set/dp/B005PX7QV8/ref=sr_1_2?ie=UTF8&qid=1518316761&sr=8-2&keywords=coaxial+center+finder       or  you can try this     http://www.penntoolco.com/27-000/?_vsrefdom=adwords&gclid=EAIaIQobChMItNXdj-uc2QIVgx2BCh1jkwDREAQYASABEgLWAPD_BwE. The second is a cheaper option the the blake co-axial but I don't know how much machine you want to really d=get into. I would get one from penntoolco personally. You can look for others out there just make sure they are certified.

Good luck

Pat


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