3 jaw questions

calstar

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See the pics for reference. The chuck backing plate has a broken thread at the thread start, I can get it to thread on but it tightens after about 1 1/2 revolutions at which point I stop. Thread is 2 1/4 x 8.I am thinking about using a dremel and removing the jagged broken edge and see how that goes. Any suggestions on how to repair this?
I don't have access to a tap, but that may be what I need to clean it up. Again, the only part of the thread damaged is at the start.

thanks, BrianDSCN1039.JPGDSCN1041.JPGDSCN1041.JPGDSCN1039.JPG
 
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Assuming you're not cross-threading the chuck, the fact that your chuck tightens up (jams?) at less than 2 turns make me think that you have some foreign material (maybe some of the chunks that broke off the damaged threads?) in the chuck's thread grooves.

I've seen chuck thread cleaners (not sure of the exact nomenclature) that look like a large hairpin made of 1/16" to ⅛" wire, with the ends turned outward and sharpened. You compress the U so the ends of the wire go into opposite sides of the female thread, then release and rotate the hairpin to clean out the thread grooves. Hope this helps. I recall seeing a picture recently, but just can't figger out where.

And yes, it would probably be a very good idea to grind down those nasty burrs.
 
A broken thread in cast iron should not be extending above the original surface to cause a binding. I would be more concerned about the nick at 3 o'clock in the second and third photos. A nick will displace metal usually with metal above the original surface. Any defects below the original surface will not cause an interference. That is why we stone out nicks on machined surfaces. They may not be pretty but they don't intefere with use. The surface at the edge of the bore is a registration surface on two sides. Usually th fit is quite close and this would be enough to cause binding.
 
"I think I would just chuck it up and bore the broken thread out."

I have a 4 jaw, I can hold a piece of round stock in it, then mount the 3 jaw to the same bar stock, then turn the thread out. Is that what you mean?

thanks, Brian
 
A broken thread in cast iron should not be extending above the original surface to cause a binding. I would be more concerned about the nick at 3 o'clock in the second and third photos. A nick will displace metal usually with metal above the original surface. Any defects below the original surface will not cause an interference. That is why we stone out nicks on machined surfaces. They may not be pretty but they don't intefere with use. The surface at the edge of the bore is a registration surface on two sides. Usually th fit is quite close and this would be enough to cause binding.

I didn't even consider the nick, I'll give it a try tomorrow.

thanks, Brian
 
I didn't even consider the nick, I'll give it a try tomorrow.

thanks, Brian

Brian, I would carefully inspect the entire registration surface, both the bore and the face. A nick on the face should affect the ability to thread the back plate on but it will affect the runout. Some Dykem will help to find any high spots.
 
I'd remove the damaged portion of thread and a Dremal will be perfect,clean,(scratch brush rotated),and clean out and just see if it screwed on better then sort out other problems. Looking at where the damaged thread is in relation to the start the male may be trying to 'start' twice. Also it does not take much deformation on a tight tolerance thread to cause binding. Do you have or can borrow a thread file? At least you have room to move.
 
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