# 3 jaw questions



## calstar (Nov 13, 2017)

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,  Brian


----------



## Jimsehr (Nov 13, 2017)

I think I would just chuck it up and bore the broken thread out.


----------



## hman (Nov 13, 2017)

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.


----------



## RJSakowski (Nov 13, 2017)

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.


----------



## calstar (Nov 13, 2017)

_"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


----------



## calstar (Nov 13, 2017)

RJSakowski said:


> 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


----------



## RJSakowski (Nov 13, 2017)

calstar said:


> 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.


----------



## Chipper5783 (Nov 14, 2017)

Can you track down an 8 tpi tap (1" national coarse, also a lot of industrial bolting in sizes above 1" is 8 tpi)?  Then you can scrape through the threads - you use the tap like a "form scraper".


----------



## NortonDommi (Nov 14, 2017)

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.


----------



## NortonDommi (Nov 14, 2017)

hman,
          I rember that tip for cleaning chuck threads it came from amoung other sources this:


----------



## dlane (Nov 14, 2017)

Has the backing plate ever mounted on the spindle correctly? , if not have you checked it with a thread gauge, hardening has 2 1/4 x 10 tpi spindles they look close. Been there with a spindle thread protector, just sayin


----------



## calstar (Nov 14, 2017)

dlane said:


> Has the backing plate ever mounted on the spindle correctly? , if not have you checked it with a thread gauge, hardening has 2 1/4 x 10 tpi spindles they look close. Been there with a spindle thread protector, just sayin



The lathe is 1980s Jet(Taiwanese) 12x36 belt drive, 2 1/4x8 threads; 2 chucks and a face plate I have install easily.  I picked up the chuck in question when I just got a 1991 12x24 Chinese lathe, same thread as measured with a thread gauge. The chuck mounted correctly(slightly snug ) on the lathe I was given as does a face plate(easily), which also mounts correctly on my lathe.

I'm thinking the threads may be slightly dimensionally different. If I had a tap that size I'd use it, I think that's the best option but don't have one. I'll get in my shop this evening and try some and try some of the suggested fixes.

thanks, Brian


----------



## Silverbullet (Nov 14, 2017)

You can remove the backing plate from the chuck . It'll be easier to do then spinning all that on your lathe . It's only about 4-6 bolts usually. Plus you can clean the chuck repair and relubed as needed. I wouldn't mount it in the four jaw with the chuck mounted . Just my opinion.


----------



## kvt (Nov 14, 2017)

Also, you can put the Dykem on it then work it on to where it binds.  Then look at it closely to see where it is binding.,   But agree with Dlane as unless it has fit prior it may be a little diff thread,   I know the Jet 10x24 I have is a Metric size with a 8 tpi Kinda diff and hard to find.


----------



## calstar (Nov 14, 2017)

Silverbullet said:


> You can remove the backing plate from the chuck . It'll be easier to do then spinning all that on your lathe . It's only about 4-6 bolts usually. Plus you can clean the chuck repair and relubed as needed. I wouldn't mount it in the four jaw with the chuck mounted . Just my opinion.



I'll remove the backing plate, marking/indexing the plate and chuck for reassembly.  



kvt said:


> Also, you can put the Dykem on it then work it on to where it binds.  Then look at it closely to see where it is binding.,   But agree with Dlane as unless it has fit prior it may be a little diff thread,   I know the Jet 10x24 I have is a Metric size with a 8 tpi Kinda diff and hard to find.



Dykem is a good idea. The thread is definitely 8tpi as measured by a thread gauge, the difference(if the problem is thread size) is most likely a few .001s.

Brian


----------



## Kernbigo (Nov 15, 2017)

after you remove the backing plate try to tread it on back wards, more than likely the thread is a little under size in the chuck. You may be able to lap it in with valve lapping compound, i had on years ago that did the same thing.


----------



## Silverbullet (Nov 15, 2017)

I think I have a tap and die in my shop that size , I watch for them on eBay the die I got for $20.  If I remember right new not used hss too. I bought them to help make adapters for my logan . I want to make several , er collet 32-40 , Jacobs lathe chuck from an atlas #20 size it's threaded 1 1/2 - 8 . I have it so why not use it.


----------



## calstar (Nov 17, 2017)

Kernbigo said:


> after you remove the backing plate try to tread it on back wards, more than likely the thread is a little under size in the chuck. You may be able to lap it in with valve lapping compound, i had on years ago that did the same thing.



Backing plate will not go on farther than a few turns from the back, about the same as from the front.  Given this it seems most likely that the chuck is undersize. Any one have a 2 1/4" 8tpi tap they want to rent me? On the other hand it may be best to instal a new backing plate.

thanks, Brian


----------



## kvt (Nov 17, 2017)

Do me a favor and measure the openings etc on that backing plate.   It almost sounds like it fits my 10X24   If so I would possibly like to purchase it off you.   It sounds like it is the metric size X8 that fits mine.    Then you could put a new one on the chuck.


----------



## calstar (Nov 18, 2017)

kvt said:


> Do me a favor and measure the openings etc on that backing plate.   It almost sounds like it fits my 10X24   If so I would possibly like to purchase it off you.   It sounds like it is the *metric size X8* that fits mine.    Then you could put a new one on the chuck.



I googled *metric size X8  *but didnt turn anything up. Where can I find info on it?

Brian


----------



## Tozguy (Nov 18, 2017)

Brian, what did the dykem show?
There are quite a few nicks on the backplate so it might have been mistreated. Or maybe the backplate was not machined well enough to fit the spindle properly.
Either way why not remove the back plate and chuck it in the 4 jaw. Check concentricity and square of the register surfaces and threads. Single point the internal threads a bit to get better clearance and concentricity. The thread fit is not critical. The register surfaces are. To me this a great learning opportunity for you.


----------



## kvt (Nov 18, 2017)

calstar said:


> I googled *metric size X8 *but didnt turn anything up. Where can I find info on it?



That is one of the problems it is off sized and cannot find anything about it I finaly found some information on some of it by searching the web. But found nothing specific.    I made a plug to test with,  It is not real acurage but enough for me to check ones I find.   The  regiser area is about 51or 52 mm or 1.8 something inches.   The threads are under that when measured.   Some people call it a 2 inch x8 tpi also.   I have two Taiwan made 10X24s one is a Jet 10X24 PY that has this spindle.   

If you have anything that screws on,  Then check things against that, then try the Dykem and see where you are hitting.   

Ken


----------



## Bamban (Nov 18, 2017)

What is the thread minor diameter measurement of the chuck backplate in question?


----------



## calstar (Nov 18, 2017)

Tozguy said:


> To me this *a great learning opportunity* for you.



Yes it is, I've already learned a lot!  I do not have any thread wires, might need to get some. The lathe itself is still at the sellers place(10 minutes away, I took only the accessories home), I'm going over there later on to check the spindle size on it and take a face plate that fits my lathe spindle and check with that.
Brian


----------



## kvt (Nov 18, 2017)

Sorry I though you had them both at the home already.   YOu are doing smart taking one that fits your other lathe that you know what it is them.


----------



## calstar (Nov 18, 2017)

I went to check out the lathe( cz300), took the back plate I removed from the lathe's chuck and a face plate from my 12x36 Jet(a known 2 1/4 8tpi). The backing plate spun right on, smooth and no resistance. The face plate would only thread on a short distance, then resisted in the same manner the back plate does on my Jet. My thread gauge showed a perfect fit for 8tpi so....sorta strange. I googled cz300 and found a lot of stats but no mention of spindle thread size in any manual or manufacturer spec sheet; I did see someone on a forum state it is 2 1/4 8tpi but that doesn't seem to be the case. If I find anything out I'll post here. I'll also watch some inter web vids on how to use thread wires.

Brian


----------



## Ulma Doctor (Nov 18, 2017)

if i'm not mistaken, i remember seeing somewhere that 58mm x 8 tpi was the spindle dimensions
i have a shenwai 1236, i should pop the chuck off and see if i can verify.


----------



## kd4gij (Nov 18, 2017)

A 3mm thread is so close to 8tpi it is hard to see with a thread gauge.  And 57mm= 2.24409


----------



## Tozguy (Nov 19, 2017)

So basically we know that the CZ300 spindle does not have the same nose thread as your other lathe. If interchanged parts only go on a turn or two then it seems to me that we have a close enough diameter but with a different pitch, although not by enough to see with a thread gauge. Chances are using wires to measure pitch diameter will not show much if the problem is actually thread pitch.


----------



## calstar (Nov 19, 2017)

Ulma Doctor said:


> if i'm not mistaken, i remember seeing somewhere that 58mm x 8 tpi was the spindle dimensions
> i have a shenwai 1236, i should pop the chuck off and see if i can verify.





Tozguy said:


> *So basically we know that the CZ300 spindle does not have the same nose thread as your other lathe*. If interchanged parts only go on a turn or two then it seems to me that we *have a close enough diameter but with a different pitch*, although not by enough to see with a thread gauge. Chances are using wires to measure pitch diameter will not show much if the problem is actually thread pitch.



Ulma Doctor, that would be great if you can do that.

Tozguy, yep, but thats better to know than thinking a broken thread is the causing the problem(I've learned a lot here).  The cz apparently has some gear issues which I knew about before purchase. I bought it  for parts, probably to difficult to sell as is, thinking I may part it out. I've not machined a back plate for a chuck so I may give that a try for the cz chuck, or find someone with the same dimension spindle and sell it. 

Brian


----------



## Tozguy (Nov 19, 2017)

Ulma Doctor said:


> if i'm not mistaken, i remember seeing somewhere that 58mm x 8 tpi was the spindle dimensions
> i have a shenwai 1236, i should pop the chuck off and see if i can verify.



Doc, I have seen Imperial threads on a metric machine and metric threads on a Imperial machine, but I have never seen a thread spec with a metric diameter and an Imperial pitch, at least not by design. Woe is me.


----------



## menace (Nov 19, 2017)

Imho I would chuck the backing plate in a known fitting chuck on the lathe , true it to zero, then set up an internal thread boring bar, true it to face of backing plate, then set cutting tip inside thread snug, then loosen till you can turn chuck by hand with half nuts engaged and quick change set for proper thread. Let thread bit tip move in as if cutting, if it stops , its probly metric, if it goes all the way thread is probably correct but not deep enough, you can check thread on spindle also, just set cutting tip for external threading TURNING by hand with NO power! Hope I haven't confused the issue and explained it enough . My 2cents , good luck!


----------



## Ulma Doctor (Nov 21, 2017)

calstar said:


> Ulma Doctor, that would be great if you can do that.
> Brian


my chuck is stuck on, it may take awhile to free it up.
i'll post my results when i get it off


----------

