# Thread Clearance On SB Heavy 10 Spindle



## Canuck75 (Nov 29, 2017)

Recently made a backing plate for a 5" 3 jaw chuck that will share duty on my RT and my SB heavy 10. Made a thread gauge to replicate the spindle nose precisely to use in making the backing plate. My new backing plate got a very good fit on the thread and gave it a .002" fit counterbore to shoulder. Only wanted .001" to .0015" clearance but polished it a bit too much and got .002".
Just for kicks I measured the counterbore on the small and large faceplates and the backing plate on the 6" Cushman chuck. All original SB equipment. What a surprise. They all had .022" clearance. I thought the shoulder behind the thread would be the major factor in centering anything on the spindle, but it appears everything centres on the thread.

Anybody else wonder about this?

Canuck75


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## markba633csi (Nov 29, 2017)

I've read raging arguments about this on other forums (I had the good sense to stay out of it) whether it's the threads or
the shoulder (register) that does the centering.  Apparently (even tho I have a hard time believing it) it is the threads. 
Flak jacket ON  
Mark


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## Canuck75 (Nov 30, 2017)

markba633csi said:


> I've read raging arguments about this on other forums (I had the good sense to stay out of it) whether it's the threads or
> the shoulder (register) that does the centering.  Apparently (even tho I have a hard time believing it) it is the threads.
> Flak jacket ON
> Mark




Hello Mark.

Sure surprised me. The OD on the thread is 2.246". the OD of the shoulder is 2.260". the IDs of the faceplates and the backing plate on the chuck all measured at 2.282", so they "register" on the thread!. I've heard one explanation that anything put on the thread that when it hits the face at the back of the shoulder and tightens "pulls up" by virtue of the angle of the thread and thus centres.

Wish it had the D1 spindle.

Canuck75


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## 4ssss (Nov 30, 2017)

Canuck75 said:


> Wish it had the D1 spindle.



My SB Heavy 10 has the L00 spindle.  Can't do better than a taper lock for concentricity. And even with that I still have a Buck Chuck mounted to be sure.


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## markba633csi (Nov 30, 2017)

Even so, Heavy tens still command high prices even with the threaded spindles, and old fashioned cast iron bearings.
But I wouldn't mind if somebody gave me one


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## Canuck75 (Nov 30, 2017)

markba633csi said:


> Even so, Heavy tens still command high prices even with the threaded spindles, and old fashioned cast iron bearings.
> But I wouldn't mind if somebody gave me one



Some further info in this process.

After the new backing plate was mounted on the spindle and turned/faced to accept and mount the chuck, I ran an accuracy check. Zero runout on the OD and face of the chuck body, and, with a respectable piece of 1" CRS it showed zero runout at the chuck, and .0015" runout at 6" away. Couldn't slip a .001" feeler under the nose of the jaws either so they seem to be made reasonably well, not bellmouthed or anything like that.

Now, for a ChiCom chuck give those kind of results is really surprising, or just pure luck. I dismantled, cleaned and deburred like everyone else has had to do. Inexplicably the centre bore was off by .012" and yet the back mounting recess was perfectly concentric, so I rebored the centre hole because I use it against a pilot to centre the chuck on my RT (backing plate gets changed out to a holdown plate). With all the potential variables the results are more than acceptable. In any case, I don't like buying offshore stuff, but if you're always budget driven, it just becomes a reality.

Back to the subject at hand, I just don't get using the threads for centreing, but it seems to work. I always trusted my old Cushman chuck to be accurate, and wouldn't have thought a thing about if I hadn't started poking my nose into worrying about getting a close fit of the new backing plate to the shoulder - go figure!


Canuck75


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## 4GSR (Dec 4, 2017)

Dad always told me that the chuck backplate should register on the unthreaded area behind the thread on the spindle, with a tad of clearance.  Of course, he also mentioned the thread fit needed to be close, too.


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## markba633csi (Dec 13, 2017)

I suspect that, on many lathes, both play a part especially those with worn threads. Certainly doesn't hurt to have a close register fit.
Mark


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