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South Bend 11, 1931, cross feed nut screw installation

Joeflaps

Plastic
Joined
Oct 7, 2017
I bought an new ebay cross feed nut because I figured it was cheaper than buying a 1/2-10 L/H ACME tap and a 3/8-24 tap and making one.
My original nut appears to be cast iron and has a .50x diameter by .1x deep counterbore on top with a 3/8-24 thread below, all concentric. The cross feed nut screw is a .500 diameter steel shoulder bolt with 3/8-24 threads.
The brass nut I received has a counterbore that is not concentric to the 3/8-24 thread and is not 90 degrees to the ACME thread. When I install the nut it runs free on the crossfeed screw until I install the shoulder bolt. The more threads I engage in the top of the nut, the more binding I get when cranking the cross feed screw. If I snug up the bolt to the bottom of the counterbore, the cross feed takes two hands to turn.
The seller does not think the bolt should be tight to the bottom of the counterbore and insists the bolt head should be resting on the top of the cross feed slide, pulling the nut up tight against the bottom of the slide.
I disagree. I believe the original design is to torque the shoulder bolt into the counterbore in the nut, leaving a small gap under the head. this allows the nut/bolt assembly to rotate in the .500 hole in the slide, thereby aligning itself with the ACME screw.
Dose anyone have a manual for this configuration that includes assembly instructions? There is also a version of the nut that has a .500 diameter post sticking up from the top. I do not know what the mating shoulder bolt looks like. If it too has a shoulder bolt that allows the nut to align itself, this would also prove my point.
I'd like to be able to throw something from South Bend back at the seller to coax him into making me a new nut that has accurate features. He does not seem to be concerned with the defects he has sold me.
To salvage this nut I plan to fill the outside of the counterbore with JB weld and let it cure. Then install a 3/8-24 bolt in the threads to find the center in my mill and run a 1/2 inch end mill into the counterbore to clean up the sides and hit solid brass at the bottom making sure I am 90 degrees to the ACME threads. I don't really know if the 3/8-24 threads are normal to the ACME threads, but I'm assuming the 3/8-24 brass threads will deform when I torque the shoulder bolt into the bottom of the counterbore. This should make it usable as long as the JB weld doesn't chip out of the counterbore.
Thanks for any comments.
 
I can tell you with this old stuff, you are never going to get a perfect bolt on item. Its 88 years old . . . And God knows what he has to work with as far as drawings and such. Plus he doesn't have your other parts to check fitment, alignment etc. So they do the best they can, and the end user nearly always has to modify or adjust it.

The cross feed nuts should be secured tightly. There are shims and other means to get the height correct, many of which you can find on ebay if you type in the OD, ID, or various combinations, but they'll come from a hardware seller, not under South Bend listings.

The greatest trick to getting smooth operation is having as close to perfect alignment of cross feed screw from the nut through the dial. Not only up and down of nut mounting height, but also side to side.

If an engine lathe, to see alignment pull the handle off of cross feed dial and the outer portion of dial. Feed CF screw into nut and have dial shaft of CF screw poking through the bushing of dial. Is it centered ? If no, get it centered. Sometimes tightening CF nut securely will rotate nut ever so slightly causing mis-alignment making is super hard to turn. Same with mounting height of nut.

Just pull CF handle and outer dial off, then shove cross slide back till shaft comes out of the hole. On your knees looking through the hole, how well centered is it ? The CF screw will have some play on the threads, gingerly lift it up and down, plus side to side to get your best centered position on the dial bushing hole.

For alignment if you have taper attachment, same principle, but you need to remove whatever bushings hold CF screw on taper attachment side. Then get CF screw centered in taper attachment hanger.
 
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All kind of ways the nut will be out of alignment with the crossfeed screw. Time for you to look close:

1) The new nut might be machined wrong.
2) the new nut may be for the wrong lathe.
3) older lathes have wear. The fixed ways on the slide wear, and the crossfeed ways on the saddle wear. This has the effect of lowering the nut axis and moving it to the right. This can be a substantial amount.

The nut is supposed to mount hard to the underside of the slide - no float.

As mentioned above: mount the nut solidly. Remove the crossfeed screw, dial, and bushing from the saddle. Slack the gib slightly so you can slide the slide all the way to the front. Find or machine up a shaft that's a snug fit into the minor diameter of the nut, long enough to exend out the front. Using whatever measurement capability you have, determine the net offset between the axis of the nut, and the axis of the crossfeed bushing. Remember you need to have the axis of the nut parallel to the way travel.

Report the numbers back here.
 
If you have the original cross-feed nut retaining bolt, the one that sits on top of the slide and hits the compound as you swivel it around, he is correct. the 1/2-inch shoulder should not bottom out in the counter bore of the nut and the thread length should not interfere or touch the acme threads in the nut for the cross-feed screw. The most important thing is that the replacement nut top surface that contacts the slide when tightened up has the same center distance to the acme threaded hole as your original nut. If you could post photos of the old nut and the new nut that would help. Does the new nut have the same recessed shoulder and depth to allow the nut to tighten up against the slide? Does the shoulder on the bolt fit without force inside the shoulder of the nut, or does the bolt threads bottom out in the nut before the shoulder does, allowing it to not pull the nut tight against the bottom of the slide. You are correct that the center line of the retaining bolt recessed shoulder and threaded hole should be concentric and 90 degrees to the center line of the acme threaded hole. There can sometimes be an optical illusion of this being not right if the outer profile of the nut is off to one side by finish grinding.
There is an improvement repair you can perform which eliminates the bolt. Just counter bore the top of the slide to accept a flat top shoulder that is larger than your .500 bore and sits flush with the top of the slide so the compound clears. Slot the top of the flat top to accept a screwdriver blade like the later model lathes. Use the same .500 diameter inner bolt shoulder and thread size and fit to the nut to pull it tight against the bottom of the slide.
 








 
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