Craftsman 101.28940 Half Nut Skipping Teeth

Pulled the half nut off, it was packed with gunk and debris (it was quite silly of me to think the nut was clean when I attempted to clean it installed on the lathe). You can see that some of the thread peaks look rolled over and the overall nut is very worn. Regardless, I cleaned them up and reinstalled the half nut but saw no difference. I've placed an order for a new half nut and am crossing my fingers this solves the issue and that the leadscrew isn't the problem.

PXL_20210422_023550566.jpgPXL_20210422_024557117.jpg
 
Those look like aftermarket or shop made half nuts- I didn't think Atlas ever used brass inserts
Maybe I'm mistaken
 
markba633csi said:
Those look like aftermarket or shop made half nuts- I didn't think Atlas ever used brass inserts
Maybe I'm mistaken
Click to expand...

I think you're right. This lathe was my grandfathers and was handed down to me when he passed. I have found a dozen other parts on the lathe that are not factory original, so I wouldn't be surprised if the half nuts shown above were replacements.
 
Your Grandfather probably did that then, and it probably lasted a long time
There is a person on Ebay that makes all brass replacements for those lathes, they are pricey but so are the original zamak ones
It looks like the half-nut mount plate is worn too, you might want to replace that also
 
The replacement that I ordered is all brass. I purchased from the link posted earlier in this thread, and I believe it's the same vendor selling them on Ebay.

The half nut mount plate is in OK condition. If the new half nut ends up fitting loose in the mount plate, I'll machine a new mount plate.
 
Your machine has a 3/4" diameter lead screw. Machines made prior to the 10F and 101.07403 all had 5/8" diameter lead screws. The two sizes of half nuts are NOT interchangeable. And neither are the guides. If I was reading the correct eBay ad, those particular half nuts were for the early 5/8" diameter lead screw and will not work on your machine, which is one of the first-version 1/2" bed machines, and which has a 3/4" diameter lead screw. Probably the same vendor is also selling the half nuts for the 3/4" diameter lead screw.

I don't particularly agree that brass half nuts will outlast Zamak ones. But in any case, last time I bought half nuts from Clausing, their price was quite a bit cheaper than the $85 that most of the after-market ones seem to go for these days. In any case, you should check your lead screw for wear near the headstock. The width of the thread crown when new was the same as the width at the bottom of the groove. Compare the dimension to the dimension near the right lead screw bearing, where the half nuts almost never run.
 
I ordered the 10F-12 as per what the manual specifies (3/4 diameter).

I also measured the width of the thread peaks on the leadscrew as best I could with calipers. I found a .009" difference in the worst spot. Seems like a lot to me, but I don't have much experience analyzing thread wear.
 
Well, yes and no. In the practical case, it depends upon what you are threading on the machine. Obviously you would not want to try to make a new lead screw with that lead screw. But most threads are less than an inch long. So from a practical standpoint, it depends upon delta-pitch per inch. Which is the change in pitch per inch. And to a lesser extent, in the wear on the thread Outside Diameter. So go back to the spot where you found the 0.009" wear and measure the OD of the screw. And repeat both measurements about every quarter rotation of the lead screw for the foot of lead screw nearest the headstock. The OD wear will probably be negligible as normally nothing but oil should touch the OD at least until the half nuts get a lot of wear on them. Write these measurements down and when done, calculate the delta-P per inch for each set of measurements. I wouldn't be too surprised if most delta-P's per inch are less than 0.001".

Beyond that, the decision on whether to replace the lead screw or not is up to you and the state of your bank balance. One in-between solution is to flip the lead screw end for end and modify it by cutting off the 1/2" part that runs in the right bearing and splicing on extensions for both ends. Fortunately, the lead screw will pass through the spindle. You will need to provide a support bearing for the part sticking out of the left end of the spindle. And the part that sticks into the QCGB will need a key way cut in it.
 
This is great information, thank you! I do single point thread quite often, but as you said, 99% of my threads are less than an inch or two long and so I've never noticed any variations in the pitch. I would be interested in calculating the delta-P for different sections of my lead screw - at least so that I'm aware of the deficiencies and where those areas are located along the carriage travel.

The bank balance could support a lead screw replacement, but I have definitely outgrown this lathe and would like to upgrade to a 14-16" swing with more rigidity in the near future. If I can get away without purchasing a new lead screw that would be ideal. Flipping the current lead screw and machining the ends is a great idea if I determine the wear to be excessive for my needs.
 
OK. I'll hate to see you leave the Atlas Forum but if you need a 14" or 16" machine, you do. Unfortunately, you already have one of the largest machines that Atlas built, so for anything larger, you will have to change brands. I think that if you go to any 14" or larger, you will find that other than measuring tools in general, nothing else that you have in the way of tooling or accessories will be usable. For example, a 14" will need a BXA tool post and I think that a 16" might need a CXA. Clausing built larger machines as did their subsidiary Colchester. And of course there are a lot of other badges. If this change is going to happen in the fairly near future, I would not invest the time required to flip the lead-screw. as it is unlikely that doing that will make much difference in what you sell the Craftsman for (I would, however, probably go ahead and plot out the wear on the existing screw for your own temporary use unless you decide to go ahead and purchase a new screw). You might or might not re-coup some of the cost of a new lead-screw when you sell the Craftsman. Unfortunately, most of today's hobbyists are going to be looking for a bargain and have no idea what new parts cost today.
 
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