# Craftsman 101.28940 Half Nut Skipping Teeth



## tkalxx (Apr 21, 2021)

I have a craftsman 101.28940 that has recently started skipping teeth in the ~10" of travel closest to the chuck. I can only assume this is either due to leadscrew or half nut wear (or both). When I take a look at the half nut while it's engaged, it looks like the upper portion of the half nut is jumping around. Initially I thought it may be due to some run out on the leadscrew, but I measured the run out to be .005". I was able to get into the tight space and clean the half nut, along with the leadscrew but this made no difference. 

Before I tear this thing apart, is there any adjustment for the half nut? It feels to me like the upper portion of the nut is loose. There is definitely some wear on the leadscrew, so I assume the half nut is pretty worn as well but I'd like to avoid replacing the screw if I can. 

-Adam.


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## T Bredehoft (Apr 21, 2021)

Just a suggestion, I don't know the answer.

Remove the saddle, determine if a spacer can be put behind the upper half (the looser one) to  move it closer to the lead screw. If so,   do so. 

Also it may be possilbe to replace both halves of the half nut.


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## Janderso (Apr 21, 2021)

https://www.ebay.com/itm/Atlas-Craftsman-9-12-9-10-12-Lathe-Half-Nuts-Split-Nut-/202639054831 

I found these on the Bay.
Are they correct?


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## tkalxx (Apr 21, 2021)

T Bredehoft said:


> Just a suggestion, I don't know the answer.
> 
> Remove the saddle, determine if a spacer can be put behind the upper half (the looser one) to  move it closer to the lead screw. If so,   do so.
> 
> Also it may be possilbe to replace both halves of the half nut.



That seems like a decent temporary solution. I would need to make a spacer that pushes the upper half down towards the screw. I haven't taken the half nuts a part yet, so I'm not exactly sure how they function or if this is even feasible. The exploded view in the manual that I have is a little confusing and doesn't show adequately how the half nuts function. 



Janderso said:


> https://www.ebay.com/itm/Atlas-Craftsman-9-12-9-10-12-Lathe-Half-Nuts-Split-Nut-/202639054831
> 
> I found these on the Bay.
> Are they correct?



Those are incorrect. My lathe has a 3/4" ACME screw. Half nut part# from the manual is 10F-12. It looks like this is the correct nut:









						Atlas / Craftsman 10F-12 10" & 12" Lathe Half Nuts / Split Nut - BM Parts Tech
					

To place an order please email: info@bmpartstech.com



					bmpartstech.com


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## tkalxx (Apr 21, 2021)

After a little digging, it looks like the half nuts engage using some pins and a scroll wheel when the lever is engaged. I don't think adding a spacer is an option.


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## macardoso (Apr 21, 2021)

I do not know about your particular lathe, but I do know that in general, there are several adjustments that can be made to the half nuts.

You first need to determine if they are worn out and need replacement. If not then you can start figuring out what adjustments to do.

My import lathe has half a dozen adjuster screws as part of the half-nut assembly


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## tkalxx (Apr 21, 2021)

macardoso said:


> I do not know about your particular lathe, but I do know that in general, there are several adjustments that can be made to the half nuts.
> 
> You first need to determine if they are worn out and need replacement. If not then you can start figuring out what adjustments to do.
> 
> My import lathe has half a dozen adjuster screws as part of the half-nut assembly



I'll pull the half nut off the lathe later today to inspect it; it's a 35 year old lathe, I can only imagine it's worn and needs replacement. From the various images I've found online, there doesn't appear to be any obvious adjustments available. The manual doesn't state anything either.

You can see the pins (one on each half) that seat in the scroll wheel. When the scroll wheel is spun, the two halves come together.


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## macardoso (Apr 21, 2021)

Hmm, looking at that design, perhaps no adjustments are possible


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## markba633csi (Apr 21, 2021)

On Atlas lathes there really aren't any adjustments. When it's worn you replace.  I'm guessing both the leadscrew and the half nuts are worn
-Mark


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## Manual Mac (Apr 21, 2021)

Couple months ago I purchased rebuilt half nuts for my SB9. 
They were probably original. 
I’m back to cutting threads, etc.
I’d be inclined to buy new/rebuilt half nuts if it were me, they wear more than the leadscrew.
of Course, YMMV
Cheers


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## tkalxx (Apr 22, 2021)

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.


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## markba633csi (Apr 22, 2021)

Those look like aftermarket or shop made half nuts- I didn't think Atlas ever used brass inserts
Maybe I'm mistaken


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## tkalxx (Apr 22, 2021)

markba633csi said:


> Those look like aftermarket or shop made half nuts- I didn't think Atlas ever used brass inserts
> Maybe I'm mistaken



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.


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## markba633csi (Apr 22, 2021)

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


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## tkalxx (Apr 22, 2021)

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.


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## wa5cab (Apr 22, 2021)

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.


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## tkalxx (Apr 22, 2021)

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.


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## wa5cab (Apr 23, 2021)

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.


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## tkalxx (Apr 23, 2021)

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.


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## wa5cab (Apr 23, 2021)

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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## tkalxx (Apr 23, 2021)

The Craftsman has been a wonderful machine. I did the metric threading change gear modification, added a DRO and for a long time it did everything I needed it to do. Lately I've been finding myself turning larger off center pieces and attempting to counterweight a lot more often. After getting more familiar with a 1340 and 1660 lathe that we have at work, the desire for a heavier and larger machine is pretty high. I am in no immediate rush to upgrade though. In the next year or so, if a decent condition Standard Modern 1440 popped up for sale local to me, I can't say I wouldn't jump on it.

My craftsman doesn't have a quick change tool post, it has a custom 4 way tool post. Most of my 5/8" and 3/4" tooling should still be usable with a 14" machine but you're right in saying that I would want a BXA/CXA tool post. In addition, my lathe is missing the steady and follow rest. It seems like the value of these lathes automatically get cut in half by hobbyist's when those accessories are missing. Maybe I'll keep a look out for some good Ebay deals on steady/follow rests so that when it comes time to sell I can get double the asking price


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## wa5cab (Apr 23, 2021)

OK.  If most of your cutters are 5/8" or 3/4", they would still be usable with the larger lathe.  I've had an AXA on my 3996 and almost all of my cutters are 3/8".  Which would probably all be too small for a 14" or 16".

For the last several years, my rule of thumb for pricing if someone asked has been about $150 for most of the accessories except for the Tool Post Grinder which I have always figured @ $200.  The other two common accessories are the Taper Attachment and the Milling Attachment, which I also usually allow $150 for.  Chucks, collet chucks or closers, and collet sets are too variable to have a fixed figure for estimation purposes.

I have an Atlas 3996 bought new 40 years ago this year.  Although I have reason to know that it isn't the last one built, so far it has the highest serial number reported.


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## tkalxx (Apr 23, 2021)

All of my insert tooling is 5/8"+ other than a few small boring bars. Custom/form tools are HSS but I tend to stick with 1/2" or greater if I can.

 I had no idea these lathes came with a taper attachment!

That is very cool that yours is among the last built, what is the difference between the Atlas 3996 and Craftsman 101.28940? From the various images I've seen online it looks like the motor mounting/stand are the only differences.


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## wa5cab (Apr 23, 2021)

The half-inch way Atlas machines have two major versions.  The first version, of which yours is one, were introduced in 1957 and 1958 and were made until 1966.  The first version used the same lead screw, right lead screw bearing and cross-feed actuator as the final versions of the earlier 3/8" way 10" and 12".  They were available in two lengths (24" and 36" Between Centers), with Change Gears or QCGB, and as a bench model with horizontal countershaft behind the headstock, or (except for no change gear models) as a cabinet model with a vertical countershaft mounted in the cabinet below the lathe.

In 1962, the change gear models were discontinued.

In 1966, the eight (four Atlas and four Craftsman) new models got a new right lead screw bearing with needle bearings and not designed to break away in the event of a crash while under power feed.  This function was replaced by shortening the left end of the lead screw and inserting an adjustable slip clutch.  The QCGB was modified slightly by changing the output shaft.  Finally, a lever operator was added to the carriage apron to engage or disengage power cross feed.  The cross feed knob was deleted and the cross feed drive shaft was shortened slightly and modified to connect to the end of the cross feed actuator lever.

In 1972, the four 12x24 models were discontinued.

In 1974 the two 12x36 bench models were discontinued.

In March 1981 the final two 12x36 cabinet model was discontinued.

The only differences between Atlas and Craftsman models were the headstock nameplates and the serial and model number plates on the right end of the beds.


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## tkalxx (May 30, 2021)

wa5cab said:


> OK.  I'll hate to see you leave the Atlas Forum but if you need a 14" or 16" machine, you do.



Well, that time has come a lot quicker than I had anticipated... Had a fellow on a local forum message me about a Standard Modern 1340. Although not a 14" swing, I couldn't pass up the offer. The lathe is in exceptional condition with original paint and came with many accessories - taper attachment, steady rest, 5C collet closer, 4J, 3J, QCTP, acu-rite DRO, chip tray and backsplash. It's a metric machine, but does imperial threads as well. 

I had been searching local listings for an SM lathe for about a year with no luck so when this opportunity arose I couldn't say no. The previous owner says the machine was built in the early 80s.







I'm sad to see the atlas go, but I just don't have room in the shop to keep it.


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## wa5cab (May 30, 2021)

Good luck with it.  It appears to be a nicely built machine.


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