Sears/Atlas 6" (101.21200) Direct drive coupler problem

[bsorrell]

Hey folks,
I just joined to get some expert advice on a problem that I am having.

Over memorial day weekend, I reacquired my dad's old lathe. He bought it used sometime before 1996 and I got it when he passed away. It sat in my shed/shop for a few years, then sat in my step dad's shed for a few years, then sat in my brother-in-law's shop for a few more years. It was developing a bit of surface rust on the ways, the cross-feed and the chuck. I have the ways mostly cleaned up but I'm still working on the rest of it.

The problem I'm having is the the direct drive coupling is in the back gear position and will not move in or out. it does spin freely most of the time but binds on occasion. I think the binding is due to the woodruff key being loose and floating around back there and getting caught up on something at times. I know it's there because I can see it shining away at the bottom of the pulley. The outer retaining is missing, as well.

Looking at the exploded view, (I found a manual online that I am referencing), it seems that with the outer retaining clip gone, the coupler should come off the spindle and with the woodruff key loose, the coupler should go in, even without being exactly aligned. Neither assumption is true , though. After some studying and finger nail chewing and WD-40 spraying, I decided to whack the coupling with a rubber hammer, thinking that maybe the retaining ball and spring were stuck and that could break them loose. No luck with that. I also tried getting a couple small screw driver blades in between the pulley and coupler to see if I could pry it out. No luck there, either.

I'm sure this has been going on for some time as the coupler looks like it got intimate with a pair a channel locks more than once. I will try to attach some picture so you all can see what I am seeing.

I would really like to get this up and running since I have a project that is just a little too big for my Sherline lathe to handle well.

Thanks in advance for any and all help and advice you can provide.

Bill

 

[markba633csi]

Hi Bill, I have one of those. You may end up replacing the headstock. Check Ebay.
The coupler may have shattered inside letting the key loose. You'll have to try to brutallize the coupler off but the pulley may suffer in doing so. There may be a burr from the outer retaining ring which could be the main reason the coupler is stuck. Take a close look and maybe a file to the spindle ring groove.
Try heating the coupler with a propane torch while you pry. Use several screwdrivers. There's a second retaining ring in there so you can't just use the pulley to lever it off. You have to get the coupler off first. Worst case drill holes in the coupler and break it apart in pieces. Maybe you could drill and tap holes in it and rig up a puller of sorts? You can't reuse that one now anyhow
Looks like from the pulley wear the lathe was run at the highest speed- that's unusual, most people use the lowest ones. Something may have flown apart from the forces involved. It's not a really strong drive system, even when it's working.
-Mark
PS don't try melting the coupler off it is zamak alloy and contains zinc, don't want to breathe it

 

[markba633csi]

By the way, there are two different versions of the headstock; ball vs. roller bearing. Yours looks like mine- the ball-bearing version which actually has a fatter spindle and is a bit more robust as far as the coupler and key portion, more meat and potatoes. There are two different couplers and two different spindles. I believe the large step pulley is the same casting except they put two different sizes of bushings in them to accommodate the different spindle diameters. The coupler system is a weak point on these. They could have put two keys 180 degrees apart or used splines, but this redesign was supposed to save money. I doubt it did by the time they got done fiddling around with all the bits and pieces- they dropped the model shortly after. I like the concept and the convenience of the backgear lever/collar but they didn't make it beefy enough-should have let me design it
-Mark

 

[wa5cab]

The 101.21200 is the Sears badged and model numbered version of the 3950, so it is the ball bearing model. Sears did not sell an equivalent to the 10100 roller bearing model.

I tend to agree with Mark that destructively removing the coupling may be the best solution.

 

[bsorrell]

Just circling back around to let you all know what happened with my lathe. I did end up destructively removing the old coupling but in a manner that I could get the important measurements off of it and make a new one. I made one out of some plastic that I had laying around and, even though I misread my own drawing and had to add screws to replace the missing stubs, it worked really well. I think with the type of work I do on the lathe I could have left it at that. However, I had a piece of steel bar stock in about the right diameter so I turned and milled a real replacement from that. I've done some turning with it in place and everything is working really well. The drawing is both metric and inch because it seemed like a good idea to me.

 

[RockyCreek]

I'll add to this for the thread history having just disassembled the coupler, it was frozen solid in the position disengaged from the pulley. Heated with a torch and it came off pretty easily. Plenty of hardened greasy gunk to clean off.

Maybe or maybe not related to my stuck coupler, looks like the woodruff key height-above-shaft is not to design. I didn't take a precise measurement, but it looks like it's maybe 0.030 high. Standard design height found on the internet is around 0.060 and the coupler has capacity for almost 0.080 height. The key is a standard 1/8"x1/2" dia woodruff, so it looks like a shaft machining error - too deep a key cut. There are allegedly "full half round" woodruff keys, but I was unable to find one without a large order - I'll make one from round stock. My guess is that I'm not the unlucky one, there are probably others with this low key height which might start locking the coupler as it deforms the coupler keyway. Looks like the original poster could have had this problem too as the key got free somehow.

If you pull yours apart to check, cautionary note: somewhere in my shop is the 1/8 ball bearing for the axial position detent (2 grooves you can see inside the key area). I heard it richochet off something to the west when the coupler came off.

 

[56type]

That captive ball & spring was what was messed up on mine. IIRC, the ball had escaped somehow under the previous owner's care and the spring was jammed in there. After getting it apart I drill the hole for the ball & spring complataly thru the coupler and tapped the hole for half it's length for a set screw to maintain tension on the ball & spring.

This solved two problems...The first being the tendency for the ball & spring trying to attain low earth orbit upon removal. The second being able to increase/decrease tension after the coupler was installed to obtain optimal lock-up with the ball in it's groove. I've since bought an entire 10100 Mk.2 headstock as a spare in case something should go wrong with mine as well as a coupler from Clausing that appears slightly different than the OEM coupler and appears to be made of aluminium.