Sherline Thread Cutting Attachment Slipping

I'm wondering if there isn't some quality control issue with the threading attachment arms. I have never had mine slip or give any problems whatsoever but mine is an old one. Have you considered contacting the factory to see if they have any input? Have either of you guys checked the arms to see if they are actually flat? Please let us know.
 
I haven't contacted the factory, but will once I rule out the possibilities I mentioned. And I'lll check for flatness. My lathe and screw thread assembly are about 6 years old. It seems to be well made to me. Thanks for the ideas. I'll report back.
 
Okay, I've got my lathe reassembled and also discovered a few problems leading to the slippage. The overall issue is too much friction. Here is what I did to reduce friction enough to get the threading working and also a quick but dirty way to create more friction at the "elbow joint" where the 2 arms that hold the gear train in place join. These are called the "Primary Support Shaft" and "Secondary Support Shaft" in the Sherline documenation. They are held at the desired angle by a 10-32 socket head bolt that threads into the primary support shaft.

1. There was some friction inside the leadscrew support that houses the pieces that engage/disengage the threading action (fixed shaft and sliding shaft) due to some burrs I cleaned off the burrs, cleaned and regreased with silicon grease. Leadscrew was cleaned and relubricated as well. There were also some wear spots on the fixed shaft, so I lightly filed them because they seemed to be rubbing a bit inside the support. Once deburred, cleaned and relubed, the shafts rotated nicely inside the support.

2. The gib strip for the saddle was cleaned, relubricated, and backed off a bit to decrease the pressure it was applying.

3. The bushing that holds the 40 tooth gear (Gear E in the Sherline documentation) had a rough surface that the gear rotates on. It looked a bit like galling. I cleaned and lubricated.

3. Leaving the gear train in the arms, I detached the assembly from the lathe and spun the gears. The 40 tooth gear (gear E) had an area of about 4 or 5 teeth where it would catch with it's neighboring 20 tooth gear (Gear C). The same location on the 20 tooth gear seemed fine on the other side of the 40 tooth gear during rotation, so I am assuming the 40 tooth gear has some tooth shape issues. The teeth may be too thick at some elevation within the tooth. There was also some funny wear in this area. The engagement of this area when everything is put back on the lathe is the area that causes the arm to move, disengaging the gears. As a sanity check, I replaced the 40 tooth gear with a 36 tooth gear and found that it worked fine.

4. I put the 20 tpi gear train back on the lathe and rotated it both directions for a bit. It seemed to be working fine, but finally failed once again. I tightened the bolt holding the arms together a bit more and added a small 1" C-clamp near the screw to apply more pressure without over tightening the bolt. It is now working without failure. I can still feel a bit of periodic friction in the system, so I need to look at the 40 tooth gear some more for a more thorough fix. It could be that my current setup will simply wear the other gears out in time. I like the idea of using a locking bolt too that was offered. I'll give that a try too.

I checked the flatness of the arms and found that the secondary shaft (the one on the outside, away from the headstock) does have a bit of a bend in it. It bends about 0.003" over 1.5", so the angle change is very small. It could be a contributor to the the gear meshing issue, but it seems like there is more play in the system than the angular change due to the out of flatness. The surfaces of the arms that are engaged when bolted together appears to be flat.

I'll give the factory a call about the gear. They may know about some issue and have a fix.

Oh, this is a bit off subject, but I discovered why the thread stripped on my base t0 leadscrew support attachment. Sherline used a 3/8" long 10-32 socket head bolt to attach the base to the support. Sometimes there is an added spacer, which my lathe uses. Once you add the space, lockwasher and thickness of the base and support metal, there are only a couple of threads on a 3/8" long bolt. The fixed shaft has a groove in it and is held in place by the screw. I found that a length a bit less than .470 would pass through all the pieces and hold the shft in place, so I ground down a .5 long socket head bolt to size and used that. I also replaced the bolt holding the leadscrew thrust on the other end with a longer bolt because it appeared not all threads were engaged at that end as well.

Thanks for the ideas! It's great to have other brains thinking about a problem and also hearing other experiences when trying to come up with a solution to a problem. Sully, I hope something here helps out your slippage. I'd be interested to know if something helps or not.
 
Outstanding! This sort of thing will help someone, for sure. I am concerned about the lack of flatness in the arm and the poor surfaces on the gear bushing and on the 40T gear - that should not be the case. I would contact Sherline, too, and ask them what gives. I just looked at my 40T gear and it is perfect. I have used this threading attachment heavily over the years and it performs flawlessly for me. If it did not I would be on the phone fast!
 
I used a gear from my Sherline threading set last night that I had not used before. Trying to thread was jerky and took a lot of force, and caused the idler gear to 'jump' out of position just as the OP described. After disengaging the lead screw and applying a liberal dose of way oil (vactra #2), and turning the spindle many times in both directions, the motion smoothed out very nicely. The Sherline aluminum gears don't appear to mesh nicely when new, but after some 'break in' the attachment worked well!

John
 
I just used my threading attachment to turn a Class 3 fit M8-1.25 thread. I've found that if the gears are engaged too tightly they may bind or feel "jerky" as you mentioned. This goes for any of the gears but especially the A & B gears. These gears are like any change gear set - they do not need to mesh tightly. A few thousandths clearance between all meshing gears will provide smooth movement.
 
No matter what I did with the clearance, I could not get the 20 tpi arrangement to work without the work I did to remove the friction, although other combinations worked fine. I do have a useful update that follows John's and Mike's recent entrys. I cut a 20tpi thread in a piece of O-1 rod as a test. By the end of the test, my gears were running much more smoothly. I was careful to make sure that the meshing was engaged, but not too tight. Perhaps I caused a bit of damage to the one gear by meshing too tightly the first time I tried it. I think that if I find myself using a new combination of gears in the train, I will spend a bit of time running them in with the back and forth motion John describes before making a cut. Thanks for reorting back on your discoveries.

Hey Mike,
Are you the same Mikey that wrote up the back-side hoizontally oriented cutoff tool project over on machinistsblog a few years back? I'm in the process of making one, and want to say thanks for the writeup if you're the author.
 
Hey Mike,
Are you the same Mikey that wrote up the back-side hoizontally oriented cutoff tool project over on machinistsblog a few years back? I'm in the process of making one, and want to say thanks for the writeup if you're the author.

Yeah, that's me. I hope it works out for you because it is one of the most useful tools you will have for your lathe. The one in that write up was a prototype but I'm still using it because I can't figure out how to improve it. A new one would look better but wouldn't work better.

By the way, I just used some gears I haven't used before to cut that metric thread I made and had no binding once I got the clearance right. Something has changed in their production or design.
 
I'm wondering if my problem was a combination of a slightly misshapen gear compounded by initially having the mesh too tight and the slightly out of flat arm. The odd wear I mentioned earlier could be attributed to the lack of flatness issue, since the wear is squewed to one side. I am on a trip right now, so I can't check some of the other gears. There are small discrepencies among individual tooth profiles though, perhaps enough to add to the issue. Bottom line is that fiddling around in the direction of the info posted in this thread resulted in getting some decent threads cut. Forums get problems solved!

I'm looking forward to getting the backside cutoff done. I had some bad crashes early on getting cutoff figured out with the Sherline setup, bad enough the the jaws in my 3 jaw chuck need work and I flinch whenever I have to part off. Some new blades are waiting for me at the post office and I need to finish up the mill work when I get back. I've got a bunch of parting off to do for my main prject, so I'm pretty excited to get a copy of your tool made. I'll let you know how it goes. The good news about messing up the 3 jaw is that I am better at centering stuff in a 4 jaw than I would be if I wasn't forced into using it so much.
 
Yup, been there, done that. Major dig ins on a Sherline can happen in an instant and it is amazing how much a cobalt parting tool can bend before it snaps. Its what made me go looking for a better option and the rear mounted tool is it. I haven't had a bad moment or problem since I built it.

If I may, let me offer one bit of advice. Make SURE the bottom of the blade slot is on the dead center of YOUR spindle. As long as you do this the tool will work. The ledge also helps keep the tool aligned under load and lets you mount the tool on the cross slide in seconds.

Do let us know how your tool turns out.
 
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