Sherline Thread Cutting Attachment Slipping

sully2161

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So I've finally gotten around to needing to cut some threads on the lathe. The installation guide was fairly straight-forward and configuration of the gears was no problem. The issue I'm having is that when backing off the tool and reversing the handwheel direction, the arm holding the gears together is loosening and the gears slip. Sometimes it happens almost immediately and sometimes only after several cutting passes.

I'm wondering if anyone else has had this issue and what they've done to resolve it. I'm debating a couple of modifications but I don't want to do anything drastic because the arms have to support multiple gear configurations (which means a mod that doesn't block a gear today might tomorrow).

So far I've been thrilled with the lathe (4400) and all the other accessories. The thread cutter falls a bit short, with loose brackets, a cramped fit, and having to remove the motor.

Any suggestions appreciated. Thanks!

- Steven
 
Sully, I've been using the Sherline thread cutting attachment for a long time now and never had any of it slip or loosen, not even a little bit. Are you meshing the gears really tight together? They should have about 0.003" of gap between the teeth (paper thickness or so). It might help to post a pic of how you're setting up the gear train and we can maybe help more.
 
Mike, thanks for the reply. Glad to hear you haven't had any problems -- maybe there's hope for me. I am meshing the gears tightly. I make sure everything is aligned and meshed before tightening the last 2 screws (the arm screw and the sliding gear screw). The problem seems to be the direction reversal puts pressure on the sliding gear at a different angle and forces the arm away, allowing slippage. I've even added lock washers to the arm screw and it still slips.

I will definitely post a photo. It'll probably be Monday before I can, though. I'm setup in a friend's huge shop and he's out of town until Monday (still owes me a door key!)

- Sully
 
We'll iron this one out, Sully. Off the top of my head, I would guess that you're trying to mesh the gears too tightly and that may be causing the RH screws that hold the gear train in place to loosen as you reverse. If you mesh the gears too tightly it becomes hard to turn the leadscrew handle as well. The leadscrew should turn freely and without a lot of effort when you aren't cutting and just a little more effort when you are cutting. It's been a long time since I read the instructions but I seem to recall Sherline doesn't tell you anything about how tightly to mesh the gears so its not your fault.

Oh, when you get the gear train set up just the way you want, put a single drop of oil on one of the gears and another on the 100 gear. That is enough to lube the entire gear train.

Another thing to remember is that cutting forces increase as you feed in deeper. I prefer to take 0.002" deep cuts for the first 5 passes for general thread cutting, then I use 0.001" deep cuts for every pass thereafter. This is slower, granted, but it assures accuracy and optimal thread form on a hand-driven lathe with an aluminum gear train.

Stay with it, Sully. It takes awhile to get used to removing the motor and setting up your gearing but it will become simple and easy as you progress. I don't even think about it anymore - I just do the swap, cut the threads, replace the motor and I'm done very quickly. The Sherline thread cutting attachment looks sort of cheesy but it will cut more threads, both Imperial and Metric, than most full sized lathes with a full set of change gears and like just about everything else with a Sherline, it will cut those threads with precision.

One last thing. Your lathe tool should have a very accurate 60 degree included angle and I suggest a 15 degree side relief angle on both side. Stone a tiny flat, just visible, at the very tip to prevent tip breakage and keep your tool razor sharp. Keep your tool overhang minimized and be sure your tool is on center and perpendicular to the work. If you haven't made a tool height gauge for your lathe then I suggest you make one calibrated to the centerline of your specific lathe. Use a fishtail to align the tool to the work. If any of this sounds like Greek to you, let us know and someone will clarify it.

Welcome to the Sherline lathe and to the HM Forum!
 
Thanks for the welcome! Well, you were definitely right about meshing the gears too tightly. I was a bit concerned about the aluminum gears and so I'd meshed them pretty tight. Loosening them up a bit made for much less effort in turning the handwheel.

Unfortunately, this didn't solve my problem. I tried various degrees of meshing and it always jumped out in reverse. In the photo, it's the arm in the foreground that twists slightly clockwise (against a tightening screw & lock washer no less) so that the 40t gear separates from the 20t on the leadscrew. (Handwheel removed for photo)

IMG_0088.JPG
 
Your set up looks correct. Be sure all the SHCS are tight except for the one on the end of the leadscrew - that one is just hand tight. If done as above, the set up should not be able to move. The only other thing that could remotely do it is if the hole at the end of the arm that is currently holding the 20T gear is somehow not drilled straight. That might allow the gears to cock and move out of alignment. There really isn't anything else there, Sully. Give it a try and let us know how it goes.
 
Steven,
I'm curious.
It looks like, from the 127 tooth gear on the spindle shaft, that your Sherline lathe is the Metric edition. Is that correct? I was looking at my Sherline Gear Selection Chart for Thread Cutting Attachment and the only application for the 127 tooth gear on the spindle shaft is if you have a metric 4100 or 4410 lathe. Your original posting indicates you have a 4400 series lathe. Which do you actually have? What pitch thread were you setting up to cut?
 
Sharp eye. I actually have a 4400 AND 4410 lathe. The attachment works on both and has the same problem on both, so I was testing & taking photos on the metric lathe to leave the 4400 free for use. I'm selling the 4410 shortly so I didn't include it in my equipment list. The gear train is setup for 20 TPI.
 
Some Thoughts.
The design makes it difficult to tighten the gear that is mounted in the slot. The screw tightens into a short stub with nothing but a thin flange on the back side, providing no way to back it up when tightening. Is that one tight enough?
At the end of the pivoting arm holding that same gear there is a 10 32 SHCS that threads into the mating arm. Perhaps a longer bolt with and the addition of a locking nut behind it might provide additional holding power at that joint.
Also, it would be good to make sure the arms, where they mate, are perfectly clean and dry. Any lubricant there would not be good.
 
I've had threading success in the past, but recently ran into the same problem. Oddly enough, I am also experiencing it trying to cut a 20 tpi thread. I've had some luck using the normal leadscrew dial to reverse rather than the screw attachment dial. I also took the lathe apart to clean and chech for any problems. I did find a problem. The threads were stripped on the leadscrew support hole that the screw used to fix the base to the leadscrew support goes in. Once I get this fixed, I am going to continue to study the problem. I also have installed the anti-backlash mechanism and need to see if that might be creating excessive friction for the system when returning the cutting tool to its starting position to begin another pass. I really hope it does not have anything to do with the geometry of the 20 tpi gear train. That's a pretty useful pitch for a lot of things. Once I get the lathe reassembled and some more testing done, I'll report back.
 
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