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Can't Thread Finer Than 28TPI on LMS3540/Sieg SC4 Lathe?

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MontanaAardvark

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#1
I've only threaded a couple of pieces on my LMS3540, and never experienced the problem I've run into this weekend. A few months ago, while making a steam engine kit, it needed a 1/4-28 thread cut on a quarter inch shaft. I did a couple of practice pieces, and a 1/4-20, which is pretty similar.

This weekend I had a small stud on a part I was machining that needed to be threaded 6-32. The only die I have wouldn't get close enough to the part body to thread it (too much taper), so it seems like a natural to do on the lathe. When I put the proper gears on, there's mechanical interference and the driven gear won't engage with the change gears.

These lathes, like other Sieg lathes, use a set of four gears, A-D, as pictured here:

InterferenceSpot.jpg

To thread something, you change the gears over to the required set:

GearTable.jpg

I post this to point out that there is one minor difference between the gears I used successful, the 28 pitch and the 32 pitch: on the 32 pitch the last gear is 80 tooth vs 70 on the 28 pitch.

When I put the 80 tooth gear on, I have to move the B/C gears farther out an arm (left in the picture) they're mounted on, and then it interferes with a bushing on the driving gear (circled in red). The B gear (inner one) never meshes with the driving A gear. The 70 tooth gear works fine, and I also changed over to the smaller gear used for the 20TPI screw (50 tooth) and it works. It seems as the D gear gets bigger, it mechanically doesn't fit.

I put on the 100 tooth gear required for a 40TPI screw and the interference moves from that bushing over to the enclosure you can see on the left. The B/C gears hit that enclosure and never get within a few inches of the gear on A they're supposed to mesh with.

That makes it seem like it's impossible for the SC4 to cut these pitches.

Web searches turn up nothing about this, but this is probably the place to ask. Does anybody know about this? Been able to thread 32 TPI? Can you see anything stupid I'm doing?
 

MontanaAardvark

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markba633csi

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#4
I would call LMS and ask them about it, sure seems like the arm is the problem. Maybe the factory one wasn't designed correctly? With Chinese stuff anything goes (out the door)
Mark
 

MontanaAardvark

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#5
I would call LMS and ask them about it, sure seems like the arm is the problem. Maybe the factory one wasn't designed correctly? With Chinese stuff anything goes (out the door)
Mark
Yeah, it's looking like that's the only alternative. I must be doing something stupid, but I don't see what.

I've looked up the lathe on Sieg's website and they say it turns 8-24 TPI threads, so everything I tried - 32 and 40 TPI won't thread according to them. I looked up other companies that sell the SC4 and they quote the 8-24 also. In fact, LMS is the only place I can find that says it will cut pitches that fine. Which means they're the folks to ask.
 

MontanaAardvark

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#6
I did a video demonstration of what I'm trying to describe. What I didn't do a good job of explaining was that I started out going to the 32 TPI gear set,
A B C D
30 120 127 80
then went to the 40 TPI
A B C D
30 120 127 100

I talk like everyone knows that.

Video on YouTube

I have an off the wall idea to try and will update here if it works.
 

stupoty

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#7
Can you swing the banjo clockwise and engage the b-c gear on the right hand of the B gear? you would have to run with the side door open.
 

MontanaAardvark

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#8
Can you swing the banjo clockwise and engage the b-c gear on the right hand of the B gear? you would have to run with the side door open.
That's exactly the idea I mentioned.

TheTrick.JPG

It took me, my wife and a friend about three days of looking at the normal arrangement to see that. None of us are machinists, although the friend did some work on a machine tools in the Navy, so maybe this is normal practice? Normal for this machine? I need to put up a video to show how to resolve the problem, or take my video down.
 

markba633csi

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#9
Looks like you found a work-around. So it seems that both Seig and LMS are not entirely sure what their machines can do/not do. Maybe they will pay you for pointing this out to them- :D
 

stupoty

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#10
That's exactly the idea I mentioned.
sorry my bad , I must have been skim reading again :)

Extra carful when running with the door open , especially with watch straps or similar as you have a new hazard in a place where before their was none :)

Stu
 

MontanaAardvark

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#11
No problem, Stu. I didn't say what my idea was, only "I have an off the wall idea to try and will update here if it works. "

The open door is a little spooky, but thankfully it's still a bit far from the controls, and you ride the control panel on the lathe while threading.
 
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