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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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#3

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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9t8z28

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#12
MontanaAardvark I didnt know you were a member of this forum. My name is Brandon. I messaged you on Youtube then through email. Sorry I have not replied. I have been pretty busy. Glad you got this figured out as did I as well.
 

BaronJ

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#13
Hi Guys,

Many smaller lathes use a banjo arrangement to allow different sized gears to be used. Normally you would try and keep everything inside the cover. Running a lathe with those gears exposed like that is very bad practice. Sometimes you have to use a smaller gear as a spacer. Or use one as an idler.

Unfortunately some manufactures don't include all the gears that you may need to cut some threads.
 

MontanaAardvark

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#14
Hi Guys,

Many smaller lathes use a banjo arrangement to allow different sized gears to be used. Normally you would try and keep everything inside the cover. Running a lathe with those gears exposed like that is very bad practice. Sometimes you have to use a smaller gear as a spacer. Or use one as an idler.

Unfortunately some manufactures don't include all the gears that you may need to cut some threads.
I'd be more than delighted if someone could show me how to thread those finer pitches while keeping everything inside the covers. User 9t8z28 has said he thinks he sees how by making a different banjo - or making a series of them.

Unfortunately, there is no "customer service" help to call for these lathes. I bought it from Little Machine Shop and asked them in addition to posting here back on July 1. LMS told me nothing. They didn't comment when I sent that picture of how I did it.

In the mean time, I use a temporary cover over the gears. I don't wear long sleeves in the shop, and the only thing it needs to do is block me from bumping into it. While threading, I'm watching the cutter with magnifiers on and my hand on the stop button.
 
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BaronJ

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#15
Hi Guys,

I'm sure that I've seen somewhere that someone had made a new banjo/banjo's for his lathe. I know that there is at least one design for a replacement banjo for my lathe. The slot is about 12 mm (1/2") too short to fit certain gears. To be honest, I don't remember what the gear size was, other than it was one in the chain. I do remember that in order to use the 127 tooth gear, you can't close the cover, but it doesn't protrude like the one in your picture.
 

MontanaAardvark

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#16
Hi Guys,

I'm sure that I've seen somewhere that someone had made a new banjo/banjo's for his lathe. I know that there is at least one design for a replacement banjo for my lathe. The slot is about 12 mm (1/2") too short to fit certain gears. To be honest, I don't remember what the gear size was, other than it was one in the chain. I do remember that in order to use the 127 tooth gear, you can't close the cover, but it doesn't protrude like the one in your picture.
As I pointed out in the original post and the video (post #6), I was able to thread 28 TPI, but when I went to 32 TPI the 80 tooth D gear pushed the 127 tooth C gear into the bushing on the drive and kept A and C from meshing. When I went to the 100 tooth D gear for 40 TPI, that pushed the C gear so far out that it hit the plastic enclosure on the left. It wasn't within two inches of meshing.

Like I said, if I'm being stupid, I want to fix it. I'd love to see how to get threading to work without the gears hanging out beyond the cover.
 

9t8z28

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#17
Hi Guys,

Many smaller lathes use a banjo arrangement to allow different sized gears to be used. Normally you would try and keep everything inside the cover. Running a lathe with those gears exposed like that is very bad practice. Sometimes you have to use a smaller gear as a spacer. Or use one as an idler.

Unfortunately some manufactures don't include all the gears that you may need to cut some threads.
I believe MontanaAardvark and I both have the full set of available gears to thread any TPI from 4-80 and .2-8 metric. I know I do and then some. I have more gears than are needed. Maybe they accidentally gave me 2 kits. Anyway, the only way to thread these finer TPI’s is to do exactly as MontanaAardvark did. I am working on a solution so that we can keep the banjo inside the cover. My idea is to use an idler that will be a part of the banjo but will be adjustable the same way the banjo is. Another option is to relocate the plastic box that has some kind of electronics in it. If this was out of the way the 40 tpi gear set would fit inside of the cover, but I am not sure on the 32 that MontanaAardvark had to cut. I would preferred to make another banjo rather than relocate the electronics box

BaronJ, I have never heard of an issue where the banjo was not long enough. In regard to seeing someone make a longer banjo for their lathe; yes I saw it as well on youtube but its for a 9x or 10x lathe. I can’t remember which one it was. The banjo is almost identical but there are a few slight differences. Let me see if I can find the link
 

9t8z28

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#18
EDIT: I posted right after MontanaAardvark and he states that relocating the electronics box would not allow the 32 TPI gearset to fit on the inside of the cover.
 

9t8z28

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#19
What kind of lathe do you have?
Hi Guys,

I'm sure that I've seen somewhere that someone had made a new banjo/banjo's for his lathe. I know that there is at least one design for a replacement banjo for my lathe. The slot is about 12 mm (1/2") too short to fit certain gears. To be honest, I don't remember what the gear size was, other than it was one in the chain. I do remember that in order to use the 127 tooth gear, you can't close the cover, but it doesn't protrude like the one in your picture.
 

9t8z28

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#20
Here is the Youtube video series by Dans-Hobbies. Its titled “A better lathe banjo”.
This mod is for a Harbor Freight 8x14.
 

MontanaAardvark

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#21
Hi Guys,

I'm sure that I've seen somewhere that someone had made a new banjo/banjo's for his lathe. I know that there is at least one design for a replacement banjo for my lathe. The slot is about 12 mm (1/2") too short to fit certain gears. To be honest, I don't remember what the gear size was, other than it was one in the chain. I do remember that in order to use the 127 tooth gear, you can't close the cover, but it doesn't protrude like the one in your picture.
I don't see how that could be my problem. It's not that the gear wasn't far enough out on the banjo, it's that there is mechanical interference out there. If the gear couldn't reach where it's going, that's one thing, but on the SC4 the problem was the gear was hitting something else, the drive gear (for the 32 pitch) or that enclosure (for the 40 pitch).
 
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BaronJ

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#22
Hi Guys,

Sorry I didn't say ! I have a Myford S7LB with a gearbox and power cross slide.
Myford supply a new modified banjo for use, it comes with the metric conversion kit. Basically it is longer in the curved slot to allow it to move slightly further round.

I've done a number of searches on the SC4 without finding the info that I wanted to pass on.
I wonder if ARC Euro could help. They seem to be the Sieg UK experts.
 

9t8z28

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#23
Hi Guys,

Sorry I didn't say ! I have a Myford S7LB with a gearbox and power cross slide.
Myford supply a new modified banjo for use, it comes with the metric conversion kit. Basically it is longer in the curved slot to allow it to move slightly further round.

I've done a number of searches on the SC4 without finding the info that I wanted to pass on.
I wonder if ARC Euro could help. They seem to be the Sieg UK experts.
I am curious to see how the Myford banjo looks. You mentioned that it is curved in the slotted section. Do you have a picture by chance or maybe you can find one online. I did search for one but cannot find any. Maybe I’m using the wrong search terms
 

mikey

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#24

BaronJ

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#26
Hi Guys,

I've taken mine off so that I can photograph it.

Banjo-002.jpg
You will notice that the pins for the gears are in fixed positions. Myford made another banjo for this model lathe that is part of what they called "Metric conversion Kit".
 

9t8z28

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#27
This is exactly what my idea is. I didnt even know this existed. In my opinion, this would fix the issue of having the change gears sticking out of the cover. The lower slot would have the change gears and one of the extra smaller gears would be used as an idler. Sieg does not offer such a part.
I wonder if a dual-arm banjo would solve this for you. Of course, you would have to make one (unless Seig has one) but it might solve this issue. Might ask LMS if such an option is available. Looks sort of like this: https://www.ebay.com/itm/NICE-ORIGI...565840?hash=item4683c6f8d0:g:wZcAAOSwgq9bgbpd
 
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9t8z28

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#28
I like this idea. So, in the first link you posted, the gears on the bottom is where the banjo goes ? The banjo fastens behind the large wide gear ?
Hi Guys,

I've taken mine off so that I can photograph it.

View attachment 274508
You will notice that the pins for the gears are in fixed positions. Myford made another banjo for this model lathe that is part of what they called "Metric conversion Kit".
 

MontanaAardvark

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#29
Sieg does not offer such a part.
Doesn't look like it would be hard to copy, or change slightly and copy.

I'd need to do some accurately dimensioned drawings to see how it would allow everything to work and keep all the gears inside the enclosure.
 

mikey

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#30
This is what the Emco banjo looks like:

Emco Banjo.PNG

Hope it gives you some ideas.
 
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