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PM1340 Threading dial quesition

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sanddan

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I have a PM1340GT lathe and have a question on the threading dial. My previous lathe had 4 numbered hash marks and 4 nonnumbered hash marks between them for a total of 8 marks. You could also engage the half nut in between the 8 hash marks. This gave you a small section on the arc you could use light pressure on the handle to help engage on the mark. My PM lathe has an additional 2 engagement points that split the standard 16 points. here's a pic IMG_0921.JPG

The narrow "window of opportunity" between marks is very hard to hit with consistency. Is this typical for the PM1340 lathe? Is there any way to modify the setup to eliminate the "extra" marks? I hate to admit I've had this lathe for quite a while and haven't tried single point threading until now. Looking for feedback.
 

mksj

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When I used the threading dial on my 1340GT I would just hit the marks at the numbers, and yes it seems more sensitive as to hitting it at just the right spot. It could have something to do with the TPI of the leadscrew and the threading dial gear. There was a previous post on this problem and sometimes the actual engagement point is either just slightly +/- of all the marks and you always need to engage at the same point. I only used the threading dial a couple of times early on, after that I went to a proximity stop system. I do not disengage the half-nut until I am finished threading, each pass is electronically stopped at the same position. These days I only use the crosslide for threading and lock the tool post slide, works fine with laydown inserts that seem a bit more robust then the GTN type.
 

BtoVin83

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I think the thread needs to be divisible by the number of hash marks on the dial. A 24 thread is divisible by 8 so you can use all 8 marks, a 20 TPI is divisible by 4 so only the 4 marks can be used (opposite each other of coarse). You can engage the half nut in between the marks for sure but the tool won't line up every time.
 

petertha

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My (non-PM but Taiwan) 14x40 has a similar characteristic where it likes to be fed the engagement leaver a 'line width' ahead of the actual intersection. Not sure if that's just reaction time or mechanical. Looks like you have a rivet for a center so you may have to experiment till you find the special spot LOL. I think its classic offshore machines that they put a half decent graduated scale on & then just hammer down the opposing center mark TLAR +/- a wide margin. I wish they would just put an adjustable scale on one side & leave the tuning to us.

Another thing I'll throw out. I had to disassemble my apron I had the threading lead screw, power feed & fwd/rev switch bars all off. When re-assembling I thought hmmm.. now what if the threading lead screw were rotated a half turn before being pinned in the output coupler, would that not shift the threading dial phasing? Fortunately my roll pin hole looked like it was was drilled off center 'by eye' so there was no chance the roll pin would even engage if rotated 180-deg. It could only go in the one way. So maybe this was by design intention? Normally I would give the benefit, but seems like every shaft with a roll pin was a similar off center unique hand job, which makes parts replacement a real PITA. Anyway, something to check if you ever pull your lathe apart.

Oh, and I discovered I had a steel roll pin instead of a brass shear pin so I made one. Make sure its a decent fit, not hammer in tight but also not loose so you don't introduce the equivalent of backlash. I've heard of other stories where its a taper pin drill but they have a steel pin in there because brass taper pins are hard to find & 50/50 chance its its metric.

I'm kind of like you, been using the lathe for a while but just not that much threading. I bought some inexpensive Ebay internal/external tool holders that look to be based on Carmex. They seem pretty decent. A few practice runs & parts came out pretty decent. Now I am actually quite excited by all the things I want to make with threading features. It is rather contagious.

A few amateur tips I can pass on:

- you must have a reliable way of measuring pitch. I have the 3-wire kit but I seemed to be fumble fingers holding the buggers on, at least fine threads. I have an older model Fowler screw pitch micrometer with the anvil kit. Personally I find that much easier & more reliable. If I did it again I would buy a Shars digital because then you can buy the metric anvils, switch the display to mm & now can do both imperial & metric with same unit. The big name thread micrometers like Starrett or Mitutoyo are brutally expensive new. Used ones are always a crap shoot, maybe sure teh anvils are included or you are confident you can get them.

- I found threading is a bit like boring with respect to taking a few spring passes to be sure the depth is consistent before measuring. There probably is a progression formula somewhere but if DOC is say .025", the first .015" is nothing, very little removal. Its the last 5 thou where more material is coming off = more depth on the face of point tool = more propensity to deflect = reason for spring/cleanup pass. Good luck!

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sanddan

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Thanks for the input. My main issue is this lathe has more actual engagement points than is typical. Most all lathes have 8 main spots. 4 are even and 4 are odd numbered, makes a difference which you use based on even or odd numbers of TPI. Mine has 3 additional spots in between each of the typical 8 spots. When you divide that small of a circle (threading dial) into 32 distinct points the dial will engage, the resulting tiny window of opportunity is hard to hit with consistency. I guess I will just have to run it at a slower RPM to have any chance of getting a good thread.
 

sanddan

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I did order up a set of thread wires so I can actually measure the thread vs using a nut to check fit. I doubt I will ever do enough threading to justify getting a thread micrometer but you never know.
 

bretthl

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Thanks for the input. My main issue is this lathe has more actual engagement points than is typical. Most all lathes have 8 main spots. 4 are even and 4 are odd numbered, makes a difference which you use based on even or odd numbers of TPI. Mine has 3 additional spots in between each of the typical 8 spots. When you divide that small of a circle (threading dial) into 32 distinct points the dial will engage, the resulting tiny window of opportunity is hard to hit with consistency. I guess I will just have to run it at a slower RPM to have any chance of getting a good thread.
I had the same question with this lathe when I began threading with it. I could not hit a consistent mark on the dial. Then I stumbled across an ancient Chinese proverb in the factory manual that came with the lathe. I paraphrase: As the desired numbered line beomes aligned with the rivet, push down on the lead screw engagement lever UNTIL full engagement with the lead screw. The carriage will progress and the numbered line will come to rest slightly past the rivet . On my 1340GT (and it may be different on yours, these are not "precision" machines) when I throw the lead screw lever when the line hits the rivet, the split nut rides the lead screw and then closes to full engagement. Don't force it, just steady pressure until the spit nut engages. Using this technique I have achieved consistent lead screw engagement.
 

petertha

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Sorry but that doesn't sound right to me. The whole idea of the engaging by the lines is to synchronize / align the internal threads of the clam shell to the threads of the leadscrew. If you are riding on top for a while before it engages, that means the marks are out of phase. You risk wearing or even mangling your brass clam shell threads. Now to be clear, if its out say a line width either way, or your reaction time is something you have to practice a bit (like start downward motion 2 line widths before it comes around) that is probably normal. But if the lines are not matching up like engagement occurs in between the numbers or you are having to ride on it for a while routinely, I suspect something is amiss. See my comment in post #4. Coarser threading and or higher rpm means everything is kind of sped up a bit, but you might not have much optionality from that perspective.

Another thing to check, and this may be lathe specific. The rotating engagement mechanism we are talking about can be rotated out of position so its not always rotating with the leadscrew. Its there just to save wear on parts if you are not threading & something you should do, Now if its not quite rotated back in position correctly to leadscrew, like partial engagement to leadscrew, that might be giving a bit of false position reading. It shouldnt be much but something to check.
 
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bretthl

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Sorry but that doesn't sound right to me. The whole idea of the engaging by the lines is to synchronize / align the internal threads of the clam shell to the threads of the leadscrew. If you are riding on top for a while before it engages, that means the marks are out of phase.
You're right and this has been bugging me. Off my arse and off to work.

Remove the "precision" offending part:

P1000293.JPG

Break the gear loose from the shaft (4mm and 5mm Allen):

P1000294.JPG

Disassemble, clean, grease, etc.:

P1000295.JPG

Reinstall the thread dial with the the split nut (clam shell) closed, gear engaged to the lead screw and a timing line aligned with the rivet then tighten the gear retaining screw. I put a dab of blue Loctite on the screw.

P1000297.JPG

Now engagement is dead on the timing lines!

As Dan noted in his original post there are 32 possible points of engagement. The only way to change this is to reduce the circumference of the thread dial gear by a a factor of four but then the dial would turn 4 times as fast and that would okay with me (would have to re-design the housing too).
 
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petertha

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Good job. Glad you persevered to the heart of the matter. Happy threading.
 

sanddan

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With mine the lines do line up so it doesn't need adjustment like yours. Looking at the cosmoline build up I should take mine apart just to clean that up. I didn't take the carriage or compound apart when new, just cleaned up as best I could with it fully assembled. I bet it would run better if it got a complete disassembly and cleaning. I know some of the oil ports will not accept oil so they could be plugged with cosmoline. I just oil the ways on those areas right now. I'm 66 so I doubt I will wear anything out during my lifetime, next owner can deal with it if they so desire.
 

Djl338

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Glad I opened this thread, I removed and cleaned my dial, the brass gear coated with paint and hard, dirty cosmoline or grease-oid material, real mess Cleaned up lightly polished and re-greased
F5EFFEC3-B69A-4471-9553-C836D1BCE43C.jpeg
 

rherrell

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GREAT thread! My 1340GT dial was only off a little but it's always bugged me. I followed your instructions and now it's dead on! The only difference is the shaft on my dial didn't have a hole in it, but I took care of that!!:cool:
 
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