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Logan Threading Dial: What did I do wrong?

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ErichKeane

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#1
I've got a Montgomery Ward version of a Logan 10" lathe. Last night, I was cutting 20 TPI threads in some brass (1/4-20). According to pg 8 here (http://vintagemachinery.org/pubs/2093/3353.pdf), I shouldn't have had to pay attention to the threading dial at all!

However, I found that if I didn't line up to the numbered hashes, that I was getting a 2nd set of threads cut! Am I misunderstanding the text there, or did I do something wrong?

Additionally (hopefully not important), I recently took the apron apart in order to install the power-cross-feed mechanism. I had to take the half-nut out in order to do this, but didn't mark a top/bottom. I'm unsure if I got it on in the correct direction. I DID notice that when engaging the half nut, it didn't engage until about 10 degrees past the hash mark (similar to the image in Figure 10 of the above page 8). Could I have messed up doing that?

Thanks in advance!
 

JimDawson

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#2
When threading I normally use the same number on each pass even though the book says you don't have to, saves confusion on my part. :) To adjust the dial, engage the half nuts, and loosen the dial mount, and rotate the dial to a mark, and then re-engage.
 

ErichKeane

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When threading I normally use the same number on each pass even though the book says you don't have to, saves confusion on my part. :) To adjust the dial, engage the half nuts, and loosen the dial mount, and rotate the dial to a mark, and then re-engage.
Ah, thats great news about the dial mount. I thought I'd potentially misaligned something re-assembling. I DO keep the dial out of the way most of the time, but didn't even figure that it could be aligned differently :) I'll take a mental note to make sure thats about right next time!

I tried to thread some small brass parts, and ruined 2 parts before figuring out what I was doing wrong! The first one I got about 2/3 way down before I 'lucked' into being in the wrong position on the dial. Because it was so heavy of a cut, it snapped my part in half. The second one I ended up picking wrong early enough that it CUT (just not nicely!), so it was clear what I'd done wrong. 3rd times the charm :)
 

ErichKeane

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Watching a video (
) about the threading dial got me thinking... The manual says "When cutting even numbered threads the half nuts may be engaged at any point on the threading dial". I took that to mean "whenever you want". However, I suspect that it really means "at any of the hash marks". I suspect I was lucking into the hash marks for most of the time, since the threads were working.

Since it is a 8 TPI lead screw (and the dial rotates 1x per _2_ revolutions) I actually had 8 chances to engage. I was cutting a 20 TPI thread, so I was actually fine engaging at each hash mark (numbered and unnumbered), PLUS I was fine engaging at the half-hash-marks.

I realize now, that last time I was cutting threads, I was cutting 16 TPI (3/8-16), so that actually evenly divides at the quarter hash-marks, so I would have been able to engage it at literally all 8 opportunities. That explains why I believed I didn't have to pay attention.

I was in back-gears on the slowest speed since I had a hard-stop shoulder (I didn't have a gullet cut), so waiting for the dial was painful for my last part, at least I'll know better now!
 

Bob Korves

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#5
Engaging the half nuts must be in the exact same position relative to the usable number(s) every time. If the line does not line up exactly the same then you are one thread off. It is not important if it might be meshing slightly off being perfectly lined up, but it DOES matter if it is the same picture every time, even when you can use any number. I find that it helps to engage the half nuts SLIGHTY early, to make sure the half nuts catch the proper thread. The half nuts will not engage right away because they are riding on top of the thread. Keep the pressure on and they will drop in when they reach alignment. Some more things to do as a novice: Start well away from the work if possible, that way you will have time to disengage the half nuts and start over if you don't like the pointer position on the threading dial -- no damage done. Also, as a novice, practice the entire threading sequence with the cutting tool well away from the work, over and over, until you have the muscle memory of the entire sequence firmly in mind, with no mistakes on the sequence and you are quite satisfied with your practice performance. Also, have the panic procedure firmly in mind at all times. For instance, if you disengage the half nuts, the feeding will stop. Leave your hands on the half nut handle and infeed handle, and know that when anything goes wrong, the first maneuver is to immediately be ready to disengage the half nuts and back out the tool. One more thing. When you are ready to engage the half nuts, get everything out of your brain except the task at hand. Know exactly what you will do and when and where, and focus on the task, with the escape plan at the front of your consciousness. That is a good approach with running most machine tools for most operations. With time you will be able to relax, and just do it, but your focus on the task must always remain on top, no daydreaming.
 

Briney Eye

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#6
Watching a video (
) about the threading dial got me thinking... The manual says "When cutting even numbered threads the half nuts may be engaged at any point on the threading dial". I took that to mean "whenever you want". However, I suspect that it really means "at any of the hash marks". I suspect I was lucking into the hash marks for most of the time, since the threads were working.

Since it is a 8 TPI lead screw (and the dial rotates 1x per _2_ revolutions) I actually had 8 chances to engage. I was cutting a 20 TPI thread, so I was actually fine engaging at each hash mark (numbered and unnumbered), PLUS I was fine engaging at the half-hash-marks.

I realize now, that last time I was cutting threads, I was cutting 16 TPI (3/8-16), so that actually evenly divides at the quarter hash-marks, so I would have been able to engage it at literally all 8 opportunities. That explains why I believed I didn't have to pay attention.

I was in back-gears on the slowest speed since I had a hard-stop shoulder (I didn't have a gullet cut), so waiting for the dial was painful for my last part, at least I'll know better now!
You are correct about "at any of the hash marks." I don't think you need the dial at all if the pitch is divisible by eight. The pinion gear on the threading dial has 16 teeth vs the 8tpi lead screw, which is why the thread dial has four hash marks instead of eight. It takes sixteen revs of the lead screw to turn the dial one complete rotation. Lathes with 32-tooth thread dial pinion gears and 8tpi lead screws (like South Bends) have eight hash marks on the dial. Then there are the dials with five hash marks, and 12...

Your difficulty has made me think that I need to add a thread dial hint to the user interface on my electronic lead screw project. I should have thought of it sooner, since it confuses me too, and having an explicit prompt will save having to remember. I have to think through how to present it clearly.

Here's an interesting thread dial that I just found on eBay, from a "Cadillac" lathe (Taiwanese, I believe):

 

Janderso

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#7
I learned a lesson. When I took apart my thread dial to clean and oil I did not pay attention to how I put it back together clocking wise.
It would not lock in on a numbered slot.
I mention this because some of us buy these dials used from Ebay etc and the clock may be off.
Just my 2 cents.
The practice part is something I do every time I thread because I can screw them up every time, no problem :)
 

P. Waller

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#8
When threading I normally use the same number on each pass even though the book says you don't have to, saves confusion on my part. :) To adjust the dial, engage the half nuts, and loosen the dial mount, and rotate the dial to a mark, and then re-engage.
I have done the same thing for the last 30 years, have never found that using any other position on a thread dial saves much time in a manual operation.

If you are a hobbyist making 5 parts at a time for your own amusement why do you care if it takes 5 seconds longer for the threading dial to rotate to 0? I realize that if you are making 5000 parts the seconds add up, in this case 7 hours over the part run.
Engage the 1/2 nuts at the same dial position every time and you can not go wrong, if indeed you need to make time then split the dial as required.
 
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