Single Point Threading Issue - might be gearing?

WobblyHand

WobblyHand

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Thought I'd try some threading. I have a small rod I'd like to thread in 8-32. So, having never done this before, looked up the appropriate gear ratio and other things to do an 8-32 on my LMS 7350 high torque mini-lathe. The gear chart on gear chart says to use the following set up:
ABCD
20Any gearAny gear40
I used 20/80/60/40. (Default setup is 20/80/20/80) 60 seemed to be the only gear that would mate up with the 40. "Any gear" is kind of confusing to me. Does "any gear" mean pick whatever might properly engage the gears?
mini-lathe_gears20806040.resized.jpg
Used a piece of 1/2" aluminum for practice. It cut like 6061, but since it was unlabeled, not sure of the alloy. Used sulfur cutting oil since that is what I had. End result was not 32 TPI. The threads seem well formed, but seem to be around 21-22 threads/0.490" or 45 TPI! (OK, they aren't pretty under 60x, but due to the slow speed that I cut them at, they aren't bad.) I'm surprised about the wrong TPI. I engaged the lead screw at the same mark every time.
tpi_is_off.resized.jpg
So what am I missing?

The mini-lathe has a 16 TPI lead screw. (And I did physically count 16 threads in an inch.) Maybe a red herring, but I never could engage the lead screw on the mark (1) of the thread dial. I could engage just before, or just after, but never on the mark. I practiced this a lot, and it seems the dial is misaligned?
thread_dial.resized.jpg
I'd appreciate any help that you could give.
 
Confusing to say the least. If B and C are the two gears on a common stud, changing the ratio of those two will definitely change the TPI.

Edit: Reading the change gear page, I find this

"In the change gear results, many of the combinations have "Any gear" in columns B and C. This means that you can use a gear with any number of teeth in position B. It is an idler and does not affect the overall gear ratio. Use a gear that makes it easy to properly engage the gears. For these combinations, you can use any gear for position C; this gear acts only as a spacer and does not engage the other gears. "

Your D gear should engage the B gear, C is not actually used in the drive train.
 
A is the 20T metal gear. B is the 80T plastic gear that mates with A. C is coaxial with B and on the top of B. C is 60T. D is 40T and attached to the lead screw. Still don't know what "Any gear" means. But it's clear that 20/80/60/40 isn't correct.
 
WobblyHand said:
A is the 20T metal gear. B is the 80T plastic gear that mates with A. C is coaxial with B and on the top of B. C is 60T. D is 40T and attached to the lead screw. Still don't know what "Any gear" means. But it's clear that 20/80/60/40 isn't correct.
Click to expand...
Read my previous post. C gear is not used at all, it is just a spacer. Both A and D engage B.
 
A cup of coffee, staring at the change gear chart with pencil and scratch paper, and I'm confused as well.

I opened the manual but cannot figure out the tooth count on the spindle gear (#4 - p/n1320).
Thought I might work out the math on the gears.

Pull off the thread indicator and count the teeth on it's gear. It should be lining up . . .
Count out the teeth on 'A' just to be certain. And double check 'D' as well.
 
Further searching this site, I found the answer. Post #5 of gear change for 18 TPI has a video, which clears up the issue. Yeah, it's obvious once you've seen it! The write up on LMS is incomplete for a newbie, in my opinion. Sure left me puzzled. Seeing the video clears up my confusion. I'll try it again on aluminum to see that I got it right. Then I'll try it on some O1 at the correct diameter for a #8 screw.
 
middle.road said:
A cup of coffee, staring at the change gear chart with pencil and scratch paper, and I'm confused as well.

I opened the manual but cannot figure out the tooth count on the spindle gear (#4 - p/n1320).
Thought I might work out the math on the gears.

Pull off the thread indicator and count the teeth on it's gear. It should be lining up . . .
Count out the teeth on 'A' just to be certain. And double check 'D' as well.
Click to expand...
From viewing the video, I was just doing the setup wrong. It's a miracle that it was even close. (If one calls 40% error close!) I'll try it the way it was suggested in the video and report back.

I will check the thread indicator and count teeth. How many teeth should the indicator have? 16?
 
The problem is that you are creating a compounded gear train, the gear on the QC box should mesh with the large gear in the middle, not the gear that is on the same shaft with it. Likely, the gear on the thread dial can be re positioned to make the marks line up properly.
 
On the dial, with the machine NOT running, engage the half nuts, loosen the screw and rotate the dial till the marks line up, tighten the screw.
 
T Bredehoft said:
On the dial, with the machine NOT running, engage the half nuts, loosen the screw and rotate the dial till the marks line up, tighten the screw.
Click to expand...
Which screw, the one that holds the threading dial housing to the apron? I did that, and it seems to help some.
 
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