I have cut several single point threads on my enco 13x40 successfully in the past. Including metric. For some reason, as i cut 1.5 mm pitch threads now, I get a good scratch pass and then on 2nd or 3rd pass, I'll get a thread cut between the previous pass. At first I thought it was me, still may be. I have slowed it down to make sure I close the half nut on the exact same spot. I disassembled the thread counter to make sure the gear was not slipping. I checked the lathe gears and have correct gears and they are keyed. I'm really frustrated at the point as I have wrecked three attempts at making a part and then numerous attempts on samples. Any ideas what might be happening?
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Lathe cross threading
- Thread starter Clunker1
- Start date
- Joined
- Feb 1, 2015
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- 9,625
You can't disengage the half nuts when threading metric with an Imperial lead screw and vice versa. You may get lucky and hit it just right, but it is pure luck.
As stated, Need to keep half-nuts engaged, then reverse feed. Or if you have to disengage half-nuts to keep from over running. Stop the spindle immediately after disengaging the half-nuts, then manually reverse the spindle until the same thread indicator mark lines up. Re-engage the half-nuts. Then continue running the lathe in reverse until it comes back to start location. Rinse & Repeat.
- Joined
- Apr 23, 2018
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- 6,567
Are you SURE you can't disengage the half nuts? I mean, really really sure?
Because if you start at "1" on the dial and return the carriage to the same spot for every chase, you can disengage the nuts and start on 1 every time, regardless of system. You could be cutting threads per furlong or cubit for all I care, because you are establishing the origin (unitless) between the carriage, lead screw, and dial. The origin is the origin is the origin. Anything downstream has to agree with your measuring system, but the origin is fixed. You CAN unclamp, you CAN re-engage, as long as the carriage is in the same spot and the dial reads 1.
We learn rules to threading, but there is no substitute for understanding what you are trying to do.
Because if you start at "1" on the dial and return the carriage to the same spot for every chase, you can disengage the nuts and start on 1 every time, regardless of system. You could be cutting threads per furlong or cubit for all I care, because you are establishing the origin (unitless) between the carriage, lead screw, and dial. The origin is the origin is the origin. Anything downstream has to agree with your measuring system, but the origin is fixed. You CAN unclamp, you CAN re-engage, as long as the carriage is in the same spot and the dial reads 1.
We learn rules to threading, but there is no substitute for understanding what you are trying to do.
Yes I am sure and I have tried it also without luck.
I think that:
- if one rotation of the threading dial = one rotation of the lead screw
- if you reposition the carriage at exact the same position every pass
If both conditions are met, than it could work. I don't have an imperial lead screw to try and since I CNC converted my lathe, can't do manual threading any more.
- Joined
- Feb 25, 2021
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- 3,134
Hmm, I'm skeptical. That doesn't mean I'm right!
Seems to me you need the carriage, lead screw, dial, AND spindle clocking to all be the same. Or else you get a misthread.
Let say I have a hypothetical lathe with a 1 TPI leadscrew, just to make the math easy. And my thread dial counts 8 leadscrew turns (or pick a integer). And I use metric change gears to cut a 1mm thread pitch. That means I slow my leadscrew so that 1 spindle turn gives 1/25.4 leadscrew turns. Or, I need 25.4 spindle turns to get 1 leadscrew turn. Now, I fire up my lathe in one direction with the half-nuts disengaged without moving the carriage. My thread dial will come back to the same exact spot every 8 leadscrew turns. To get 8 leadscrew turns, I need 8*25.4 =203.2 spindle turns. So when my thread dial comes around again to the same spot, my spindle is off by .2 turns, or 72 degrees.
My understanding is the thread dial works because imperial threads are all some nice multiple number of turns in 1 (or 2, or 4) inches. So as long as the thread dial makes one turn in that number of inches, the spindle and leadscrew align on that period.
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