[w0mbat]

I just got a Microlux 7x16 lathe (my first lathe experience!) and I'm trying to cut a 14 tpi thread in aluminum. I changed to the appropriate gear and the first thread has the correct spacing, however, it rarely cuts on the same line when I do multiple passes.

I am starting on number 5 of the thread counter and release the carrier at the end of the cut. If I make 10 passes, it just makes a mess of everything. Everything on the lathe seems tight (except the gears seem to have a little play) because I spent 2 days adjusting all the play out, so I can't figure out why it would be starting at a different point every cut.

Could it be the play in the gears that is causing this or something I'm not seeing?

Any help would be appreciated!

[SCzEngrgGroup]

Sounds like #5 is not the correct place to start. How did you arrive at that? What pitch are you cutting?

Regards,
Ray L.

[RTTIM]

Manual says use either 1 or 5 - have you tried it at 1?

I'm assuming you've done all the checking proper position of the thread dial, checked the change gears to insure there isn't excess play, etc.

You have to be really precise when engaging the half nut - might help if you use a slower speed.

The other thing you could try is to stop the lathe, move the carriage manually until the thread dial is at the right mark, engage the half nut then turn the lathe on.

[kawazuki]

I always thought the 'drop in' number had to be a multiple of the TPI you were trying to cut? If 'drop in' dial numbered 1 - 5 then 4TPI could use 1, 2 or 4, 7TPI could use 1 only 9TPI use 1 & 3, 14TPI - 1 only?

[HorridHenry]

As RTTIM said its precise timing in engaging the half nut.

I don't think it will be a gears problem but I'd wager a bet its your timing out on engaging the halfnut.

I guess you need to try different numbers on the dial.

The chart says engage at 1.5 on the dial for 14tpi

[Robin Hewitt]

Apologies for stating the blooming obvious, but you did say it was your first attempt

If you always go for the same number on the dial you should not be able to go wrong.

You have to start and stop the transit by engaging - disengaging the half nut.

If you have a dog clutch in the lead screw, that is fine and dandy for fine feeds but you mustn't touch it when you are screw cutting

[RTTIM]

......The chart says engage at 1.5 on the dial for 14tpi

Actually it says "1,5"not "1.5" - means you can engage the half nut at either 1 or 5 on the thread dial.

If push comes to shove, he can try the method you would use to cut metric threads on a machine with an inch lead screw.

1. At the end of the pass use the cross slide to rapidly retract the cutting tool and STOP the machine. DO NOT disengage the half nut.
2. Select reverse direction for the motor then run the carriage under power back past the starting point.
3. Stop the machine, set motor to forward direction, reset cross slide depth for next cut, then power up machine & make the next cut.
4. Repeat from step one until desired thread depth is reached.

What this does is eliminate the thread dial from the equation by locking the carriage motion and tool position to the spindle position. If it works, then the lathe most likely is fine and the issue is probably with precisely engaging the lead screw. If there still are problems, there is something wrong with the lathe.

Hope this helps....

Tim

[txcncman]

I had a cheap lathe once that would allow the half nut to be engaged even when not properly aligned with the lead screw. I had to learn to engage the half nut when the number on the dial was bang on and even learned to feel when the half nut did not engage properly. I am sure I could have corrected this problem with a repair. Eventually just got rid of the lathe.

[pete from TN]

I used to have a cheap lathe awhile back that would do that. I wound up not using the thread dial anymore. I just would engage the feed and leave it in for the duration of the thread cutting operation. When you get to the end of the cut with the spindle going slowly I would just back the cutter off with the cross slide. Then reverse the spindle to feed the carriage back to the beginning and then engage the cutter to depth again and make another pass. It is not the best or the RIGHT way to do it but it will result in a good thread without fear of cutting off the previous thread passes. Fortunately my 12x36 I have now does not have that problem.... Good luck and peace

Pete

[w0mbat]

If push comes to shove, he can try the method you would use to cut metric threads on a machine with an inch lead screw.

Actually, I am trying to cut an M12x1.75 and the 14 seems to fit my thread gauge perfectly. I did use your method yesterday and the threads cut fine. I also tried using both 1 & 5 and noticed that when I have the machine turned off, the carriage engages exactly on 1 or 5 but with the leadscrew running, it engages about 1/16" past those numbers every time. I'm not sure if that is normal but I will try turning the machine off before I engage the carriage and see if that works.

[SCzEngrgGroup]

I would check for axial movement of the leadscrew, and/or backlash in the gears of the threading dial....

Regards,
Ray L.

[w0mbat]

Well, I've been at it for a few hours again today and found the following:

If I keep the carriage engaged with the leadscrew and just stop the head at the end, backup and repeat, everything threads fine. I can probably use this method but it bugs me that the dial method is not working! It makes me wonder if something else is not setup correctly.

I rechecked for any backlash and found that I was getting about .003 on the cross-slide. I tightened that as much as possible (there is still a little movement - .0015 or so) so it takes a little force to operate and tried to thread using the dial. I made about 8 passes that were really close, then the 9th pass cut a new thread in between the original threads. I was careful to stop the number at 5 every time.

Next, I tried to stop the lathe, engage on #1 and then start the lathe. It cut a new thread on every pass.

Funny thing is, the instructions list 2 different ways to cut threads - one by stopping and reversing the machine, the other by using the thread dial. I would assume that if there was excess play in the gears, the threads wouldn't cut whether the carriage was left engaged or not??

I'll keep working on it and post if I can find the solution.

[txcncman]

With the spindle not turning, while moving the carriage (Z) back and forth past different numbers, try engaging the half nut and see if it will "engage" just anywhere. See if there is a "sweet spot" where you can tell it is fully engaged versus a partial engagement.

[LongRat]

The amount of play in the gears really shouldnt matter much. Like backlash, you'll always engage in one direction only so the effects of play are eliminated. Much like the cross-slide. There's really no problem with a bit of backlash as long as you always approach from the same direction. I don't think that's your problem here.
The key is that the reversing and leaving the half nuts engaged method worked.

[photomankc]

Actually, I am trying to cut an M12x1.75 and the 14 seems to fit my thread gauge perfectly.

Wait.... what? Are you cutting an M12x1.75 thread or are you cutting a 14TPI? If the machine has an inch lead-screw then you can not disengage the half-nut to cut metric threads. It will destroy them. The thread dial can't be used on most of these machines for a metric thread.... at least as far as I understand it.