Lathe threading random start

So finally yesterday got to make a propper thread. Cheers to robh that got to hit the point.

Connected all my lathe electronics to my MESA 7i76E and the problem was still there. But, since the pitch was OK, went to install the simulated index component that andy posted a while ago. But before that I decided to try to adjust the gains pots on the encoder when I notice one of the pulleys was rubbing on the casting.

Seems that the although the pulleys were press fitted and I can not move them by hand, the rubbing friction was enough to move it just a few encoder counts per spindle turn, getting it out of syncronism. Separated the pulley half millimiter and now did a perform well. Installed the lathe electronics again and was able to chase the threads at 700rpm without problem.

I've been scratching my head with this problem for quite so long time before I decided to ask for help. Thank you so much for everybody!

Now I can think on getting some extra upgrades. Really dislike the original BLDC motor driver and V pulleys, so I'm thinking on getting a 3.8 kW AC servo to drive the spindle. Will make some HTD pulleys with 1:2, 1:1 and 2:1 ratios. I will use the servo encoder. On high gear I will have 1 index per two turns so I think it will work, on low I get two per turn so I will have to add an external index or simulate it.

Yes, I was thinking something like this. Although to be fair a 1:1 ratio with that motor is going to be more than enough for threading most things. I don't like too much the idea of having a slow inductive sensor bypassing a encoder. On the other side, having a sensor on the spindle is safer in case the belt fails. I think the optimal would be to attach the spindle sensor to the index-enable signal with an AND in HAL, and getting it mechanically syncronized with the encoder index. This way, no matter if the ratio is 1:2, 1:1 or 2:1, it will read the encoder signal in the right moment, once per revolution.

I know that a induction motor wiht a VFD will be way cheaper and easier to setup, but is not about money. To be fair, the original BLDC 1.5kW motor is better than a induction one, plus a similar power one will not fit on the space my lathe has. I don't like the drive because it has pre-programmed some really slow acceleration and deceleration ramps. The drive lacks any kind of documentation so I'm stuck with this parameters. I could also replace the drive, I could probably even use a servo drive to run the motor, but I don't have the time to do experiments.

The 3.8kW servo happens to be the biggest motor I can install without any mayor modification. To be fair, the thing that stops me more is the lazyness of having to deal with making the pulleys. If I happen to deal with it, I want to have provisions for further improvements: spindle brake, pneumatic collet closer and maybe even a worm gear.