G0766 Operation: Having to Reset to Get Lathe to Start Again

[Matt Schrum]

I'm not sure if I am doing something wrong or if my lathe is acting up.

I'm roughing out a large bowl, and I'll have the lathe going at 200-400 RPM. Without hitting any buttons, I turn the speed control knob down to 0 to check my work. Again, without hitting any buttons, I then adjust the knob to increase the RPMs. Nothing. It won't move. I've tried removing tension from the belt (so the motor can spin freely), giving the blank a push in the right direction before increasing the speed, turning the direction switch to neutral then back to Forward, etc. Nada.

The only thing that works is hitting the red power button on the lathe, waiting until it fully shuts down, then starting it up again. It'll run great without issues or doing anything odd.

I've only noticed the issue on the two big bowls (20 inches+ diameter) I've done and never on any of the smaller ones. it doesn't happen all the time, but enough it's annoying as all get out.

The only thing I can figure is that on these large blanks, they really continue to spin after I turn the knob to zero. I'm not sure if this is causing the motor controller to freak out or anything-- but I can't really explain why I haven't seen the issue on smaller pieces.

Thoughts?

[Matt Schrum]

UPDATE: If I slow it down gradually, I don't seem to have the above problem. That is, instead of turning the knob from 300 RPM straight down to 0, if I turn it down slowly as the piece slows, once it hits 0 RPM I can immediately start it back up.

It's got to be some "feature" of the motor controller or something. Odd.

[Roger Chandler]

Probably not a bad idea to ramp down the speed with a really heavy blank instead of going all the way to zero in an instant. I always ramp down......not sure why, but always have done it that way without any problems, on all my 4 different lathes I have owned.

I guess you probably know that the reverse/foward button is designed as a safety as well.......any time you hit the red off button, you must go to the neutral position, and turn the speed dial down before it will turn back on....a safety feature that prevents the lathe from starting with the rpm's on high........dangerous!

[Dale Miner]

This is normal for what you are doing.

When you decrease the speed of the spindle by turning the control knob, you are asking the inverter to absorb the energy stored in the spinning mass. When the knob is turned down rapidly, the energy exceeds what the inverter can handle, and the inverter 'trips' and shuts down to protect itself. When you turn the knob slowly, the energy can de dissipated within the capacity of the inverter, and no 'trip' occurs.

In just about every case of the people I know that have had issues with their inverters, they were in the habit of stopping the lathe by using the speed control knob, and the failure occured during/after stopping a large blank. Too many near 'trips' and the inverter gives up the ghost.

I have heard of one incident where the lathe was left overnight stopped by turning the speed control to zero, and the next morning the lathe was running wide open with the blank thrown from the chuck.

Most inverters are programmed to ramp the spindle speed down within the inverters capacity when the stop switch is used.

My advice is to use the stop switch to shut the lathe off, and get in the habit of checking the speed control setting each and every time before gurning the lathe on. Your inverter will (in my opinion) last much longer.

[David C. Roseman]

Dale, interesting. Might this also be affected by the presence/absence of a braking resistor? The inverter on Matt's lathe is the Delta VFD-M series inverter. It has terminals to add a braking resistor, but the Grizzly G0766 (like the smaller G0733) does not ship with one. A BR can be added for around $30, although it involves tweaking some parameters on the drive. I've added one to my G0733 drive, but only to give me a little more flexibility in adjusting the rate of deceleration when turning big bowl blanks. Even before I added it, I had not experienced the issue Matt and you describe when using the speed rheostat to stop a big blank.