Series II 2hp spindle slows down while taking cut

rybern · Jan 29, 2020

Hey guys, we bought this Series II BP new in 2002. It's variable speed with soft start, HC ways, ballscrews and has been a great mill. Lately, the motor seems to slow down when taking more than a light to medium cut(even on 6061AL). We just finished going through the head to see if it could be something mechanical like a belt or sheared key, etc. While we were in there, we replaced a few bearings and bushings but didn't find anything odd. As I suspected, we got it back together and turned it on. It sounds nice a smooth but I can slow the machine down by grasping the spindle with my hand. I can't stop it but can slow it down.

Any thoughts appreciated.

Joe Miranda · Jan 29, 2020

Grabbing a 2hp spindle by hand you will notice a slight increase in noise from loading but not much. If you are seeing appreciable difference when cutting I would next suspect the motor. It may be getting tired. Is the motor hot to the touch after running a while, and if so - how hot?

Joe Miranda · Jan 29, 2020

Also, what are you grabbing when you are doing this? Are you just grabbing the spindle nose? If so, then it should be pretty hard to slow it down visible but you will hear an increased loading.

That being said, I can stop a 3hp lathe with a 8" chuck by grabbing it with my hands.

rybern · Jan 29, 2020

Thanks Joe. We figured it out. Funny thing is that it's been wired for 460V all these years(18 years). It never gave us any issues until we swapped power from 240v to 208V. I remember when we bought the mill back in 2002 and we all thought it was cool that the mill had "soft start". Apparently, wiring the motor for 460 on a 240V supply caused it to act that way.

We connected the motor wires for 208V and all is well. Kind of weird.... but it is resolve.

Randalthor · Jan 29, 2020

Joe Miranda said:
That being said, I can stop a 3hp lathe with a 8" chuck by grabbing it with my hands.

I wonder if your motor is actually putting out the rated hp? Using a torque calculator, a 3hp motor should produce 250ft.lbs of torque geared at 60 rpm. Pulling 250 lbs. with a one foot lever is a pretty good chore. Using an 8" round to pull 250 lbs. would seem extremely difficult for most men. I would think grip strength would be the limiting factor.

It's been posted here before, but here is a kid trying to stop a lathe using what looks like about a foot long lever from the center point. Of course the lathe in the video is larger than 3 hp.

YouTube

housedad · Jan 30, 2020

Damn lucky kid, and that was a real stupid move on his part.

CarbideBob · Jan 30, 2020

rybern said:
Thanks Joe. We figured it out. Funny thing is that it's been wired for 460V all these years(18 years). It never gave us any issues until we swapped power from 240v to 208V. I remember when we bought the mill back in 2002 and we all thought it was cool that the mill had "soft start". Apparently, wiring the motor for 460 on a 240V supply caused it to act that way....

That comes under the category of just "wow". 18 years and no bad smell.... you must have very nice to it.

I don't think I'd stop any lathe no matter how small by grabbing the chuck even with power off and coasting. :eek:

On SGs without VFDs kids try to do this at wheel change time with a rag or other on the hub since the stop is so very long.
You have to spank them soundly and they may not understand why or think you a safety fool.
No touchee the rotating parts. Even when no longer powered humans don't see inertia until it bites you hard.
Freely admit to learning this like sticking two sides of a bent out paper clip into an outlet at grandma's house when young. (bad idea but a good lesson)
Bob

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Series II 2hp spindle slows down while taking cut

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