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Author Topic: Handwheels stall the table easily  (Read 2474 times)

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Offline bdmsb

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Handwheels stall the table easily
« on: November 25, 2009, 10:45:41 AM »
I'm adapting a knee mill conversion to Mach3. The motors are size 42 steppers with about 1500 oz/in of torque. With velocity set to 40 you can't stall the table to save your life. But a feather touch on a spinning handwheel stops it cold. I understand that this is because the wheel is a big lever arm. But it's shocking to see how little torque the motors have at speed when tested in this way, and how easy it is to stall the table with a bump to a handwheel.

Is this typical? It's not something you'd notice on a mill without hand cranks.

Also, is 40 or 50 ipm a typical top end for a Bridgeport conversion with stepper motors and without ball screws?
Re: Handwheels stall the table easily
« Reply #1 on: November 25, 2009, 11:46:43 AM »
i am not a expert, but with the steppers going that fast to move the table at that speed you might need a faster pitch screw to keep the motors slower with more power.
just a thought

Offline bdmsb

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Re: Handwheels stall the table easily
« Reply #2 on: November 25, 2009, 12:19:02 PM »
i am not a expert, but with the steppers going that fast to move the table at that speed you might need a faster pitch screw to keep the motors slower with more power.
just a thought

It's not practical to change the lead screw pitch on a knee mill. 5 turns per inch conforms to the manual markings. But I'm using 1:1 toothed belts on the motors. I could change the coupling ratio which would run the motors more slowly. Does  anyone know if this is a net improvement in operation of a knee mill conversion?

I could retrofit ball screws, but I'm not exactly qualified to do the work myself and know of no one locally who is.
Re: Handwheels stall the table easily
« Reply #3 on: November 25, 2009, 01:55:48 PM »
What stepper controllers are you using, with what current limit, and what power supply voltage?  Limited current, or low voltage will seriously degrade high speed performance.  Even with steppers, properly setup you should get at least 100 IPM, though using leadscrews instead of ballscrews will decrease performance significantly, due to the greatly increased friction.  You will also likely find the nuts wear, and develop backlash, very quickly.

Regards,
Ray L.
Regards,
Ray L.
Re: Handwheels stall the table easily
« Reply #4 on: November 25, 2009, 01:57:37 PM »
BTW - Leaving the handwheels on also works very much against you, as it creates an imbalance that can excite resonance at certain speeds (and resonance is already a problem with large steppers), and cause the steppers to stall.

Regards,
Ray L.
Regards,
Ray L.

Offline bdmsb

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Re: Handwheels stall the table easily
« Reply #5 on: November 25, 2009, 03:44:55 PM »
I'm using Parker Hannifin OEM 650s matched properly to the motor current as best as I can tell, though the motors never get more than warm to the touch. The drivers do power down at idle, so it's hard to judge drive power from motor temperature.

I find that, when milling a 1 inch hole, I get a lot of jitter in the X axis unless I feed slower than 10 ipm.