Author Topic: Gear cutters in CNC  (Read 1280 times)

Offline dieselpilot

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Re: Gear cutters in CNC
« Reply #15 on: September 26, 2021, 05:48:01 PM »
Estimate the cut area in CAD and do some math to find the cut torque. Does Tormach provide a tq/power curve for the spindle? When I had the Emco Mill 125 1HP I calculated cuts I thought would run into the torque or power limits. It really save a lot of headaches.

Offline steamer

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Re: Gear cutters in CNC
« Reply #16 on: September 26, 2021, 06:11:59 PM »
Estimate the cut area in CAD and do some math to find the cut torque. Does Tormach provide a tq/power curve for the spindle? When I had the Emco Mill 125 1HP I calculated cuts I thought would run into the torque or power limits. It really save a lot of headaches.

Thank you dieselpilot.    That is a very reasonable approach, and is something I could do for sure.  Unfortunately, I don't have nor can I find a torque curve for this spindle.   It's really optimized for 10000 rpm, though with a belt change, it will run at 300.    I put a blank holder in the spindle and started the spindle at 300, and grabbing the spindle firmly, I couldn't stop it.....so that gives me a "warm fuzzy" which is very far from science and engineering, but better than faith based Engineering...

It's a 3/4 HP spindle by the way, so I could just do the 3 Pi N T/33000 and back out the torque...but I very much doubt this motor puts out that much power at that low an RPM.....I suspect it will be a "try and cut"

 :DrinkPint:

Dave
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Offline dieselpilot

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Re: Gear cutters in CNC
« Reply #17 on: September 26, 2021, 06:31:03 PM »
Does the motor have a name plate? That's the best place to start. Characteristics of induction motors are well understood.

Offline jadge

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Re: Gear cutters in CNC
« Reply #18 on: September 26, 2021, 06:45:49 PM »
I assume you have the Tormach 440 or 770 with an induction motor and VFD drive? Below base speed a VFD should maintain constant torque, while the power drops in proportion to the drop in speed. So all that needs to be done is to work out the torque from base speed (at 60Hz) and rated power on the motor plate, and that should be the torque you will get at the lowest speed.

Andrew

Offline steamer

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Re: Gear cutters in CNC
« Reply #19 on: September 26, 2021, 07:55:03 PM »
I suspected it was a VFD  and have been concerned about that very condition.
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Offline dieselpilot

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Re: Gear cutters in CNC
« Reply #20 on: September 26, 2021, 09:48:50 PM »
Power reduction below rated frequency is a characteristic of the motor, not the VFD. It doesn't matter as you don't need much power at low motor RPM, but you will need the torque. Any given cut, (Ap, Ae, Fz,) uses the same torque, but power is proportional to RPM. Rated torque of an AC induction motor is available to minimum RPM, but somewhat dependent on the drive. If this was going to be a long continuous condition you'd be concerned about motor heating.

The spare VFD on the site is a Control Techniques Commander SK (discontinued) which are very good drives. It's an open loop vector type and has excellent low speed capability. I have a couple, and just bought a newer SL to replace a failed AB PowerFlex4.

Offline jadge

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Re: Gear cutters in CNC
« Reply #21 on: September 26, 2021, 10:23:25 PM »
Power reduction below rated frequency is a characteristic of the motor, not the VFD.....

That's a moot point, given that the speed of an induction motor is set by the applied frequency (ignoring slip) and the frequency is set by the VFD. The constant torque characteristic is set by the VFD. Torque is proportional to phase current. Below base speed the VFD reduces the applied voltage to keep the phase currents (and hence torque) constant. If the applied voltage below base speed stayed constant then the phase currents woild increase as would the torque as speed decreased. But more current equals more heating and possible overheating of the motor.

I'd disagree about needing low power at low speed. The speed is set by the cutter size and work material but I want full power available even at low speeds. For full power as the speed decreases the torque needs to increase, which is what you get with a belt or gear drive.

Early on I learnt that my Tormach doesn't have the grunt or rigidity of my other mills. So I compensate by running small cutters at high speeds and feeds.

Andrew

Offline steamer

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Re: Gear cutters in CNC
« Reply #22 on: September 27, 2021, 01:06:18 AM »
Ok boys and girls   let's give this a rest hmm
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Online Jasonb

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Re: Gear cutters in CNC
« Reply #23 on: September 27, 2021, 01:11:58 PM »
Probably best to make up a test blank and run a few passes to see what you can manage, it may be a slower feed rate and therefore smaller chipload will compensate for the loads from the larger diameter cutter but not too fine that you end up rubbing.