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Topic: Maxon DC Motor for CNC? (Read 1 time) previous topic - next topic

Skoobs

Aug 30, 2015, 02:42 am Last Edit: Aug 30, 2015, 02:44 am by Skoobs
I have three Maxon DC motor 118778 just sitting around and I was wondering if they would make suitable motors for the X and Y axes of a small CNC mill. I have found very little information on these motors, but what I have found is below:





Two of them have HEDL-5540 encoders on them (the datasheet is readily available). For 50 bucks, I could put one on the third as well, and use it for the Z axis. I assume that I would want to make use of the encoders. However, I have absolutely no idea how I would make them work.

My worry is that the torque numbers look pretty anemic on these, and I am not sure how well they would fare in the application. Does anyone have any thoughts?

Finally, my plan is to use a raspberry pi with an arduino alamode, then run the drivers from the alamode. If you think these motors would work well, does anyone have any pointers as to a driver for these motors? I was looking through some arduino shields, and it seems like a 2.5A would be sufficient, so the GRBLShield might actually do the job (http://www.adafruit.com/products/1750). I was also looking at some of the drivers on Geckodrive.

Thanks!

MarkT

The specs I found are:

42V  2.15A
0.115 Nm,  8200 rpm, 90W

brushed slotless.

Very little torque for CNC mill, you would need 10:1 or 20:1 reduction gearing to get that
up to something at all reasonable.  You'd need to find very low backlash gearing / belt drive.

The power level is fine but you'd need to find a suitable H bridge and close the loop using the
encoders.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Skoobs

The specs I found are:

42V  2.15A
0.115 Nm,  8200 rpm, 90W

brushed slotless.

Very little torque for CNC mill, you would need 10:1 or 20:1 reduction gearing to get that
up to something at all reasonable.  You'd need to find very low backlash gearing / belt drive.

The power level is fine but you'd need to find a suitable H bridge and close the loop using the
encoders.
Maybe these particular motors would be better left to someone building a 3d printer, where they arent doing much beyond just moving the table.

I have some servos that have about three times the torque, but they are very power hungry. 3.5 amps rms with the same voltage.

russellz

I would forget the motor/encoder combination and go for stepper motors.  Much easier to control. 

I converted a Seig X2 mill to cnc and the motors I'm using have about twenty times the torque of those motors so Mark is right with his suggested 20:1 gearing.

The torque you need does depend on many factors though:
What material are you intending to cut?
How big a cut and how fast do you want to cut?
How heavy is the X-Y table and how fast do you want to move it?

Russell
Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.

Robin2

I can't help feeling that using DC motors for a CNC system would be a major challenge.

Reading encoders can use up a lot of Arduino resources unless the motor speed is low - and at 20:1 gearing I suspect it would not be low.

...R
Stepper Motor Basics
Two or three hours spent thinking and reading documentation solves most programming problems.

MarkT

Servos are taking over from steppers in the low-end CNC market as far as I can tell - slow rapids are
very tedious on a flatbed router (which is what most low-end CNC machines are).  You want 50cm/s
rapids and steppers really struggle with anything like that.  Cheaper Chinese servo systems are
becoming available too.

Putting the encoder on the output shaft eliminates backlash of course, but you may have trouble
stabilizing a servo loop then.  The best systems have encoders on motor _and_ leadscrew.

I'd go to cnczone forums for more specialized advice...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Robin2

Cheaper Chinese servo systems are becoming available too.
Just out of curiosity have those devices got the electronics for PID included so that the Arduino would just need to give position data?

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

russellz

Putting the encoder on the output shaft eliminates backlash of course,
He's talking about a leadscrew and nut.  The biggest source of backlash will be the nut unless special care is taken.  Putting the encoder on the output shaft won't eliminate that backlash.

He did say he's thinking about a mill not a router.

Russell.
Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.

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