Need advice and help: difference between motor drivers

Hi guys.
I must confess, I`m total newbie in CNC, so I need some advice from more experienced people here.

I`m planning to buy 4x Axis CNC kit (4x Nema 34 1200 Oz/in motors, 2x 360W,60V PSU, 4x MD882) from ebay and mount it on a 1300 x 1000 mm frame. If all goes well, I`m planning to upgrade it further.

The question is:

1. What are general differences between MD882 vs DM860a vs MA860h motor drivers? I have read manuals but not sure about conclusion except that some of them are weaker in terms of how current they can tolerate (average and peak). I`ll probably got MD882 in the kit. Should it work sufficiently? What are your experiences and advices about this? Are there any hidden issues, general and specific, that I should be aware?

Respect,
Miodrag

If you can , buy the g540 gecko drive & still use same motors , I once used similar package to you & had problems with drivers failing , I've had my g 540 for more than. 2 years now. & hasn't missed a beat.


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Originally Posted by riche543

If you can , buy the g540 gecko drive & still use same motors , I once used similar package to you & had problems with drivers failing , I've had my g 540 for more than. 2 years now. & hasn't missed a beat...

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Thanks for a response.

Yes, I would like to get it also, but sadly I`m on a tight budget in that respect. I now see, It seems gecko is max. 3.5A per channel (g250), if specs. are correct. Isn`t it little too low for a Nema34 motors? They are labeled as 3.5A, so I would like to have some offset there if possible. Nevertheless, if I got MD882, what was the reason why that driver failed? What are the general difference between all those drivers?

First, table size has nothing to do with your choice of motors. There are many other more important factors to consider when choosing motors.

Second, 1200oz and bigger motor packages from Ebay are usually a very bad choice, and you'll end up replacing them with much smaller motors in the future. In most cases, a Gecko G540 with 380oz motors will give you up to 5x better perfromance than those 1200oz kits.

How much weight will you be moving?
Will it be ballscrew, acme, or rack and pinion?
If screws, what pitch and diamter screws?

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Originally Posted by ger21

First, table size has nothing to do with your choice of motors. There are many other more important factors to consider when choosing motors.

Second, 1200oz and bigger motor packages from Ebay are usually a very bad choice, and you'll end up replacing them with much smaller motors in the future. In most cases, a Gecko G540 with 380oz motors will give you up to 5x better perfromance than those 1200oz kits.

How much weight will you be moving?
Will it be ballscrew, acme, or rack and pinion?
If screws, what pitch and diamter screws?

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Hi ger21,

Sorry in advance, I am a mechanical engineer, but English is not my native language and my technical terminology in this respect is lacking. I`m embarrassed, but what is acme, rack or pinion?:)

I`m looking to buy something llike this:
- Ballscrews RM2005 (dimensions 1500x900x400mm)
- Support linear rails SBR20 900/140mm/SBR16-400mm
- BK/BF
- couplers
- sensors, solid s relays and other support items.
- the weight should be stationary. Gantry could weight max cca 80kg, it that`s the question.

I`m not sure why 1200 oz/in (11.5Nm) holding torque is less effective then lower ones? I`m planning to build classical CNC router/cutter which could be expanded if need be. What`s your advice or recommendation? What motor should be interesting in this case and what driver from those above will do the job sufficiently. Please, don`t forget about budget...

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I`m not sure why 1200 oz/in (11.5Nm) holding torque is less effective then lower ones?

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Becasue stepper motors lose torque as rpm's increase. With larger motors, the torque drops off faster. Smaller motors will typically have more torque than much larger motors once you get over 500-700 rpm.

5mm pitch lead screws makes the problem worse, as it requires higher rpm.

10mm pitch screws are a much better choice, along with motors in the 380-425oz-in range. What's important is the inductance rating of the motor. You want motors with an inductance of3mH or less.

Quote:

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Originally Posted by ger21

Becasue stepper motors lose torque as rpm's increase. With larger motors, the torque drops off faster. Smaller motors will typically have more torque than much larger motors once you get over 500-700 rpm.

5mm pitch lead screws makes the problem worse, as it requires higher rpm.

10mm pitch screws are a much better choice, along with motors in the 380-425oz-in range. What's important is the inductance rating of the motor. You want motors with an inductance of3mH or less.

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After some read, I think understand now. Also, it`s now clear to me why low inductance is important. In layman's terms, it`s a laziness of the motor`s coil to allow full rated current in one phase (one phase is given in milliseconds and is equal to a divider of inductance and restistance of the coil). If that`s big value, in one phase there will be less and less time for current to reach it`s maximum as motor seeds-up.
So, for motors that I want to buy, I should check the stepper motor torque curve and how it`s related to ballscrew pitch. That should be adequate for a speed that I`m planning to move item or parts of the router. Is this correct?

Here is a design I`m planning to adapt and build (it`s partially built already).
What are yours (or others) recommendation for a motor and ballscrew`s pitch in this case?
Could 10mm pitch ballscrews with 425oz-in motors be sufficient for me?

There are several different 425oz motors available. Most of them have very high inductance, but I've seen some available in Europe with low inductance. A low inductance 380oz motor is a lot better than a high inductance 425oz motor.