stepper motors for cnc

My son and I are attempting to build a CNC router. We decided to go with the Arduino due to low initial investment plus being able to learn about wiring/electrical circuits etc. We are trying to determine what size motors we can use with the Arduino paired with the Gshield. I initially ordered these... http://www.ebay.com/itm/331517151618?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT They are rated at 2.8 amps and the Gshield is rated at 2.5. I have found a lot of what seems to be conflicting information on whether these would work or if they would fry the shield. Does anyone have any input on what motors to use and or how to calculate what motors will work? I would like to use the largest motors that would work well with that shield.

Nate

Unless you have big heatsinks then I doubt you'll get much above 1.5A from those DRV8818's without thermal cut out. Alternatively smaller heatsinks and a good fan to cool them.

2.8A motors would normally be driven by a discrete MOSFET driver (8 external MOSFETs with low on resistance, not the build-in DMOS devices in the DRV8818 which have a large on resistance). Discrete MOSFETs are readily available down to 0.005 ohms on resistance or better, DMOS on-chip are 0.25 ohms or so usually, meaning lots of heat when driving windings of about 1 ohm.

Mark, Thanks for the reply. I had read that I shouldn't expect to get much more than 1.5A from the shield with the built in drivers and that I should use separate drivers for larger motors. Would the motors just not be operating at optimal torque if I use them with the Gshield? I had also read about using a fan to keep everything cooler. We are both extremely new to all this and it is getting a little overwhelming. I thought about going the inventables route but thought it would be better to walk our way slowly through the entire process so we can learn as much as possible instead of just getting a plug and play system. Thanks for the help.

Nate

You may get some useful background info in stepper motor basics

Can you post a link to the datasheet for the Gshield ?

My guess is that running 2.8amp motors at 1.5amps would lose a large amount of the torque.

...R

nates02gt: Would the motors just not be operating at optimal torque if I use them with the Gshield?

That's correct. Although "optimal" isn't the right word here; you mean "peak" torque.

What usually happens when you can't supply enough power to your motors is that you just need to lower the speed of the motors. That's not such a horrible thing at this point in your build so I wouldn't let your lack of a full power driver stop you from continuing.

Robin2: You may get some useful background info in stepper motor basics

Can you post a link to the datasheet for the Gshield ?

My guess is that running 2.8amp motors at 1.5amps would lose a large amount of the torque.

...R

Here is the link to the gshield. http://synthetos.myshopify.com/products/gshield-v5

@chargin....I do see how peak would have been the proper terminology =). So if I understand correctly, using a 2.8A motor should not fry anything, it will just not operate at peak performance. Am I correct in assuming that the Pololu A4988 driver is the standard driver that is on most of the shields you can get off of Ebay? The biggest difference (as far as I can tell) with the Gshield is that the drivers are all hardwired to the board. Thanks again for the help!

Nate

I suspect the TI 8818 on the gShield should be at least as good as the A4988.

As @MarkT said earlier your motors may require a more powerful (and more expensive) driver. But you may be able to get the project working with the gShield and consider upgrading later. There should be little or no change to the Arduino code.

The correct process is

  • figure out the torque you need
  • identify a motor that can provide that torque
  • identify a driver that can comfortably manage the current required by the motor.

For a given torque requirement you may find a choice between motors with low coil resistance and high current and higher coil resistance and lower current. Choosing a lower current motor may reduce the cost of the driver. But, generally speaking, the low-resistance high current motors will work at higher speeds for a given supply voltage.

...R

Robin2: My guess is that running 2.8amp motors at 1.5amps would lose a large amount of the torque.

Torque will be down about 47 %. What will that mean? It will mean that you won't get such fast operation. A lot depends on the size and weight of the machine you are building. The bigger and heavier the gantry is the lower the maximum acceleration will be before you start losing steps. The faster the material removal you aim for the more torque you'll need.

Unless you have fixed performance specs to aim for you can just go for it and when the machine is built you can easily check the maximum acceleration and speed and then decide if upgrading is worth the cost.

Russell.

nates02gt: @chargin....I do see how peak would have been the proper terminology =). So if I understand correctly, using a 2.8A motor should not fry anything, it will just not operate at peak performance.

The DRV8818 used by the Gshield has internal current limiting (which is always active) so it's not going to burn itself out. It also shuts itself off if it gets too hot. But still I'd avoid using terms like "peak performance" at this point.

nates02gt: Am I correct in assuming that the Pololu A4988 driver is the standard driver that is on most of the shields you can get off of Ebay? The biggest difference (as far as I can tell) with the Gshield is that the drivers are all hardwired to the board. Thanks again for the help!

The DRV8818 chip has a metal underbelly you can't see that acts as the primary method of dissipating heat from the chip. When you solder that to a large board (like the Gshield, as compared to a smaller Pololu/"stepstick" style module) then you can create a lot more surface area with the PCB's copper to dissipate heat. The obvious disadvantage is that if you happen to fry any of those chips, for example by disconnecting the motor when the Gshield is powered on, then it's magnitudes more difficult to replace that faulty part. That's the real tradeoff that is happening here.

PCB copper isn't anything like as good a heatsink as a heatsink!

Wow, that is a lot of info. So I guess I am kind of going at the whole project backwards. I need to be able to determine the weight of the gantry and base the torque needs off of that, correct? I have been looking at the two plans in the links below. I understand that this is probably at least somewhat out of the realm of this forum. http://solsylva.com/cnc/18x24x5.shtml http://solsylva.com/cnc/25x25.shtml The write up on these plans is what brought the question up as far as the motors. The designer suggests to use nothing less than a 270oz stepper. When I started looking into it further and saw that Inventables discouraged using the 269oz steppers that they offer, that raised a red flag. Would it be better to build a smaller and therefore lighter frame and gantry to work with the electronics that I have? Also, would it help to put heatsinks on top of the gshield chips? Thanks again for all the input.

Nate

MarkT: PCB copper isn't anything like as good a heatsink as a heatsink!

You're twisting what I said, but the copper pour thermal resistance is around 40oC/W + 2oC/W (junction to bottom of the chip). Add 16oC/W (junction to top of package) to whatever heatsink you're thinking of.

nates02gt: Wow, that is a lot of info. So I guess I am kind of going at the whole project backwards. I need to be able to determine the weight of the gantry and base the torque needs off of that, correct? I have been looking at the two plans in the links below. I understand that this is probably at least somewhat out of the realm of this forum. http://solsylva.com/cnc/18x24x5.shtml http://solsylva.com/cnc/25x25.shtml

It's not so much the weight of the gantry but how quickly you accelerate it (again: speed). But looking at that leadscrew-driven Solsylva I think you'll be OK with what you have. If you compare it to Inventables', belt-drive Shapeoko with sub-2.5A motors your leadscrews will provide much more reduction and the peak torque of your motors matters less.

As an aside, try to find a power supply as close to the maximum 30V that the gshield is spec'd for. That will give you the most speed.

Chagrin: It's not so much the weight of the gantry but how quickly you accelerate it (again: speed).

It is both. Force = mass x acceleration.

Russell.