3-axis CNC router project

[ulfh2012]

Hi.
I am new to this forum, and new to the world of CNC routers, but have some electronic background.

I have been gatehering parts for a 3-axis CNC router project. I have got most of the mechanical stuff, but I don't understand all consepts about stepper motors. I have found a controller on ebay using th TB6560, and have read articles about how to modify this Chinese unit to make it work. This unit can supply 1-5 to 3 A, and does not support lower current than 1.5 A. Don't understand why, but that's another story.

When I look at stepper motors, I find that they mainly specify current, and not voltage. This controller will support 12 - 24 volt input voltage. My project will have a working area of about 10" x 12" (may be a bit less) and I want to use it for making PCBs and maybe small parts of aluminium.

My main problem now is to find a stepper motor I can use and hopeflully not blow the motor (or controller) by supplying wrong voltage. I have found some motors at Chinatown in Bangkok (I'm in Thailand on a vacation), but they are 24 volt .21 A, and thus cannot be used with this controller. Maybe they also have to litle power? They are incredible cheap, but that does not help, if I cannot use them.

Can someone give me advice on what motors to use?

Best regards

Ulf

[GeoffS]

Welcome to the forum.

You need to choose motors that have the same, or higher, current rating as your drivers. In your case 3A.
Driving at the maximum rated current will ensure maximum torque from the motor.

Voltage on the other hand can be a LOT higher than the rated voltage. A good rule of thumb for maximum voltage is

32 * ?L = VMAX

where L is the inductance of the stepper in millihenries.  Any voltage lower than VMAX means your stepper is not running as fast as it could. Other factors will limit the maximum speed.

For a good overview of steppers and power supplies, take a look at Gecko's website. No connection with them other than as a customer.

[cthree]

I love dealing with a vendor that knows what they are doing and doesn't mind helping you be a well educated customer. It makes me WANT to buy from them! Great link

[GeoffS]

I have a number of their drives. They work great and are almost un-killable.
If you do manage to destroy one, they will repair/replace it, no questions asked, at no charge.*



* First time only.

[Kremmen]

Gecko is a reasonably good motor controller for hobby use, but if you get the 540 (the quad motor unit), then one caution: You _must_ solder the current limit resistors in the specified pins of the D9 connectors for individual motors. The documentation indicates the resistance vs desired current. Failure to do that will most likely cause the individual motor driver module(s) to blow inside the unit.
I was asked some time ago to fix just this problem and turns out the problem module had wasted half of the H-bridge fets and both half-bridge drivers so there is little or no internal protection in the modules themselves. The motherboard does have protection features but they won't help if you forget the limit resistor.

[poorchava]

Price of Gecko drivers is absurd for a hobby machine. Especially if you live outside of the US.

As for the chinese TB6560 driver boards I have analyzed a few types and built my own 1-axis drivers based on that. All chinese boards I have seen have an ihnerent problem: power supply sequence. TB6560 datasheet states clearly that you should connect the logic power supply before any voltage is applied to output bridge supply pins. What chinese do is that they route the high power supply straight to the bridge, feed a 5V regulator from that supply and then use it to power logic. It creates a situation, where you can get a H-bridge shoot-through during power-up and the chip blows up.

Also, the TB6560 is not that good for rapid movement, because it's specified only up to 13kHZ of step frequency. With typical drive arrangement being a leadscrew with pitch of 4 or 5 mm, and typical motor having 200 steps/rev, at 1/16 microstep rate you will be getting about 4 rotations per second, which translates into roughly 16-20mm of travel. Even less (~6-10mm) when you use ordinary threaded rods with 1-1.5mm pitch. With normal 4-5mm leadscrew you will get up to travel speed of 1200mm/minute. And that is slow.

Those chips can run much faster, but this depends on particular specimen. I've tested like 20 of those and some would go all the way up to 29kHz or so, and some would crap out at 15kHZ. And when operating outside of specification you will occasionally get missed steps which completely ruins the precision of such machine.

I would consider using something like A3986 or A4989 as they operate at much higher frequencies.

As for the voltage, Geoffs is right - the higher voltage you supply, the faster your motor will go. On the other hand TB6560 goes up to 35V only (normal operation, maximum taring is 40V iirc). For most small motor this should be fine. This is because a motor winding is an inductive load and it will make current ramp up. Current rise rate is proportional to voltage applied, so the higher the voltage the more times a second you can rise the voltage to the max and back again. On the other hand higher voltage requires faster control logic to keep up with current changes.

[KuchateK]

For stepper theory I recommend this FAQ:
http://homepage.cs.uiowa.edu/~jones/step/

If you want to buy something better than TB6560 or Allegro based drivers but not as expensive as Gecko I suggest this:
http://www.leadshine.com/productdetail.aspx?type=products&category=stepper-products&producttype=stepper-drives&series=M&model=M542

Ebay clones go for less than $40. They are usually marked as 2m542 (I have clones) and they also work very well.
Single supply, built in opto-isolation, current and microstep selectable with dip switches (no G540 resistor crap), quite fast, rated at 200kHz.

[KTP]

Go brushless 

I picked up these Adept linear  robot slides on ebay for $100 each including a 300 watt brushless motor.  I bought a few EN-204 or EN-208 industrial drives that happen to take step/dir for $20 to $30 each and threw together this gantry router that can move a 50 pound Z axis at over 2000 IPM.



I do like the cleaness of the Geckodrive stuff for steppers though.  They sometimes put the 4 axis unit on sale for $225 on cnczone

[ecat]

For stepper theory I recommend this FAQ:
http://homepage.cs.uiowa.edu/~jones/step/

If you want to buy something better than TB6560 or Allegro based drivers but not as expensive as Gecko I suggest this:
http://www.leadshine.com/productdetail.aspx?type=products&category=stepper-products&producttype=stepper-drives&series=M&model=M542

Ebay clones go for less than $40. They are usually marked as 2m542 (I have clones) and they also work very well.
Single supply, built in opto-isolation, current and microstep selectable with dip switches (no G540 resistor crap), quite fast, rated at 200kHz.

+1 for the Leadshines.
I have three M752s (good for 70V I run at about 50V) and so far they have performed without fault, this includes continuous runs of 16 and 20 hours. They are not the cheapest but, and this is where many people go wrong in my opinion, while there is nothing wrong with saving a bit of money on your CNC build it is a big mistake to skimp for the sake of 10% or 20% on the basics.

At the end of the day your main costs will be tooling, materials and time. How much will a couple of lost steps cost you? At £5 to £10 for the tool, about the same for the material and say 5 to 10 hours lost time it soon adds up.

Anyway, just my 2c.