Electronics > Projects, Designs, and Technical Stuff
Stepper motors and control for CNC router
(1/3) > >>
cowasaki:
I'm going to start building a CNC 3 axis cutter/router/etc machine over the next few months.  I'm happy enough with the mechanical side plus I've found a HEX image for the Arduino that turns it into a Gerber device and controls the three steppers.

What I'm not sure about and what I want to avoid spending too much on or buying the wrong stuff is the stepper motors/control boards etc

I've not used stepper motors before but I had 3 tiny motors and 3 control boards that came with my original Arduino years ago.  I've messed about with them but clearly, they are just toys.

What I'm looking for assistance on is what size motors I'm really going to need. I'm seen a video of a CNC 3 axis machine about the size I want to build which used 3 Nema 17 motors and I've looked at these on Ebay but there are lots of different ones with different specs.  I'm hoping to be using a 1/4" or 3/8" router as the main tool but will be building it capable of changing the head for other devices.

Any pearls of wisdom ?
awallin:
if you want to go with small motors then you need to minimize friction, i.e. linear rails (with ball-bearing blocks), and ball-screws for the drive.
To minimize force on the Z-axis some designs use a counter-weight (like in an elevator) for 'neutral bouyancy'.
the usual advice is to direct-couple steppers as they are good at turning at low rpm but the torque falls off at high rpm.

the usual problem with stepper CNCs is lost steps which ruin the part. if you plan on milling hard materials (aluminium, steel, etc) then think hard about servos with encoder feedback instead of steppers.
for servos gear/belt reduction is usually used as the motor likes to turn at a high rpm.
cowasaki:
My design is a direct drive.  I don't mind getting motors rated above what I need if it makes the design more reliable but the problem I have it not knowing what force I realistically need. 

I'm looking at a design running fully on bearings and from a video of a similar device it is very smooth.  As I said the one I've seen was using 3 Nema 17 steppers but there seem to be so many different specs for it!

I suppose the best way would be to build it minus the motors and actually measure how much force is required but where is the fun in that!
H.O:
You don't say how big of a machine you're building or what materials you intend to cut, you don't say if it's belt, screw, or rack & pinion drive, what type of spindle you'll be using or any other details which greatly impacts the selection of axis motors.
With that said, a 1/4" tool, IMHO, sound pretty big for a machine with NEMA17 size drive motors, let alone 3/8" but it all depends on the available spindle power, depth of cut, type of material and so on.

For example, if you intend to build a machine using threaded rod from the hardware store the efficiency of that threaded rod is around 1/4 (or even less) compared to a ballscrew (of the same pitch). This means that your drive motor needs to put 4 times the amount of torque into the screw at any given speed in order to get the same amount of linear force. The thread pitch of that threaded rod will also be much less that what it is on a ballscrew meaning the motor not only has to put more torque into the screw but also turn a lot faster - and as it happens with step motors, the faster they turn the less torque they're able to deliver.

Then, if you go "the other way" ie belt drive - where one motor revolution might equal 30, 40, 50mm worth of travel you don't need the motor spin as fast but you do need 10 times as much torque compared to a 3, 4, or 5mm pitch screw drive in order to get the same amount of linear force.
 
beanflying:
1/4 or 3/8 router bits generate a lot of forces in a lot of directions so start there in your design and generally wont be put in a conventional router under 750W. Think about that mass you then need to shove around (including your gantry or bed), change direction and push into your job. Nema 17's are not what you need IMO and will lose steps and fail, Nema 23's and fairly gutsy ones at that or even Nema 34's for preference. Doing this by trail and error gets expensive so at minimum find a complete design that works with the bits you want or do the numbers.

Look up some data sheets on 'Nema' steppers and there are very large variations in length, wind, step angle ......... evilbay is not a place to get truth in spec's ;) That said some dedicated CNC sellers do exist.

In evilbay terms you would be talking a 6040 spec or better for those bits. Spec list for what it is worth is on this listing and may help you get your head around so parts if you are determined to roll your own https://www.ebay.com.au/itm/New-3-Axis-Desktop-CNC-Router-Engraver-Milling-Drilling-Machine-Engraving-6040/232705187439?epid=12016920160&hash=item362e4f5e6f:g:ne0AAOSwi0xaNJXX:rk:3:pf:0

Good Luck  :)
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod