Monday, 31 January 2011

HobbyCNC stepper kit

The HobbyCNC Pro driver package I had on order arrived last Friday, so I have spent the week on and off assembling the driver board into the old UPS case that I had. It's a nice fit. I had planned on falling back on using a PC case, but with a bit of magic and luck, it all went into the smaller UPS case. I'm glad I persevered, as it's a really neat little unit when the cover is on.

The package includes everything needed, except the case and transformer. Luckily, the transformer from the UPS was suitable (not ideal, as the final DC supply to the driver board is ~ 22V. Ideally, it would have been about 35V or so, but the board will work down to 12V ). I don't know the exact specs of the transformer, but from the UPS spec (600VA output) and judging by the thickness of the low-voltage leads, I think it should be good for at least 10A of current. I'll be monitoring the DC voltage for droop once the CNC is up and running.

I got the largest steppers that HobbyCNC sell in the package - they are a lot heavier than I expected. I think there will be more than enough torque for most applications. Here's the first run, driven by Mach3 (took a while to get it configured to use the right parallel port address. Laptop was confusing it apparently)...

Having got the steppers running, I had to work out how to connect them to the threaded rod. I had expected to turn down (in a lathe) the end of the threaded rod to 1/4 inch ( the same size as the stepper shaft ) and then use a 1/4 inch shaft coupler. My mate Gordon (who has the lathe) came up with these instead...
Stepper - lead screw couplers. Thanks, Gordon!
8mm tapped on one end, 1/4 inch hole with grub-screw on the other. Perfect! Great work, Gordon.

Next post...should be the CNC machine assembled. I hope.

Tuesday, 18 January 2011

Just got 7oz in the mail

That's 200 grams.

Turns out the package wasn't the CNC steppers, but a pair of MSP430 development tools that I had forgotten that I had ordered from TI ( got them for free - thanks Dangerous Prototypes). They were dispatched from the Netherlands, surprisingly.
The MSP430 is an ultra low power, 16 bit microcontroller. These development kits are an integrated USB programmer/debugger with a tiny 14 pin MSP430F2013 attached to the end of the board. The board with the target processor can be separated from the end of the debugger, and the pins are bought out to reasonable sided pads.
USB case opened - close up of microcontroller end
There's also a LED on there, just top-left of the chip. There doesn't seem to be much on the micro side of the header, so you could probably use it as an in-circuit programmer too.

Now, what to do with them.... I'm thinking something solar powered...

Monday, 17 January 2011

I've been framed

The frame of the CNC machine is almost complete. I've gone with a "2 axis-of-movement table with a fixed Z axis" design.
It's basically two rectangular frames with laterally fixed threaded rods, which are threaded onto a central plate. The central plate is where all the adjustments for lash between the two axis are made with aluminium L-section glides. This reduces the number of bearings required down to just 2 for each axis. If it turns out there's too much friction for the steppers (yes, I've decided to go back to steppers), I can always replace the glides with bearings later on.

Here's the assembly before the z-axis verticals went on
The top frame will have a timber panel attached to it that will be the "work" surface; so it will move in the X and Y axis, relative to the fixed Z axis. 

This is a bit later on...
The router is attached to a timber frame that moves up and down the Z axis frame. This needs to have some aluminium slides added to it yet.

One of the hardest parts of this so far was working out how to attach the central plate to the threaded rod using just ordinary hex nuts. The problems here are: 
  • that the threaded rod is a little bowed, so the attachments cannot be totally rigid
  • I wanted to be able to disassemble the device reasonably easily
  • there needed to be some means of adjusting the lash of the nuts on the rod
So some device was needed that prevented the nut from turning, but allowed it move up and down as the rod rotated. Here's the evolution...
1st attempt - cut and bent L section.  
Ugly. And you can't get it apart without threading the rod right through it. And no lash adjustment.

2nd attempt. Three small L sections screwed together.
Better, but a pain to make. The nut slides in (tightly) between two of the L's, the third one takes the lateral (?) force. Seemed like that wasn't going to be rigid enough - the L section was wanting to bend when a reasonable amount of force was applied. But it was good in that the lash could be adjusted by sliding the rod out of the holder, turning the nut 1/6 of a turn, then sliding it back in.

3rd try. Channel with a cut and bent in 12mm section.
This is what I settled on. It's aluminium channel with a 13mm internal width. The nut is a snug fit in there. The walls of the channel are cut 6mm deep, 12mm apart, then bent in slightly. These are where the lateral force is applied from the nut to the channel.

So that's it for now. The stepper motors and control board have been ordered (actually, FedEx tried to deliver them today). I got the HobbyCNC PRO kit with 305 oz-in motors. That's almost 22 kg-cm of torque, if the online calculators are right. That seems like a lot. Guess we'll see...