Monday, January 17, 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...

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