Sunday, February 26, 2012

Building a CNC Machine - Linear Bearings & X-Axis

I've been back and forth between buying the parts to build a RepRap off eBay or designing something myself. I've also been concerned that the Prusa frame wouldn't be strong enough to handle pcb routing. So I decided to build a somewhat heavier duty cnc machine to do pcb routing, but make it able to run an extruder. I may just use it with the extruder or use it as a RepStrap to print the parts for a Prusa at a later date. I intend to use the Sanguinololu board I built to control it.

Linear Bearings

The linear bearing were a hangup for me, all the best ideas were either expensive (v-groove bearings, 1/2"+ linear bearings), or looked flimsy for my application (8mm linear bearings).

I've seen this design on the internet before, and couldn't come up with a better solution that didn't cost a small fortune.

I bought 100 608zz bearings, some angle iron and a big bag of 5/16" nuts, 5/16" x 1" bolts and 1/4" washers. I made a jig to ensure the holes were in the right place, clamped it to the drill press and here's what I came up with.

I will weld 1 1/2" angle to the backs to make mounting brackets. 

  • 1" angle iron cut to 4"
  • Holes drilled at 1/2" and 1 1/8" in from each end (roughly)
  • Clamped to a jig to ensure holes are perfectly aligned in relation to the center
  • Each bearing has a 1/4" washer on each end of the 608zz bearing, a 5/16"x1" bolt and 2 nuts (one on each side of the angle iron)

X Axis
With the bearing design done I started the X-axis
I wanted a bigger work area that the RepRap's 8"x8"x5.5" My X-axis has 11 3/4" max travel. I was going for 12", but the frame interferes. I'll probably limit it to 11-11 1/2". the Y-axis should be about the same, haven't decided on the Z axis yet, I have 8" in mind, but probably between 6-10"

  • Slides are 16" long, allowing 12" travel (without the cross pieces). the slides are structural frame parts. they have 1 1/2 angle iron welded to the back of each end to attach them to the cross pieces
  • Cross pieces are 18" wide, allowing slides to be bolted at 15 3/4" wide. It will work out to 16" when I do the Y axis mounts (1/8 added per side)
It seems very sturdy, I stood on it, and it was still easy to move. I'm out of 1" angle iron again, so that's it for now. I still need to attach a table, mount a stepper motor, and it hook up a lead screw of some sort.

Closeup View
 Weight Test