Wednesday, 8 January 2014

Converting an A3 printer into CNC machine - Pt3

The Y Axis:

The original print head carriage got a hack and slash so I could re-use the shaft runners.  These aren't linear bearings...more like a bronze bush.  I plan to replace them with linear bearings because there is a small amount of lateral movement in them.  However, since the shaft is only supporting the back side of the Z carrier platform, it's not such a this stage.

I used some 40x40x1mm aluminium angle to make up a stepper mount, the mounting bracket of an 8mm smooth rod (recovered from another printer I pulled to bits) and the threaded rod mounts.  I had a bunch of LMUU8 linear bearing + mounts from ebay, which I put to use with the 8mm rod.  I had thought about a vertical Y axis arrangment, but there wasn't much point since the gap down through the top platform is only a couple of inches, and there is heaps of space to tuck the second rod into the front edge.  My thinking is that is also spreads the Z axis load across both shafts and keeps the moving mass a bit lower down.  I'm not a physicist/engineer/mathematician, and that argument probably flies in the face of all good CNC designs..but works.

Everything was screwed down with 8G hex head self tapping screws.  Having quite a few flat and level surfaces to measure from helped with the alignment..although some was done with the old "eye-o-meter".

The 40mm angle is the perfect height to rest one edge on base of the Y carriage chassis.  It also ensures that it is parallel to the rest of the top assembly, and provides a great mounting point for both the Y axis screw rod and the endstop microswitch.

Again with the 25mm angle, I made up a screw rod bearing mount.  The acrylic is retaining a 6mm flange bearing.  The small pieces of white plastic are from one of the discarded printer parts (one of the flap hinges) and just the right size for bearing bushes.  Yes...I should double-nut the bushes, it's on my "enhancements to do" list.

The nut for the screw rod is a self tapping MDF machine insert.  It's ugly, but it works!  Because of the flange at one end, it butts up against the face to the mount, ensuring it is straight.  The length also removes all discernible backlash, and therefore no requirement for an anti-backlash nut setup.

The 2 small self tapping screws are there to anchor the insert from rotating.

I do plan to pull everything back off at a later stage and tidy up my rough-as-guts hacksaw cuts, remove barcode labels and pen please don't give me any grief about it :-)

And finally...the business end!

The 40mm angle has a bit of flex in it, so I braced it with a couple of 50mm tri-corner steel brackets, which made the whole thing rock solid.  In theory, the steppers shouldn't be subject to any linear pressure since the bearing at the other end should be stopping that.  The brackets helped with the torsional twist (although if you have the bearing, carrier nut and stepper all aligned should be as smooth as a baby's bum!)

Here's a close shot of the bearing arrangement that the Z axis rides on.  The 8mm rod and bearing was close enough to be a good point for mounting the Y+ endstop switch.

Next..the Z axis!

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