Saturday, April 21, 2012

Low Cost CNC Part IV - Cheeseplates and belt-drives

I started by making some cheese-plate, aka optical breadboards.  You can buy these--and they will be far more accurate than I can make--but they are expensive.  This is probably because they are precision ground, anodized and 1/2 inch thick.  Half-inch plates are too heavy for this application, so I set about making some from 1/4 inch plate.

I scored out a grid pattern and then center-punched the intersections.  Actually I center-punched them twice, once with a very fine punch and then again with a much larger punch.  I've found this results in better-centered holes.  I also purchased a 1/4-20 tap-drill.  This simultaneously drills and taps a hole and, when used in a drill-press, dramatically speeds the process of making these plates up, as well as results in more accurate holes.  You can see the grid of center-punched holes and tap-drill below:

It's kind of weird using these with a drill press.  First you have to lower the spindle to drill into the material, until the threads begin to catch.  At this point it pulls the spindle down at the correct speed, you basically have to just stay out of the way.  However it is critical to get the stop of the drill-press set at the right location, particularly with the 1/4 inch plate, since the tap-drill is only intended for thinner plate stock.  If the stop is set too low, it will drag the countersink portion of the bit most of the way through the plate, leaving almost no thread.  Too high and it will presumably either strip the hole or break the bit, since the drill-press will stop while the threaded portion of the bit is still in the plate.

After a few false-starts, I had it down and started cranking out the holes:

The tap-drill was a life-saver.  Previously each hole would take about 5 minutes by the time center-punching, clamping, drilling and hand-tapping was done.  With the tap-drill it is closer to one or two, and the results are way more accurate.  A catch is that with the non-reversing spindle of the VHS drill-press it is necessary to unscrew the spindle manually.  This is a bit of a workout for the wrists, but less so than tapping by hand.  A tap-head could fix this but costs more than I care to spend.

After a bit over an hour, I had my first cheese-plate:

Then I made another one.  Then added a few rows of holes to the existing plate I'd been using as a base for the first stage.

Following up on the previous post, I've printed out fixed-end bearing blocks for the leadscrews.  These support axial loads on the leadscrew and take the load off (flimsy) motor mount.  I also tested the completed stage at 30V, hoping to reach the 100 mm/s that I'd set as a target.  I was able to reach about 40 mm/s, far short of what I'd hoped.  Switching to a proper leadscrew would fix this, however would fly in the face of 'low-cost'.  I'm pretty sure that the current feedrates are more than any spindle I plan to use can handle, however they're way too low for 3D printing.

This is where the beauty of the design-concept comes in.  With all part mounting on 1" centered, 1/4-20 holes, it is trivial to modify the machine design just by unbolting the parts and shifting them around. So I rotated the motor and fixed-support by 90 degrees and shifted some of the linear bearings a bit and added some timing pulleys.  The end result was this:

The bottom axis is belt-driven, while the top-axis is still screw-driven. All the parts are common, except a few spacers that I pressed into service as clamps for the belt so I could tension it correctly. You can see the setup below:

The fixed-end screw supports were used to hold the timing pulley opposite the motor.  The picture below shows the support with the top-axis screw sticking through.  Backing nuts onto the bearings locks the axial position of the screw.  It currently uses 608 (i.e. skateboard) bearings but should be modified to use angular contact bearings:

So how fast is it?  Much faster.  Like, a lot faster.  It's not quite what I'd wanted (a design that handled everything), but I'll settle for a set of parts that can be CNC or 3D printer with only a wrench and some re-jiggering separating them.  And what's more, it's still pretty beefy.  I loaded up the stage with (in addition to the top-axis), 16 lbs (i.e. all) of my girlfriend's free-weights.  It had no problem flinging them back and forth, shaking my coffee table vigorously in the process.

That's all for now.  I'll probably convert the other long-axis to belt-drive since I think I'm more likely to get the full machine up and running as a 3D printer rather than as a CNC, at least in the initial stages.

1 comment:

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