CNC Mill >
Anyway, things are still coming together- now I just need to tap a bunch of holes, and machine some giant plates for the main structure of the machine. And then probably spend far too long re-aligning things to get them as close to square as possible.
Speaking of the ballnut... that had to be reversed on the ballscrew. Now, I know there are ball bearings in there, so I was being smart, and turned a piece that would fit on the end of the ballscrew to keep the balls inside while I twisted the nut off. That... didn't work. Somehow the balls got out of their tracks anyway. Fortunately they were covered in grease, and many tedious minutes later I had them stuck back in the right places.
After screwing up a couple things, the Z axis is mostly done. I got the front milled flat to accept the new spindle adapter place, a hole bored through it for the leadscrew, and a cutout (plus mounting holes) for the ballnut. Oh, and a place for the bearing beneath it, of course. One little piece needs to be fabbed yet (a bearing clamp for the top side of the leadscrew), and some holes need to be tapped, but otherwise it's basically set.A couple things really went wrong when doing this work. First, the cutout for the ballnut was too big- I screwed up the gcode offsets. Anyway, that meant that the six mounting holes for the ballnut couldn't be drilled. So... with some help and the use of a press, a new chunk of metal was machined to size and pressed in the hole. Then I drilled the holes, and machined the ballnut cavity to the proper size. There shouldn't be a lot of force on those screws (the flange is at the bottom of the carriage), so I think it's all good.
One more piece- this time the mounting plate to attach the Z axis to the X axis rails. It came out... hopefully OK. The part shifted a bit while trying to elongate the holes on the lower right. And of course I screwed up the gcode and forgot to drill the other holes (leftmost) before that. I think I got it realigned (ie, they should be square to the rest of the holes), but who knows. At least the rightmost holes are non-critical. I was hoping the left holes would be a good alignment reference, but it's not the end of the world if I need to drill one of them out a hair. I'd rather not, but that's the way it goes.
Fortunately everything else came out well enough. The edges need to be trimmed a bit, and there's a pocket on the back that still needs to be cut, but it's getting there.
Anyway, it's progress, with some frustration. On the up side, I think there's only somewhere in the mid-teens of parts left to machine...
Long story short, those holes will need to be drilled out, and I'll just stick nuts on the back side. Fortunately I can do that with these holes, and haven't tapped any other M5 holes yet. Which is really good, since most of those are blind, and I really couldn't recover from having the wrong threads in them. M5-0.9 screws are nearly impossible to find, and stupidly expensive (like $2/screw), so that's not really an option.
Back to the drilled/tapped holes on the gantry... evidently an M5-0.9 tap is a thing, and I have one (the "normal" M5 thread is M5-0.8). As far as I can tell, it was only ever used on some pre-1972 cars. And when I went to the hardware store and asked for a normal M5 tap, that's what I got. I'm really no sure why they even had one in the first place, and I probably should have double-checked the taps before I left, since the guy fishing them out for me didn't seem terribly with it.
A whole batch of new parts. The big one (literally and metaphorically) is the gantry. It's been squared up to the right length and had holes drilled (and tapped). More on that in a bit.The second-largest part is the plate that the Z axis stepper sits on. And the little parts are shims to go on the leadscrews. The bearings I got are shorter than the non-threaded portion of the leadscrews, so in order to clamp down on them, I need a shim in there. That's what all the little round things are. Strictly speaking, I really have no reason to need to do anything to the ends of the leadscrews, since they're going to be held in place by the bearings (which will be clamped at either end, so the leadscrew can't slide anywhere). It's more a matter of principle... And I already got the (oddly-sized) nuts for it, so I might as well.
Obviously most of the structural parts still need to be made, but this is a good chunk of the little pieces.
Well, kinda. The CAD is getting done. There are still a lot of screwholes to put in (plus some other little details to work out, like not having leadscrews magically go through bricks of aluminum), but it's looking good overall. After that, it's a run past a real MechE, getting a whole lot of metal and screws, and then some quality time machining parts.
(I also got some new couplers to connect the leadscrews to my steppers, but those aren't quite as exciting)
I need to secure the ballscrews somehow, and that means bearings. [Un]Fortunately, the ballscrews aren't the same size as my existing leadscrews, meaning I need new bearings. And that lead to a whole new can of worms, namely "what kind of bearings?"Long story short, it looks like they don't make spherical roller bearings small enough, so I just went with some angular contact bearings. Those seemed to be decent for both axial and radial loads, though I imagine only the former is really relevant, and only marginally at that. With the loads this thing is likely to see, I suspect regular ball bearing would have been good enough. I figured if I was going to the effort of putting ballscrews on, I might as well at least mount them with nice bearings.
Well, that happened. I got trained in on the CNC mill at work, and they talked about having ballscrews and how great they were. So... I got jealous and decided my mill needed them as well. If I'm upgrading the thing, I might as well do it right. Plus it's incentive to actually work on it, though actually being able to use the mill at work to fabricate the pieces is also a huge help.
I finally decided that enough was enough, and added real depth probing to my CNC mill. The BeagleBone has plenty of inputs to support it (I even have a spare with a pull-up resistor pre-soldered), and since copper-clad board is conductive (along with my spindle and tools), it was really easy to add a few wires, tweak the configuration, and have it working.
So now I can tell the machine to depth-probe a chunk of copper-clad board, then compensate the PCB's milling depth based on those heights. This isn't entirely automatic (you need to run the depth-probing gcode, then run a converter script to modify your gcode, THEN run the PCB milling gcode), but it's pretty painless. And the results are pretty darn good- I just milled a board, and things overall appear to be pretty darn even. There was one edge that seems to be cut too shallow, though. Maybe an aluminum chip got under there or something... there are certainly enough of them around.
I even improved my code to be slightly more flexible, and stuck it up on Github in case somebody stumbles on this page and finds it useful.
There would be pictures here but all there is to see is a couple of wires, which I didn't deem worthy of an image.
Keywords to help people find this: PCB milling, depth probing, depth compensation, linuxcnc
Big news... big chunks of aluminum! I finally got the plates (and channel... and other chunks) needed to upgrade my machine yet again. The plates are 3/8", which I think should be enough to eliminate vibrations and resist bending. They certainly FEEL hefty enough.
Anyway, the big pieces will form the base, sides, and back of the updated machine, while the channel you can see buried under the long thin pieces will be the gantry. Those smaller pieces are to be basically the glue to hold everything together.
Now I just need to get back into the shop with the big mill for several hours.
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