Brain-dump lest I forget...

A project log for CD/DVD mechanisms and cartesian thinggie[s?]

DVD-laser-etcher, dremmel-router, possibly a 3D printer? Who knows!

esot.ericesot.eric 11/13/2015 at 10:500 Comments

Kinda burnt-out, been working toward some sort of functional PCB-CNC/rotary-tool system, all day.

Decided to use the groovy microscope-slide mechanism my Pops found for me, but not exactly sure *why*, as it's been difficult to come up with a driving-solution. The thing is, the way its *designed* to be mounted results in the knobs *moving* *with* the motion of the slide. So I spent some time trying to figure out how to use stationarily-mounted motors and still allow it to move as-designed. End-Result.... Mount the dang thing differently. I did come up with a couple solutions for the mobile "knobs", one not unlike CoreXY. The first method requires *FOUR* motors for the two axes. The second method requires two motors, but one is dependent on the travel of the other, AND, this method requires three pulleys that are *exactly* the right diameters. If they're off just a little, the "pulling" motion of the motor will only work in one direction, there'd be a lot of slop, the belt would be loose in one direction. Oh, I think there was another 2-motor solution that requires "idler pulleys" that follow the motion of the knobs, but these are *really difficult* to attach, due to the geometry of the axes. The end-solution is to mount it in a way that it wasn't designed to be mounted, but it keeps the "knobs" stationary. Actually, this "mount" is the same sort of mount necessary for the idler-pulleys, but the idler-pulley method was difficult because it relies on the "mount" being *entirely* free-floating, whereas the stationary-knob "mount" (which, again, is the same) gets added-support from being stationary...

Anyways, Lucked-Out that I just happened to have a bunch of long bolts (with nuts already on 'em!) that have the right threading to replace some bolts already on the system... So they extend outward and I can attach them to a surface. Awesome.

Then the next difficulty is that the original mounting-method now becomes a moving axis, that axis happens to be the one that will support the Z-axis (the rotary-tool, in this case). And, again (previous log) there's the issue that the "slide" travels *under* this axis. Alright, PCBs are thin and can do this. Regardless, the actual cutting-bit needs to be offset by quite a bit from the mounting-point, which means I'll probably need to add another "glider" to support it.

So, let's just pretend that's all solved... what's next...?
I need a friggin' rotary-tool. @Stefan Lochbrunner's project suggests using a "chuck" designed to attach to a regular-ol' DC motor. And, being broke, I decided it'd be easy to make my own rotary-tool chuck (since rotary-tool-bits always use 1/8in shafts)...


The Good: This'll leave my rotary-tool available for other purposes. Excellent. This is also quite a bit lighter, which is a good thing. It's also *smaller*, which makes mounting easier.

The Bad: Unless you have *exactly* the right-sized drill-bits, this is darn-near impossible. The end-result is about 1/8th inch "wobble" at the end of the bit. Whoa. There's no way to make a PCB with that. I've thought of a few things, but the end-result was a remake, then a total redesign.

Too bad, 'cause the whole process went amazingly smoothly until the wobble-thing at the very end. First, I needed to find a "dowel" of some sort, so I eventually came across a couple really-long threaded aluminum standoffs, leaving plenty of room to grip both shafts. The threads had to be drilled-out for the shafts, which is great, 'cause it was basically pre-drilled. Then I grabbed my tapping-tool to add set-screws but realized the smallest tap was too large, not only in diameter, but it would only have about 1-2 threads in the shaft-coupler. I had this crazy idea to cut sharp slots into regular ol' bolts (with much smaller threading) to *make* taps... (Hey, we're working with Aluminum, here, who knows...). And, lo-and-behold, I actually found the perfect bolts with a really tight threading *AND* apparently designed to be self-tapping (!?). I've certainly seen self-tapping wood screws, self-tapping sheet-metal screws, but I don't know that I'd ever noticed a self-tapping *bolt*. Wild. Cool. They're now *in* my tap/die set. I had no idea what size bit to use, but tried one, and it worked great. Oh, before that I predrilled the setscrew holes with the only drill-bits I have (besides 1/8th-inch) that will mount in my rotary-tool, so I could make use of its drill-press stand. Excellent. Those bits are *TINY*... smaller than most needles. Managed to drill both holes straight through, so decided two set-screws might not be a bad idea. Redrilled with my regular-'ol hand-drill with a size better-suited for "tapping" and despite the tiny holes that were barely big enough to have an effect on the larger bit, the redrilling and tapping worked perfectly.

I mean, seriously, the whole thing went so smoothly... Then I started to assemble it with the motor and realized that it was *just slightly* larger in diameter than the bits, and it happened to have "knurls" (?) on it... so why not press-fit the blasted thing? Oh yeah.

Oh and a few other bits and pieces here and there that just seemed to flow together so dang well, it was shocking.

Then... The wobble. Seriously, I'd say it's nearly 1/8in at the end of the bit. I tried bending, and various other things... I just can't get it right. I think what happened was I drilled once from one end, and I used one of the set-screws to keep the thing from spinning in the vice (didn't want to clamp it so hard as to warp the thing!). Then I switched the set-screw to the other side and drilled from the opposite side. I think, maybe, the drills didn't align perfectly. I don't think anything could be done about that, as far as bending goes.

Fought it for a bit. Gave up. Finally regained some motivation. Tried again from the beginning. This time *one drill* straight down the center. But, this time, decided to use a different motor, with a smaller shaft. Not sure why, exactly, but I think it's higher-quality and certainly much quieter (though probably slower). And I have several... And I haven't quite figured out how to remove the press-fit shaft-coupler from the other motor...

So, drill out the smaller diameter first... all the way through, then drill the other end (with the nearly-identical-sized predrill) for the 1/8in rotary-tool-bit shaft... Simple. But, I don't have a drill-bit that matches this motor's shaft. I have one that's slightly smaller... So, right, brilliant, 'cause I didn't get the point of wobble before... I drilled it out, then widened it as best I could... and it was just wide enough for the motor at the end I was working, and apparently *too wide* for the 1/8in bits at the other end. Whoa. Alright, I give up, wobble's not gonna be avoidable with my tools.

How do they make rotary-tools so precise that that *tiny* bit could drill a perfectly *tiny* hole?! I bet it has something to do with a long body and bearings at each end. Well, I don't have a long body to work with, here... and I can't get alignment in my shaft-couplers to the tolerances most anyone (and certainly a tool-manufacturer) could do with the right tools...

How can we avoid this...? Well, I don't have any U-Joints that'd handle these speeds and could easily be attached-to at both sides... So... here's where we're at right now...

I have ball-bearings that are *almost* perfect for a 1/8in shaft. They're a *tiny* bit loose, but not much. So, two bearings where the bit normally mounts in the chuck, but that would fall-out. So, a third (easily dis/reattached) bearing under the bit's plastic size-indicator. This, also, serves the benefit of preventing up/down slop, which would have to be considered in the directly-coupled design, since apparently many motors have axial slop (interesting). Also, it serves to prevent the bit from torquing while being dragged along the PCB.

So, in all, I think this'll be a good route, but it means I've got to figure out how to support these three bearings. First-Go, I have a hollow-rod (a pipe, really) that's almost exactly the right size, but needs to be drilled-out slightly. Fine. I have a drill-bit that is almost exactly the right size... Drill it out as best I can... Turns out, it's easier to drill more precisely *in reverse* where it won't *grab* the material. Excellent. Drilled out enough for the two bearings surrounding the plastic size-indicator, then planned to cut the "pipe" to drill-out the same just long-enough for the third bearing at the end of the shaft. Looked *everywhere* in my home for a matching screw to mate with the end of the pipe (preferably an end-cap sorta thing, which I'dve drilled out if necessary to allow the bit to protrude as well as be changed easily), but couldn't find any. Fine... I'll figure something out... Not sure when I decided to drill it out just a little bit more, but I did... and lo-and-behold, apparently the inner diameter of the pipe is actually *wider* than the bearings, once you get past the threaded-portion (of the pipe). Now... well, we have wobbling bearings.

So, where we at now...?

Did I mention that my plan is to spin the bit with a rubber belt? Yeah, that's of some questionability...

So, I think I'm going to drill out a block of wood to support the bearings. (it *just* occurred to me, maybe I could set-screw the bearings? Hmm, maybe even in the pipe...?)

That's basically where I'm at... THIS part IS relevant to [my] CD/DVD mechanism project... Lasers are cool and all, but I think it'd be more useful if I attach a rotary-tool. I chose to work with the other mechanism mostly 'cause it's larger... But as @John Pfeiffer pointed out, 40mm x 40mm (DVD-mechanism) is actually a pretty decent size for a PCB. It's much bigger than #The Square Inch Project! Heck, the last several boards I ordered were 50x50 and had lots of free space. So, I'm kinda tempted to revisit this. The last experiments with the "Secret Project" were finally repeatable with microstepping (which seems to help reduce missed-steps due to loading). (I think the lack of repeatability was due to using the wrong supply voltage). And, if I recall correctly, single-stepping gives 1055 steps end-to-end, which didn't seem like much at the time, but is actually something like 0.04mm precision, that's pretty dang good for a PCB, if I can fix this wobble-problem!

Oh, and I finally figured out how to squeeze a few more fractions-of-an-inch out of my cheapo cordless drills... I've had an old (wooden-handled) right-angle drill-attachment in storage for longer than I can remember. It has a 1/2in chuck! So I took a Hex-standoff and chucked it up between my drill's and this thing's chucks, and spun the drill so the right-angle-thing's shaft spun... Its shaft acting as though it was chucked up in a lathe, I filed that shaft down until it fit in my drills, and now a chuck-size-adapter.

Shoulda done this years ago. Not only does it make for a larger chuck, but it also mounts easily in a vice to use as a lathe of sorts. Much more stable than just a bare drill. Excellent.