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<wpwrak>
nicksydney: as long a your boards have no bottom-side components, you can indeed get away with such simple PCB-only fixtures :)
<wpwrak>
nicksydney: else, you'll see that mine are pretty much equivalent, with the differences being in a) using acrylic instead of pcb (because i need more depth), b) my registration being particularly poor in the case of ybpgm (not sure what went wrong, but it seems that i'm almost 1 mm off between acrylic and pcb. ironically, i made an
<wpwrak>
extra effort here to keep those coordinates perfectly matched. in that older MDF fixture it was all manual ... and quite accurate), and c) the holes for the pogo pins are much wider. that's because i'm too lazy to change tools and/or couldn't be bothered to buy a drill of exactly the right size. the pogo pins survive a bit of abuse, especially if they're as short as the ones i'm using, so not having mechanical support at the top is no
<wpwrak>
t a cause for immediate concern.
<wpwrak>
last but not least, yes, designing production testing is important. i skipped that for my firs "mass-produced" product, UBB. since it's hard to get that one wrong but i did it for my second one, the atben/atusb pair. here's the process:
<whitequark>
the one on the right is incompletely milled
<whitequark>
the one on the left is much better. all faults are due to the fact that I only had corrugated cardboard for a sacrificial material and it is not exactly flat
<wpwrak>
whitequark: yes, i'd reduce the feedrate or increase the spindle speed. if this doesn't help, let the mill run the path twice (the second time you can go faster since it does almost no work)
<wpwrak>
burr can also be cause by worn bits, but that would be a bit early soon your case
<wpwrak>
grmbl. the high art of editing. s/early soon/soon in/
<wpwrak>
other things to look for: a loose fixture, an unstable table, and lack of lubrication (if the material is hard)
<whitequark>
plastic :)
<whitequark>
it's polycarbonate sheets for laser printer.
<wpwrak>
okay, polycarbonate is actually quite hard. but not quite bad enough for that :)
<whitequark>
actually, polystyrene, not polycarbonate
<wpwrak>
with plastic, there's also the issue of finding the right speed. if you go too slow, you may melt it. but you seem to go fast in general, so that's probably not the issue
<whitequark>
ok. 1.4mm/s feedrate is better. let's try 1mm/s, and I'll add another 1/4 pass per pad to mill out the loose bits
<wpwrak>
so as a first step, it's go a little slower. you can also make a test pattern of parallel lines or such that will allow you to test parameters more rapidly
<wpwrak>
aah, still the stencil. i see.
<wpwrak>
flexing of the plastic is then probably the main issue. so yes, minimizing the forces should help.
<whitequark>
yea, it flexes in a nasty way
<wpwrak>
i guess that's why they use steel ;-)
<wpwrak>
maybe try to glue it down, possibly via adhesive tape. that would restrain the movement. then you could look for a way to chemically weaken/dissolve the glue.
<wpwrak>
(customizing the cnc mill) it may get a little tedious. you need to add air pumps, an elevated table, a feeder system, a fairly complicated head (with a motor for rotation and also the transport needle, although that needle probably doesn't need to be motorized)
<whitequark>
you do have a point
<wpwrak>
nicksydney: that's a photo plotter ;-) we've had that technology for just how many centuries now ? ;-)
<whitequark>
combines the worst of wet and dry processes?
<whitequark>
slow, requires chemicals
<whitequark>
wpwrak: ok, I've figured out the problem with plastic flexing.
<whitequark>
see, it actually results in burr being pushed *under* workpiece
<wpwrak>
hmm, what do you use as support ?
<whitequark>
I've duct-taped it to a PCB
<whitequark>
the bare side, to be more specific
<wpwrak>
that ought to be stiff enough
<wpwrak>
maybe try reversing the spindle ;-)
<whitequark>
mmhm
<whitequark>
it actually pushes material to both sides of the sheet, so it won't do anything
<wpwrak>
not sure if this won't just make things much worse, though :)
<wpwrak>
ah, i see. lower speed didn't help enough ?
<whitequark>
I think the problem is that I'm using an engraving bit, but ought to use a real endmill with flutes
<whitequark>
no point in lowering speed under 100mm/min. no difference in results.
<whitequark>
maybe I should run spindle slower?
<wpwrak>
(engraving) that sounds suspicious, yes
<whitequark>
according to datasheet right now it should run around 6000 rpm
<wpwrak>
you can try that. not sure what it does. mine if fixed-speed
<wpwrak>
have you tried running it twice on the same path ?
<whitequark>
that won't really do anything. see, if you look at the bare sheet from the thin side, it looks like this:
<whitequark>
----------
<whitequark>
and after milling, it looks like this:
<whitequark>
-----III----
<whitequark>
where III is the material pushed to the sides of the sheet
<whitequark>
it's not *inside* the holes, meaning the bit won't touch it
<whitequark>
it's basically a bevel now
<wpwrak>
try it anyway :) at least with endmills, this works wonders
<whitequark>
nah, it's worse this way
<whitequark>
& spindle speed (50%) does not noticeably affect result
<wpwrak>
ah, pity
<wpwrak>
hmm, lowering the spindle speed is what i would have suggested next
<wpwrak>
well, if you want to fix the plastic very solidly, you'd cut another pcb such that it has slightly larger openings, then make a "sandwich"
<whitequark>
from what I understand, material being pushed under the sheet has quite a leverage for pulling it out of the fixture, or flexing the plastic & fixture
<whitequark>
mainly fixture, PET doesn't stretch at all