Best material for spoilboard beneath PCB etching? Best hold-downs?

I'm flattening/leveling out my spoilboard prior to sticking a PCB against it with double sided Nitto tape.

https://www.cnczone.com/forums/attac...d=447004&stc=1

The first spoilboard material I tried was plywood, but it turns out that the knots in the plywood don't smooth as well as the rest of the wood, so I'm rejecting plywood.

I'm presently, as I type this, leveling out some particle board, and it seems to be leveling out quite smooth. The only downside is that it's loaded with formaldehyde resin, so the dust probably isn't healthy. I presume MDF would have the same issue.

So, for that reason, I may also consider Advantech, which allegedly has no added urea formaldehyde in it. It's more of a chipboard, though, so I'm unsure how well it will mill flat.

What material are you all using?

Presently, as you can see from the photo, I'm using aluminum holddowns, but I think I may switch to plastic bolts so that I'm not blasted with metal shrapnel if the cutterhead should ever slam into it (Murphy's Law, right?). Even then I would sink the plastic bolt-heads below the surface of the spoilboard, to make a collision even less likely. Also, that way I could mill the entire surface flat.

What kind of hold-downs do you guys use?

Been there, done that. I used to level the bed, make special fixtures for the PCB, use double-sided tape, masking tape, silicone, some special glue ($10 a bottle)...

None of that was worth the hassle. Trying to mount a PCB perfectly flat and level is a useless exercise in futility. If you look at commercial prototyping machines such as LPKF, you will see that they are not relying on the PCB being flat. They are using springs and adjustable rings to control the depth of cut.

Another option - probably the easiest for a hobby machine - is probing the surface with software like AutoLeveller. It works amazingly well even with severely warped boards. So I just mount the PCB any way I like and let the software do its magic.

https://i.ibb.co/HKmbNPy/IMG-20200810-094756.jpg

https://i.ibb.co/C8dykqN/IMG-20200715-121619.jpg

OK, I'll give the two stage autoleveler a try. I previously tried the single stage autoleveler's built into chilipeppr and OpenCNCPilot,, and just by themselves those didn't seem to be good enough. Perhaps in those instances maybe there were other factors in play though. I'll try harder this time around.

On the other hand, even with a good autoleveler, I'm pretty sure your PCB needs to be held down fairly tightly against a substrate of one kind or another, or else a difference arises between the height that's CNC probed and the pressed down height that occurs during milling. In cases like that, you end up needing some kind of gizmo like:
https://www.cnczone.com/forums/attac...d=447008&stc=1
to press down on the board prior to the probed height being measured (cf. https://blog.thegaragelab.com/seven-pcb-milling-tips/ ).

Anyhow, I like the look of yoru PCB. Looks as though you somehow even managed to apply a nice solder mask to it. :)

That Shane guy has a lot of valid points, but I do not agree with the "pressing down" part. If your endmill makes enough force to press the PCB down, you are probably feeding it too fast. The PCB should not be springing up and down during milling, even if it has a void under it.

For the soldering mask, I use the same procedure as for the traces. Same V-cutter and same probe data file. Of course, precise height control becomes even more important.

Here is an example where the cutter went too low and some copper pads were removed along with the mask:

https://i.ibb.co/cYPbGnp/flashlight-pcb-01.jpg

Nice. To handle your "cutter went too low" scenario, Bantham says they use a spring-loaded tip to etch away the solder mask over the pads:
https://www.youtube.com/watch?v=vaqOFH0Te5Q

Apparently Wegstr uses a spring loaded v-bit to do it: https://hackaday.io/project/35101-cn...pring-engraver

Where does one get spring loaded bits like that? What are they even named? Exactly which tip is ideal?

And, in regards to your earlier point: why do they both rely on springs rather than autolevellng? Actually, for all I know maybe they do autoleveling as well but use the springs in addition(?). However, judging from just the videos, it looks like they utilize the spring-bits without autoleveling, as no autoleveling is shown.

Aha! In the hackaday comments I see that at least one prototype for the spring-bit is posted to thingiverse: https://www.thingiverse.com/thing:4252330

Anyone here tried either that spring-bit device or some other one and gotten it to work?

Quote:

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Originally Posted by WhiteHare

Aha! In the hackaday comments I see that at least one prototype for the spring-bit is posted to thingiverse: https://www.thingiverse.com/thing:4252330

Anyone here tried either that spring-bit device or some other one and gotten it to work?

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That looks like a diamond drag tip, not a V-bit (which would rotate). The 3018 community uses them, eg https://www.amazon.com/RDZ-DEGREE-DI...dp/B082S332GZ/

I have not tried any spring-loaded cutters yet. Would be interesting to know how well they discriminate between epoxy and copper on a not-so-flat PCB (I have done some boards with as much as 0.7mm from the lowest to the highest point - that's 20 times the thickness of the copper layer).

Quote:

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Originally Posted by gtoal

That looks like a diamond drag tip, not a V-bit (which would rotate). The 3018 community uses them, eg https://www.amazon.com/RDZ-DEGREE-DI...dp/B082S332GZ/

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Would a drag bit rotate? I honestly don't know, but I'm guessing not. Therefore, have a look at the Bantom Tools youtube video below at time index 2:29. Whatever bit it is that Bantam is using to remove the solder mask, at least to my eyes it definitely looks as though it's spinning.

I tend to use HDF as a base. Usually levelled. Most of the time PCBs have mounting holes anyway, I just bolt them down through them, trying to warp them as little as possible.

Good news! I think I may have found not only the particular drag engraver that they're using to remove the solder mask, but also video instructions on how to use it for that purpose:
https://twitter.com/_MG_/status/1179815920396451840

:banana:

Here's a link to item itself: https://boringresearch.com/store/drag-engraver/
According to the video, it's the adjustable one.

According to the video, the default bit that comes with it is shown here:
https://www.cnczone.com/forums/attac...d=447016&stc=1

It does indeed look like some kind of v-bit, which perhaps explains why it appeared to be rotating in the videos below.

This is good timing, as I just recently received both UV solder mask as well as a UV light for curing it. Therefore, I just now ordered .one. If anyone else here orders one as well, then after receiving it let's compare notes on how best to use it for selective solder mask removal.

Well, answering my own question on hold-downs, I ordered these two things to safely hold down the spoilboard to the extruded aluminum platform:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

and

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

TL; DR: I'm wadgering that M8 plastic bolts will be strong enough.

As for the spoilboard itself, I finished flattening the particle board. It's now fairly smooth, but at the same time kinda perpetually dusty/gritty. I suppose I could try sealing it with a light coat of spray lacquer. Bah, humbug. Hopefully autoleveler renders it moot.