Underlay/Backing Material for milling PCBs

[gtrdude]

What kind of material is suitable for backing when drilling through holes into PCBs?

Are different types of material used for vacuum and non-vacuum machines?

Paul

[vladdy]

Most of the time I use some 'surplus' 1/16 thick single sided board that I had won on ebay, phenolic side up helps the bits last a little longer..

On boards that are larger that my surplus PCB material, I use 1/8 MDF board, pretty cheap, and 'reasonably' accurate in thickness, for a backer board..

enjoy

[gtrdude]

Originally Posted by vladdy Most of the time I use some 'surplus' 1/16 thick single sided board that I had won on ebay, phenolic side up helps the bits last a little longer.. On boards that are larger that my surplus PCB material, I use 1/8 MDF board, pretty cheap, and 'reasonably' accurate in thickness, for a backer board.. enjoy

Yes, after thinking about it a little more I came to the conclusion that I could use some spare blank boards as backing. But what about vacuum machines? Do you use some kind of material that lets air through?

Paul

[vladdy]

My little mdf critter doesn't have vacumn, just hold down strips...etc..
for a vacumn table you could always do a simple pattern generation to do a 'grid' of vacumn through holes, I suppose..
Only draw back to that as far as I can see is the possibility of bit damage if you need to drill a hole through your 'work' piece and the bit comes through at some point perhaps half way into a vacumn hole..messy..
a bit of pre planning would be required to indicate 'non drilled' portions that could be used for vacumn ports...ok for production, time comsuming for smaller lots..

not sure.. can't call this an 'educated' guess as I've never had to do it..

enjoy..

[gtrdude]

Hi vladdy,

Thanks again for the replies. I have been trying to think of some kind of porous material that would serve as an underlay but still give me accuraccy. I haven't come up with anything yet. "Messy" as you say.

I would definately not consider creating custom underlays for each board. That pretty much defeats the whole purpose of milling pcb's in the first place.

If I can't solve this problem then I will give up the idea of vacuum and just go with duct tape as a work holder. I haven't had much of a problem with warped boards at the moment anyway. Having said that ofcourse I'll probably be proven wrong at some stage.

Paul

[vladdy]

Saw something the other day at the local Windsor Plywood store that 'might' work for you. It was a sheet [about 12x24 inches] of what seemed to be a fairly fine grained MDF maybe 1/8 inch thick. The part that was of interest was that it was already preforated in a fairly dense arrangement of holes, each hole was maybe .080 in diameter and looked like the spacing was around double that..
I asked what it was for, and the clerk said it was for people making jewel case boxes and tabletop curio boxes, and he thought they were laser cut, the surface seemed very smooth with no bumps or shavings of any kind...cost was $7.95 CDN for that sheet...
A fairly simple premade standoff for a vacumn table sacrificial board, and MDF would be a lot easier on the drill bits as they pop through the FR4 when you are drilling..
Not sure of the dimensional accuracy of MDF in general, but it may be worthwhile to pick up a piece and try it.. pretty cheap and nasty solution maybe..?

enjoy....

[c-c-cncboy]

Understanding that you want to drill the component lead holes and consider using vacuum hold-down, for prototype boards I generally do the routing for track/pad boundaries, then center-pips for all drilled holes, with the same tool in the same pass. If I'm only isolating tracks (without removing the copper oceans by hatching) then I'm done with the routing and the vacuum table. I move on to hand drilling, perfectly located in the center-pips just provided.

If I need to remove the copper oceans, there may be a tool change (subject to spacing issues) or not, followed by hatching out the oceans.

Don't laugh, but if I use 3-ounce copper the oceans can be hand-skinned off like a rabbit -- O0o°UuCcHh!! -- instead of hatching them out with the cutter.

The overall result is, I never puncture the PCB while it is on the vacuum table gasket.
Terrence

[gtrdude]

Originally Posted by c-c-cncboy The overall result is, I never puncture the PCB while it is on the vacuum table gasket. Terrence

This is a good point Terrence. Vacuum could be used for isolating tracks and clearing oceans in order to maintain uniformity on the z-axis. However, absolute uniformity is not needed for drilling, nor is an underlay needed for milling.

Paul

[leo.dearden]

I came across this stuff: Vac porous diffuser. Not all that cheap, but perfect.

http://www.pacersys.co.uk/acatalog/S...ser_Sheet.html

I haven't tried it, but I may well one day.

Leo

[keebler303]

A piece of particle board is surprisingly porous. MDF may work too but not quite as well. You can pull a good vacuum straight through it. You just have to make sure to seal all the edges which aren't covered or it leaks too bad. This you can drill right into and you won't have any problems.

http://en.wikipedia.org/wiki/Particle_board

This is the stuff, you want it without any veneer or laminate on the surface.

Matt