PCB hold-down methods

[yngndrw]

Hi,

I'd like to both mill, drill and cut out PCBs. (Engraving, drilling, profiling.)

My CNC has an aluminium T-Slot table and I was thinking of bolting some 6mm MDF that I have spare to it. I'd then surface it with a 10mm router bit so that I have a nice flat surface. My issue is how do I then hold the PCBs down on to it.

I've seen people clamping them by the edge, using double sided tape and using super glue on to an aluminium plate.

I'm not sure about edge clamping them as that could bow the board. I'm also not sure that I could get double sided tape flat. Finally I think that MDF would be far to pours to use super glue for it.

I haven't got a vacuum pump and can't afford one so a vacuum hold-down is out of the question.

Any ideas as to what I could do ?

Thanks.

[Eclipze]

I've tried a few different methods, such as corner bolts drilled through the PCB, or as part of the PCB or as tabs. Very good for high accuracy with double sided boards, but bowing is an issue near the corners. Same as with clamps, particularly when you have a soft backing board to drill into. Vacuum has limited use for boards where you have to drill holes.

The best method I've found, and the one I've been using, is the doubled sided tape. I face mill the MDF backing board to get it nice and even as you do. The double sided tape I make sure to stretch out so there are no small wrinkles. This method is easy for using off cuts! You have to have a board of a certain size to make sure it won't move while routing the board profile. If the tape is super sticky, just use less where this is fine pitch tracks (double sided) going face down. Otherwise, just stick the tape to the MDF, pull it off (now less sticky) and then stick to the PCB. I usually do this with the doubled sided boards, just to avoid the tape being too strong and peeling tracks off.

Hope that helps!

[morphious69]

any one thought of using a vacuum cleaner or wet dry vac as the vacuum pump? that should be strong enough to hold it even with drilled holes.

though I would think the "getting things lined up" issue might be a problem unless you use a jig and make sure you are tight to the "zero" axis corner and sides.

..I have thought about using a 4th motor and a clamping assembly to hold the edges and then rotate it back and forth to get the sides right since it would not come out of the clamp it would be where I wanted it and centering on the rotation axis is really all you would need to do to be sure to be dead on with both sides.

...I also am considering using a double set up with a pair of lasers from high speed DVD burners to burn epoxy spray paint (like used to touch up appliances like fridges and stoves) then etch the metal off with ammonia or potassium persulfate. Have to look up which because I know one is reusable indefinitely with a decent filtration system and some sort of cooling unit.
...(metal precipitates out so enough liquid and a not so fast pump will let it stay at the bottom of a tank with out a filter to clog)then use a second set up with a single laser to drill the holes as the metal will protect the parts of the board around the hole from being cut since the laser is not powerful enough to cut the metal.)

[yngndrw]

Thanks for the ideas and help.

I will have to try the double sided tape method as it seems to be the easiest method to get results from. How much tape do you use ? Do you cover every last bit of the board or just so much ?

I do have a second vacuum cleaner which I could use (The first is a Dyson for dust extraction.) but I'm not sure how good of a vacuum it can actually pull. It's also very noised so I'd like to keep away from that if possible.

For lining things up, I intended to use pins like this guy has done:
http://www.millpcbs.com/index.php?op...d=21&Itemid=65
I do intend to move to that kind of setup (Aluminium and super glue.) But I don't feel like buying and facing a sheet of aluminium as my first use. I also don't want to have to re-face the aluminium every time I remove it, but with MDF I don't mind as I have enough of that spare.

I'd love to be able to use a laser to "etch" the copper off and cut holes in the boards, but unfortunately with the way PCBs are I don't think it can be done. Using a laser to etch a resist off is an interesting idea but I have a laser printer so could just use toner transfer for that kind of method. The reason I wanted to go with milling PCBs rather than etching them is so that I could get away from the acid altogether. Having said that, etching them is probably much quicker and neater.

[Jay C]

Vacuum:

YouTube- PCB milling

YouTube- PCB milling

Carpet tape is what I've used. You level the spoil board first and make sure you have adequate support of the PCB where you will mill.
CA works on MDF after you seal it. Use a 1:4 ratio of wood glue (or white it's the same stuff) and water. Brush it on and go.

I recently bought a vac clamp system from joewoodworker.com. So far I have just cut wood (just got it last week and had some gifts to mill out) and the clamp is worth the money I spent. The ease of placement, clamping, and then milling is huge to me. I got the Excel 1 kit with jig option.

When I first started using it, I drilled the hole all the way through the jig so it would clamp itself to 1" MDF spoil board. On top I put from 3/16" hardwood. The pressure gage read 5. I then sealed both (the top of the MDF and the bottom of the hardwood plank) with the PVA water mix. and the vac went to 21. I have no doubt that I could drill a PCB full of holes and the 1 CFM would keep the board in place.

I do plan to test this in the near future and will post my results in the PCB forum.

Jay

[yngndrw]

I would love to try a nice vacuum system but haven't got the money currently. I will defiantly look into that in the future though.

Thanks for the advice on the sealing, I will have to try that. I'm guessing that it would also be better to seal MDF when using double sided tape as well.

I'm looking forward to see your results.

A slightly different question, does "tramming" of the spindle matter when milling and drilling PCBs ? I only aligned mine by eye and it has been loctite'd so I'm not sure if it's worth re-doing it. I suppose I should measure it first.

[Jay C]

Not so much. The thing is, you skim your table with the spindle and the table is now trammed to the spindle. I do this every time I swap spindles. It is not required to seal the MDF for carpet tape.

[yngndrw]

Ah okay, thank you.

I just measured it for the first time and found that the Z axis was out by 0.2mm over 50mm of travel. I managed to adjust that to 0.08mm, however once I'd tightened it up again it was back to 0.13mm. I only adjusted the spindle mount as the Z axis to Y axis mounting is covered and this is probably out. The spindle mount is mounted on T slots so the T nuts pull it back out of line due to the whole Z axis being out. Might as well set it up as square as I can while it's not in an enclosure and 0.13mm over 50mm will do.

I don't know why I had it in my head that skimming the table doesn't fully tram the spindle to the table, but your post made me rethink this and realise that it doesn't matter at all so thanks. I would have been disappointed at the 0.13mm otherwise.

[Eclipze]

If you are just using tape, don't bother sealing the MDF. Otherwise it might make it harder to pull the tape off. If you do seal the MDF, level it after you have sealed it, as the water/glue will make it swell.

I use double sided tape over most of the board... gives nice uniform support and good grip for routing out the board profile. But I don't go overboard in covering everywhere. The important thing is for smaller board that you still use a big enough piece of PCB... so there is enough tape to make sure it doesn't move while cutting the board profile. To be accurate, I usually do two or three passes to keep the cutting load very low. Milling and drilling is no problem though.

I tend to have plenty of small MDF offcuts and do a lot of small PCBs. So when it comes to double sided boards I tend to go the extra effort to avoid alignment problems. I route the PCB blank out first. I then pocket into the MDF and push the PCB into the pocket. Then I drill, mill, flip the board and mill the other side. As long as you setup your cam right, you'll get perfect alignment for top & bottom milling

[underthetire]

I have used our New Hermes engraving machine to do PCB. It has a vac table on it, I think it would require more suction than a shop vac can provide. I think this http://www.harborfreight.com/cpi/cta...emnumber=98076 might work ok.

[Jay C]

Originally Posted by Eclipze If you are just using tape, don't bother sealing the MDF. Otherwise it might make it harder to pull the tape off. If you do seal the MDF, level it after you have sealed it, as the water/glue will make it swell.

I used sanding sealer first and that didn't stop the air pulling through the MDF so I applied the water PVA mix. That might be why I didn't see any swelling.

Jay

[yngndrw]

Thank you Eclipze that's great.

Also thank you for the heads up about vacuums, I could do with getting a refrigeration vacuum pump for refrigeration stuff but haven't got the money currently.

How deep does the PVA mix go into the MDF ? If seems counter-productive to seal it then skim it if the sealing layer is cut off as well.