PCB Workholding Problem

[djh82uk]

Hi Guys

Im having problems holding down pcb's

I have a Taig mill with a normal T-Slot bed. Now my problem is that I just cannot hold the pcb's flat enough. I spent ages getting it so that I can have known points on the fixture for doing double sided boards, but when I come to mill it, it is never flat.

At the moment I have a piece 1/4" alumimium that I have run a surfacing gcode on with an 8mm end mill, I then used the mill to mill holes that I later tapped around the perimeter of where the board would be. I then made 4 strips of 1mm thick aluminium with holes in the right places. So I place the board, bolt down the strips and it's always out by about 0.002 - 0.003", it's higher in the center as if it is buckling. The board are not really flat when I get them, but I was hoping the strips would help flatten things out.

I also had to cut a section out of the baseplate as my pcb micro drills kept snapping when going through the board.

My initial plan was to make a plate out of 2 pieces of the 1/4" thick ali, a top and bottom piece that had a recess of half the thickness of the pcb's on each half. i would then bolt these together, but my mill's Y axis travel is not big enough due to the column.

I then though to make the strips onto one rectangular piece but faced the same problem to a degree plus I ran out of the alu sheet.

So before I buy more, does anyone have any suggestions?

I think it may be buckling due to the fact that there should be something the same thickness as the pcb on the outer edge of the strips, but again due to my travel issues, the surfacing gcode was not able to reach there, so it is difficult to guage what thickness that should be.

I have also though about making the strips out of the 1.4" thick ali, perhaps in two parts.

I have also thought about vacumn holding, but when drilling/milling the pcb's i would be damaged the vacumn bed all the time.

Please, any advice appreciated

Regards

DJH

[Bubba]

DJH,
I agree vacuum is the way to go. I read one persons way to NOT gouge the surface was to drill the holes half way through from the bottom and then before flipping to the second size, he would put on a piece of self adhesive paper to prevent loss of vacuum when he drilled the rest of the holes through. This way, the drill never drilled the vacuum plate.

[neilw20]

METHOD 1:
Use a piece of 1/4" fine MDF.
Rout a vacumuum grid in it. I assume the machine is CNC? It doesn't take very long with a stubby broken 1/8" drill bit router (or aircraft type panel drill bit for off the shelf) to make some vacuum grooves on a 1" grid.
Drill just through with normal PCB typ carbide drills.
They have a very flat tip angle, so it takes a while to destroy table.
You could put some vacuum groves under the MDF to match your table to keep the MDF flat or use super glue in a few spots.

METHOD 2:
PCB manufacturers often place a piece of scrap PCB top and bottom so that break through and edges stay perfect.
PCB drilling machines/routers have an arrangement on the drill head that presses down on the PCB near the hole being drilled. They often only locate the PCB with a few pins and a bit of tape. All the Z axis control is done by the head. An array of three or 4 castering sping loaded rollers will hold the PCB down for routing.
4 small air cyclinders (10mm OD) with off centre rollers running very low pressure will work nicely and they are easy to retract.
This is much less work than vacuum method. Vacuum systems and fiberglass dust are incompatible anyway.

Now the only thing that sucks is the dust extractor.

[wimwam]

Hiya,

Since you are in the UK check out the Mega UK website http://www.megauk.com. They sell a PCB drilling exit board. Cheap. It seems to be made of SRBF and is glazed white both sides. It is about the standard pcb thickness.

I am also looking at the holdown problem. If a get a cheap foolproof solution in the next few days I will post.

Martin

[abasir]

Will something like this works for you? It's basically a non-permanent sticky pad (think Post-It note) meant to hold down plastic sheet for engraving...
More details here

[pminmo]

CA glue. A few drops of thin CA will hold the board. To release it use acetone which will soften the CA. Since you have a mill the ideal solution is a piece of AL plate that is either drilled or milled with grooves to only leave a percentage of surface to bond. Put index pins so you can repeatedly orient the board correctly so you can flip to do double sided boards. Drilling can be accomplised by scap pcb in between the plate, and since it all aligns with index pins and is repeatable, you can do it in different sessions if you like.

[keebler303]

I surface a piece of MDF and then stick the board to it with good quality crepe paper double stick tape. If you get some good tape. mine is from Spec Tape, it will hold like hell but still releases easily with a hair dryer on it. The adhesive softens from the heat and you can pop it right off with the corner of a putty knife. The mdf can be reused over and over, until there are too many holes in. Then you can just face off another .030" and you are good to go again.

Matt

[yuliany]

Originally Posted by djh82uk Hi Guys Im having problems holding down pcb's I have a Taig mill with a normal T-Slot bed. Now my problem is that I just cannot hold the pcb's flat enough. I spent ages getting it so that I can have known points on the fixture for doing double sided boards, but when I come to mill it, it is never flat. At the moment I have a piece 1/4" alumimium that I have run a surfacing gcode on with an 8mm end mill, I then used the mill to mill holes that I later tapped around the perimeter of where the board would be. I then made 4 strips of 1mm thick aluminium with holes in the right places. So I place the board, bolt down the strips and it's always out by about 0.002 - 0.003", it's higher in the center as if it is buckling. The board are not really flat when I get them, but I was hoping the strips would help flatten things out. I also had to cut a section out of the baseplate as my pcb micro drills kept snapping when going through the board. My initial plan was to make a plate out of 2 pieces of the 1/4" thick ali, a top and bottom piece that had a recess of half the thickness of the pcb's on each half. i would then bolt these together, but my mill's Y axis travel is not big enough due to the column. I then though to make the strips onto one rectangular piece but faced the same problem to a degree plus I ran out of the alu sheet. So before I buy more, does anyone have any suggestions? I think it may be buckling due to the fact that there should be something the same thickness as the pcb on the outer edge of the strips, but again due to my travel issues, the surfacing gcode was not able to reach there, so it is difficult to guage what thickness that should be. I have also though about making the strips out of the 1.4" thick ali, perhaps in two parts. I have also thought about vacumn holding, but when drilling/milling the pcb's i would be damaged the vacumn bed all the time. Please, any advice appreciated Regards DJH

Dear friend,

A new clamping technology based on adhesives that can fill the gaps you don't need good surfaces, you can fix your PCB with a tensile clamping force of 300 kg/cm2.
Please find it at: www.mts-rnd.com

Yulian.

[neilw20]

Make a spring loaded pressure foot which is a ring supported on 2 legs, so that when the Z comes down the pressure foot holds the PCB down just before the drill or the cutter touches. Now you ask how can I engrave with the pressure foot against the PCB? It's not against the PCB.
The bottom of the pressure foot has some disposable brushes attached which do all the pressure work, and can even be the springs.
Cut up tooth brushes work really well. Now while you are routing the brush just slides around on the PCB holding it down perfectly.
When changing directions, lifting up at a corner then going down again will help the bristles settle nicely (if need at all).
All you you need in the PCB is one alignment pin hole, another at the edge at the end somewhere and a few bits of tape.
MDF or a an old scrap PCB is perfect underneath.
If you get carried away, you attach a vacuum hose to the ring, and a broken drill detector opto beam.

It took longer to write this than make one (almost)

Neil.

[pminmo]

Simple see: http://www.millpcbs.com/index.php?page=liquid-holding

Results see: http://www.youtube.com/watch?v=lhdqBrciQZU

[yuliany]

Originally Posted by pminmo Simple see: http://www.millpcbs.com/index.php?page=liquid-holding Results see: http://www.youtube.com/watch?v=lhdqBrciQZU

Dear friend,

Please visit at: www.mts-rnd.com

NEW CLAMPING TECHNOLOGY.

Yulian.

[samco]

I use vacumm also. (scarifical wood top)

I have heard that double sided carpet tape works well also.

sam