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Vaccuum fixtures

Posted by Anonymous User 
Anonymous User
Vaccuum fixtures
October 16, 2008 10:13AM
Wow things have been slow around here! I thought a nice lively discussion about vacuum fixtures might liven things up?

To date I have been using 3M double sided scotch tape to hold down my PCB stock to a sacrificial piece cast acrylic that is bolted to the table. This works OK, but is is sometimes hard to keep it stuck down evenly all the way around. I have seen pictures of vacuum fixtures that some folks have made to hold down the boards and see some commercial machines even employ a presser foot to push the PCB down.

What do you guys use? I like the vacuum fixture idea as it is simple but it requires a vacuum pump. I'm not to keen on the presser foot as it seems that it would be pretty easy to scratch up your board that way. I curious how everybody else holds their stock in place.
Anonymous User
Re: Vaccuum fixtures
October 29, 2008 03:50PM
Hi Jeff and all, new guy here
I have also pondered the same question. I built a router table for exactly that, but use it for other things also
like you I use bouble sided tape to keep the PC board down and fond it to be the most flexible way of working. I also had had problem with keeping the precise distance between the cutter and the PC board. I resolved the problem by using a foot as you mentioned. Also the depth of the cut is adjusted by foot position and not the Z axis, getting good results that way. Had scratches also got away from that by adding a nylon trip on the bottom of the foot or using a plastic scrubbing pad to remove them. So I'm happy with that.
Many times I've thought about building a vacuum table, but keeping it from getting chewed up when cutting out the perimeter of the board, hole drilling and masking holes keeps putting me off. Maybe other people have had more success. Read elseware that someone was using soft plastic to plug up the holes that are not covered and a sheet of protective material between the PCB and the vacuum table, there would probably vacuum leak problems.
In building one
How many holes per square inch would be needed?
What size of hole should it be?
Would the surface have to me machined to maintain vacuum?
Re: Vaccuum fixtures
October 31, 2008 11:17PM
I use vacuum to hold all sorts of jobs that I cut so because I already have a vacuum source it follows that PCBs are held with it also.

Making a vacuum hold down is easy. I have several different types for various purposes. The bigger part and expense of it is a vacuum source.

Naturally I rout the traces before I drill. For traces the surface must be very flat, as you know, routing only 3 or 4 thou deep.

Vacuum does this very well. Providing the vacuum table is flat.

For drilling even though the vacuum gets less as more holes are drilled, it does not take much holding down and flatness is not so important.

If I did not already have a vacuum source then most likely I would go with the "super glue / acetone" method.

Greg
Anonymous User
Re: Vaccuum fixtures
November 03, 2008 11:26AM
Hi Greg
Do you have a picture of what your vaccum table air channels or holes look like, how do you prevent damge when drilling
I also route first then drill
Thanks
John
Re: Vaccuum fixtures
November 11, 2008 08:14PM
Sorry I have not replied earlier. The traffic on this forum is a bit light. smiling smiley

The way I do it is probably a bit different to most others. I have no seal on the plenum (correct name?)

I just grab an offcut of acrylic or HDPE or anything else lying around and mill a recess 1mm deep the size of

the board I wish to machine. Then cut a few quick air tracks and drill a hole. Fit a Tyco quick release and away I go.

The vacuum easily holds it down. It is dead flat because I machined the recess.

I don't care if I drill into it a bit. Makes no difference to how it works.

I now have a few of these in various sizes. They get a different size recess on each side.

Any boards I do that don't fit the size I just cut down to size after milling and drilling.

Once you have a vac pump you find all sorts of ways to hold things with it.

Having a CNC router makes plenum manufacture of all sizes and shapes really simple and quick.

Greg
Attachments:
open | download - PCB1.jpg (58.2 KB)
open | download - PCB2.jpg (69.6 KB)
Re: Vaccuum fixtures
November 11, 2008 08:26PM
Further to the above here is a link to a YouTube video I made,

[www.youtube.com]

This is that same plenum temporarily clamped to my bigger router.

I needed to show some interested people that a R&P driven machine can do smaller fiddly jobs too. smiling smiley

Greg
Anonymous User
Re: Vaccuum fixtures
December 09, 2008 03:58PM
Cool video!
That spindle sounds cool too, how many RPMs ?
Re: Vaccuum fixtures
December 17, 2008 10:33AM
That spindle runs up to 24,000 rpm.

Greg
Anonymous User
Re: Vaccuum fixtures
March 15, 2009 07:35PM
We've been milling boards without a vacuum table for twelve years, over a thousand boards. We do a lot of prototyping. We use the regular double sided Scotch tape #665. But there are a few issues to consider...

Issue 1) Keep the dispenser in a drawer. Not only does dust make it less sticky, but particles can cause real problems.

Issue 2) Use 0.031 board stock. The tape does will not pull the warp out of the 0.062 stock like it will with the thinner material. I get 0.031 stock from McMaster-Carr in 24 x 48 inch sheets and cut it up on a table saw using a narrow kerf blade. A typical board uses less than 50 cents worth of material plus about 50 cents worth of cutter wear ($4 to resharpen a 15 degree cutter)

Issue 3) Use a knife blade to lift it off, especially if it's a two sided job. If you're too rough you'll warp it. Also, don't use tape that is two aggressive. Again you'll wrap it getting it off.

Issue 4) I have a program which "planes" the sacrificial acrylic. That program also leaves a "fence" parallel to the x-axis so I can cut two sided boards in perfect registration without the need for tooling holes. My post processor puts a single 0.015 in. dia. hole at the origin (0,0). I use that to line up the second side. Works first time, every time! But the x-axis must be square with the y-axis otherwise the vias won't be in registration. I use 0.056 square vias and lots of them, so I need the holes to be centered on the solder side.

Issue 5) My post processor cuts the periphery twice - once before the traces and then again after the traces. The first time it cuts at the same depth it's going to use for the traces. This gives the operator a chance to adjust the z-axis as the tool makes one pass all the way around the board. If the cutter is dull that will show up on the test pass, also. After the traces are milled it does the periphery for real because if the cutter is going to slip in the collet it will do so on the more aggressive cut. I clean my collet with alcohol before starting a serious job. Fiberglass dust can cause the cutter to slip in the collet changing the z-axis.

Issue 6) If I have an IC with a really fine pitch, say 0.5 mm, I cut it as a separate daughter board with gold connecting pins. There are two reasons for this. First, I don't have to be as exacting about the z-axis and, secondly, I can reuse that daughter board on the next revision or next project. That's less soldering under the microscope. Eagle takes care of the bookkeeping for me. The daughter board is simply a different "package". I usually call it variant "A" for "Adapter".

Attached is an example of what we call a one-cup-of-coffee project. This board went from pencil sketch to a working board in about 45 minutes. Two sided; about ten vias; total cost - less than 60 cents.

So, yes, you can get good results without a vacuum table if you follow the above procedures.

-Charlie Faddis
Attachments:
open | download - 1cup.jpg (167.2 KB)
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