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Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:23 pm
by o2photo
I was requested for more info on the vacuum jigs we are using for repetitive parts. I'm pretty new at this, so take with a grain of salt. This is shown on another machine, but it's machine agnostic.

At my day job, I'm senior photographer and hack fabricator. I develop product holding fixtures for when we have to cycle through thousands of one type of product (fans, lights, door knobs, etc). We got this big nice 5x8 table a little over a year ago with a vacuum spoil board setup. It's nice for sheet goods down to 2x4' zones, but once the parts hit a certain size, tabs are a must, especially stuff that likes to climb the bit like polycarbonate.

But when we are making 100s of something in a run, cleaning off 100s times 3 or 4 tabs is a real pain. So I started developing vacuum jigs to hold everything down for full through cuts, no tabs. This wouldn't be necessary if the assemblers didn't treat the fixtures like disposables as I make them to last forever, but I'm glad for the opportunity to learn something new.

The first was a test run. I sealed the MDF as best I could with multiple coats of poly inside and out, but even with aluminum tape around the edges, it's still not as strong as it could be. However, it works. I chose this one because of the relatively large surface area being vacuumed (you'll see why in the other example). Leakage wasn't nearly the factor with this much area. Here is the assembled test jig.

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:23 pm
by o2photo
Mid Cut

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:24 pm
by o2photo
All done!

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:31 pm
by o2photo
So with that working, then came the real challenge, a 4" and up to 3/8" thick polycarbonate disc with 7 holes to route around. Very little area to pull vacuum.

So this time we used HDPE. I cut the plenum and the channel in the lower half. In the channel is rubber X-gasket available from McMaster Carr. I opted not to use the in-board foam used up top here as I wanted this to last as long as the jig as it would be largely inaccessible after assembly. A small notch and route out the center end for the barbed fitting is at the other end.

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:34 pm
by o2photo
Here is a shot fully assembled and cutting a sheet. I used locator pins to assure the same placement every time, and the file is written from absolute 0,0, so it's just load, Z and go. As you can see, these were some interesting parts to direct vacuum to, so I used 1/8x1/4 in-board gasket from AllStar adhesives rather than the heftier 1/4" width.

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:38 pm
by o2photo
50 small parts, all cut. Look ma, no tabs!

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:39 pm
by o2photo
50 parts straight off the machine.

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Re: Vacuum Hold Down for Repetitive Shapes

Posted: Fri May 05, 2017 10:57 pm
by o2photo
So, some more info/improvements for next time:

* All we had on hand when I did this was an oiled rotary vane vacuum (5 CFM) we use to de-gas urethanes and silicones. These work great for THAT and anything else you are at full vacuum fairly quickly. It holds the parts brilliantly, but with the leakage that is there, it creates a cloud of oil the entire time it's on. Not fun to deal with or replenish. With that in mind, we are going to go with a dry rotary vane like a Gast, probably in the 10 CFM range for this in the future.

* The barbed fitting is melted in place through an undersized hole. Works perfectly!

* Getting initial vacuum on 50 separate parts is quite the challenge, a real game of whack-a-mole. A vacuum accumulator could help provide a boost at the start, but that is complicated. I ended up using masking tape around the edge to start the initial vacuum. Next time, I would just cut a continuous channel on the outer edge and run more foam gasket. This would provide the initial seal on the full sheet until each and every piece achieved vacuum and would make starting it a lot easier.

A bit of work to get up and running, but it pays for itself in no time.

Re: Vacuum Hold Down for Repetitive Shapes

Posted: Sat May 06, 2017 5:23 am
by beltramidave
Thanks for the details, Marc. I may have to try to make one someday. I would mainly be interested something smaller, say 16x16 max. Coming up with a vacuum source is my dilemma. I have seen some single pod types on the web.

Dave

Re: Vacuum Hold Down for Repetitive Shapes

Posted: Sun May 07, 2017 9:21 am
by o2photo
You can do it out of one piece - much simpler in cutting - but you pay for it in having to drill a line to each piece and the extra hardware. It may be easier on smaller jigs to do this method:

https://www.youtube.com/watch?v=ryJ56ESAHZQ

The caveat to this is that you have to have a straight, relatively short path from the sides to all pieces or it won't work. And you have to be good at tunneling point to point with a drill.

It does make life so much easier in the end, though!

Marc