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Hey all, I've got some experience in making vacuum fixtures, but I still have a lot to learn.

This is the first revision of a fixture that I designed to hold down the top and back plates to machine inlays, rosettes, etc. I am using a cheap vacuum pump with it right now, but I have a venturi system that I will move over to when I get the right connections.

Anyway, the fixture pictured below isn't sealing at all, but I wanted to see if anyone here had an idea as to why.

It's made out of MDF, I've got quite a few coats of shellac on it. The vacuum goes thru a plastic line all the way to the port you see in the middle (if i put my finger over the port, i feel vacuum and the pump bogs down to indicate a good seal).

Possible issues that I see, let me know if you agree with any and/or all.

-MDF is basically trash with vacuum no matter how much you seal it.
-there is possibly a section of the fixture that is not sealed as well, thus leaking at that spot
-the total area of vacuum is too large for the pump to manage
-the overall design is terrible and I should feel very very bad



I feel like a plastic version of this might be better, but I don't want to spend more money and time on a design that sucks.

Any ideas would be very welcome, thanks!

_________________
-Marcus
http://www.bailieguitars.com
Instagram | YouTube

I do a fair amount of CNC work in support of my guitar builds but have not ventured into vacuum hold-down, though I have considered it. So take my comments on your hold-down issues as wild speculations based on no direct experience.

A few thoughts:
-- I've found it challenging to get an entire top or back to lay flat on my spoilboard. Might that be true for your vacuum hold-down. How do you get the seal to be "good" at all points? (even a quarter inch of blue foam that's not in firm contact with the guitar top (or back) will be a big enough leak that you'll never get suction.
-- Can you put another piece of MDF on top of the fixture and get good sealing? If not you'll never get a guitar top to "stick." If you can, can you put a guitar top on the fixture with the vacuum pump off, then lay a sheet of MDF on top of the guitar top, then turn on the vacuum pump, wait a moment, and then remove the MDF sheet.
-- Looking at your fixture, I'm concerned about details like the sharp edges of the blue foam where it bends around the corners at the verticals of the "D." That corner won't compress nearly as easily as the straight line or the gentle curve of the D. That means that the guitar top (or bottom) may rest on the folded corner and not compress the rest of the foam. And that means leaks.
-- Can you (dare you?) put soapy water or colored water on the blue foam, put your material on the fixture, turn on the vacuum, wait a bit, turn off the vacuum, remove the material, and see, from the pattern of the water, where the vacuum leaks are? As an engineer, I can't imagine that any significant volume of air is coming through the shellac-sealed MDF base. Of course, if your leaks are massive (and I expect that they are), this experiment will just make a mess and not tell you much.

And a question and a few of observations.:
-- how do you expect to use the fixture? For inlays or rosette channels, it should work fine (once the hold-down issues are resolved). But for soundholes, you'd best have very accurate (and uniform) thickness measurements for the guitar top because the vacuum fixture doesn't allow a spoilboard. I have had tops whose thickness was specified by LMI and other vendors, tops whose thickness was set by using a friend's full-width thickness sander, and tops whose thickness was set by me using planes, scrapers, and sandpaper. None were particularly uniform (the tops that I thicknessed were the least uniform of all; lack of uniform thickness seemed to have no negative impact on the tone of the completed guitar.)
-- I believe that leaks in a planar vacuum system come, almost entirely from the peripheral seal. The area under vacuum is not particularly important. In your fixture, the "D" foam in the lower bout and the ovalish shape in the upper bout may be causing some of your problems. If you want them as spacers to keep the soundboard flat from edge to edge, I'd suggest using only short pieces of foam in the straight and gently curving sections of your upper and lower bout shapes. It's all under vacuum. See if you can pull a vacuum with those foam sections removed.
-- I usually cut both the rosette channel and the soundhole after gluing a second layer to the back of the soundboard. This layer only extends a bit past the rosette. I don't know how that could be accommodated by a vacuum fixture.
-- I find that cutting on the soundboard is an important use of CNC in lutherie, but is probably only about 10% of the CNC work that goes into my guitar builds. I also use the CNC for cutting headstocks and headplates (some with bas-relief carvings), bridges, slotting fretboards (including fan fret), cutting bas-relief tail wedges, cutting mortices for bolt-on necks, and cutting nuts.
-- I use double-sided tape for most hold downs.

Just some ramblings, but they might spark some ideas for you. Your fixture is a beautiful piece of work. The next step is getting it to work. Best wishes.

I made a vacuum box with MDF and no amount of shellac would seal it. I finally got it to hold a vacuum coating it with epoxy. I had a good pump that could move a lot of air.

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http://www.Harvestmoonguitars.com

I couldn't get MDF to hold vacuum.

Pat

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Pat

Thanks for the feedback, folks. I might try to seal the MDF with some epoxy.

Hans, you've definitely given me a few things to think about. I did try the "sandwich" method you mentioned above with another piece of flat MDF on top of the test board, I even used clamps on all sides to ensure contact with all the seals. It still didn't work.

The idea behind the foam in the middle was to try and create balance in the "push back" on the work piece created by the foam. I can see now there may be some design flaws in the tighter radii.

I probably wouldnt use this specific fixture to route the soundhole for as soon as you puncture the soundboard all vacuum would be lost.

I will probably start with removing some of the inner foam cords and test again. After that, I might try to seal with some epoxy, although it will be a challenge to not clog my channels. I may just set the project aside for a bit too.

Thanks again!

_________________
-Marcus
http://www.bailieguitars.com
Instagram | YouTube

Probably just spraying with polyurethane would do the trick and be much easier than epoxy.

Mine's not as pretty as yours but works well.

Pat

_________________
Pat

Some woods are porous enough (like mahogany) that you can’t use vacuum to hold a thin plate.

I'm also designing an MDF vacuum work-holding system and I've picked up two important points from the treasure trove of older posts here:

1 - Rick Turner has talked about using MDF extensively for vacuum fixtures, but always using a specific type of thin epoxy for sealant.

2 - Bob Garish has emphasized that there needs to be lots of empty space below the workpiece. viewtopic.php?f=10106&p=553191&sid=99f2656118206ef81f1822be2976392e#p551026

Looking at your jig I think that you're only generating any atmospheric pressure directly above your grid lines. Following Bob's advice you might consider routing away the majority of your squares so that they don't contact the underside of your workpiece and thereby create vacuum pressure. Bob would say leave just enough support so that your workpiece does not deform.

Ditch the MDF. You might be able to get it to work with epoxy sealant but that's a pain in the butt. I use a lot of vacuum fixtures in my work and MDF really only works with flow through fixtures. Flow through fixtures also usually need high volume vacuum pumps so I don't use them very often. When I do use a flow through setup I seal the edges of the flow through with some sort of glue which doesn't seem to help at all. The link that Durero posted has a lot of info

The best stuff to use is foam tooling board, corian, or some other closed cell plastic or rigid foam.

I remember talking with Bob about the large vacuum volume and I've found that the opposite works better. You do need some high flow channels to help get to the extremities but I've found that minimizing the volume leads to better vacuum/hold down.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I briefly had access to a large format CNC with two massive vacuum pumps. MDF was used as a conduit for air to draw. It is the opposite of air tight. On my personal machine I use UHMWPE (the stuff of plastic cutting boards). I get off cuts from a local plastic supplier. It’s still not “cheap”.

Does 1” rigid insulation foam work? That machines well and costs much less. I should try that.


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I do a lot of prototyping, so making dedicated fixtures usually doesn't make sense. The times I've have made vacuum fixtures using mdf, I always sealed with epoxy and they worked fine. These are only for a few uses and not long term as that material is not very robust over multiple uses. Sealing the gasket channels is really necessary also. Pretty much seal as much as possible as mdf is very porous. The epoxies I've used are West System and even Z-Poxy Finishing Resin. Pretty much anything that will stop the airflow will work. Even super glue works, but that can get nasty due to the fumes.

Bob