Vacuum Hold-Down Fixtures

[Bruce Johnson]

I wonder if silicone from a tube could be used to make a seal around the perimeter. You’d want to go over it with a purpose made “tool” to provide the optimal height (think grooved trowels used for ceramic tiles for inspiration).

Yeah, I've thought about that as another approach to the sealing. Rout a groove around the perimeter, then fill it with a bead of silicone sealant taller than the surface. Then push it down to almost flat with the surface with a sheet of aluminum and clamps as it dries. That might work. My biggest hesitation is getting sticky silicone residue on the back side of a fingerboard. That could be a problem when gluing it.

 

[REV]

Do you know if the MDF is flat? Any gaps between the surface of the fingerboard and the surface of the MDF fixture will reduce the effectiveness of the vacuum. Also you will be pulling on the fingerboard and conforming it to the surface of the MDF. I'm assuming that the purpose of the fixture is to either flatten the fingerboard or to produce a fingerboard with a radius, right?

 

[Rabidhamster]

the DIY vacuum hold down jigs I remember seeing all had smaller holes, like an air hockey table with bad pores.

Or they have the grid patterns which I've never really understood

 

[Bruce Johnson]

The first thing that comes to my mind is that MDF is actually quite porous. Air goes right through it when there is a pressure difference present across it. The factory top and bottom surfaces are heat sealed, but air will pass through the cut sides. You will have to heavily seal the edges, or use a different material.

You can, however, because of this, probably use MDF as the top surface of your vacuum jig, to "spread" your vacuum – as long as you machine the factory surfaces off the top and bottom. It's a fine choice as a spoil-board material anyhow, so it would seem to be great in this role.

Below the MDF spoil-board/vaccum "spreader," I would think about a fine array of holes or channels, rather than just a handful of 1" ones. You can get perforated metal sheet stock from McMaster-Carr, or use a table saw to cut a series of channels.

You could use loose sheets of thin neoprene to seal the areas that are not covered by the work, as needed (also available at McMaster-Carr). Theoretically, the vacuum will hold them in place, wherever you choose to place them.

Also, I looked up that air pump, and it pulls only 3.5 CFM. While I don't intimately know the details of vacuum engineering, I did have to learn basic principles of fluid dynamics, for my first career. And my gut says that 3.5 CFM just doesn't seem like enough flow to me, for an object that size. I believe you will have to upgrade your vacuum pump, or use it to experiment with smaller vacuum jigs.

Thanks, that's all very interesting.

Maybe the leakage through the MDF is really my biggest problem. I'm definitely going to try sealing up this first fixture completely with polyurethane before I start experimenting with rubber seals.

I like the idea of a cluster of large 1" holes, rather than an array of small holes or a fine mesh. I think that gets me more surface area, and more force pulling down. But that is an interesting thought about using strips of MDF turned edge-grain.

I think my vacuum pump has enough CFM. Rob Allen's vacuum setup uses two of the little diaphragm-type pumps. I think the two of them together are less CFM and vacuum level than mine (which is an oil-filled rotary vane type pump). Rob's fixtures hold down bodies and necks with no problem. He said that the reason he put the two pumps in parallel is to speed up the time it takes to lock the parts down. But his fixtures don't leak like mine. They have Viton O-ring seals in grooves around the parts. Most of the bottom surface of the part is active surface area being pulled by the vacuum. Some of his fixtures are machined from phenolic sheet. Others are made from MDF, but are heavily sealed with polyurethane. I should have paid attention to that.

 

[ardgedee]

MDF is chunks of wood, glued together. I would think, like Asphalt, it would leak air - I don't think there's enough glue in there to make it air tight. You can probably decrease the leaks through the MDF with a decent finish - poly or something like that.

That's one thing I was wondering -- whether the MDF itself is porous. And whether you could fix it with any old paint (polyurethane, latex, whatever) as long as the entire air passage was coated.
(edit: Oh dang, Bruce came to the same conclusion while I typed that.)

 

[Bruce Johnson]

Do you know if the MDF is flat? Any gaps between the surface of the fingerboard and the surface of the MDF fixture will reduce the effectiveness of the vacuum. Also you will be pulling on the fingerboard and conforming it to the surface of the MDF. I'm assuming that the purpose of the fixture is to either flatten the fingerboard or to produce a fingerboard with a radius, right?

Yes, that was one of the first things I checked. I ran a flat sanding block over the top surface to make sure there weren't any fuzz or lumps sticking up. And the fingerboard blank that I'm using for the test is nice and flat, right from LMI. They seem to be tight surface-to-surface.

Yes, if there's a small amount of warpage or cupping of the fingerboard blank, I expect (want) the vacuum to pull it down flat.

When I get this fixture figured out and working, I'll make up several of them. Different lengths and widths of fingerboards. Some to hold bare fingerboards in my router radiusing fixture, others for the fret slotting saw.

 

[fdeck]

I can't tell the type of pump, but it looks like it has a sight glass, so I'm assuming it's oil bathed. Those pumps can produce an oil mist coming out the exhaust, that's not good to breathe. When I worked in a lab with several of those pumps, we had to vent them to the outside.

 

[Bruce Johnson]

I can't tell the type of pump, but it looks like it has a sight glass, so I'm assuming it's oil bathed. Those pumps can produce an oil mist coming out the exhaust, that's not good to breathe. When I worked in a lab with several of those pumps, we had to vent them to the outside.

Yes, this is an oil bath, rotary vane pump. I had to fill the main chamber with a special oil when I unpacked it. The first time I started it up, it blew a puff of oil smoke for a second or two. But now I've had it running for ten minutes or so, and it doesn't seem to be emitting oil mist or any real odor. The exhaust port does have threads to fit a large vent hose, if I have a problem.

To start out, I plan on keeping the vacuum pump portable. Once I get the fixtures figured out, I can see using them on 4 or 5 of my machines, and a few specialty bench routing fixtures. Normally, I'd set the pump on the bench next to the machine I'm using, and plug it in.

If the fixtures work really well, and I'm using them a lot, I could mount the pump in a corner and plumb rubber hoses around the shop to the different machines. Or, buy a couple more pumps. They aren't expensive.

 

[rudy4444]

Since we're discussing vacuum systems, in some cases a user may want to consider the venturi units which produce vacuum by using your existing air compressor system. They are widely available, and how they produce high vacuum from an air compressor is explained Link Removed.

I'd consider them a poor choice, since small vacuum pumps are easy to source and the venturi vacuum is a noisy solution. They may be a good option in some shops, though.

 

[Beej]

This is great stuff and I'm watching with excited interest, hoping to glean enough to adapt some things and improve a few operations of my own. There were several vacuum fixtures in the old cabinet shop I used to work in back in the early 90s but I didn't work in that area, so they've remained a little unclear. This is making all sorts of things come back from the recesses of memory.

 

[Arie X]

fwiw, ime, aluminum plate and gaskets/o-rings are the way to go or you need to completely seal the MDF. otherwise you'll be chasing annoying leaks all the time.

McMaster sells an o-ring bonding kits to make any size seal you want. for example:

McMaster-Carr

Phenolic, G10 and Delrin are other material options that are a lot flatter than MDF.

yeah, that oil mist is an issue. it could migrate onto bodies and other instrument parts causing finish problems. one solution is to run the exhaust through a fiber micro filter and change it out often.

at the day job we've tired these guys to machine golds alloy braze foil to various shapes. made in Germany from a machinable ceramic at around 3K each, you have to seal off the areas that are not in use with a rubber membrane. i can't say just how much these suck for this application -and not in a good way. pulling 15 in/hg of vac, they won't even hold down an .008" thick piece of foil:

 

[Bruce Johnson]

An update on the continuing R & D:

Well, I slopped polyurethane all over that first fixture, edges, surfaces, pockets, everywhere. Two heavy brushed coats.

No difference. It still barely pulls 2 in-hg. So, leakage through the MDF isn't the big problem. I guess that indicates that the leakage is through a poor seal between the fingerboard and the MDF.

Next up, I'm going to try the soft round O-rings idea.

 

[MVE]

Bruce,
Maybe you could try wiping the thing with vasaline and using a piece of scrap as a test.
Then you would at least know for sure that the seal is the problem.
If it is the problem, I still think a flat 1/4 inch sheet of hard rubber would be your best bet for making that seal. I can’t imagine it being too squishy to route on.

 

[Bruce Johnson]

Bruce,
Maybe you could try wiping the thing with vasaline and using a piece of scrap as a test.
Then you would at least know for sure that the seal is the problem.
If it is the problem, I still think a flat 1/4 inch sheet of hard rubber would be your best bet for making that seal. I can’t imagine it being too squishy to route on.

The point of the O-rings is that they are set into a groove, leaving maybe 0.030" sticking up above the surface of the MDF. And they'll be soft, probably 50 Durometer. When the vacuum is applied, it'll pull the fingerboard down into hard contact with the MDF, squishing the O-ring down into its groove. So it will be sealed and have hard contact.

That's the way Rob's fixtures work. He's using a long Viton O-ring cord going all around the outside of the part, about 1/2" in from the perimeter. He uses his CNC machine to cut the groove around that path, to an accurate depth that leaves the O-ring sticking up about 0.020". The vacuum pulls the wood part down into solid contact with the fixture surface, squishing the O-ring into its groove. I think his fixtures pull down to about 20 in-hg.

I know that design works. I'm trying to come up with something simpler to make.

 

[MVE]

Yeah you definitely need some sort of gasket, or maybe something with a surface flatness tolerance better than the mdf, something like glass, but good luck finding a piece of glass the right shape with a bunch of holes in it.

Have you tried a liquid gasket?
Maybe all you need is to spray some soapy water on there to help make a seal.?.?.
Like, you have to wet a suction cup before it will stick.

 

[Bruce Johnson]

Yeah you definitely need some sort of gasket, or maybe something with a surface flatness tolerance better than the mdf, something like glass, but good luck finding a piece of glass the right shape with a bunch of holes in it.

Have you tried a liquid gasket?
Maybe all you need is to spray some soapy water on there to help make a seal.?.?.
Like, you have to wet a suction cup before it will stick.

I'm sure I could squirt a bead of some kind of liquid gasket (automotive Form-A-Gasket?) and it would seal and work....but I don't want to have to do that on every part, and have to clean off the back of the board. It wouldn't be worth the trouble or time. I need a seal that's dry, doesn't mess up the board, and attaches and releases in seconds.

 

[MVE]

I'm sure I could squirt a bead of some kind of liquid gasket (automotive Form-A-Gasket?) and it would seal and work....but I don't want to have to do that on every part, and have to clean off the back of the board. It wouldn't be worth the trouble or time. I need a seal that's dry, doesn't mess up the board, and attaches and releases in seconds.

Yes, that’s why I suggested just try spraying it with a mist of water, or soapy water. The water might act as a gasket the way it does for a suction cup.
(Note, Im sure you probably already know this, but mdf hates water. So, if it actually works, you would need to seal it up with something, or it would turn to mush after a few months of use.)