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I get only partial vacuum holddown when using the UHMW fixture shown without the cardboard shim.
When using the cardboard shim I get full vacuum holddown up to 20"+HG.
I don't understand why the vacuum doesn't grab as tight without the shim.
I thought perhaps the wood blank is deforming without the shim but that doesn't seem to be the case.
The entire fixture has been machined flat as installed.
Any thoughts, guys?
Thanks
Nelson

Nelson. are you sure the blanks not deforming and creating a leak? Technically it has to deform, right?
It only take a little gap to create a large leak as I'm sure you know.

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Jim Watts
http://jameswattsguitars.com

Jim--I layed a straight edge across the top of the blank in an attempt to detect any deformation but couldn't see any.
It is possible, though, that it's deforming a little as you mention.
After posting this query, I got to thinking that maybe the UHMW fixture is deforming without the shim. I need to check that but it's hard to do when covered up by the part blank. May need to add a couple hold down bolts out in the middle area of the UHMW as all I have now is a bolt in each corner.
The blank is about 1 1/8" thick whereas the UHMW is only about 7/8" thick so seems logical that it may be sucking up out in the middle and causing vacuum leakage.
It's beginning to sound like I'm trying to answer my own question here but I do appreciate any and all input. Maybe someone else has experienced this?
Thanks
Nelson

Nelson, try a small shim, like a washer or something, in the middle of your fixture and see if that helps to hold things with less defelction. If it works then superglue the washer down and you are good to go.

Shane

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Canada

Nelson, If you push down on the edges above the gasket does it seal?

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Jim Watts
http://jameswattsguitars.com

Nelson the idea that James had about a washer in the middle gave me an idea. Could this problem you are having have anything to do with too little vacuum volume area? I have seen several vacuum fixtures that seem to have deeper slots, and multiple vacuum holes.

I am speaking from seeing....not doing, so keep that in mind!

Mike

OK guys, thanks for the replies!

The seal I'm using is 1/4" diameter foam with approx. 40% of it protruding above the UHMW fixture plate.
The vacuum pump pulls about 5CFM with about a 5 gallon reservoir.
I'm running a 1/4" ID hose about 6' in length to the fixture.

Shane, I tried the washer idea using a couple different thickness of washers but didn't seem to help.
Jim, I'm not able to push down, at least not hard enough, to make it seal without the cardboard shim.
Mike, your question about too little vacuum area makes we wonder but it does seem like with the seal protruding about .100" above the plate that the vacuum would apply to almost the entire area of the part blank and immediately suck it down.
I get a good enough seal with the .045 cardboard shim to hold about 20"HG.

Thanks
Nelson

Dumb question...but what are the dark spots in the middle of the jig? I assume one of them is the inlet?

In general, you should not bolt from inside the vacuum area to the mounting plate as you'll basically be making another way for the vacuum to escape. If you're worried about flatness, bolt from the plate into a blind hole on the bottom of the vacuum jig.

Have you tried any other pieces of wood? Maybe there is something screwy with that piece you have...

Nice looking jig though!

Trev

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http://www.PeakeGuitars.com

I forced the work blank down with a heavy gobar and found that the vacuum then "grabs" and holds it down tight.
The vacuum is then maintained after the gobar is removed.
Apparently the vacuum hose doesn't let enough volume thru to grab the large part without assistance.
The length of the 1/4" diameter seal is about 34" so I suppose there is a fair amount of leakage before the foam is compressed.
The reservoir figures about 10 gallons capacity rather than the 5 I mentioned above.
All thoughts and suggestions very much appreciated.
Thanks
Nelson

One last thing you should try. Add some self adhesive abrasive strips. I use 100 grit. These help hold the part steady and give the vacuum somewhere to "live".

Trev, the dark spots are blind threaded holes that I used to mount a temporary fixture one time.
The inlet is seen at the left end in the top photo.
Thanks
Nelson

Hi Nelson,

I have a couple more thoughts. Is the foam made for vacuum use? I tried some once that looked like it would work, but the vacuum actually pull through the foam.

Second.....is there any chance you board is ever so slightly warped?

I will make a deal with you. Send me ....... oh 9-10 of those junker blanks of fiddle maple, you know for testing, and I will figure this out for you!

I do hope you get it correct, because I know the feeling all too well of watching a prime piece of wood get prepared for the scrap pile!

Mike

Mike, you're too kind!
The foam seal that I'm using came from a vacuum clamping supply hose.
It is a fairly "open cell" and I suppose that is a happy medium between too much "closed cell" to compress and "bite" and too open to prevent leakage when compressed. And all the above probably hinges on how much of the seal is buried in the groove and how much is exposed above the clamp surface
I've tried several different seal materials over the years including plastic tubing.
I think my issue with the workpiece not wanting to bite is due to the 1/4" hose not "passing gas" quickly enough. (air is a gas, isn't it?)
May change the valve and line to 3/8" and see if that doesn't faciliate the clamping "bite". Using a gobar isn't that big of a deal though actually.
I can't pry the workpiece off even with a cheater bar once the vacuum has "bit".
Just having way too much fun!
Nelson

Edit: Forgot to mention in regard to your question of whether the piece is flat----I use a 24" disc sander to sand the clamping surface flat as possible but it's not perfect if we start talking thousandths.

Jim--I'm not sure that I understand your comment about the blank having to deform.
Can you elaborate on that?
I can see where it would if the seal material was stiff enough to keep the blank from sucking down tight against the fixture plate.
I have tried vacuum clamping a rough carved (inside and outside) plate using controlled vacuum and that can sure introduce distortion and cause vacuum loss.
Thanks
Nelson

A reservoir can work for or against you. It maintains the status quo which, if you missed the initial 'grab', is low or no vacuum. Remove the reservoir from the system, hold the blank down well, and see how long it takes to grab.

The cardboard might be covering a leaking hole up long enough for the vacuum to stick?

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Nelson, what I was getting at, is that unless the groove that you're gasket is setting in has room for it to collapse into it will leave a space between the bottom of you're work piece and the top of you're fixture. The pressure created by the vacuum will flex the work piece down (or the fixture up) until the two touch deforming your nice flat surface into a curved surface, possibly creating a gap at the seal. I'm not really saying this is your problem however. It would take an extreme gap and a flexible material to create this situation, it could happen however.

As a diagnostic, you might try taking something pore free, flat and stiff like an aluminum plate, perhaps a laminated countertop or such and try to suck it down. If it sucks down you probably have a problem with your work piece, if it doesn't your leaking in your fixture.
jim

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Jim Watts
http://jameswattsguitars.com

Do you have a Fein vac? They have gobs of volume and hold plenty tight to secure large pieces.

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tim...
http://www.mcknightguitars.com

The thing that I have found critical with vacuum fixturing is to have plenty of flow with absolute minimum pressure drop. As I am sure you know, you have very low PSI so you loose flow very quickly if you have an pressure drop at the air flow. Even pulling air through parts can be an issue as they get thinner. I tend to have fixture channels of .25" width on aobut 1" spacing in a grid. I have a master vacuum plate that my "daughter" fixtures sit on. The master plate is laid out into zones each with a vacuum hole in it. Also, the master plate has registration pins sticking up so the daughter fixtures always go on the master plate in exactly the same location. I set my machine zeros to the registration pins on the master and then the router "knows" where the parts are. Hmm, sorry for the slight diversion. The master plate zones are each gasketed and depending on the size of the daughter fixtures I "cork" off the unused zones with 1/4" rubber cork devices from Thermiwood. On a fixture like you are showing, I would end up with probably four vacuum ports feeding it and there would be a lot more air channels. My master plate has a 5/8" ID hose feeding it from roughly a 10 gallon tank. This way, I minimize the losses and when I turn on the vacuum flow, it can handle a quick high volume, low pressure air surge and will suck a part down and compress the gasket. Also, around my gasket groove, I have small reliefs so as a gasket compresses, the gasket cant pinch under the part.

Good photos would help.... but we have been traveling the last few months and won't be back home until December. I checked my laptop and these are about the best photos I have with me. Hope this is of some help...

Jim--Thanks for the suggestions on a flat mockup for testing

Tim--I do have a Fein but never dreamed it would have enough "suck" but will certainly give it a try.
I have two Feins actually with HEPA filters and love their low noise level.

Bob--I didn't realize that the reservoir could work against me but can sure hook it direct and compare the grab.

Terry--I've often wondered if I should have more vacuum channels and I can try that also.
Multiple inputs would be worth a try also.
I had always assumed that since the workpiece was initially sitting about .1" above the gaskets the total area should suck and pull down.
That may be assuming too much. (ASS U ME )

Keep the good ideas coming, guys. Much appreciated!
Nelson

Bob--I've been thinking more about your comment regarding the reservoir. I'm having trouble wrapping my little brain around why the reservoir can be a negative. (pardon the pun) I was thinking that the outrush of negative air pressure would be aided by the reserve in the tank if it temporarily exceeded the pump output.
I do sometimes have a little trouble analyzing vacuum vs positive pressure. Have to think backwards, I guess.
Is there any advantage to having a reservoir other than maybe acting as a temporary backup if the pump should quit in the middle of a job?
Thanks
Nelson

One other thought - at PRS we used to have to use a big rubber mallet to "coax" some bodies onto the vacuum jigs. They don't lie flat initially...you have to use the mallet to set them well onto the jig and to allow the vacuum to grab them. On thin parts that easily deform, you don't have this kind of issue as they conform just due to the vacuum pressure.

edit - maple is particularly prone to warping, mahogany not so much...

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http://www.PeakeGuitars.com

Was that an index pin fit/alignment issue or a flatness issue? We run into that. Less so lately.

[quote="terrymcc"] On a fixture like you are showing, I would end up with probably four vacuum ports feeding it and there would be a lot more air channels.
quote]


Nelson this quote I clipped from Terry's post is what I was talking about in my first post about not enough volume.

My offer to help was strictly from the heart!

Mike

ps what is a Fein? Std type pump, high volume????

http://www.mcfeelys.com/product/FPT-800/Fein-Turbo-Dust-Extractors-HEPA-Filter

Mike--The Fein is a high priced shop vac.
I'm convinced that you guys are right about not enough volume.

Lots of info on Fein vac on the ShopBot forum:

http://www.talkshopbot.com/forum/messages/28/38128.html
http://www.talkshopbot.com/forum/messag ... 39359.html
http://www.talkshopbot.com/forum/messag ... 39924.html
http://www.talkshopbot.com/forum/messag ... 28690.html

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tim...
http://www.mcknightguitars.com