Official Luthiers Forum!

First the set up. I'm using one of the vacuum generator kits you build from the Joe Woodworker site. It's fully assembled and will pull AND HOLD 22 inches of mercury.

I can't get the LMI clamp to hold the vacuum. I've taken the pisco connector out...and tested that....it WILL HOLD the vacuum with my thumb over it..so it's not that.

To LMI's credit their customer service was FANTASTIC! They are immediately sending me 3 feet of additional gasket material and another pisco 'just in case'. They think it's the gasket material. It seems too porous to me for the job. There is definitely a leak because my vacuum generator settles at 9 inches of mercury. Problem is that it keeps the venturi valve open...which cycles my compressor on way too often.

Anybody got any ideas?

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There is no difference between the man that thinks he can....and the man that thinks he cannot.

Those of you that read this post are not going to believe what I am about to say.

If I turn the guitar body over (on it's face - western red cedar) it holds the vacuum like a champ. As soon as I place it on it's back....won't hold the vacuum.

Is it possible the vacuum is pulling air through the pores in the wood? It's EIR. I know the glue joints are good. Another contributor might be that currently it's only sanded to 80 grit. I pre-sanded the top with 100 grit on a ROA before attaching to the rim assembly.

Crazy way to discover something like this.

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There is no difference between the man that thinks he can....and the man that thinks he cannot.

Do you have another piece of EIR you can try?

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Steve Smith
"Music is what feelings sound like"

Steve the answer is yes and I am on my way out to the shop right now to test it.

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There is no difference between the man that thinks he can....and the man that thinks he cannot.

Well the results are as follows:

I took 8 other back pieces of EIR out to the shop and tested them on the clamp. All of them leaked vacuum...some more than others.

The one thing I didn't do was sand to a higher grit...my thinking being that a higher grit level would give a better seal...and therefore reveal whether or not it was the pores in the wood or not. All my back sets are sanded at 80 grit. The test had one exception and it was a back set of EIR from LMI. It leaked too...but I don't know what grit level they sand up to.

There you have it.

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There is no difference between the man that thinks he can....and the man that thinks he cannot.

You can definitely suck air through wood. I learned about it when I was CA-filling some inlays (and found my headplate glued to the vacuum fixture...from the back!).

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Chris,

Try scraping the back with a card scraper then if that doesn't work, sealing the back with some shellac.

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Bill Hodge


One does not simply, own enough guitars!

It is not unusual for backs to be porous enough to leak vacuum. EIR has been one of the worst for me. I have occasionally used plastic wrap on the "upside" of the wood to seal it enough to hold on the clamp. Shellac is much more effective, but you must let it dry for far longer than you can imagine - at least overnight. I have suffered an unpleasant mess several times when the shellac in the pores remained gooey enough to suck through, even though the surface was very hard.

Epoxy could be used also, though I have not done so.

I am not sure why fine sanding or scraping would help with this issue - are you trying to fill the pores with dust?

Brook

Man that would really suck (pun in tended)

Regarding the scraper, filling the pores with dust, no! He mentioned that he had a surface finished to 80 Grit and I was picturing too rough a surface on such a porous material to seal properly. I always sand to 150 then scrape prior to vacuum clamping and have never yet had a problem with a seal. Not even on EIR ir Mahogany.

On a serious note however, does the bracing not cause some difficulty in trying to get the plastic to seal well enough? Seems as though that would be a very difficult process to get the plastic wrap into place and have it stay well enough while trying to add suction. I suppose if the jig is in the upright position and the suction is on while positioning the plastic that it would draw down. Did I just answer my own question? I'll have to try it next time when waiting is not an option.

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Bill Hodge


One does not simply, own enough guitars!

Chris,
I have had more problem with Mahogany than with EIR, but like everyone agrees, it varies from piece to piece. I just seal it with vinyl sealer (just my choice for sealing) and move on.

Chuck

I don't want to hijack the thread but wanted to ask a quick question since you folks are obviously using these clamps. I was initially impressed with these clamps and had thought about building one. Do you find it to be a very useful clamp or do problems like these limit the utility?

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Steve Smith
"Music is what feelings sound like"

Steve,
I built my entire vacuum system using the fixtures developed by Charles Fox as he uses in his shop. There's a great article on the subject by him in issue #88 of "American Lutherie" (the GAL publication). You can order it on the GAL website (www.luth.org).The LMI fixture looks handy, but is too pricey for me.

Chuck

Steve,

I built one of my own and it is one of the most used tools in my shop on the workbench as far as assembly and preparation in my own work flow process. This link has pics of the one I built. http://luthiersforum.com/forum/viewtopic.php?f=10101&t=21873 As per Chuck's statement, my system is set up much like that of Charles Fox's too with the exception of the clamp I built which replaces two of the jigs I used at ASL. Below is a pic of those two holders in his shop. Note the round one on the side of the workbench. If you have the space, these two are far less complicated to build than the one I made. Charles has developed an awesome work flow system during his 40+ years in lutherie. Oh I forgot to mention the two jigs are the one at the top of the pic and the one at the bottom.

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Bill Hodge


One does not simply, own enough guitars!

Guys I'm not to the pore filling stage yet..but that would most certainly fix the issue. I will try some Saran Wrap this afternoon on the inside of the body and just suck that down to seal it. That will allow me to do what I need and then proceed to pore filling.

With regard to the utility of the clamp I think it's the best money I ever spent. Once clamped you can tilt and rotate the body of the guitar in any direction you wish for access to any portion of the top, back, or side you wish. It's exactly what I needed for my style of binding channel routing and anything else that uses 5 sets of arms and hands.

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There is no difference between the man that thinks he can....and the man that thinks he cannot.

The article in Al by Charles Fox is what got me interested. Later I ended up with a good vacuum pump. I've already built a bridge clamp. This will go on my list next. I'll start a new thread if I have any other questions. Thanks!

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Steve Smith
"Music is what feelings sound like"

Regarding my plastic wrap tip - yes I only do this in the upright position! It does take some careful fitting to work around braces, and I forgot to say that you should use tape on the edges.

I almost finished my clamp. I have to counter sink the screws yet but wanted to see if it works. I figure it cost me about $6 in parts. I found some black pond liner at Home depot this morning that was the end of the roll. A 2' X 15' piece that I talked the manager down to $5. Seems like it will work. The frame is corian type stuff that I have . I think I'll finish the fridge compresser hook up now that I know it works. It was $20 and is surprising fast and strong and quiet as can be.

Chris did you make that frame out of corian?

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Bill Hodge


One does not simply, own enough guitars!

Chris, that looks good. What's the fitting on the right side for?

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Steve Smith
"Music is what feelings sound like"

Corian like stuff not the Corian brand. I went into a local place looking for some sink or range cut outs and they said I could have this pile for $40. I couldn't pass that up even though I don't have a good place to store it.
The valve is to release the vacuum. I bought some yesterday at the local Fish Farm Store. They're a $1.98 each.

Never mind Chris, I just re-read your post. Guess I better go brew up some coffee.

If you don't have a place to store that stuff just ship it to me and I'll store it for ya.

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Bill Hodge


One does not simply, own enough guitars!

On the valve thing, I can also use it to series up another clamp/jig if need be too.
I have to get in gear too, trying to figure out if I want to make a neck centering jig or to just center it the old fashion way.
I've never seen the the great rocky mountains except from 10,000 ft.