Vacuum clamping bridges

[Bob Gleason]

Just wondering whether to continue vacuum bridge clamping. When I vacuum clamp veneers in a large bag, my vacuum system quickly evacuates the air and shuts off. It will stayed sealed for at least a half hour or longer before the vacuum pump starts up again. When I vacuum bridges my pump never shuts off. It hovers around 22-23# of vacuum. The noise is annoying and I'm not sure the clamping results are any better than just using regular clamps. I've never really had any trouble with bridges coming off using my regular clamps, but the vacuum set-up is a little less dangerous. No clamps to fall over on the top or come loose. Just wondering if others are able to get the vacuum pressure high enough for their pump to cycle qnd shut down. I definitely have more line connections on my bridge clamping system, but have not been able to find obvious leaks. Thanks, Bob

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[simso]

I am a fan of vacuum clamping.

Just to clarify, you have a vacuum operated power switch, one that you can adjust for power on and power off?

Have you got a photo of your bridge clamp setup, I only have one vacuum line in use when bridge clamping, so curious about your multiple line comment

Steve

[Bob Gleason]

By line connections, I meant connectors required to reduce the size of my vacuum line down to the small size that LMI supplies with their bridge clamp. I've tried to make a clamp, but I am currently using a standard LMI vacuum bridge clamp. Yes, my system does have a sensor switch that controls the pump when it reaches the desired vacuum. I have asked this question on other forums, and so far, I've gotten no replies that say they have a vacuum system that will shut down during instrument vacuum clamping. Normally I do my other kinds of vacuum clamping in an outer room, so even if the pump continues to run it is not annoying, but the instrument clamping I do in my dry workspace. Regular bridge clamps have worked fine for 45 years, but I like the vacuum technology, if I could just get it to work right. May not be possible.

[Allen]

I use a vacuum clamp for the exact reason of so much less likely to have a unfortunate ding from a slipping clamp. My pump will cycle on- off far more than other forms of vacuum work. Annoying yes, but my set up is in a dry room that I can close the door and not hear it.

[simso]

I have the lmii unit as well, i think your problem is you need a larger vacuum reservoir, i have three vacuum pump units in the shop, the generic one used for bridge clamping cycles on and off, its connected to a small air tank, 20 litres and then that connects to the clamp assembly, typically it runs for around 45 seconds and then switches off for about 4 minutes and then cycles on again

The one i have for vacuum clamping tops, or veneers etc, cycles around 6 minutes off 30 seconds on

Steve

[Steve.Toscano]

I started vacuum clamping bridges about 6moths ago. Circa 150 odd done so far in my workshop and wouldnt go back to clamps. Love it!!

[Bob Gleason]

Thanks for the replies.-Bob

[lamanoditrento]

We use a venturi valve with an air compressor at the BNE school, the air compressor is in another room but the valve still hisses. I cannot imagine going back to clamps. Mind you, bridge glue-up is with HHG and we only clamp for an hour and then not strung up for at least 24 hours. How long are you leaving it under clamp that the noise is much of a problem?

[Bob Gleason]

My pump is not all that noisy. It's just irritating. The process only takes an hour. Vacuum clamping is usually a very quiet process with the pump rarely running and it would be great if that can be achieved for bridge clamping.-Bob

[Bob Gleason]

Just to wrap this up. Never did figure it out. Returned my clamp to LMI. -Bob

[johnparchem]

a sponge gasket on 2.x mm thick wood is not as air tight as a vacuum bag. So sure, the vacuum needs to be continuously applied. Setups with reservoirs will cycle on and off as a lot of the time the vacuum is maintained by the reservoir.

[TallDad71]

Vacuums are powerful things. How do you stop the guitar from imploding? Do you put in violin style sound pegs?

[johnparchem]

Vacuums are powerful things. How do you stop the guitar from imploding? Do you put in violin style sound pegs?

The vacuum is not strong enough to rip through the top; it does pull the membrane down to the top. With regards to leaking the vacuum will pull air through the sound hole. If the guitar box is sealed it can implode.

[Bob Gleason]

John, I do have a reservoir on my system. The unit will quickly pull the air from a bag when I use it to veneer lap steels. When I stuck the LMI clamp on the back of an instrument, it would seal in about 20-30 seconds, shut off, and hold a vacuum. Just was not able to ever get it to shut off when used as a bridge clamp. I'm sure I'll go back to trying it with a shop made clamp when I get the time. There are enough builders doing it, that it must work. Just have to figure out what I'm doing wrong.-Bob

[johnparchem]

John, I do have a reservoir on my system. The unit will quickly pull the air from a bag when I use it to veneer lap steels. When I stuck the LMI clamp on the back of an instrument, it would seal in about 20-30 seconds, shut off, and hold a vacuum. Just was not able to ever get it to shut off when used as a bridge clamp. I'm sure I'll go back to trying it with a shop made clamp when I get the time. There are enough builders doing it, that it must work. Just have to figure out what I'm doing wrong.-Bob

Were you using index pins to locate the bridge? I missed that it clamped on to the back. Were there any holes in the top under the bridge?

[Bob Gleason]

John, I do normally use a couple of 1/16" pins through the saddle slot. I tried replacing those with tapered wood pins that I think would have plugged the holes, but maybe not. I realize now that I never tried the clamp on the top with the pins in and no bridge. Should have done that.-Bob

[jeffhigh]

I use 2mm side dot material to pin the bridge for clamping. You can install these to the soundboard with CA protuding 2-3mm with the upper end tapered slightly, These will not leak with vacuum clamping.

[kiwigeo]

My method for holding a classical bridge in place during glue up. I hammer matchsticks through a 2mm hole drilled in a cabinet scraper to make them round. 2 matchsticks goes into hole drilled through bottom of saddle slot and into top.

[johnparchem]

John, I do normally use a couple of 1/16" pins through the saddle slot. I tried replacing those with tapered wood pins that I think would have plugged the holes, but maybe not. I realize now that I never tried the clamp on the top with the pins in and no bridge. Should have done that.-Bob

I speculate as these things have happened to me. I do use index pins, I push them into the saddle slot and tap the saddle slot to seal. Some times with my falcate builds the lateral arch of the top is not maintained as there is not a lateral brace close to the saddle. I radius the bridge to spec, 10 M, and use the bridge as a brace to hold the 10 M radius. The index pins can still leak without accounting for what could be a small gap under the center of the bridge (it is also possible to suck a lot of the glue out.) I help seal the bridge by applying a bit of pressure to the center of the bridge to make sure it is sealed when I turn on the vacuum.

[Bob Gleason]

As soon as I finish my current projects, I plan to return to the bridge vacuum clamp issue. I would be interested in seeing, or getting a description of, any shop made vacuum bridge clamps that you have that work. Enjoy your building. Thanks, Bob

[ArmstrongLutherie]

A few years ago I bought the LMI bridge clamp and drove it with a very expensive (circa $1k) Festool vac pump. It worked great but there were a few things about it that I didn't like. After some thought, research, and several prototypes I came up with my own version. It is driven by compressed air via a venturi and if you have a basic workshop air compressor you're in business.

We tested the clamp over several months and quite a few guitars and a few ukes (we made two sizes) at the Brisbane Guitar Making School and realized we were on to a good thing and we've not looked back. Now full disclosure: Once I realized that it was a good thing I branded it and began selling it via our online lutherie supply store. If you look at it there you'll see a full video demonstration.

Finally, I was amused to see that at around the same time I launched my clamp, LMI launched their new version (in 3 sizes) and bugger me if it isn't remarkably similar to my design!!

Andrew