LMI Help Center

Vacuum Fixtures

Robbie demonstrates how to use the LMI Vacuum Bridge Clamp and Vacuum Work Holder.
Length: 9:40

Video Transcript

Today I would like to talk about the use of vacuum fixtures in the shop and show you two fixtures that I use a lot. I can’t imagine how I ever got along without these. First, let’s talk about ways to create a vacuum in your shop. An inexpensive way of doing this is by using what’s known as a venturi value. This neat little invention connects to your compressor. The escaping air then creates a vacuum on one of the lines. This line can then be connected to your vacuum fixture.

Another way to create a vacuum would be to use a vacuum pump. Small, inexpensive pumps like the one pictured here are readily available. By using either the venture valve or the vacuum pump, you should be able to create enough vacuum in your shop to use the fixtures I’m about to demonstrate.

This brings up another question. How do I know how much clamping pressure I’m generating with my vacuum? Vacuum is normally measured in inches of mercury. You will need a gauge to measure how many inches of mercury your vacuum is pulling. With this information, we can calculate clamping or holding pressure. For example, my gauge now reads 19 inches of mercury. Multiplying the width times the length of my clamping fixture gives me the clamping area in square inches. In my case, it is approximately 24 square inches. Now I multiply 24 square inches by the inches of mercury, in my case 19, and I get 456. If I divide this by 2, I come up with 228. This is how many pounds of clamping pressure I’m able to apply with this jig if my vacuum is pulling 19 inches of mercury. For purposes of guitar building, somewhere between 15-25 inches of mercury should be sufficient.

The first vacuum fixture I’m going to demonstrate is the bridge clamping jig. Like all vacuum fixtures, this is an elastic membrane stretched between a frame. When place over the bridge with the vacuum on, the membrane collapses, applying clamping pressure as the air is sucked out. Before acquiring this jig, I had to use bridge clamps inserted into the soundhole to clamp the bridge. I did this for many years, but I found I had to be extremely careful to not scratch the finish while inserting the clamps. I then had to position each one individually on the bridge to apply the pressure. Now let me show how much easier this step can be by using a vacuum jig. With the bridge location and outline marked, I applied glue to the bottom of the bridge and use my high tech glue spreading device to spread it. Running your high-tech glue spreading device around the edges of the bridge at a 45-degree angle helps push the glue back from the edge thus reducing or eliminating squeeze out. Next, place the bridge on the soundboard at the correct location. Then I place a small piece of masking tape on each wing to help keep the bridge in place. I only do this on my classical guitar bridges.

On the steel-string bridge, I can either screw the bridge in place or use the bridge pins to hold it in place. The guitar I’m working on here came into my shop for a complete refinish. Therefore the bridge pin holes in the bridge and in the soundboard had to be covered with masking tape to help create a seal for the vacuum. Make sure that there’s no dust or debris on the fixture because this can dent your top. Place it centered over the bridge and turn the vacuum on. Adding a little light pressure to the fixture helps create the seal. The vacuum then removes the air and collapses the elastic membrane, applying clamping pressure to the bridge. In my case, 228 pounds of clamping pressure.

If you’re using a French polish, make sure it’s sufficiently cured before using the vacuum jig. If not, you have a nice footprint of the jig in the finish. For the French polish method that I use, I have found that about three days of cure time is sufficient. Lacquer finishes also need cure time. Not so much because of the vacuum jig, but because the masking tape can sometimes react with your finish.

Watch now as I repeat the process on a steel-string guitar. After about ten minutes I release the vacuum and remove the clamping fixture. If I used tape to mark out the bridge location or to help secure the bridge, like I do while clamping my classical bridges, I go ahead and remove it. Now I can clean out any glue squeeze-out that may have leaked out from under the bridge onto the soundboard. A small piece of wood with the edge sanded on it works well for this. A damp paper towel or soft rag also works well for cleaning up glue squeeze-out. Once all the squeeze out is cleaned up, once again place the clamping fixture over the bridge, making sure there’s no debris like pieces of dried glue on the foam edges. Since glue dries much quicker in a vacuum, it’s only necessary to leave the jig on for about another 30 minutes. I then release the vacuum and remove the jig. It really doesn’t get much easier than that and it sure beats fighting the clamps to the soundhole. I now let the glue cure for at least 8 hours before stringing up the guitar.

Another vacuum fixture I have become dependent on in my shop is a hold-down fixture. You will be amazed at how many uses this fixture has. The top plate rotates 360 degrees and locks in place. You can also rotate the jig a full 90 degrees and lock it in place as well. The top plate therefore can rotate horizontally or vertically 360 degrees. Please enjoy the music while I’ll show you some footage of the many ways you could use this jig. You will probably even come up with some of your own ways you can use the jig.