I wanted to post a picture of a vacuum from Joe woodworkers website. It is shop made and it vacuums on the top and bottom. So it is locked to your table. Each one of the four areas in the block has a ball bearing valve. When you put on your piece it depresses the ball and pulls a vacuum. I use vacuum clamping and holding in my guitar making. I have a venturi system I built as a kit that runs off my compresser and a continuous run Gast pump I got on ebay for about $60. Both work great a lot faster that hand clamps or screws to hold things down.