• Vacuum Clamping as seen in Plastics Distributor and Fabricator Magazine Nov/Dec Issue 1997
• Vacuum Clamping as seen in Plastics Distributor and Fabricator Magazine Sept/Oct. Issue 1999

Vacuum is the absence of matter, a space that contains air or a gas at less than atmospheric pressure. Our junior high physics taught us that a 1 sq. in. column of air from sea level to outer space weighs 14.7 pounds (for general calculations and discussion 15 psi is used). When we apply this force to a larger area like 1 sq. ft., then we generate more than 1 ton of holding force.

Vacuum is normally measured in "inches of mercury" and written as "Hg. This is the same term the weatherman uses when talking about the barometer reading. "The barometer is 29.52 "Hg and falling." The maximum barometer reading on a standard day is 29.92 "Hg and the maximum force or weight of 1 sq. in. of air is 15 pounds. From this we develop a 2:1 ratio between vacuum level and holding force. Thus, calculating the holding force generated by a vacuum clamp is easy.

For example, if we are templet routing an 8" diameter circle (about 50 sq. in.) and achieve 26 "Hg (always use a vacuum gauge) with our vacuum pump, then we have 650 pounds holding our work piece to the templet (50 sq. in. x 26 "Hg/2). Normally, this is more than adequate to hold the piece for routing. The amount of holding force required for a cutting operation depends on several factors such as material hardness and thickness, speed and sharpness of cutting tool, etc.

Why use vacuum clamping instead of mechanical clamps, screws or double sided tape?

Speed, it is faster than other methods, drop your workpiece on a vacuum template and it grabs it instantly. No pulling off and cleaning up double sided tape, etc.

Strong, it generates more than 1800 lbs./sq. ft. of holding force. A mechanical clamp typically generates 200 to 400 pounds of force and can get in the way of the cutting tool.

Even and uniform pressure over the entire area where vacuum is applied. A mechanical clamp applying 300 pounds, applies it to a point and the workpiece could shift or pivot when a cutting tool is applied.

Safer than mechanical clamps as there is no metal clamp to get in the way of cutting tools.

More accurate when templet routing. You have the ability to go completely around the workpiece in one pass without having to unclamp and re-clamp the piece. This is especially important when several pieces are being routed and must fit together.

Does not mark or mar the work piece. The soft foam gasket material used with the vacuum jig keeps the piece clean and mark free.

More versatile than other clamping methods.

A vacuum clamp can be placed anywhere on a surface. For example, to hold a straight edge to a 4 x 8 sheet, the mechanical clamps must be at the edges in order to grip the sheet and straight edge. With a vacuum straight edge, it can be placed anywhere on the sheet and when vacuum is applied it will be pulled straight down. When routing, the workpiece is held off the table top, therefore, you can rout completely around the edge of the piece.

All these features add up to reduced fabrication costs. Most improvements will pay for a vacuum system in one week. One user who was routing a large piece on a pin router needed two men to handle the piece because of the size of the mechanical clamps and jig. With a vacuum clamping system he reduced the jig size and eliminated the second person. Reducing the templet routing time by one half or one third is not unusual when going from a mechanical clamping system to a vacuum clamping system.

What kind of vacuum pump should you be using for vacuum clamping? It depends on the task. There are two types to choose from; electric or air-powered (Venturis'). The electrics have the advantage that they can be plugged in anywhere. Most electric pumps used for vacuum clamping are the oilless rotary vane type and they usually have a thermal overload switch built in. This shuts the pump off when it overheats and prevents it from burning out. Cycling an electric pump on and off will cause Overheating. Therefore, if an electric pump is used, additional hardware is recommended to allow the release of your workpiece without shutting the pump off.

The air-powered pumps have the advantage of being more reliable (no moving parts), less expensive and with some manufacturers, they can be upgraded later for more performance without losing the original investment. Also, some give a lifetime warrantee. Air-powered pumps can be powered with a 3/4 H.P. compressor or larger and, like electric pumps, come in different capacities. When holding plastic, the difference between a low capacity vacuum pump and a larger unit is the speed at which it gets to maximum vacuum level.

Once you decide on the type of pump, you need to decide on what specifications are needed. Both types of pumps are rated by maximum vacuum level (the maximum force it can generate) and vacuum flow. Vacuum flow is the volume of vacuum air that can be pulled in at a given vacuum level. Usually the pumps will max out in the 26 to 28 "Hg range. The cost of a pump that reaches 29 "Hg and above is not worth the extra money. Vacuum flow is important when holding porous material like wood. Wood is porous and the larger pumps can pull in more vacuum air, thus giving a higher vacuum level which means more holding force. Vacuum flow can become important when holding large plastic pieces because you may need an initial large vacuum flow to grab the piece and pull it to the vacuum jig to create the vacuum seal.

Vacuum clamping is ideal for the plastic fabricator as it will save time in many fabrication operations. It grabs fast, it holds tightly with up to 1800 lbs./sq. ft., it doesn't mark the workpiece and it's very affordable. As an investment, the payback in most cases will be in one to two weeks.

Once a fabricator starts using basic vacuum jigs and understands how it all works, then they start to make special or unique jigs to solve specific problems. Our next article will be on designing and making vacuum jigs and fixtures. They are simple, straight forward and easy to make and a creative fabricator will find many uses for vacuum in the shop.

Ed Ferri is the founder of Quality VAKuum Products, Inc. and has more than ten years experience working with vacuum applications. Q.V.P. specializes in vacuum equipment for the plastics industry and can be reached at 1-800-547-5484.

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Vacuum can be used very effectively in the plastic fabrication shop. It's fast and grabs instantly; it provides up to 1800 pounds/sq.ft. of strong, tight holding force; it doesn't mar the work piece; there are no mechanical clamps to interfere with the routing process; you can go completely around the template in one pass, and it releases instantly.

Unless you are routing very small and thick pieces, a vacuum clamp will double or triple the output of template routing work when compared to using mechanical clamps or double sided tape. This can pay for the vacuum equipment in as little as one day of use.

Plastic is nonporous, smooth and flat; therefore, plastic fabrication shops have a significant advantage when using a vacuum clamp, no leakage. So the maximum holding force can be achieved. In addition, plastic scraps are available for making jigs, fixtures and templates, so the cost of material is minimal, especially when many templates have to be made.

Making a vacuum template is quick and simple. We have used a template made from wood for better contrast. It's cut and sanded to shape and then a hole is drilled through the center. This allows vacuum to be applied to both sides. Next, the piece is outlined with a vacuum foam tape on both sides (top and bottom). The foam provides a seal between the template and the workpiece on top and the table on the bottom.

In this example we have a drilled hole in the solid tabletop with a vacuum line connected underneath. When we place the template over the hole, and put the workpiece on the template, it creates a vacuum cavity. Turning on the vacuum pump sucks all the air out from under the piece. The vacuum holds the template and piece to the table with about 400 pounds of force.

Calculating the holding force is straightforward. First you determine the area under the vacuum. In this case, about a 5" X 6" area or 30 sq.in. With the pump on, we reached 26 inches of mercury ("Hg); about the maximum. For every 2"Hg, we get 1 pound/sq.in. (psi) of holding force; a 2:1 ratio. Therefore, 26"Hg equals 13 psi. The 13-psi is multiplied by the 30 sq.in. of vacuum surface area. This equals 390 pounds of force holding the template to the tabletop and workpiece to the template.

One nice feature of the vacuum is that it holds with even and uniform pressure unlike a mechanical clamp, which holds with a point force.

And here is the completed piece. Because we were able to rout completely around in one pass (no mechanical clamps to interfere) we get a very accurate reproduction. When several router pieces are needed to fit together, vacuum template routing is the best option.

Other vacuum jigs that are useful are Straight Edges and Tee Squares. They can be placed anywhere on a sheet and the vacuum sucks them down exactly where they are placed. Another advantage of vacuum jigs is that they do not have to be at the edge of a sheet in order for the clamps to grip like with a mechanically clamped straight edge.

Those with pin routers will find vacuum templates significantly improve production. When routing large pieces, the vacuum template will be lighter since the weight of mechanical clamps is eliminated. One user required 2 men to shape 3' and 4' diameter table tops because the template was too heavy for one man to handle safely. By switching to a vacuum template, one man is able to handle the job.

Prices for vacuum clamping systems vary from $100 to $1000. You have a choice between air-powered or electric pumps. The air-powered units are good for hand template routing since they are reliable and less expensive than the equivalent electric unit. Since plastic is nonporous, air-powered systems are generally more than adequate to meet most hand and pin routing needs. Our final photo shows manual and foot operated air-powered units, an electric vacuum pump, and vacuum tape and quick disconnects. All that is needed to make a template is a piece of plastic.

All units shown provide up to 1800 pounds/sq.ft. of clamping force. The manual unit (left) offers low cost while the foot operated unit (center) offers vacuum control with the pedal. This is about 20% faster for production work. It leaves your hands free to handle the router and workpiece. The electric pump with a foot pedal (right) offers the convenience of plugging in anywhere since no compressor is required.

Written by Ed Ferri, the founder of Quality VAKuum Products. Mr. Ferri has over ten years experience working with vacuum applications. Q.V.P. specializes in vacuum systems for the plastics and woodworking industries. They can be reached at 800-547-5484. Or see their web site: www.qualityvak.com.

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