By Ed Ferri
Ed Ferri is the founder of Quality VAKuum Products, Inc. and has more than fifty years experience working with vacuum applications. QVP specializes in vacuum pressing and clamping equipment for the woodworking and plastics industries. For more information visit the QVP website at www.qualityvak.com or call 800-547-5484.
Vacuum is one of the least expensive and most helpful tools that a fabrication shop can use. Simple production operations such as templet routing can be doubled or tripled with the addition of a vacuum clamping system costing as little as $99. Before discussing vacuum applications in the shop, we should understand how vacuum works, how it's measured and how it's created.
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., 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 work piece 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 work piece 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 work piece 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 work piece is held off the tabletop. 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 use 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 oil-less 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 work piece 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 work piece 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, straightforward 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 fifty years experience working with vacuum applications. QVP specializes in vacuum pressing and clamping equipment for the woodworking and plastics industries. For more information visit the QVP website at www.qualityvak.com or call 800-547-5484.
