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Let’s face it, the easiest and cheapest way to hold down a piece of material to cut a part is using a simple mechanical clamp. You know, the way we usually do it - on a Bridgeport manual mill with steel clamps, threaded rod and nuts. Although this is simple and effective, it’s actually not the most efficient way to hold parts for high production CNC applications.
The first problem is that the clamps get in the way. They prevent you from accessing the complete part. So, you end up machining around the clamps and later, in another setup with different clamp locations, machine those missed areas. Very inefficient. In addition, the time required to remove the finished part and replace with new material, re-tighten clamps, etc. is also excessive. Time is money.
What if there was an easy way to hold raw material with accuracy and strength? A solution without pesky mechanical clamps, that allows for very rapid part changes?? There is, and it’s called vacuum workholding! The principle is rather simple. A vacuum pump powers a specialized chuck, which holds onto your parts simply by evacuating the air pressure. The resulting force against the part surface equals 14.7 pounds per square inch. For a part that was 10” x 10”, that would be 1,470 pounds!
A vacuum chuck, holding a part of reasonable size (we’ll talk about small parts later), is an ideal method of holding raw part material - especially non-ferrous parts machined from large, flat plates. Vacuum chucks are very effectively used in the aircraft industry today to hold aluminum, titanium and composite parts.
The Heart of the Operation
The first requirement for a vacuum work holding system is a strong vacuum pump. There are many types available for various applications as follows:
- Mid-level vacuum capacity at high flow rates - most commonly used on large routers that hold very large pieces of wood, particularly for the furniture industry.
- Full vacuum at capacity low flow rates - for metal cutting applications, the best vacuum pump should be able to provide full vacuum (defined as 29 inches of mercury) at low flow rates (leakage). This is ideal for most vacuum chucks that will be holding a metal, plastic or composite part.
- Heavy duty vacuum pumps - utilize oil or water to provide lubrication, cooling and sealing. In addition, there are low cost venturi vacuum pumps that generate vacuum by using compressed air. Each type of pump will match a particular application.
Chucks and Seals Count Too
The vacuum chuck is where the rubber meets the road. If it doesn’t work, the part will end up out of tolerance or worse yet, fly off the fixture during the cutting. A good chuck must be designed to support the part material to the level of accuracy required, expose all the material to the vacuum provided by the pump, and have a gasket that will provide a seal between the chuck and the part. The most common type of gasket used is the O-ring. The O-ring seal prevents the vacuum from escaping and is critical. Without a good vacuum seal, you lose vacuum to the atmosphere and reduce your part holding force.
Small Parts? No Problem!
So, what about holding small parts? Sometimes there are small plastic, aluminum or even glass parts that must be held for processing. There are unique ways to approach this problem and some good solutions out there. The key is to carefully limit the cutting forces on the part. Custom vacuum chucks can be designed, and machining processes written that will optimize the manufacturing for small parts.
Vacuum chucks are available in many forms and sizes. There are grid style chucks which can be used for various part sizes, and custom chucks that are made to hold specific parts.
Sometimes Vacuum Workholding is the Only “Choice”!
Vacuum is sometimes the only solution for certain types of parts. Glass, ceramic, and precision optics production generally use vacuum chucks. Semiconductor wafers are also held using extremely flat vacuum chucks for inspection and other processes. A vacuum chuck can be made with very high accuracy by grinding the chuck surfaces to very tight tolerances. Porous ceramic and even porous aluminum materials are available for use in flat or 3D vacuum chuck designs.
The Ideal Solution - In Most Cases
Vacuum sounds like the ideal work clamping solution for any part, right? Well, not exactly. Some parts have too many through holes and window cut outs. Vacuum only works on solid surface areas to create holding force. For instance, it would be impossible to hold a screen with vacuum.
Vacuum is a great time and money saving approach for many machining applications, particularly with high volume jobs. To determine if vacuum is the best solution, each part should be evaluated by an experienced application engineer. If you need any assistance, we’re happy to help!