IBAG North America

The ABC’s of Part Holding Using Vacuum

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One of the most basic decisions that must be made when machining a part is how the part material will be held. There are several options, including mechanical fixtures, vises and chucks. The part shape or configuration will generally dictate how it will be held, and, all part holding methods will have advantages and disadvantages. Using a vacuum chuck is one option that deserves consideration. So, how is a vacuum work holding system configured and how do you determine if it makes sense for your parts? Let’s take a look at the “ABC’s” to find out.
 
A Stands for “Area”
The first question to answer is how large are your parts? This is needed to calculate the amount of required holding force, which is based on two fundamental facts: the degree of vacuum provided by the pump and the surface area of the part.
 
Clearly, when part areas are large, it is very simple to generate huge amounts of holding force with vacuum. For example, let’s say your part is made from a flat 10” x 10” plate. That’s a total of 100 square inches of surface area. When we apply full vacuum under that plate, the top of the plate is subjected to full atmospheric pressure (14.7 PSI). That generates a vertical force, over the part, of 1,470 pounds! That is almost 3/4 ton of force. Impressive.
 
However, we all know that most parts are smaller, and usually have either holes drilled through them, cutouts, windows or other features that will reduce the affected surface area. Most of these features can be supported using the proper vacuum chuck design and pump. Vacuum can still be the best solution, even when the parts are complicated.
 
“B” is for Benefits
Vacuum work holding has many benefits over other methods. One particularly attractive advantage is that the part is completely exposed for cutting operations. Mechanical clamps often block tool paths and must be removed or relocated to allow complete part cutting. This requires additional labor and extends overall processing time. Vacuum chucks are also rather uncomplicated and do not require much maintenance. Depending upon the type of vacuum chuck, most use a simple sealing gasket that must be replaced on occasion. Vacuum chucks are very accurate.
 
In addition, chips and cutting fluid can be managed in a well-designed vacuum system. Rugged, reliable vacuum pumps are available that can run for long periods of time. Vacuum storage tanks and coolant separators can also be included in the system to protect the pump from damage.
 
“C” is for Chuck
The key to the vacuum work holding system is the vacuum chuck. That is literally where the raw material meets the fixture and turns into the part. There are many types of vacuum chuck designs available. The optimal one depends upon the part configuration. For parts that have large exposed surface areas, grid type chucks are very common. The chuck has a series of slots milled into the surface. The sealing gasket, typically an o-ring rubber material, is placed in the slots surrounding the outside edges of the part. When vacuum is applied under the part, the seal forms, capturing the part onto the chuck surface. It is critical that the seal is not broken or the part holding force will be reduced.
 
This is the most common type of vacuum chuck, ranging in size from small to very large. A single vacuum chuck used to mill wing skins for the Boeing 777 is 25 feet wide and at least 75 feet in length! That is one big part.
 
It is also possible to build small part vacuum chucks or a vacuum chuck to hold smaller parts. Typically, quantities of smaller parts are produced from a large sheet. Most of the machining is done on the full sheet except the final depth cut which is done as the last operation. The larger sheet takes advantage of the high holding force to do almost all of the cutting. Other types use plastic pads and configurable mats to produce small parts. Many of these chucks are available off the shelf. For other parts, a special, dedicated chuck design is used to support vacuum holding a specific part.
 
“S” is for Suction
The heart of any vacuum system is the vacuum pump. In an ideal world, all vacuum chucks would have perfect seals and never leak. However, no chuck is perfect and leakage does occur. So, it is imperative that the vacuum pump is robust enough to provide full vacuum and sufficient flow to compensate for normal chuck leakage. The holding force is ultimately determined by the amount of vacuum under that part. Check the specs on your vacuum pump and make sure it has the chops to do the job.
 
Conclusion
Does it make sense to use vacuum to hold my part? Well, is your part big enough (larger than 2” x 2”) and flat? If yes, it probably makes sense to consider a vacuum chuck. If it is made from a flat plate, again, probably yes. However, what if it has 1,000 holes drilled through it? Or, it is 0.50” x 0.50” in size? Those parts probably won’t work well on a vacuum fixture. However, a special chuck design may still be a viable option.
 
The only sure way to determine if your parts should be held with vacuum is to contact an experienced vacuum work holding systems supplier to carefully evaluate your application. They will be able to quickly determine if the parts qualify for vacuum work holding. The best solution may be a simple grid-style chuck, or perhaps a custom chuck is required. However, a careful application analysis should be made before investing in a complicated system.