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In our previous blog, we discussed how the need for very small parts is increasing, as the miniaturization movement continues to accelerate. As with all parts that are made, we must first figure out how to hold the raw material. The easiest method is to use mechanical clamps. Clamps are simple, provide a huge amount of holding force and are inexpensive. However, the clamps can block machining paths and require multiple setups to complete the part cutting. Many small parts can be held with vacuum chucks, which eliminate the need for clamps and allow complete access to the part for cutting. However, there are some challenges that must be dealt with when using vacuum for small parts.
On a typical vacuum chuck, the holding force on the raw material and the part will be directly related to the surface area of the part and the amount of vacuum it is exposed to. On planet Earth, our home, we are exposed to an atmospheric pressure of 14.7 PSI. When we evacuate with a vacuum pump, we remove that pressure under the part. The result is atmospheric pressure of 14.7 PSI pushing down on the material area of the part. This force can be quite high for even medium sized parts.
For small parts, however, the actual part area may be much smaller. Let’s say, for example, you needed to make a 1” diameter part from aluminum sheet. With full vacuum under it, a vacuum chuck will only generate a total clamping force of 11.5 pounds on each part. Although this may appear as too low to hold the part to completion, it may be satisfactory if other techniques are used. And, for many parts that small, there are not many other alternatives to use.
Selecting the Right Pump is Critical
So, what can we do to effectively hold small parts with vacuum? The first consideration is a good vacuum pump. There are many types of vacuum pumps available, and they are not all identical. Some vacuum pumps are available that utilize compressed air and a venturi system to create vacuum. Although lower in cost than electric pumps, these pumps typically cannot provide full vacuum and do not tolerate leakage very well. For a small part the vacuum source directly affects the holding force which must be maximized to ensure a good clamping solution. Therefore, for small parts production, make sure you have a vacuum pump that can provide close to full vacuum (27 ~ 29 inches of mercury) and tolerate reasonable flow rates due to leakage. Many electric vacuum pumps can provide these capabilities.
How Does Vacuum Hold the Part?
Now that we have maximized our vacuum source, how does the vacuum chuck actually hold the part? There are basically two concepts used when designing a vacuum chuck for small parts: a dedicated design for a specific part and modular concept that uses plastic pads that can be customized.
Dedicated Design - feature grooves milled into an aluminum plate to provide the vacuum to the part and o-rings to provide the sealing. These can be very effective solutions, however, they are limited in that they will only be useful for one specific part. When the part changes or has modifications, it may be necessary to build a new vacuum chuck. In many cases, this may be the best, and only solution.
Modular Concept - allows the use of a base vacuum slotted chuck, covered with a prepared elastomeric plastic pad. The pad must be modified to match the part layout and have holes or grooves machined into it to match the part shape. The base chuck provides a vacuum source to hold the part and the pad provides the seal between the vacuum and the part material. The main advantage of using this system is that the base chuck, with a new pad, can be used for many different parts. For a new part, it is not necessary to build a completely new vacuum chuck, only modify a new pad.
Other Techniques - benefit the production of small parts on a vacuum chuck. Most small parts are produced in large quantities, so, it is common to nest a large number of parts on one sheet of material. Since the holding force is directly related to the surface area affected by the vacuum, it is better to machine with full sheet exposure. In other words, do as much cutting as possible on all parts except for the final break through. Leave a small “floor” on the perimeter of each part. This will maximize your holding force and prevent any part movement until the last operation. At the last operation, carefully cut through the perimeter of each part at a conservative feed rate to complete the process. For small parts this may take some trial and error, but worth it.
In some cases, for very small or complex parts, it may be necessary to stop and add a mechanical clamp prior to the last operation. This may be necessary if the part requires multiple holes drilled through. Drilled holes will reduce the vacuum and may also create “leakage”. Although this may seem like a hassle, it can still be the best solution.