IBAG North America

As the Part Turns – Holding Rotating Parts with Vacuum Chucks

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We have spent a lot of time in past blog posts discussing how parts can be held with vacuum chucks. In all of the examples we shared, the parts were held in a stationary position on a CNC milling machine. However, plenty of parts are not made on milling machines. Many are made on lathes. Many of them can be held using vacuum chucks. So, how do we make sure a rotating vacuum chuck will work? What are the differences?

What Type of Rotary Vacuum Chucks Are Out There?

First, we start with your part. Is it made out of steel, aluminum, titanium, plastic or glass? Vacuum doesn’t really care about the part material. Vacuum forces apply the same to all materials. What about accuracy? Vacuum chucks can be made very accurately, and that accuracy is translated directly to the part. 
Does the part have cut-outs or holes drilled into it? That will require the vacuum chuck to have special sealing around those areas. Can we build the chuck so it can support a variety of parts? Lots of questions to consider.

Once we figure out what we are trying to hold, and what special requirements (accuracy, voids, etc.) we need to accommodate, we can consider a few different types of vacuum chucks. What about the vacuum pump? How do we get the vacuum to a rotating chuck? That sounds pretty important. We’ll get to that soon! 

The Grid Gets the Job Done

A simple grid-style chuck, similar to flat grid chucks used on milling machines, will provide great holding force and accuracy. That concept also applies to rotating chucks. A grid pattern is cut into the chuck and the sealing is provided by an O-ring gasket material. The grid spacing and O-ring diameter can be tailored to fit your part surface roughness, too. 

However, a grid style chuck holding force relies upon having a completely sealed area between the part and the chuck. Voids, holes and open part features can expose that sealed area to the atmosphere. So, the chuck will need to have special areas sealed separately by O-rings or isolated sections to support these open areas. 

Basic grid type chucks can also support a variety of parts. If the part surface is flat and simple, a general grid type chuck may work. It is easy to build a vacuum chuck that can support a range of diameters, as long as the parts are simple, like a flat plate. The sealing O-ring material is placed in the proper grid slot to seal the part.

Aluminum is the most common vacuum chuck material as it is light and can be easily machined. If the chuck requires a high degree of accuracy, it is also possible to fabricate the chuck out of steel so it can be finish ground to a high tolerance. 

Let’s not forget that anything that rotates will create vibration. So, keep in mind how balanced the rotating chuck + part will be. It may be a good idea to measure and balance the finished chuck!

A Dedicated Approach  

As with some stationary parts, sometimes a dedicated vacuum chuck will provide the best answer. The part design just doesn’t lend itself to using a general purpose vacuum chuck.  Special areas on the parts will be supported, offering the best accuracy and holding force. A careful analysis by an experienced vacuum chuck supplier can help determine if a custom chuck is the best solution. There are many examples of dedicated circular vacuum chucks in use today. Think of aluminum alloy wheels. How do they hold them for turning?

There Are Always Special Cases

Circular, grid-type vacuum chucks are widely used for the grinding of glass and ceramics, as well. These parts generally require higher accuracy, especially for the optics and semiconductor industries. Vacuum chucks are commonly used for these parts.

There are other special applications that require holding thin plastic films for turning operations. The plastic is thin and cannot be “dimpled” during the clamping process. This requires the use of a very flat, porous clamping surface. Vacuum is pulled through the ceramic porous material and holds the part firmly without any distortion.

How Do We Mount The Chuck?

Now that we have decided what type of vacuum chuck we need, selected the chuck material and finalized the features required, how do we connect this chuck to our lathe?  Depending upon the size, a circular vacuum chuck can be clamped to an existing hydraulic chuck. If the chuck is large, it can be bolted to a rotating face plate. Another choice is to have the vacuum chuck produced with the appropriate DIN mounting option included in the design. In that way, the vacuum chuck will attach directly to the lathe spindle nose taper, secured by screws. This simple and reliable mounting method will provide the highest accuracy.
 

Finally, How Do We Get Vacuum to the Chuck?

Now that we have our vacuum chuck designed, fabricated and mounted to our CNC lathe, it’s time to connect our vacuum source. Like all vacuum work holding systems, we must select the proper vacuum pump, considering capacity, leakage and maybe some coolant management. Once that is in place, how do we feed our vacuum to the rotating chuck?

What is a Rotation Joint?

Most modern CNC mills today have coolant through the spindle options. To do this, the coolant is supplied under pressure to a rotary coolant union. The rotary union is comprised of two parts: a stationary part and a rotating part. The rotating part is attached to the spindle shaft. The stationary part is attached to the machine and includes seals that prevent the pressurized coolant from leaking through the rotating joint. In a similar way, to feed vacuum, we use a special rotary vacuum coupling. It is designed to reliably pass our vacuum suction from our vacuum source (pump) to a spinning chuck.
 
The installation is simple. The vacuum rotary joint is mounted to the rear side of the lathe spindle supply tube. A plastic tube is inserted to the rear of the vacuum chuck. This tube is also attached to the rotating part of the union. The fixed part of the union is attached to the machine with a bracket. The vacuum source is connected to the rotary union, controlled by an electric solenoid valve. Standard rotary vacuum unions can also support speeds up to 6,000 RPM and different flow capacities, depending upon your needs.
 
One important note regarding any solenoid valve used for vacuum. Unlike pneumatic valves, valves used for vacuum should always be used in a “fail safe” condition. That is, the valve should be normally closed and default to a power-off clamped condition, ensuring that vacuum is on the part if the system loses power. The last thing we need is for the part to be released when the chuck is turning! Vacuum valves also require venting back to the chuck which is not included in a standard pneumatic solenoid valve. Without correct venting, the part will not come off.

Don’t forget to include an electric vacuum sensor that is interfaced into the CNC. Any loss of vacuum during cutting time should result in a fast emergency stop.

Conclusion

By this time, we are all convinced that vacuum work holding is a viable, and sometimes only, option to holding parts for milling. However, there are also many turning applications that can benefit from vacuum work holding. The principles are the same. The logistics are just a little more complicated. We still need to design and fabricate a vacuum chuck to hold our parts. This may be of a general grid type design, or a dedicated chuck. 

The key to using vacuum for turning applications is the need for a reliable vacuum rotary joint, or union. This rotary union supplies the vacuum source to the rotating chuck. It must be capable of rotating at high RPM and maintaining the vacuum without leaking for a long period of time.  And, the solenoid valve used should be the proper type, set up in a “fail safe” mode with vacuum sensor monitoring.

Keep turning those parts out!