Published 11/30/2007

Hold On With Vacuum Clamping

Because of the possibility of workpiece deformation, clamping using three-jaw chucks is not always suitable for final machining. In cases where tight machining tolerances must be upheld, this Danish cutting specialist relies on vacuum clamping technology to hold its parts.

Chris Felix

Former Editor-in-Chief, Production Machining

Since the company’s founding in 1969, Madsen Machine Technology (Horsens, Denmark) has specialized in turning removal of deep-drawn and rolled parts in metal, including stainless steel. Such parts are relatively thin-walled and exhibit particularly high material tension because of their manufacturing process. A certain level of knowledge and expertise is needed to control forces that arise during milling and machining of finished products with tight tolerances. Through the years, Madsen has become known for specializing in turning such difficult parts, even to the point that renowned companies from Scandinavia, England and Germany enlist the company’s service.

Morten Helding Madsen, who runs the company together with his brother Claus Helding Madsen, explains: “We have here near our main plant in Horsens a partner company called Oestergard, which co-owns 40 percent of our company. Oestergard lets us do fine turning of many deep-drawn turned parts ourselves, but more than 70 percent of our turnover goes to export. We mainly export turned parts for automotive, medical and pump industries.” The situation with incoming orders is so good that 15,000 square feet of space available for production is no longer enough. By the end of 2007, Madsen plans to increase its machining facilities by 50 percent.

One of the contributing factors to the company’s growth has been the considerable number of special orders it has received. For example, an automotive supplier has ordered 100,000 dampers per year, supplied as rolled, semi-finished products. These dampers weigh about 18 pounds and have an OD of approximately 13 inches. It is Madsen’s job to not only machine the reverse flange inside and out, but also to carry out milling of the face ends. Tight dimension and position tolerances are required. Before this job, the company found that three-jaw chucks were sufficient for part clamping, but this method met its limits for the dampers. Mr. Madsen describes the problem: “For face milling on one end we can clamp the flange on the ID using a three-jaw chuck. But for the other end, the part is too big. Also, the surface would become convex because of the tension, and we could not maintain the required tolerances.”

So the cutting specialists went in search of alternative clamping methods. Tests with magnetic chucks did not produce the desired results because of the accumulation of iron chips. During discussions with the company’s machine supplier, Gildemeister, the idea arose to try a vacuum chuck from Witte (Bleckede, Germany). Vacuum would allow an even and tension-free part clamping process.

According to Mr. Madsen, “The people at Gildemeister talked favorably about their positive experience with Witte as their partner, which is why we were happily prepared to go for an innovative solution. Already, initial tests have shown that our skepticism regarding holding forces was unfounded. We machined these parts with approximately 2,000 rpm and clamping was 100-percent reliable. I am convinced that we could machine even larger parts with the Witte vacuum chuck.” He also emphasizes that being able to easily clean the system with compressed air or water gives it another significant advantage over magnetic alternatives.

Vacuum systems are available in different versions, designed to address the needs of particular clamping applications. The principle setup, though, is always the same: Vacuum conditions are generated via a modular vacuum unit with an integrated liquid separator, vacuum tank and pressure controller. Vacuum distributors channel operating vacuum to the chucks. Gages, manual or magnetic valves, pressure controllers and safety switches take care of monitoring the process.

For applications on turning machines, circular vacuum chucks are the best option. Witte offers a range from 100 mm (4 inches) to 630 mm (25 inches) in diameter, with custom solutions for larger or smaller parts. Circular chucks allow fixation of parts with different size inner and outer diameters. Freely adjustable stops ensure accurate positioning of workpieces.

Madsen is now using four such chucks from Witte. Two are permanently installed on Gildemeister CTX 420 linear universal turning machines. These machines are used exclusively for manufacture of the previously mentioned dampers. The third system is mounted when required on a CTX 400 to turn a lid insert that is required in smaller quantities. Changing the chucks is a fast process. Mr. Madsen states that setting up a vacuum on a machine takes less than 2 hours.

The 30 employees at Madsen consider it important to keep an eye open for innovations that will optimize quality, price and delivery time for their products. For Mr. Madsen, the vacuum workholding systems are an important component of his modern machinery: “Without this vacuum clamping system, we could not machine these dampers to the required tolerances without taking a lot more time. Also, the support from the supplier during installation of the system was excellent. Their know-how with regard to vacuum clamping technology was invaluable.”

For the additional production space, Madsen has already ordered four more Gildemeister machines. “These can be equipped with vacuum chucks for corresponding orders,” Mr. Madsen adds. A further innovative step is currently in the planning process. In the future, a robot arm will load one or more machines. Mr. Madsen explains, “We want to relieve our staff of simple, heavy work so they can do more important work, such as quality control.”

Vacuum Clamping Advantages

Reduced clamping time
Universal clamping system for different parts
Clamping of non-magnetic parts
Oscillation-free machining
Clamping of thin parts and foils without deformation
Problem-free machining of contours and cutouts
Broad range of machining and materials applications

How It Works

To clamp a part using a vacuum, the atmosphere is removed from underneath the part that is to be held. The atmosphere around the outside of the part is what actually holds it in place. At sea level, the atmosphere provides 14 psi of holding force, but the higher above sea level the operation is, the less holding force there is. In Denver, Colorado, for instance, with 26 percent less atmosphere than at sea level, holding force is only about 10.4 psi.

While vacuum workholding is not necessarily as secure as a jaw chuck, a vice or an electromagnet, it can be used to hold any material. Most vacuum applications are for flat type of work—anything from composites to steels that is hard to hold with any other method. When holding a flat workpiece, it is supported completely, all the way around, including the center of the part. This method avoids the oil-canning or bouncing effect that is typical when using some other clamping methods.

The down side of the vacuum is that the smaller the part, the harder it is to hold, even under perfect conditions. For example, a 1-inch by 1-inch piece is generally not advisable with vacuum because only a maximum of 14 pounds of pressure holds it in place. Fortunately, there are products on the market that can help to contend with these situations.

According to Bruce Thomsen, applications manager for IBAG North America (North Haven, Connecticut), vacuum chucks are not necessarily custom made for the part. “Most suppliers have what’s called a grid-style chuck,” he explains. “Custom chucks are just a variation of a grid-style chuck. A grid-style chuck consists of a pattern of squares with grooves in between that distribute the vacuum. You put material between the grooves to seal the vacuum on the outside of the part so that when you apply the vacuum within the seal, it sucks it down into place. You need to be careful not to break through at any point, though, or you will lose the vacuum and the part will go flying around.” This seal-type system is common for most vacuum applications.

The two-part, slot-style chucking system consists of the base, where the vacuum is drawn through, and the top rubber mat, which is supplied blank, allowing custom placement of holes for holding the workpiece. This system is an alternative for workpieces as small as 1 inch by 1 inch with certain machining techniques.

Another style of a two-part system is the Vac-Mat. With this system, the mat has a set array of suction cups to hold the part securely in place. According to Mr. Thomsen, “The unusual benefit of this system is that it actually leaks. You can put a part on it that’s smaller than the mat size, or you can put on a part and cut it completely and drill and tap or do whatever operations you want and come off with a completely finished part. This system is good for a one-up or a production type of operation.” The down side to this system is that the part needs to be at least 5 inches by 5 inches to cover enough of the suction cups to hold it in place. If the size requirements are met, the part will be secure during manufacturing, even with vacuum leakage. The key to an open-type system such as this is having a pump that has enough air flow to overcome any leakage that occurs.

The two main considerations for vacuum pumps is power and flow rate. Power is defined by the inches of mercury (Hg) that can be pulled under best conditions. The maximum power is 29.6 inches at sea level under perfect conditions. Some pumps are limited to about 20 inches of mercury or less, even under perfect conditions. Without enough power to create full vacuum, considerable holding force is lost.

Flow rate is a measurement of cubic inches per minute of free-flow air the system will pull through when it is not connected to anything. In a seal-type system, although loss always occurs to some extent in the line or around the seals, high flow rate is not necessary. In an unsealed, open system such as the Vac-Mat system, flow rate is very important because outside air is constantly being sucked in. A pump with high flow rate is required to overcome any leakage and retain a certain amount of holding force.