When Clamps Aren't the Answer
Workholding with magnets, vacuum, and glue
By Robert B. Aronson
Workholding techniques using a magnetic field, a vacuum, or an adhesive can be effective alternatives to clamps. When these techniques are used, more part area is available for the cutting tools, thin parts can be held, and initial setup can be fast and simple. Plus, there is a potential for smoother surfaces and a shorter overall production cycle.
Magnets and vacuums work best when the holding force is perpendicular to the part. Side loading with aggressive tools can be an issue with some systems unless there are stops on the work area that rest against the part.
When stops are magnetic, they generate a magnetic field perpendicular to the base magnet and can hold a part in two axes. They may also act as part locators, which reduces set up time.
One side effect of magnetic holding is that the fields of the holding magnet may magnetize the part. If this is a potential problem, it's necessary to degauss, or demagnetize, the part by passing it though a neutralizing field. However, explains Brad Evans, product manager, stationary workholding, for Schunk (Morrisville, NC), less than 5% of the parts machined on a magnet need to be demagnetized.
Small parts can be a problem for magnetic workholding systems because holding power is a function of the part area next to the magnet or vacuum. Depending on the process being done, one rule of thumb for a magnet workholder is that a part should have at lest 4 in.2 (25.8 cm2) of surface contact.
Magnetic systems work best with ferrous parts, but nonmagnetic materials require special fixturing, such as a design that sandwiches the part between the magnet and a ferrous holding plate.
The Magnos permanent electromagnet from Schunk has a square-pole design so the magnet's surface is made up of Alnico and neodymium magnets in a checkerboard pattern. A part has to contact two squares (one positive and one negative) to be held.
The magnetic field generated has a controlled height, which can be a major advantage. "In one of our smaller units, the field height is about 12 mm," says Evans. So a part 25 mm above the magnet would not be attracted.
"Another plus is that apparent part weight greatly increases. For example, a 10-lb [4.5 kg] workpiece would have an apparent weight of 3000 lb [1361 kg]," says Evans. The result is minimal vibration and chatter so the part surface is smoother. Tools also last longer.
"Sales of magnetic workholders are going well for us," comments Michael Harris, Vice President Earth-Chain USA (Indianapolis, IN). "US manufacturers are finally realizing the two main advantages of magnetic workholding: shorter setup time and greater access to the part for machining operations."
Earth-Chain offers the MagVISE line of permanent and electro-permanent units ranging in size from 17 x 9" to 32 x 30" (432 x 229 to 813 x 762 mm). Specials are also available. Generally, holding force is 2000 lb (907.2 kg) per each four poles of a magnetic chuck.
Magnetic workholding is most practical in operations required to machine multiple part families. In these cases, it is critical to minimize setup and changeover time. "Magnetic workholding also offers advantages in large plate work, mold and die work, and use with large boring mills," says Harris.
Although the company specializes in hydraulic workholding, Pascal Corp. (Elk Grove, IL) also offers four sizes of electromagnetic workholders. Holding power is 14.7 kN per 100 mm2 of workpiece contact area. Worktables range in size from 450 x 315 mm to 1030 x 600 mm.
A degaussing system is built in, and the residual magnetic field can be reduced to 20 gauss. This is so small that chips can be cleared with an air hose. Magnet coils are epoxy-sealed to prevent penetration of cooling fluids that could destroy a magnet.
"Sometimes potential customers aren't aware that they can't take aggressive cuts on a part with a magnetic holder the same way they would with a part that is mechanically clamped," explains the company's Kevin Branstrom. "But although this may reduce the production of a given machine somewhat, the customer has to look at the total picture. Setup time is reduced, more of the part is accessible to the tools, and surface finish is improved. Added up, these positive factors can cut total cycle time and improve part quality."
"The workholding industry is seeing a shift away from clamp-type workholding to magnets and vacuum applications," says Dave Nordman, president of Magna-Lock (Rockford, IL). "There are more delicate parts these days that can't take the pressure of clamps."
Magna-Lock makes electromagnetic chucks for rotary and surface grinders. The chuck has a low magnetic force after degaussing. This is a particular requirement from bearing manufacturers who don't want stray material attracted to the bearing surface.
The company also offers a vacuum system for nonferrous and delicate parts. Holding force is about 30 lb (134 N) and the worktable measures 96 x 144" (2438 x 3658 mm). These vacuum chucks can also be put on grinders.
Several double-sided tapes are available from 3M (Minneapolis). These tapes are used chiefly to hold smaller parts for machining processes that apply light loads.
Tape 410B is a 0.005" (0.13-mm) thick tape used to hold parts in place temporarily. It has good shear strength and peels off easily.
Tapes 401B and 9589, both 9.0-mil-thick double-coated tapes, are more suitable for textured or irregular surfaces.
Bond strength depends on the amount of adhesive-to-surface contact developed. For optimum adhesion, the bonding surfaces must be clean and dry. Ideal tape application temperature range for most applications is 21–38° C. The company does not recommend application to surfaces at temperatures below 10°C.
Light Activated Adhesive Gripping (LAAG) is a workholding system from Master Work-Holding Inc. (Morganton, NC) that relies on UV-cured adhesive to hold a part. The workpiece is positioned in a patch of adhesive on a worktable, and the adhesive cured by UV light. After machining, the part is removed from the fixture and the process repeated.
Another new development is the application to larger parts. "In the past, we have dealt with smaller parts that were fixtured using off-line carrier plates, says Dustin Clark, LAAG project manager. "However, there has been a recent demand for LAAG solutions for very large parts set up directly on the machine." This work has included 10' (3-m) wide titanium billets, and thin, concave, 8–12' (2.4–3.7 m) aluminum-skin sections for an aerospace application. To handle these larger parts, the company has developed larger LAAG grippers.
Mitee Bite Products Inc. (Center Ossipee, NH) has an adhesive product available in several forms called Mitee-Grip. This wax and resin compound enables a billet to be glued to a flat surface which is parallel to the X-Y plane of the mill. It is then possible to machine the top surface of the billet and all the way round its profile. The billet and flat surface must be heated to 85–90°C for the adhesive to hold the two together. After the adhesive sets, the part can be machined. Heating separates the part and flat surface.
Mitee-Grip is available in three styles. A paper-thin 0.001" (0.03-mm) thick version is used for small to medium parts and precision work. It's an alternative to double-sided tape, which some find difficult to remove after machining. A thicker mesh-like version holds heavier parts and those with more complex shapes. Additional holding forces can be obtained by capturing the wax in the mesh webbing.
In use the adhesive is treated much like conventional wax. The user places the adhesive on the worktable, heats it, then places the part on top. After a few seconds, the adhesive sets, holding the part.
The company also offers a venturi vacuum system. Vacmagic VM300 operates from standard shop air and produces about 13 psi (65 N/cm2) of holding force. The unit also doubles as a precise pallet changer that can be changed in seconds. "This is a simple vacuum system. Within an hour of receiving the unit the customer can be making chips," says Dave Bishop, company general manager.
Vac-U-Lok (Rockford, IL) specializes in custom workholding systems. "Educating the customer is our biggest challenge," says Doug Green, company president. "Vacuum is the most underutilized holding system available.
"Often I see job shops expending a lot of effort repetitively machining single parts," he says. "A more efficient method is to machine multiple parts from a single sheet of material held down by vacuum.
"For example," explains Green, "instead of machining a single 2 x 2" [50 x 50-mm] workpiece, you could use a vacuum chuck to hold down a larger plate and then machine a number of parts with one quick setup. The idea is to let the machine do the work and avoid the saw-cutting operation."
Vacuum chucks are used in many applications, including horizontal and vertical machining as well as turning and assembly operations, drilling, milling, grinding, welding, painting, routers, and laser cutting.
A typical system consists of a vacuum chuck with manifold, gasket material, vacuum system, safety system (which sends a signal to shut down the machine if a loss of vacuum is detected), and an automatic coolant return system for wet applications.
The 2L Corp. (Hudson, MA) got its start in vacuum workholding by filling a need for an aerospace customer.
The company's vacuum-table consists of pallets measuring 8 x 12" (203 x 305 mm). Up to eight pallets can be linked together in Lego-like fashion to handle larger parts.
Vacuum is drawn by a conventional pump or with a venturi attachment.
"The venturi is preferred by customers that don't use the vacuum plates frequently, have a supply of shop air available, and desire the lowest up-front cost to begin using vacuum chuck systems," says company president Lance Nelson.
Average holding power is about 10 psi (0.7 kg/cm2) at each of the tapped holes in the plate. Holes are spaced 1" (25-mm) apart. So a 4 x 4" (10 x 10 cm) part which has a 16 in.2 (100 cm2) area would experience 160 lb (72.5 kg) of downward holding force.
Vacuum calculations are based on part size. So the larger the part area exposed to the vacuum the more holding force. This means that machining can be more aggressive on the larger parts.
"Side loads are the Achilles heel of nonclamp workholding," says Nelson. "With some parts it's enough to rely on friction to keep the part from sliding, particularly if you are working with a heavy part. But there are other tricks. Light, shallow cuts is one, and we offer a line of tools for that need.
"Another is using side clamps or stops. That's why we have the tapped positioning holes. With the sides constrained, you can make a full-power milling cut," Nelson concludes.
This article was first published in the May 2008 edition of Manufacturing Engineering magazine.