Vises Used As An Alternative To Custom Fixturing

Impact Forge, Inc. (Columbus, Indiana) is comprised of four companies that offer a range of forging technologies, including producing hammer dies requiring large milling operations. The company reports that positioning and clamping these dies in a machining center can be challenging because of characteristics such as heaviness and bulkiness.

Because the dies can be as heavy as 1,500 pounds and as large as 24 inches by 26 inches by 12 inches, positioning them in a machining center fixture requires an overhead crane and skillful maneuvering so as not to damage the part, fixture or machine. Equally challenging are holding and clamping these dies because many manufacturers rely on some form of custom-constructed fixtures. These are designed with the appropriate amount of mass and clamping points to securely hold the die throughout long, complex milling cycles. Although standard holding devices such as vises have not traditionally been regarded as a practical method for positioning and clamping the hammer dies, the company decided to explore them as an alternative to its expensive, custom-designed fixturing.

"Our custom fixtures worked well, but they were slow to assemble," reports Doug Lynch, applications engineer at Impact Forge. "So, we examined various types of vises, as well as modular fixturing. With the array of standard products available, we wanted to investigate all of the alternatives and ascertain if reducing die setup time and front-end costs were possible. The expansion of our hammer die volume also necessitated a more cost-effective workholding method."

The company ultimately chose tooling offered by Kurt Manufacturing Company (Minneapolis, Minnesota) because of the availability of multiple vise sizes. "We were familiar with the Anglock design for accurately centering and gripping heavy parts for machining, and this also contributed to our decision," says Mr. Lynch.

The company selected two vises, Kurt 640 XLS models, with an open jaw capacities as large as 14 inches. All sides of these vises are machined to accuracy requirements within 0.001 inch, which was acceptable for aligning the company’s setup. Mounted vertically on a die plate, the jaw openings of the vises also provide a clearly defined path for the hammer die as it is lowered and moved into the jaw openings by the overhead crane.

"The crane raises the die on its side and then lowers it into the dual vise fixture," explains Mr. Lynch. "A round dowel centered between the two vises acts as a locator, which matches up with a dowel pocket in the hammer die, guiding and centering the die in front of the two vise jaw openings. Once the vises are moved into the open jaws, the operator manually tightens them and drops the load. After re-tightening, the operator removes the crane from the machining center work area. At this point, the milling operations begin."

With the previous custom fixture, positioning the die entailed adjusting and tightening several knobs to ensure that every component was aligned "just right." The company reports that the new method is not only easier for the operator, but it is also 50 percent faster.

The company considered hydraulic clamping for the setup but decided against it because hydraulic failure might cause a heavy die to become loose.

Mr. Lynch says that the manual vise setup is simple to operate, and the testimony to this is the fact that the setup has worked problem-free for more than 2 years.

Delivering as much as 6,877 pounds of clamping force, the Anglock design aids in automatically positioning the die in the vise jaws. As clamping force is applied, the die is pushed down and onto the base, which is also said to align it accurately. The Anglock design also resists the tendency for part lift during milling, the manufacturer says. Other potential benefits derived from this setup are the elimination of chatter and vibration, which can be attributed to the stability provided by an 80,000-psi ductile iron vise body.

An MA60H machining center by Okuma America Corporation (Charlotte, North Carolina) was also integrated to meet the company’s requirements. The system features feeds and speeds of 150 ipm, with the spindle speeds as high as 12,000 rpm. Roughing the larger areas with a 3-inch face mill at 700 rpm (50 ipm) put the stiffness of the fixture to the test. Most operations are finish milling, all are performed using 1/8-inch to 1-inch ball cutters (with carbide inserts in the larger sizes). One to one-half of the hammer dies are completed per 8-hour shift with the system operated 24 hours a day, 6 days a week.

"The results of changing to Kurt vises from custom fixtures have been favorable," reports Mr. Lynch. "We’re now operating at maximum capacity with the present system. Plans to install a second machining system with a similar Kurt vise setup are on the horizon."