High speed machining can be a risky proposition. At high speeds, an out-of-balance toolholder can cause costly damage to valuable spindles and workpieces. With spindle re-builds running as much as $25,000, a damaged spindle can mean a severely injured balance sheet. But trial-and-error balancing can also be costly, using up valuable shop time—especially critical in short-run environments where time is of the essence.
Cummins Aerospace is a Southern California shop that specializes in complex parts for the aerospace industry. Because many of the company's runs are very short and involve a multitude of operations, fast setup times are critical to keeping costs under control.
Recently the shop acquired a Deckel- Maho DMC 125-U machining center that's capable of multiple operations at speeds up to 18,000 rpm. This high speed capability has significantly increased Cummins' ability to take on the most advanced projects, but it has also placed greater demands on equipment and personnel. Says Sean Cummins, president, "At the speeds we're running, precise balance is absolutely essential."
At such speeds, an out-of-balance condition presents some very real dangers: burnished and burnt spindle surfaces, destroyed spindle bearings, ruined cutters and, of course, a poor finish and ruined workpiece. In longer runs, it's even possible for an imbalanced toolholder assembly to deform a spindle so badly that it must be rebuilt or replaced.
Before Cummins Aerospace acquired the high speed machining center, pre-balanced toolholders were adequate to ensure a decent finish and prevent spindle damage. But now, with speeds between 8,000 and 18,000 rpm as part of the daily routine, many operations require a balance grade up to G2.5. Most toolholders are pre-balanced to a grade of G6.5, good enough at conventional speeds, but not adequate for high speed machining. And even a perfectly balanced toolholder can be thrown out of balance by an imbalanced cutter. So Cummins had to find a way to quickly and precisely balance an entire tool/toolholder assembly as a single unit.
As machining speeds have increased, some shops have tried to "get by" by grinding or drilling toolholders to reduce vibration. But Cummins has a strict policy against it. "It's poor shop practice," says Ian Teale, manufacturing supervisor. "We're not willing to damage our equipment like that. If you over drill one side, you need to offset it with another hole on the opposite side. Drilling and grinding quickly eats up time and toolholders."
After evaluating several balanceable tooholders using rings and adjustment screws, Mr. Cummins and Mr. Teale settled on the Vectaron balanceable toolholder from Tecnara Tooling Systems (Santa Fe Springs, California).
The Vectaron uses a ring and weight system. "It's quick and very precise," says Mr. Teale. "Once you know where the imbalance is, the entire tool and toolholder assembly can be rebalanced in a matter of seconds." A balancing ring covers 36 pockets in two layers around the circumference of the holder. To compensate for any imbalance, the operator uses a magnetized pickup tool to place a ball bearing in the appropriate pocket. (The balancing balls come in three weights.) A collet built into the balancing ring prevents any movement during abrupt starts and stops; the cover prevents the weights from being ejected during operation.
"The system can be used with almost no training," says Mr. Teale. "The weights are easy to place, and they stay put until we remove them. It's faster and more accurate than a ring system, and it's completely trouble-free." Even with the Vectaron, imbalances must be located and measured before they can be corrected. Without a balancing machine, there's no way to do this with any precision. Of course, the operator may feel vibration. Poor workpiece finish is another indicator of an out-of-balance condition. But those clues don't tell the operator where—or how much—correction is needed. In that situation, the only solution is trial and error, which is no way to run an efficient shop.
With many of Cummins' work products requiring multiple high speed operations, the top priority was to find a system that could quickly balance an entire tool/toolholder assembly to balancing grade G2.5. The company's system of choice was the German-made Haimer FH-1 Tool Dynamic balancer, also sourced from Tecnara. The FH-1 consists of the balancing machine itself and a PC with Haimer's tool balancing software installed.
To locate any imbalance, the mounted tool/toolholder assembly is placed on the FH-1. It's given a spin, then rotated 180 degrees on the mount and rotated again. When the assembly stops spinning, the Haimer software vividly displays the location and mass of the imbalance. At that point, the weights are placed in the indicated position, completing the balancing operation.
The FH-1 can read in two planes along the toolholder axis, providing an extra degree of precision when balancing a long milling chuck or collet extension. Once a toolholder is balanced, the Haimer software stores all of its parameters.
Total time for the entire process is usually about 1 1/2 minutes. Although it's possible to run a confirmatory test with the weight in place on the Vectaron, Cummins' experience has been that the initial placement is invariably correct.
The FH-1 can be used to balance any toolholder, so Mr. Teale has instructed shop personnel to balance any job over 5,000 rpm. Says Mr. Teale, "It's a small investment in time, but it means one less wild card in the process. And it greatly reduces the worry factor."