Benchmark Manufacturing now does its own precision grinding on drive-shaft rotors.
Benchmark Manufacturing, a division of Attica Hydraulic Exchange Corp., once farmed out its production grinding work. However, doing so grew impractical as the shop's manufacturing environment transformed to just-in-time deliveries. Benchmark Manufacturing needed a grinding machine.
A large portion of Benchmark Manufacturing's work is remanufacturing and reverse engineering vane and piston pumps. It runs 14 to 16 hours a day, producing lot sizes that average 25 pieces. The shop operates at 98 percent same-day shipping, and just about every day gets "hot" orders for parts that upset scheduling, so the grinder it chose had to be easy to set up and offer quick changeovers, while being flexible and reliable with good repeatability. That machine was a Studer S33 universal grinder from United Grinding Technologies (www.grinding.com).
The S33 lets Benchmark do both I.D. and O.D. grinding, and with a rotating B axis, the machine eliminates having to dress chamfers into grinding wheels. Instead, the shop manuevers a straight wheel mounted in the machine's rotating head around the part to grind all its chamfers.
Ed Vienot, production manager at Benchmark, says the S33 provides the necessary grinding capabilities in a single setup. "Most importantly is how fast we can set up the machine for a variety of different jobs. In any given day, we may be doing an O.D. job, followed by an I.D. job. Then, as hot jobs come in, we might have to interrupt a job, set up and run the hot job, then re-set up the original job," he says.
According to Vienot, one of the shop's more difficult jobs is a drive-shaft rotor that features a large center diameter (5 in. on the O.D. and 3.1255 in. on the width), and has two shafts that extend from each side of the center. These shaft rotors range in size from 6 in. to 20 in. in length and can weigh from 2 lb. to 30 lb. They have a 58/62 Rc hardness rating, and the overall tolerance for all diameters is 0.0002 in.
The squareness between the center diameter to the O.D.s that extend from the center section must be held to 0.0001 in. Vienot says they hold this tolerance tighter than required.
"We actually measure this with a 0.001-in. indicator, and if we see any movement at all, we correct the problem. However, with the S33's precision, this hasn't proved a problem," he says.
The shop does this part without a wheel change, going in at a 30-degree angle and grinding the face of the large center diameter (holding a 0.0001-in. tolerance) while simultaneously grinding the first small diameter of the center (2.250 in.). After that, it runs an automatic dress cycle, and goes on to the next O.D. (2.2455 in. and a seal surface), still at 30 degrees. It dresses again, then grinds the third O.D. (1.875 in. and the drive extension of the shaft). Dressing this often helps to hold tight tolerances and to maintain the part's demanding surface-finish requirements.
Benchmark's S33 is equipped with the Movomatic, in-process gaging accessory that handles non-interrupted diameters. The machine also has Studer's Sensitron system for automatic changeover from infeed to grinding feed to help to reduce the shop's cycle times.
To maintain the required surface finish, the S33 lets Benchmark control the speed of the wheels as they go across the dressers. This is important for maintaining the specific finishes on the shaft seals, and to eliminate leaks when the shafts are put into use.
Vienot says the machine's rigidity, wheel configuration and wheel-dressing flexibility combine to help his shop to keep the exacting micro finishes on the shafts. "We have a dresser that's on the back of the workhead we use for O.D. wheels and an optional table-mounted dresser with a single-point diamond for I.D. dressing. In addition, we have a rotary dresser used when grinding the ball end found on many piston bodies," says Vienot. These balls get pressed into what is known as a slipper or shoe, working together as a type of universal joint. Balls bind up and seize if ball roundness and finish are not perfect.