Chucking production bottlenecks

March 1, 2000
The inherent stiffness and robustness of a vertical chucker delivers precision and accuracy for increased production.

The inherent stiffness and robustness of a vertical chucker delivers precision and accuracy for increased production.

A typical cast iron stator housing bore contains a rabbet counterbore at each end of the bore and a terminal-box recess. One roughing tool and one finishing tool completely machine all surfaces, eliminating the need to index tools between operations.

Motch modified the housing on each of its chuckers at Baldor to accommodate an overhead hoist for easy loading and unloading of castings.

Baldor Electric runs a four-machine Motch Vertical Numerical Chucker line. The newest 125 VNC is in the foreground with a larger 135 VNC just behind it.

Production responsiveness is critical when a manufacturing company must react instantly to customer orders, regardless of the number of units needed. Just ask Baldor Electric Corp., Ft. Smith, Ark., a company that prides itself on offering the industry's broadest line of stock motors and drives with the shortest lead times on customer orders. To reach and maintain this production level, the company has improved its manufacturing resource planning. The result has been a quicker turnaround in products without a sacrifice in quality and reliability.

One such improvement involved unplugging a bottleneck in machining its cast-iron motor stator housings. Traditionally, these housings — which range from 8 to 14 in. in length, with bore sizes from 6 1 /2 to 9 1 /2 in. — were machined on three separate systems. They were faced on an engine lathe, transferred to a 36- in. vertical turret lathe for boring, and finally the rabbets (counterbores) were milled at each end of the housing. The manufacturing line produced about 100 housings/week and required frequent maintenance to remove abrasive cast iron chips and debris from the machines.

A vertical chucker takes over
Baldor upgraded its production with CNC vertical chuckers because, as Manufacturing Engineer Carl McBride explains, "The relatively thin wall of the stator housings makes it an ideal candidate for vertical machining." The company compared a number of systems and finally selected a standard Motch 125 VNC (Vertical Numerical Chucker). "We felt the machine's inherent stiffness and robustness could provide the precision and accuracy we needed at a greatly increased production rate," says McBride.

Motch Corp., headquartered in Cleveland, manufactures vertical and horizontal turning machines, vertical grinders, and self load-ers. Although its standard single-spindle 125 VNC is equipped with a six-position turret (up to 12 available), only two tools — a roughing tool and one for finishing — make the stator housings. A third position locates and centers the casting in the chuck. Each tool faces, chamfers, cuts one rabbet, bores, and then chamfers, cuts the other rabbet, and faces the under-side of each casting. Each tool completely machines all features, eliminating tool-indexing time during roughing or finishing. Cycle time for roughing and finishing the largest casting (9 1 /2-in. I.D.) averages four min, and because of their flexibility, the Motch 125 VNCs easily handle typical stator housing lot sizes of 40 units.

Machining the stator housings poses some difficult problems. "In addition to the notorious difficulties of machining cast iron, an interrupted cut resulting from a terminal-block recess in the stator bore generates a shock load that is transferred to the tool during boring," explains Joel Mosely, Baldor corporate manufacturing engineer. "This could affect precision at high production rates. However, the inherent rigidity of the Motch 125 VNC minimizes the effect of this shock load, making it possible to hold a tolerance of ±0.001 in. on all dimensions."

Production on the original Motch vertical chucker rose to about 60-70 stator housings per eight-hour shift — an increase of 90% over that of the three-machine manual line it replaced.

The line expands
As stator housing production requirements steadily rose during the next several years, Baldor manufacturing engineers chose additional Motch vertical chuckers to keep pace. Shortly after the first 125 VNC went into service, a second, larger machine was ordered to machine bigger stator housings. This 135 VNC is virtually identical to the original system, except for its 35-in. swing (the 125 has a 25-in. swing). Baldor then added a second 135 VNC.

Motch application engineers realized that the stator housing machining application did not fully use the capacity of the larger machines. They suggested adapting a smaller 125 VNC to the specific needs of the application as a more cost-effective solution. Thus in January 1999, the newest member of the Baldor/Motch line — a specially designed, stretched version of a Motch 125 VNC — went into service. Responding to the needs of Baldor manufacturing engineers to provide a machine that could handle the largest stator housings, Motch designers modified the riser on a new 125 VNC, which increased vertical clearance by seven inches and extended the original 18-in. Z axis an additional seven inches.

Other modifications included a 50- hp Baldor Vector Drive for high-torque output and Baldor pump motors.

In addition, the housing of each vertical chucker in the line was modified by Motch to accommodate a jib crane for loading and unloading castings.

"We are impressed with the willingness of Motch designers to modify their standard machine to meet our special needs," comments Mosely. "Al-though our newest vertical chucker cost about 25% less than a larger 135 VNC, we did not sacrifice any capability for machining stator housings."

Two operators keep the four-machine, Motch vertical chucker line operating three shifts per day, five days per week, with a capability of producing about 3,500 cast iron stator housings per week. Production has increased from 100 stator housings a week to a capability of machining 750 units per day. The Motch VNCs are flexible, and any size stator housing can be cut on any machine. However, to minimize setup and changeover requirements, Baldor groups housing sizes, assigning them to specific machines.

The vertical chuckers have required little maintenance, says McBride, explaining that the chip enclosure covers on each machine have virtually eliminated the company's abrasive cast iron chip problem. The machines have been so reliable, in fact, that the original Motch 125 VNC has been operating since 1987, requiring only scheduled maintenance.