Protomatic Inc., Dexter, Mich., specializes in efficiently machining small volumes of precision metal parts, whether prototypes or production runs. One example is a series of prototype automotive throttle plates.
A throttle plate controls airflow into an engine and thereby determines its output. In production, the plates are stamped in volumes of thousands per day. Protomatic typically makes prototype and limited-run plates in lots of 100 to 500. The prototype plates are used for testing. “The fuel system, brakes or anything that is safety critical goes through a battery of tests,” said Doug Wetzel, general manager. “It is not uncommon that one of the big automotive companies will do 200 to 500 plates in different dimensions, varying by 0.0005 " or 0.001 ". They don’t want to have throttle-sticking problems.”
Protomatic manufactures the plates out of CA260 brass or 6061T6 aluminum in diameters from 60mm to 75mm and thicknesses from 1.1mm to 3.1mm. For brass plates, squares of stock are cut from a 36 "×96 " sheet with a shear. For a plate with a final diameter of 75mm, the squares are about 75mm on a side. Then two mounting holes are drilled through the plate on a Haas or Hardinge CNC mill. “We like to buy American,” Wetzel said, “so we use domestic machine tools.”
The holes, typically 0.130 " in diameter and reamed to ±0.001 ", are drilled 1 " apart. Protomatic holds the stock with clamps or in a vise. “For the volumes that we are doing, we hold them individually,” Wetzel said. “We don’t stack them because we want to control the hole diameter very tightly.”
The next step is milling a 6° angle of a varying width around the edge of the plate. The slanted, compound elliptical shape that results ensures the plate edges are parallel to the bore walls when the plate is tilted 6° in the closed position in the throttle body bore.
Initially, the shop machined the plate edges on a lathe, using a milling head. The square piece of stock was screwed onto an angled mandrel that had locating pins. “On a lathe it was effectively a one-at-a-time operation,” Wetzel said. “Cycle time might be a couple minutes, then you have another minute unscrewing one plate and putting on the next one.”
The operator was almost continually engaged in mounting and unmounting plates. As plate volume increased, Protomatic sought to free up operator time and make the process faster and more repeatable.
The answer was a custom magnetic fixture to hold a piece of stock at an angle on the bed of a CNC mill and a special cutter programmed to perform circular interpolation around the plate edge. The shop puts eight fixtures on the mill table. The longer cycle time allows the operator to run another machine or perform another task, Wetzel noted.
Courtesy of Protomatic
This electromagnetic fixture, featuring a steel top plate that enables it to hold nonferrous parts, was engineered by Protomatic to improve process speed and repeatability when machining prototype automotive throttle plates.
Courtesy of Protomatic
Protomatic machines prototype automotive throttle plates in diameters from 60mm to 75mm and thicknesses from 1.1mm to 3.1mm.
The approximately 5 "-tall fixture has a wedge-shaped aluminum base that tilts the part at the desired 6° angle. Bolted to the base is a low-carbon steel flange with a central hole. In the center of the flange is one pole of a 20w electromagnet. The pole has pins that match the holes on a throttle plate. In use, the throttle plate stock is placed over the pins, and a 0.200 "-thick, 2 "-dia. steel cap, also with holes matching the pins, is placed over the plate. The outer rim of the flange forms the magnet’s other pole, and the cap closes the magnetic path through the nonferrous plate.
An M code in the machine tool control activates the electromagnet. Wetzel said it is a problem to hold the thin throttle plates flat, and with the magnet “we probably have 300 lbs. of force holding that plate down in a nice, uniform fashion.” He noted that the magnet coil generates heat, which can go into the part and is a concern because of the ±0.0005 " tolerance. However, flood coolant and the operation’s short cycle times minimize the opportunity for heat to change part dimensions.
Before milling the plates, the square stock is trimmed to an octagon to minimize interrupted cuts. The custom milling tool, produced by Rojo Industrial, Novi, Mich., machines the plate edge at 3,500 rpm and a 15-ipm feed rate. “Accuracy is more important than speed,” Wetzel said.
Next, two 8mm-long flats are machined on opposite edges of the plate at the point where the shaft that holds the plate pivots in the throttle body bore. A 3⁄8 "-dia. carbide endmill mills the flats at 3,500 rpm and 15 ipm.
Machining the OD angle and flats generally leaves a burred edge. Protomatic deburrs the features with a custom tool featuring a V-shaped cutting edge, which chamfers the top and bottom of the plate simultaneously.
“We don’t want to hand deburr the part,” Wetzel said, adding that the shop’s philosophy is to automate whenever possible. “A lot depends on the volume; if we are doing more than 500 parts, we automate the process.”
Protomatic has converted much of its machining equipment from 3- and 4-axis to 5-axis operation. Wetzel said: “We use multiple axes even in a prototyping environment to reduce handling and ensure consistency. We have very talented people, but even talented people occasionally get distracted. The goal is to mistake-proof the operation when doing critical components.”
After the operations on the mill, the plates go through a coining or stamping operation in a special press. “We make a small dimple, 0.020 " to 0.030 " deep on a 0.060 "-thick plate, just enough so people know how to orient it in assembly,” Wetzel said. The manufacturing operations may conclude with abrasive tumbling to impart a matte finish while removing less than 0.0001 ".
Protomatic was founded in 1971, largely to serve automotive customers. Then, amidst the massive upheaval in the U.S. automotive industry, the company converted from 80 percent automotive to 80 percent medical and also serves the aerospace industry. “Since we had a highly talented staff and we were used to doing higher precision parts, the medical and aerospace parts were a perfect blend for us,” Wetzel said. “They are very similar because they are mission-critical types of parts. Even though they are quite different, serving both of those industries makes logical sense.”
Related Glossary Terms
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
Device, often made in-house, that holds a specific workpiece. See jig; modular fixturing.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- inches per minute ( ipm)
inches per minute ( ipm)
Value that refers to how far the workpiece or cutter advances linearly in 1 minute, defined as: ipm = ipt 5 number of effective teeth 5 rpm. Also known as the table feed or machine feed.
Process of generating a sufficient number of positioning commands for the servomotors driving the machine tool so the path of the tool closely approximates the ideal path. See CNC, computer numerical control; NC, numerical control.
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
Workholder for turning that fits inside hollow workpieces. Types available include expanding, pin and threaded.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- outer diameter ( OD)
outer diameter ( OD)
Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.
Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.
- reaction injection molding ( RIM)
reaction injection molding ( RIM)
Molding process that allows the rapid molding of liquid materials. The injection-molding process consists of heating and homogenizing plastic granules in a cylinder until they are sufficiently fluid to allow for pressure injection into a relatively cold mold, where they solidify and take the shape of the mold cavity. For thermoplastics, no chemical changes occur within the plastic, and, consequently, the process is repeatable. The major advantages of the injection-molding process are the speed of production; minimal requirements for postmolding operations; and simultaneous, multipart molding.
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.