An Innovative Prototype
There’s a great publication called “Cutting Tool Engineering” magazine that has been supporting the manufacturing and CNC machining world for years. Bill Kennedy is a contributing editor of the magazine. He has selected John Prosock Machine as a subject for his text article in his “Part Time” column. He will be doing an interview with our shop manager about a machined part that we have recently made for a customer. We will post the article when its finished. In the mean time here’s one of Bill’s recent articles.
BY BILL KENNEDY,
A manufacturer developing a small electric motor with a cast aluminum housing wanted to work with a prototype before investing in tooling for producing the castings. Innovative Machining Inc., a job shop that handles engineering design and both production and prototype manufacturing, engineered a way to efficiently machine the
prototype from aluminum bar stock. Measuring 6″ in diameter x 3″ long, the complicated front half of the housing had a 1.85″-deep cavity on one end, a variety of holes and a contoured channel on the other, and an array of cooling fins around the OD. Using an IGES CAD file supplied by the manufacturer, Innovative created turning and milling toolpaths with Mastercam CAM software to machine the housing from a 6″-dia., 31⁄4″-
long bar of 6061-T651 aluminum. Initial roughing took place on a Mazak Quick Turn 20N CNC lathe. A 0.850″-dia. hole was drilled through the center of the part’s axis, and the hole was counterbored at a diameter of 1.378″ to a depth of 1.0″. A grooving tool held in a boring bar cut a 0.056″-wide, 1.464″-dia. keyway 0.30″ deep in the counterbore. Then the part was turned end-toend in the lathe chuck and rough-bored to within 0.100″ of final dimensions.
The rest of the process required three setups on a Haas VF-3 vertical machining center. Programmer Seth Cross said one of the main challenges in making the part was “timing,” or aligning, the part in the three fixturings to ensure correct relationship among the housing’s complex features. For the first setup, the part was clamped on the VMC’s table with custom-made aluminum soft jaws, and located using the center bore made previously on the lathe. First, a 1⁄2″-dia. SGS Ski-Carb endmill, run at a 7,500-rpm spindle speed and 50-ipm feed rate, roughed the inside of the part. One pass left 0.010″ of excess stock in the cavity, and a second pass finished the casting bottom. Cross said maintaining the required 32 Ra surface finish was difficult, because the cavity’s depth made it a long reach for the cutter. The solution was to “slow everything down.” Next, 18 flats, each 0.700″ wide, were milled around the cavity wall with a 5⁄16″-dia., 2-flute Garr endmill run at 2,400 rpm and 10 ipm. A 45º chamfer mill, ground for use as a spotting tool, marked locations for two holes in the bottom of the cavity and for 14 other holes along its rim. Then a 3⁄4″ drill made two through-holes in the cavity bottom at 1,146 rpm and 4.58 ipm. In a cutout area in the cavity bottom, a 1⁄4″ drill run at 4,584 rpm and 22.92 ipm started a hole that was finished to a 0.150″ depth with a 1⁄4″ flat-bottom drill applied at the same parameters.
In the locations spotted earlier on the housing rim, a 3⁄16″ stub drill made six 0.750″-deep holes at 4,584 rpm and 16 ipm. Then, after a No. 31 (0.1200″-dia.) stub drill made two 0.05″-deep holes in the rim at 7,162 rpm and 13.5 ipm, a No. 30 (0.1285″) reamer run at 3,357 rpm and 30 ipm brought them to final dimensions. Next, a long spotting drill, run at
2,292 rpm and 10 ipm, located three places where a No. 43 (0.89″) drill run at 9,048 rpm and 12.6 ipm made three 0.270″-deep holes. Those three holes were threaded to a depth of 0.220″ with a 4-40 tap at 800 rpm and 20 ipm. Cross said one of the major challenges
of this first setup was tapping these holes. They were so close to the cavity walls that there was insufficient clearance for tool extensions, so he was forced to apply an extended tap at reduced cutting parameters. To complete this setup’s operations, which took about half an hour, a chamfer mill cleaned up the bottom of the cavity at 1,500 rpm and 45 ipm. For the next setup, the housing was flipped 180º. Cross made a fixture plate
with pins to fit the 3⁄4″ holes drilled earlier. “We slipped the part down on the pins and bolted it through the counterbore in the center,” he said. The 1⁄2″ Ski-Carb endmill was applied first, at 9,168 rpm and 73 ipm, to facemill the end of the part, leaving 0.070″-wide x 0.093″-high rings of material around each of the three large through-holes. Then a 3⁄16″, 2-flute endmill machined a twisting, 0.5″-wide x 0.75″-deep channel around the top of the part at 9,000 rpm and 30 ipm. Cross said, “It took a while to machine the channel. I had to step down in light steps—0.100″ or 0.125″—so I didn’t break my tooling.”
Next, a 1⁄2″ chamfer mill, ground as a spotter and run at 6,000 rpm and 12 ipm, located six points on the rim where a 3⁄32″ drill then made 1.5″-deep holes at 5,496 rpm and 13.8 ipm. Next to each of those holes, a No. 2 (0.221″), applied at 3,885 rpm and 15.6 ipm,
drilled through the part. A 3⁄8″ Ski-Carb endmill counterbored those six holes to a depth of 0.885″. This set of operations took about 1 hour. The third milling setup was on a vertical rotary table, which served as the Haas machine’s 4th axis. The housing was clamped onto the table in a 3-jaw chuck. The fixture, featuring locating pins, was designed to hold the part about 4″ away from the chuck to provide tool clearance. The housing OD has seven flats along its axis.
Six are 0.560″ wide and one is 1.7″ wide, and they were milled with a 1⁄2″ Ski-Carb endmill at 9,000 rpm and 76 ipm. Then a chamfer mill spotting tool, run at 6,000 rpm and 10 ipm, marked one hole location in each narrow
flat and nine locations in the wide flat. A 0.261″ G-drill run at 3,291 rpm and 15.6 ipm then drilled two holes in the wide flat, located 0.400″ from the front of the part. A 1⁄4-28 STI tap threaded those holes at 800 rpm and 28.57 ipm to prepare for later insertion of helicoils. Then, between the 0.261″ holes and the front of the part, a 1⁄4″ Ski-Carb endmill made two 0.600″- dia., 0.375″-deep holes at 5,612 rpm and 42 ipm. The same endmill, run at 6,112 rpm and 42 ipm, also made a 3⁄4″- dia. through-hole centered 0.700″ from the housing’s back edge.
Around the 0.600″-dia. hole, a No. 43 (0.089″) drill run at 9,648 rpm and 12.6 ipm made four holes 0.375″ deep, which were then threaded to a depth of 0.250″ with a 4-40 tap at 800 rpm and 20 ipm. Holemaking concluded after a No. 17 (0.173″) drill run at 4,965 rpm and 15.3 ipm made a 0.370″-deep hole in each of the 0.560″ flats, 1.050″ from the back edge of the housing. The holes were threaded with an 8-32 tap at 800 rpm and 25 ipm. A final machining challenge—80 cooling fins, each 0.34″ deep, arrayed axially around the housing—was overcome with two cutters designed by Shawn Gibbs, Innovative’s general manager, and Dean Kerbs, shop floor manager. Each cutter was tooled with three carbide inserts. One cutter machined a 0.026″ radius at the fin base and a 0.027″ radius at the top, and the other, engineered to make the fins positioned next to the axial flats, produced only the base radius. The cutters ran at 1,900 rpm and 19 ipm. Run time for the operations in the third setup was 45 minutes. Postmachining operations included selective anodizing of parts of the housing service, and installation of helicoils. Three prototypes were produced. Gibbs noted that the housing “had a lot of difficult challenges,” but it’s typical of Innovative’s work. In many cases, he said, solving customer problems on specialized prototype parts leads to production-level contracts on other jobs.
For more information about Innovative
Machining Inc., Wheat Ridge, Colo.,
or call (303) 421-1006.