How do you minimize unit cost for complex parts in need of turning, milling and drilling? For Steve Hattori, the answer is simple: Machine them with just one setup--and make that setup as simple as possible.
The president of the contract shop Salinas Valley Precision (Salinas, California), Mr. Hattori used to need a minimum of three machines to produce any complex round part from solid: a bandsaw to cut the stock to length, a lathe, and a machining center to add flats and holes. Now, the shop lets a single machine do all of this work--converting uncut barstock into finished parts automatically, effectively replacing numerous setups with one.
"Anyone who would argue for the old way has never tried to machine a fishing reel," says Mr. Hattori. In fact, the first time a local sporting goods manufacturer offered him the fishing reel part, Hattori declined. Back then, the part was too complex to be profitable. Cut from solid aluminum with 90 percent of the stock machined away, the 3.25-inch diameter reel consists of two easy-to-warp flanges less than 0.1 inch thick, connected by a hollowed-out core with walls not much thicker.
The part has to be attacked from all sides. Each flange is penetrated by 12 holes, 0.45 inch in diameter, which serve to reduce weight. The core circumference includes 12 oval-shaped weight-reduction slots as well. Finally, a counterbore on one face includes 36 serrations for the fishing rod ratchet mechanism, plus a press-fit bearing bore with a total tolerance of 0.0002 inch on diameter.
"And all that geometry was only half the problem," says Mr. Hattori. "The rest was batch size. We could have machined the part using complex fixtures on the machining center, along with the CNC lathe we were using at the time. But this would have incurred a large setup cost, one that was impossible to amortize over the part's small runoften as low as 100 pieces."
Mr. Hattori did machine the fishing reel prototype, but didn't bother to bid on the production part. At least, not at first. Since then, however, the part has become a major source of revenue for the 10-employee shop. That came after Mr. Hattori reinvented his process.
The key result of Mr. Hattori's ingenuity is a process where all turning, milling and drilling operations are combined on a single machine, a Mazak "Super Quick Turn" (SQT) 15MS-Y CNC turning center. Besides carrying a complement of rotary tools for milling operations and an opposed second spindle for second-side turning, the machine is equipped with a programmable Y-axis with four inches of stroke. This Y-axis capability allows off-center milling operations to be performed while the workpiece is still held in the primary or secondary spindle.
Through coordinated, automatic hand-off between the two spindles, every surface of the part can be reached without any human intervention. Every turning, milling and drilling operation is performed without "turning loose" of the part, thus critical workpiece position references are maintained throughout the entire machining process.
Workpiece blanks are fed to the machine with a Mazak Cut-Feeder. Just as mill-drill capability lets the turning center take the place of a machining center, the Cut-Feeder eliminates the band saw from the process, and it also cuts the delay between consecutive part cycles to just a few seconds. The Cut-Feeder not only advances barstock, it cuts it to length, from 0.60 to 5.11 inches. It executes this cutoff while the SQT machines the current part, then loads the new workpiece through the chuck as soon as the lathe is ready for the next cycle.
The automatic feeder helps minimize setup time. The unit stores up to five of the 3.25-inch diameter bars used for fishing reelsand advances and cuts each one without rotation. Rather than a conventional bar feed methodology, the Cut-Feeder holds the bar stationary while the cutoff tools rotate around the stock, slowly feeding in until the cutoff is complete. Because the bars don't turn, no pre-machining is needed to make them round (even square or hexagonal profiles work). As a result, Mr. Hattori can simply drop one or more bars into the feeder and let the turning center convert them into finished machined parts. Final deburring is the only operation that is performed off the machine.
The machining process involves 20 tool changes but requires no operator, so Mr. Hattori often lets it run unattended--getting back 90 reels for each 12-foot bar he loads.
Return On Imagination
The reels come out of the turning center at the rate of one every eight minutes. Though the cycle does take advantage of the machine's high spindle speeds--5,000 rpm for main and second spindles, 3,000 for rotary tools--Mr. Hattori credits creative programming as the primary reason that cycle time is so low. A former machining specialist for an aerospace R&D firm, Hattori opened his shop 10 years ago so he could continue to devise complex CNC machining strategies after his employer closed and shut down.
"The multitasking lathe provides greater payback for a programmer's imagination than any other I've used," he says. "With so many options in one machine, there's always another way--and often a better way--to produce any feature."
For example, Mr. Hattori found an unusual method for turning the aluminum reel's thin flanges at high speed without deflection. After all of the features in one face have been machined while the part is in the main chuck, Mr. Hattori's program has the second chuck take hold while still maintaining the primary chuck grip. Gripping the part from both sides in this manner eliminates the deflection problem. As the two spindles rotate in unison, the metal between the two flanges is hogged out as if it were a single groove, 0.75-inch deep.
The Y-axis capability is used to produce the ratchet serrations. Instead of indexing the part with the spindle to cut each indentation--the way the job would be done on a traditional live tool lathe--Mr. Hattori has the main spindle hold the part still while the turret moves from one serration to the next, using X-Y interpolation. While the turret makes the move only slightly faster than the indexing spindle, when that difference is multiplied by 36 serrations, the productivity gain is pronounced.
In other cases, the Y-axis produces features that could be machined on a conventional three-axis (X, Y and C) turn/mill machine. One example is a 4340 steel ratchet wheel for the timing mechanism of a robotic assembly system. The part is 3.25 inches in diameter with a 3/16-inch thick disc and a wider integral hub, and it looks like a saw blade with 12 teeth around the circumference. The Y axis allows 12 flats to be machined around the blade's circumference. "Each flat is a simple cut," says Mr. Hattori. "We used to need a machining center for this part, but now that I can cut it complete [in a single setup], a batch of 100 wheels takes three hours less to machine."
This part is another case of the devil in the serrations. For a local medical equipment manufacturer, Mr. Hattori produces a 2-inch diameter support for the control unit of a cardiac electronic mapping catheter. One-half of a hinge joint that holds the control steady in numerous orientations, the part features a long flat along the centerline where half of the upper portion of the part is machined away. This flat forms the surface where the two halves of the joint meet, and it includes a hole for the pivot that links them. Around this pivot are 30 saw-tooth serrations. Allowing the joint to mesh and lock in several positions, the serrations form a symmetrical sun-ray pattern with the pivot at the center, and mate with a similar pattern on the other half of the hinge.
"Two linear axes are not enough to machine this feature; you have to have three," Mr. Hattori says. "Though the serration crests all lie in the same plane, the tool paths do not. We use a custom-ground end mill that matches the angle of the trough between two serrations, but even with this tool, a two-axis tool path can't cut the right shape. Instead of the necessary sharp peak all the way out to the circumference of the pattern, a two-axis tool path would produce a fan-shaped peak, because the space between the two cuts would grow thicker as the tool moved away from the hub."
To solve this problem, Mr. Hattori interpolates in three axes to make each cut. The diameter of the custom-ground tool is approximately the size of the peak-to-peak distance at the circumference. Therefore, Mr. Hattori starts at the hub--where the tool cuts on only its tip--and feeds the tool down in Y as it moves outward along the radius of the pattern in X and Z. This way, machined serrations have the same peak thickness at the circumference as at the hub, and they are machined in a single pass. The support hinge part, too, is machined from solid--starting with a bar of aluminum. Total cycle time is about 12 minutes per part.
Unlike the fishing reel, the support hinge was part of Mr. Hattori's stable of work even before he purchased the multi-processing turning center. The parts were originally cut on a CNC lathe and a vertical machining center. To produce the part's round taper, the turning came first. "The smooth profile made setup time-consuming on the vertical machining center," Mr. Hattori says. "We couldn't use a vise, so we had to bolt it into a custom fixture."
Now, that problem is gone--because the turning center chucks hold the part for both turning and milling. "The milling time on the SQT is the same as it was on the machining center, but now no one has to bolt and unbolt the part," says Mr. Hattori. "It's a big difference. With just that one improvement, we've cut cycle time for the hinge part by well over 30 percent."blog comments powered by Disqus