February 2011 / Volume 63 / Issue 2|
No penalty for holding
By CTE Staff
Bob’s Design Engineering Inc., Hillsboro, Ore., provides engineering and prototyping services for large electronics manufacturers and their smaller offshoots in the Portland area, as well as other customers.
After one prototyping job, BDE was asked to bid on the large-volume production. “We typically can submit a bid and beat Asia as well as producers in other low-cost-labor countries around the world,” said Jim O’Leary, tool engineer for BDE.
Courtesy of A&I Marketing
The transition from prototype to production did not present any machine tool problems. “We’ve always aggressively added appropriate technology,” O’Leary said, noting that, with its CNC mills and turning centers, wire EDM and direct numerical control, server-networked CAD/CAM system, the shop has the capabilities it needs to compete.
Prototype tools, however, are typically different than production ones. “When we prototype, we might apply a $30 tool. But if a $100 tool gives us two to three times the productivity and lasts two to three times longer, that helps us reduce cycle time,” O’Leary said.
A prime target for cycle time reduction in the project was machining a fluid manifold. The 6.3 "-long, 1¼-sq.-in. part was made from round 316 stainless steel bar. The bar was clamped vertically in vises for facing and axial holemaking, then mounted in a fourth-axis indexer to mill the bar sides square. A series of radial holes that met the axial bore were then drilled.
BDE has developed a progressive program using one Okuma machining center, according to O’Leary. “The program starts with a blank, and a completed part comes out of the machine every time the door is opened,” he said.
One time-consuming operation involved drilling a 0.59 "-dia. axial hole through the center of the bar. The part design specified a 6.2 "-deep blind-hole.
In earlier manifold prototypes, BDE’s customer had found drilling the hole from one end was too difficult, so it drilled the more than 10 diameters deep hole in two passes, one from each end of the bar. The open end was then plugged and welded to create the required blind-hole.
Following the customer’s lead, BDE drilled the prototype part from both ends of the generic 316 stainless bar. It used a standard cobalt twist drill gripped in an ER collet and a 40-taper toolholder, drilling at 20 sfm with a chip load of 0.002 ipt. Pecking was required to clear chips from the hole. Drilling took about 9 minutes per side, or 18 minutes per manifold.
That was too slow for volume production. “We had to produce these holes a lot faster,” O’Leary said. “And when you plug and weld a hole, your labor cost goes up and you have the possibility of the weld leaking.”
O’Leary contacted manufacturer’s rep Russ Johnson of Advanced Tooling Co. Inc., Olympia, Wash., for advice on completing the hole from one end of the bar. Johnson recommended the Titex A6589 DPP-15 through-coolant, AlCrN-coated carbide twist drill with double margins from Walter USA LLC, Waukesha, Wis. “We couldn’t justify the cost of a coolant-fed drill for a prototype job, but for production we could,” O’Leary said.
Holding the drill with a standard ER collet did not provide enough rigidity to run the drill at its maximum capacity. “There was too much flex,” he said. As a result, O’Leary decided to hold the drill with the powRgrip system from Rego-Fix Tool Corp., Indianapolis. The toolholders with press-fit collets hold a range of tool diameters. A miniature press is used to clamp and unclamp tools.
O’Leary initially purchased the powRgrip system from Dan Irish of manufacturer’s representative A&I Marketing Inc., Kent, Wash, to hold endmills. “According to all the stats I’ve read, the gripping power on the powRgrip is as good as or better than shrink-fit systems,” O’Leary said, adding that total runout is 0.0002 " about 5 drill diameters from the nose of the toolholder.
With Johnson’s help, BDE fine-tuned drilling parameters to about 175 sfm and a chip load of 0.035 ipt. Use of a through-coolant drill eliminated the need for pecking. “I am now drilling the blind-hole complete from one side in 85 seconds,” O’Leary said. Switching to premium-grade Project 70+ 316 stainless steel from Carpenter Technology Corp., Reading, Pa., contributed to the improvement in machining parameters.
After a few of the parts were run, Johnson suggested preceding the deep-hole drill with a Titex A6181 TFT-15 pilot-hole drill to a depth of three diameters. The pilot drill was about 0.0002 " larger in diameter than the deep drill, and also had a 150° point, compared to the deep drill’s 140° point. The larger pilot diameter prevented the deep drill from rubbing in the pilot hole, and the pilot drill’s larger point angle ensured the deep drill’s point, not its edges, would engage the work material first.
The result was excellent drill life. “We’ve probably made 3,000 or 4,000 of these parts now, and we’ve only had one drill failure, which was due to resharpening the drill beyond its recommended life,” O’Leary said.
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