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Technology Key to Moldmaking Success


New machines, tools, techniques boost productivity

 

By Jim Destefani
Senior Editor
               

         

“Work smarter, not harder” has become the mantra of many US moldmakers contending with an influx of low-cost tooling from offshore producers. Smart moldmakers have realized that adopting new technology can be one of their keys to survival against global competition.

Here’s a look at some new machining, finishing, and assembly technologies that can fit in with the “smarter, not harder” philosophy and save moldmaking time and cost.

High-speed hard milling can yield impressive reductions in both machining time and overall processing time. According to Gary Zurek, applications engineer, Mikron Corp. (Holliston, MA), successful hard milling is the result of implementing a system including the machine, cutting tools and toolholders, and the CAD/CAM system.

Unsurprisingly, Zurek says the machine tool is the key component of the system. “The machine must be designed for hard milling, along with having some of the same characteristics found in a high-speed machining center,” he says. “The base construction and the individual components of the machine, such as the drive train, spindle, and CNC, must be able to handle the demands of hard milling.” 

Zurek says machine construction should start with a rigid base with good vibration damping characteristics. Polymer concrete bases are a good choice for high-speed and hard milling applications, because they typically have vibration damping 6-10× that of cast iron. “Polymer concrete also has very good mechanical and thermal characteristics,” he adds.

Digital drives that can handle fast acceleration/deceleration provide good contouring accuracy while helping to minimize cutting tool wear, according to Zurek. Spindles should provide flexibility, offering high torque at low speeds and high power over a large speed range. “Hard milling requires a large rpm range, from low speed for a roughing application to high rpm for high-speed machining [HSM],” he says.

Zurek recommends the HSK spindle connection. “Toolholders play an important role in hard milling,” he says. “The HSK interface provides a very rigid and balanced tooling setup with minimal runout.” Other advantages over steep-taper tooling include good axial accuracy and low mass and stroke, he adds.

Zurek says a CNC with maximum block processing rate and a hard-drive large enough to handle complex programs is another key to high-speed hard milling. Algorithms to calculate velocity profiles and smooth machine motion, and “look ahead” features are required, but Zurek points out that programming to minimize large changes in direction and other abrupt movements is also important.  

“The greater the directional change [example: zero to 90°], the more a control with ‘look ahead’ has to slow down to maintain the programmed path,” he explains. “In hard milling, these abrupt changes in toolpath direction create dwells and slow-downs that affect tool life and part surface finish. Therefore, toolpath optimization should be an important feature of any CAM system used for programming high-speed milling.”

An example of Mikron’s machine lineup for high-speed hard milling is the XSM 400U, a five-axis machine with milling feed rates to 80 m/min and acceleration up to 2.5G. The machine’s swivel table can rotate at speeds to 250 rpm, and a spindle capable of speeds to 60,000 rpm completes the high-speed package. According to Mikron, the machine can cut hardened steel to RC 62.

Cutting tool suppliers are answering the challenges posed by high speeds and hard metals with an array of new materials and geometries. According to Roger Goble of cutting tool supplier OSG Tap & Die (Glendale Heights, IL), H13 tool steel is one of the most common work materials for hard milling. “Most people do hard milling on materials around RC 48-H13 is the most common,” he says. “S7 tool steel will go to RC 56-58, A2 and D2 are machined around RC 60.”

Goble cites rib cutting as an example of where hard milling can save shops time and money versus sinker EDM. “Traditionally, most mold shops have used EDM to burn ribs. Our company has tools with the same amount of draft that the ribs require, allowing users to mill ribs rather than burn them.”

Goble describes tests in a mold shop that involved production of two identical ribs 0.600" (15.24 mm) deep, 0.060" (1.52 mm) wide at the tip, and with 1ºdraft. The milling process-including toolpath generation, machining, bench work, and tooling costs-took a total of 70 minutes and cost $94.68 at a shop rate of $55/hr. The complete EDM process, from electrode modeling and machining through burning and benching, took 200 minutes and cost $199.87.

Users can select OSG rib cutters with 0.5, 0.75, 1, 1.5, 2, or 3° of draft. Ribs with square corners must still be EDMed, Goble points out.

He also notes that hard milling doesn’t always happen at high spindle speeds. OSG has developed new corner radius tooling that allows use of high roughing feed rates even on machines with relatively low spindle speed capability. A ¼" (6.4-mm) diam cutter, for example, would run at 7500 rpm and 340 ipm (8.6 m/min) in material hardened to RC 48, according to Goble, and a ½" (12.7-mm) diam cutter would run at 3800 rpm and the same feed rate. “So, shops with 4000-rpm spindles can now run those high feed rates, as long as their control can keep up without overshooting,” he says. Depths of cut at those speeds and feeds would range from 0.005 to 0.020" (0.13-0.5 mm), according to Goble.   

Yet another cutting tool material that is beginning to catch on in some high-speed hard milling applications is cubic boron nitride (CBN). Goble says the superhard material can provide excellent tool life and surface finish for finish machining of hardened steels.

“Not a lot of shops are using CBN, but there’s a need for it if you’re doing very high-precision molds with multiple cavities, or if you’re machining powder metallurgy tool steels such as CPM-10V,” he says. “CBN will far exceed the tool life of coated carbide in those types of applications.

“However, due to the hardness of CBN, it is more fragile than carbide and only mainly be used in finishing with proven programs.” He also provides several other tips for successful CBN use:

  • Minimize cutting force variations
  • Maintain the recommended cutting depths
  • Consider spindle thermal expansion
  • Use mist coolant.

According to Goble, CBN is particularly useful for finish machining of small, multi-cavity molds. In one comparison versus a TiAlN-coated carbide tool, tool wear of a CBN tool was <1 µm after milling 30 small cavities. Wear on the carbide tool after 10 cavities was 6.2 µm.

 
Micrographs compare (from left) ground, EDMed, and EBMed surfaces at 350x.  
A good example of the “work smarter, not harder” philosophy in action is a new approach to bushing retention that saves machining and assembly time and makes the mold stronger in the bargain.

Shops using the technique use snap rings rather than socket-head screws to hold ejector and guide-pin bushings in place in the moldset. It requires machining a snap-ring groove in the counterbore that accommodates the bushing’s lip-a single step, on the same centerline as the bushing bore itself, that requires no remeasuring or offsetting. After that, it’s a matter of inserting the bushing and snapping in the retaining ring. Time savings per moldset average four to five hours for relatively simple molds containing eight to ten bushings. Savings increase proportionately on larger, more complex moldsets.   

The previous method involves drilling, counterboring, and tapping a screw hole just at the edge of the bushing counterbore, and then threading in a screw. The underside of the screw head engages the top of the bushing to hold it in place.

Locating the retaining screw can be tricky. The offset must be accurate radially, or the screw’s underside won’t catch enough of the edge of the bushing lip to secure it. Moreover, the screw must be in the right place on its offset circle, or it could interfere with mold assembly or operation.

The shop that originated the technique started out by machining the snap-ring grooves with a four-flute, HSS grooving tool after completing the bushing counterbore. Cutting the slot was faster than opening the screw hole, but still left room for improvement.

Although the spindle and mold block remained in position in the X-Y plane for both operations, the tool change from the boring tool to the groove slotter took time. The operation also sometimes involved raising the spindle or lowering the table to make room for the exchange. And, the cut itself was slow to minimize cutter deflection.

Noticing that the two operations were coaxial, application engineer Chad Meyer of Ingersoll Cutting Tools (Rockford, IL) suggested his company’s Chipsurfer tool, which features interchangeable tips on a common shank. The new tool completes boring, counterboring, and grooving with the spindle, table, and toolshank left in position. Because everything is left in place and the interchangeable tool tips hold axial and radial positioning repeatability to datum within ±0.0005" (0.013 mm), location accuracy of the snap ring groove is assured.

The tooling switch cut toolchange time to 10-15 sec, and actual groove machining took 12 sec-half the time of the previous technique. The Ingersoll tools ran with depths of cut to 1/8" (3.2 mm) in a 13/4" (44.5-mm) diam bore. Other moldshops trying the tools report similar gains.

Ingersoll is now marketing kits containing tools for the new practice. Kits include four slotters with widths of 0.056, 0.068, 0.086 and 0.125" (1.42, 1.73, 2.18, and 3.18 mm), to cover the four most common slot sizes moldmakers need. The grooving tools feature six flutes for faster material removal and lower cutting forces. They use carbide cutters with a 3/8" (9.5-mm) HSS shank.

Moldmakers report that the snap-ring technique helps create a more reliable mold by distributing forces uniformly around the ring rather than concentrating on just one point of the bushing lip. Mold servicing is also facilitated.

Finishing is another part of the moldmaking process that can be time-consuming, and anything that can reduce “bench time” certainly qualifies under the “smarter, not harder” heading.

Here again, technology has a role to play. Case in point: the Pika PF32A electron beam machine (EBM) from Sodick Inc. (Schaumburg, IL). Said to be the world’s first EBM for polishing, the machine can quickly produce mirror finishes on mold surfaces using short bursts of a 60-mm diam electron beam. It can thus eliminate hand polishing.

After a vacuum is pulled, argon gas is introduced inside the machine’s processing chamber. Magnetic coils ionize the gas to produce a plasma, and delivery of an electron flow into the plasma creates an electron beam with high current density. An X-Y positioning table moves the workpiece under the stationary beam.

The process results in very smooth surfaces. In testing, EBM smoothed an EDMed surface of Ry (maximum peak to valley distance) 6 µm to <1 µm, and an EDMed surface with Ry of 20 µm to 3 µm. Electron-beam polishing also produces an amorphous (glassy) surface layer Sodick says resists corrosion and wear better than a conventional surface.

 

This article was first published in the October 2004 edition of Manufacturing Engineering magazine. 


Published Date : 10/1/2004

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