January 2012 / Volume 64 / Issue 1|
From stringy to sandy
By CTE Staff
Imparting a fine surface finish is usually paramount when machining medical parts. For example, at Acumed LLC, which makes orthopedic tools and implants, surface finish requirements range from about 32 to 8 rms, noted Randy Singer, machinist level 4 for the Hillsboro, Ore., manufacturer.
That’s especially challenging to achieve when chips become stringy and scratch the workpiece. That was the kind of chip control problems Acumed was experiencing when Swiss-style turning, the primary machining operation the company performs. The parts manufacturer machines various materials, including cobalt chrome, titanium and 17-4 PH, 316, 440 and 455 stainless steels. “And every once in a while I get to turn a piece of aluminum,” Singer added, but noted that 316 generates the stringiest chips.
Courtesy of Acumed
Because stringy, wraparound chips can cause unacceptable surface finishes, Acumed had to perform secondary finishing operations or put stops in the part program to allow manual chip removal, increasing cycle time.
High-pressure coolant helps break chips but needs to be precisely directed at the tool/workpiece interface. The shop’s Swiss-style machines typically used an external, tubular brake line to deliver high-pressure coolant, but Singer found it challenging to effectively and consistently aim the line. “If you’re off a couple of degrees,” he said, “it takes time to monkey around to get it to where it’s hitting perfectly.”
Even when a line was set just right for one tool, Singer explained that it might have to be repositioned for a different tool or moved and reset when indexing an insert. And moving the brake line caused it to wear and eventually break. “Then you had to make a new one,” he said.
Knowing there had to be a quicker and less aggravating way, Singer searched online but found that several options only delivered coolant up to 1,000 psi, and Acumed’s machines are capable of delivering twice that pressure. However, when a part requires gundrilling, which occurs frequently, the operation regulates the pressure below the maximum. “If I have to run the gundrill at 1,500 or 1,550 psi, I can’t necessarily go and crank the pump each time a turning tool or some other tool comes in,” he said.
ANSI neutral-rake, screw-on insert, Swiss-style toolholders from Streamliner & Associates Inc., Edinboro, Pa., however, allow Acumed to apply coolant at the desired pressure directly at the tool/workpiece interface. “With the Streamliner holders, I don’t have to mess around with aiming,” Singer said. “It’s aimed for me.”
Streamliner reports that its holders with the dovetailed distribution plates are estimated to withstand 10,000 psi.
Since incorporating ½ " and 3⁄8 " Streamliner holders, Acumed creates chips that look like sand when turning most workpiece materials, including 316 stainless, according to Singer. When the DOC is relatively deep—up to ¼ "—some materials can be stringy, but effective chip control provided by high-pressure delivery of the shop’s vegetable-based oil and the insert’s chipbreaker geometry prevent those chips from wrapping around a part.
Singer pointed out that eliminating the brake-line aiming process reduced setup time by 15 to 20 minutes. He sets up a couple times a day, and runs do not exceed 50 parts. The enhanced coolant delivery also enabled Acumed to boost feed rates 25 percent and cutting speeds 25 to 35 percent and extend insert life about 40 percent.
“The coolant is just blasting everything out of there,” he said. “The holders can make a bad insert look good.”
The holders use a distribution plate with an orifice to restrict the coolant stream, thereby increasing the pressure while the pump still moves the same volume of liquid. To further enhance the coolant’s explosive action, Singer purchased blank distribution plates and machined 1⁄16 " orifices; the smallest standard Swiss-style orifice size is 3⁄32 ". “The standard one worked well but I wanted to see if I could neck it down a little more,” he said.
Although Acumed’s Swiss-style machines have 10 coolant lines that go into a manifold, Singer bypasses the manifold and connects the Streamliner holders directly to the lines, which slightly reduces setup time.
Now that Acumed has proven the Streamliner holders are effective, Singer noted that the company will add them to additional Swiss-style machines, including ones for large production runs. In addition, the shop started using Streamliner Top Notch-style holders for grooving and threading inserts. “Insert life has skyrocketed,” Singer said.
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