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Machinists today are being asked to out- perform anything they’ve done in the past. Customers require a shorter leadtime, the parts are more complex and the drive to lower cost tremendously burdens the people on the floor who make it work, not to mention the bottom line.
It’s no secret that to compete in today’s market, a company must be a top performer. But the question is how to select and apply available technology to achieve a competitive edge. Shops must evaluate what is the appropriate level of technology that will fulfill the demands of the customer and fall within the skill set of the shop’s personnel.
As part volume runs become shorter, the pressure to reduce cycle time, while ever present, is increasingly only one factor in the total throughput picture. Setup and tear down of the machine are significant parts of the door-to-door time for a given job. Ideally, the goal is to cut metal as fast as possible and to have the ability to change over from job to job quickly and easily. Like a restaurant that wants to turn over tables to serve the most customers in a given shift, shops need to quickly turn over jobs to keep machines running parts.
One company successfully achieving this goal is 3D Machining Corporation in Riviera Beach, Florida. 3D is a 5-year-old company that is giving its machinists the tools they need to achieve the greatest productivity. While others were cutting back, 3D was investing in new Citizen machines with high speed milling spindles to address the micro-machining requirements of the company’s high-technology customers. A key to 3D’s success was the selection of a high speed milling spindle that fit the shop’s needs.
Selecting A High Speed Milling Spindle
High speed milling on Swiss-type machines is nothing new. Machine shops have been using air spindles and DC electric spindles for years to achieve higher surface footage with drills and end mills ranging in size from 0.004 to 0.125 of an inch. On smaller diameter milling tools, high rotational speed is necessary to achieve the chip load and surface speeds to take advantage of the cutter. The technological trade-off is that many auxiliary spindles must sacrifice torque for speed, which reduces the depth of cut that can be taken.
Because the work zone is relatively compact, the body diameter of self-contained auxiliary spindles must be small to fit into the small machining envelope of a Swiss-type or gang machine. The available speed can range from 15,000 to 60,000 rpm with torque delivered through a planetary gear drive. The compact design limits the size of the power drive and reduces the effective torque and rigidity for efficient cutting.
The installation of air and electric spindles can also be complicated, requiring a service engineer who possesses the knowledge of M code interfacing with the machine. Umbilicals, in the form of wires and air hoses, need to be carefully routed through the machine in a way that will not inhibit machine motion or cause damage to machine parts or the spindle’s electrical wires and air hoses.
Often, customized holders are required to fixture these spindles to the machine. They are then stacked on top of the existing tool blocks. The additional fixture moves the centerline of the tool away from the guide bushing, which can affect rigidity and, consequently, accuracy.
Spindle speeds are selected with a potentiometer on a retrofit controller or by air valves. Changeover flexibility is reduced and usually takes an experienced setup person as long as 4 to 5 hours to set and adjust the spindles in order to make accurate parts.
But the benefits of higher rpm far outweigh the shortcomings of these spindles. For shops looking to take advantage of more efficient cutting, programmers can select the proper rpm on small drills and end mills, tripling or quadrupling the feed rate to achieve increased productivity with fewer machines and even fewer people.
“The biggest problem we face is there are fewer experienced machinists, and their time is spread thin,” says Eddie Peña, vice president of manufacturing for 3D. “To be competitive, a setup must be easier, enabling less experienced operators to pick up slack while seasoned machinists work on bigger problems.” The company is working to resolve this issue with new technology and a different approach to high speed milling spindle selection.
3D manufactures parts for the medical industry and for new Cross Match technology that is emerging for personal identification. It does a great deal of engineering and development for its customers, adding value to the machining services they ultimately deliver. Many customers come to 3D because of its ability to machine small parts better than most. The parts it produces are bone screws, dental implants, medical devices and micro-medical tooling, all of which require complex milling with tools smaller than 0.062 inch in diameter.
The typical “bolt-on” high speed spindle was not sufficient for Mr. Peña’s needs at 3D. The company worked with the OEM and distributor to find a different approach for 3D’s applications.
Trying Something New
3D’s problem was resolved when its last L20VIII machines were delivered with high speed cross drilling and milling spindles jointly developed by Genevieve Swiss Industries of Southampton, Massachusetts, and PCM Willen, for Citizen users. The difference with these new spindles is that they are designed specifically for high speed milling on Citizen machines.
The PCM high speed spindles replace the standard Citizen spindle located in the gang slide without the addition of special fixtures. Wires and hoses are eliminated because the PCM high speed spindles are driven directly from the Citizen milling drive to deliver maximum power and torque.
The benefit of a Swiss-type machine is the support provided by the guide bushing. The PCM high speed spindle maintains the same distance away from the guide bushing as the Citizen spindles. This keeps the tool as close to the bushing as possible, providing maximum rigidity and accuracy.
When asked what benefit Mr. Peña has realized with the PCM spindles, his reply is: “no vibration, faster cycles, improved cosmetics and increased tool life.” Mr. Peña attributes these benefits to the rigidity of the design that closely couples the tool nose to the guide bushing, reducing overhang. Also, because the unit is gear-driven, high rpm and better torque combine to provide increased power for milling and drilling.
Using an example of a hexagon drive tang on a bone drill made of 174PH, Mr. Peña explains how the company was able to reduce the milling cycle by 50 percent by increasing the spindle rotational speed and feed rate for a 0.25-inch end mill. Mr. Peña comments that 3D expected faster cycles and improved productivity. However, the company didn’t expect the improved cosmetics of the finish or the reduction in burrs.
Mr. Peña explains that the cosmetic quality of parts the company manufactures is as important to its customers as the tolerance they are expected to hold. He adds that the PCM high speed spindles have helped 3D to hone its competitive edge by delivering an improved finish. Machinists are able to use larger tools because of the bigger collet, and they can push the limits without chatter or vibration. This is something they were unable to achieve with other high speed spindles.
When asked about setup, Mr. Peña passed the question to the company’s engineer, John Bodiger. Mr. Bodiger says that when he used high speed spindles in the past, he would avoid breaking down a job because it simply took too long to set up the next time. “These new spindles are a breeze to set up,” he says. And, he has no problem moving them from machine to machine for whichever job they are needed on.
This has created another problem, says Chipper Peña, who programs and runs the Citizen machines. Everyone wants to use these spindles for their setups, which requires some juggling because the company has more machines than high speed spindles.
One example of how 3D has used its new enhancement was profile milling a complex shape on a bone screw used to anchor a surgical suture. The diameter of the part was less than 1/8 inch, and the end mill was 0.030 inch. Deflection was virtually eliminated, with the end mill working closer to the guide bushing and the increased speed of 12,000 rpm reducing the cutting pressure. Even at 2.4 ipm, the result was a part with minimized burrs and increased tolerance control.
Another part that Mr. Peña mentioned was a medical shaft used in arthroscopic surgery. The finished part was 3/16 inch diameter 174PH stainless steel and was nearly 17 inches long. The process on the company’s Citizen machines included a 0.08-inch milled slot along 7 inches of the length. The end of the part had a 0.062-inch window milled into the side intersecting with the ID.
Using the old process, the window was generated with a 0.062-inch ballnose end mill running at 5,000 rpm and 1.8 ipm. The window alone took 45 seconds. When 3D changed the process to add the PCM high speed spindles, the speed was increased to 15,000 rpm and the feed rate was increased to 4.2 ipm, reducing the cutting time by 60 percent to 18 seconds. The added benefit was that the part was nearly clean of all burrs, and the surface finish remaining had absolutely no chatter marks.
Mr. Peña’s advice to others who want to get into high speed machining is to spend money to get the right tools and don’t hold back on the speeds and feeds. Just let the tool to its job, and you will truly maximize your investment.