A customer should be sure that the machine shop they choose is able to handle the number of challenges that may arise during the CNC machining process, and have the resources, communication, and quality control necessary to handle those challenges. If the precision machine shop can’t handle these issues, then the customer can waste valuable time and resources.
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So Worthy Hardware would like to introduce 5 factors for you when you choose the precision machine shop as your vendor.
Factor 1: Experienced and Stable Core Managerial and Technical Team
A company with an experienced managerial, technical, and manufacturing team that has a wealth of knowledge is vital. The more experienced the shop is, the more hands-on they can be with new projects. Experienced manufacturing engineers are often able to quickly identify and offer alternate solutions when problems arise based on previous ordeals in projects. They are even able to help design your product to make it "design-for-manufacture" friendly, allowing an OEM to save time and money on assembly. More importantly, assist an OEM to stay ahead of the competition by achieving faster time-to-market for their new products.
For Worthy Hardware, most of our engineers have more than 10 years for CNC machining industry and they are very stable with our company.
Factor 2:
Quality Control
Quality control is an important factor for any manufacturer's capabilities. A great way to learn about a factory's quality control process is to look at their certifications and auditing reports. Factories should at least obtain an ISO 9001 certification which confirms quality management.
It is just as important to review the shop's internal audit reports from operations to management, to warrant that all levels of quality are met. Factories should have detailed and accurate information on each equipment being used to detect specification accuracy, control standards, and response measures to keep rejection rates low and production yield high. A quality precision machine shop that is confident and proud of their products would be open to discuss their failure rate as well as their objectives and goals in reducing the failure rate to more suitable levels.
While each team works with a different process – one team for EDM, one for grinding, two of turning and milling and one for part deburring – there is a lot of project transfer between them. The team leader for one team will work with other team leaders as part process requirements dictate.
Surprisingly, at Owens Industries, the deburring department/ team is often the most critical because some parts are so small they have to be deburred while viewing them under 50 X magnification. One such part was a 0.010-in.-diameter tube with wall thicknesses of 0.003 in. This part had several machined features that created burrs needing to be removed.
“When we get a part that’s not much bigger than the point of a pen, and it has 65 to 70 part features on it, deburring is an extremely delicate and critical operation,” Plesnik said. For another job, the deburring department had to use a Q-tip cotton swab with polishing compound to generate an exact 8-micro finish on the parts.
Unlike typical job shops, Owens Industries doesn’t arrange its machines into cells. Instead, it groups its machines according to type.
Besides machining cells, Plesnik said there are other practices that are common in production shops, such as automation, that just don’t work for his shop.
“There’s really no place for it (automation) in our shop. We just don’t deal with job volumes big enough to justify its use,” he said.
Although, Plesnik said the one prerequisite every highprecision shop needs is a willingness to pay, in both monetary compensation and in-house training, for highly skilled and talented people. Without skilled people, Plesnik said shops such as his couldn’t succeed. That is the reason that Owens Industries home-grows most of its personnel.
Requirements for high precision
As both a high-precision shop and builder of high-precision machine tools, Kern Micro- und Feinwerktechnik GmbH & Co. (www.kern-mcirotechnic.com) near Munich, Germany, and its U.S. subsidiary Kern Precision Inc. in Webster, Mass., know what it takes to be a successful highprecision machining shop. The requirements encompass machine tools, measuring capabilities, tooling, shop environments, and workholding.
To categorize the work it does in its shop and to differentiate its machine tool products’ accuracies, Kern defines high precision as three levels of part accuracy.
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The first is micro-precision as defined by workpiece accuracies below 10 microns.
The second is ultra-precision, which is workpiece accuracies lower than 3 microns.
And, the third is nano-precision, defined by workpiece accuracies below 1 micron.
Kern builds machining centers for each one of these categories.
Kern’s Pyramid Nano is the company’s most accurate and largest machine. Its travels measure 20 in. by 20 in., and within that work envelope, the machine positions to an accuracy of 0.3 microns. It machines test workpieces to tolerances of +/- 1 micron.
How does Kern know this? It uses a special system from Carl Zeiss Inc. (www.zeiss.com) that has measuring accuracies of 0.25 microns. In fact, Kern is the first commercial user of this system.
Agreeing on the importance of measuring capabilities for both workpieces and machine tools, Pfister of Finecast said that shops can’t make precision parts if they can’t measure them. A shop’s inspection capability must match the tolerances it is trying to achieve.
“Just because a machine tool’s specification sheet lists a +/- 0.0002-in. positioning accuracy, doesn’t mean a hole pattern drilled on that machine will be within +/-0.0002 in. Believing so is the first indication that a shop doesn’t know what precision is, and unfortunately, a lot of shops rely on such machine specifications,” Pfister said.“
To illustrate his point, Pfister said his shop had a machine that had a +/-0.0002-in. positioning accuracy listed in its specifications. However, after the machine ran for a while at high feedrates, that accuracy specification grew to an actual 0.004 in. because of heat from ballscrew expansion. That is the reason that Finecast uses laser calibration to validate the axis accuracy of all its in-house machine tools.
Since Kern lists its machine accuracies as those achieved on actual test workpieces, those specifications take into account positioning accuracy and tolerances derived from the machine’s spindle, toolholders, and cutting tools, Burkhard Rother, chairman of Kern, said.
Kern equips its machine tools with special, vector-controlled spindles that have runouts of less than 1 micron. Vector control allows the spindles to reach full torque in lower rpm ranges, and it makes it possible for the machine’s CNC to know a spindle’s rotational position at any time.
Kern machines, through vector control, can stop their spindles at the same exact rotational position for each toolchange, and Rother said that loading tools in the same spindle rotational position each time they are used can increase machine accuracy by as much as 1 or 2 microns. Rother further recommends that shops use high-precision collets, shrink-fit holders or balanceable toolholders to hold tools.
Kern also uses vector control to load touch probes accurately into machine spindles.
During part-probing operations, Kern touches one side of a part, rotates the machine spindle 180 degrees, and touches the opposite part side to ensure that the same side of the probe is used to touch off on the two opposite sides of the part.
“These are things that shops have to do for high-precision work. We do them every day in our machine shop in Germany,” Rother said. He should know, the shop once had to mill 50-Rc steel parts using a 50-micron-diameter cutter and drilled 30-micron-diameter holes into a human hair (see “Kern’s move to machine tools” sidebar).
At Kern’s Germany shop, high-precision machines operate in a vibration-free climate-controlled environment. The highprecision machines are separated from the shop’s other machines and are kept away from other external sources of vibration. In addition, Kern keeps the machines in a working area that has a controlled temperature that fluctuates no more than +/-1-degree C.
Rother said that the machines would perform well in greater temperature fluctuations, but for repeatable high accuracy, the temperature should be held constant. Kern equips its machines with internal thermal-stabilization systems to provide more temperature control.
Another must for high-precision machines is a direct-measuring system, such as glass scales, to calculate positioning. Glass scales replace using ballscrew pitch and rotary encoders for calculating positioning. All axes on all Kern machines feature Heidenhain (www.heidenhain.com) glass scales.
When it comes to workholding for high-precision machining, Finecast, Owens Industries, and Kern all recommend the use of reference workholding systems, such as those offered by System 3R (www.system3r.com).
For Owens Industries, these modular systems streamline part processing by holding parts in setups that can transfer quickly from one type of machine to another. And, because reference workholding systems also work well for Finecast, that shop plans to eliminate the T-slots on its machines altogether.
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