on the machine tool cutting the parts. “They want shop-fl oor operators to make those measurements without waiting for the quality lab to make the mea- surements and send them back,” he said. “It saves time and provides a better product.” While the instruments
can be compared digitally with an image captured through a video camera. While available since about 2013, Arenal noted that the company recently intro- duced a “profi le-fi tting” capability that transforms the old go/no-go capability into a quantifi ed whisker chart of deviations of the part along the profi le. It is a comparator version of the deviation color map one often sees with scanning systems. “We recognize it is cumbersome to
A variety of medical implants being checked on the Renishaw Equator.
they provide seem to fi ll the need for accuracy and repeatability, there is always the inconsistency that comes from human error. “You can have quite a differ- ence in repeatability as a result of an operator,” he said. That is where the company’s new, highly portable zCat CMM comes in, using a program to perform measurements and taking the human element out of the equation. “It weighs 30 lbs and you can oper- ate it on a desk, though it is better on a surface plate,” said Petersen. It measures to 4.0 + (L / 100 mm) microns when equipped with a Renishaw TP20 touch probe, according to its spec sheet. “What our medical customers like about it is that they use it to measure parts near or even on the machining center, or in reverse engineering prototype parts created us- ing a 3D printer,” he said.
Video Comparison Another company that is refi ning an old standby with new digital technology is Starrett Kinemetric Engineering (Laguna Hills, CA), upgrading the vener- able optical comparator with a digital version. “We replaced the old mylar overlays with digital profi les created from CAD fi les provided in .DXF format,” ex- plained Mark Arenal, managing director for Starrett. The digital fi les include a tolerance band so that parts
MSS20
AdvancedManufacturing.org
match parts manually, like knee or hip joint replacements, against Mylar over- lays,” he said. “There is also the man- agement overhead of managing those overlays.” Human error is always a pos- sibility when matching parts to overlays. New overlays need to be plotted as parts change. Medical applications include replacement hips, knees, or bone screws. “All of these parts from a medical manu- facturer need inspection to 100%,” said Arenal. “The video comparator improves the processing speed as well as the quality” com- pared to the traditional optical comparator. Starrett’s series of Horizontal Digital Video Com-
parators, or HDV series, are targeted for the shop fl oor, to be operated next to the production tool. The most recent release in the series is the HDV500, released in October 2015. It boasts an X/Y accuracy of 3.0 + 1 L (33 mm) μm for objects that are up to 250-mm tall. Control is either full CNC or motorized manual joystick. Stage travel is 500 mm in X and 200 mm in Y.
Choosing the Right Device It would seem that variety in instruments is the spice of measurement. But, there are good reasons for this, especially in medical device manufacturing. “Which device and accuracy to choose is applica- tion specifi c,” explained Dean Solberg, co-founder of Exact Metrology (Cincinnati). He lists the devices his contract measurement and scanning services busi- ness uses or delivers as including metrology grade CT scanning, structured light devices, and Romer Arms equipped with laser scanners. While choosing the right device is a balance be- tween accuracy requirements, size of the part, speed and cost, he also observes that increasing concerns over product liability is driving attention towards
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