“Often times, we’ll see very tight dimensions that are not needed on a print,” Thornburgh says. “If you put James Eastwood in front of one of our spinal implant designs early in the process, and that designer would tell him where he needed the tight tolerances on that part, Eastwood would be asking him, ‘Do you really need this radius to be this tight or this slot dimension to be this tight?’ ’’
Dan Owens, Vice President of Global Advanced Manufacturing at Paragon Medical, also notes the challenges that manufacturers experience with tolerancing.
“The computer age has ushered us into an unforgiving world in the manufacturing environment,” he says. “Even though machines can hold tight tolerances, there are still factors that come into play, the two biggest variables being the operators and tool wear. That’s a constant battle with tolerances.”
Not accounting for tolerancing can drive your scrap rate up and your quality down because the product is subject to revisions between you and the manufacturer, though tight tolerances may be required in some cases, Owens says.
“These features [that require tight tolerances] require precision machines and precision machinists to repeatedly produce in a production environment,” he says. “What is important is that the design has been critically examined to ensure that 1) it is truly needed, and 2) that every dimension is not CTQ (critical to quality). When you cannot prove capability on a feature, it drives 100 percent inspection, and 100 percent inspection is only 80 percent effective, then take into consideration gauge R&R correlation and it will effectively drive escapes, which means returns from the customer.”
Owens uses a pictorial to explain to customers the size of a micron to depict tolerances.
“The picture illustrated is that of a human hair. Inside the human hair is a piece of pollen that is 30 to 50 microns,” he says. “There’s a ragweed that is 17 to 23 microns and a dust mite that is ten microns. The human hair is 50 to 70 microns in diameter. One micron is illustrated inside of the hair, the hair being a four inch diameter blown up. The other microns are depicted in a little white sphere inside that. The point with this picture is this, do you really understand the correlation between what is required and what is computer default, as it relates to tolerances?”
Creating a repeatable process to account for tolerancing in a design goes a long way, says Ken Gredick, Engineering Manager at Triangle.
“A lot of times, designers will put something down and they’re not 100 percent sure that their tolerancing is correct, whether it’s geometric dimension and tolerancing (GD&T), or just a straight tolerancing,” he says. “If they called it out just a different way, it would pass all the time, but the way they had it detailed on the print, it could fail at 50 percent of the time. This makes a big difference when it comes to compliance.”
It’s also beneficial to have some understanding of inspection techniques, Gredick says.
“Inspection, in terms of standardizing inspection techniques, is an overlooked area that needs to be investigated and approved by the quality engineers on both ends,” he says. “Streamline radiuses for your off-the-shelf radiuses or inserts that you could buy, like McMaster Carr, just to standardize processes so you don’t have to go out and customize processes. All of those processes really add up to time, and that’s where we get killed, is on the time.”