Elisabeth Eitel
Senior editor
Colleen Telling
Associate editor

"We've seen situations where designers have selected a certain level precision to incorporate in a component, only to find that some mating part doesn't allow for the same."
-Joe Beltrami

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Even if you could mathematically model systems exactly, the physical world is too messy — has too many variables — to flawlessly produce paths described by clean, exact equations. Nonlinear friction, noise, vibration, and various forms of backlash color all machine motion. But machine accuracy and precision are getting better all the time, making for improved positioning and increasingly defined motion.

"Two things limit precision: Interference from physical and electrical sources over the entire system, and the ability of the design (including manufacturability) and materials to provide highly repeatable results over time," says Robert Davis of HBM Inc., Marlborough, Mass. To illustrate: Precision ball valves require extremely tight tolerances for a consistent seal when closed. In most industries, tolerances are typically in the thousandths of an inch, if not higher. But improved hardening techniques are now producing valve balls (and, in fact, rollers for other applications as well) with diameter and sphericity tolerances within millionths of an inch. This makes for consistent repeatability — that remains over time.

How is overall ball precision improved? New electronic controls are replacing manual control of grinding pressures during manufacturing. "A pressure control motor utilizes a resistor that senses pressure reduction as the balls decrease in size during the grinding process, and readjusts the grinding plate distance accordingly. This keeps a constant pressure on the balls and allows for more consistency," explains Joe Beltrami of Hartford Technologies, Rocky Hill, Conn. "This is especially beneficial to high accuracy and precision designs, which perform better with through hardened elements of high-carbon and martensitic-type stainless steel," he adds. In contrast, case-hardened materials (of low carbon steels and softer materials — normal austenitic stainless steel or brass) can deform and don't hold tolerances as well.

Accuracy — the closeness with which a system can come to a target mark — is different for system stops than it is during motion. For example, when it comes to controlling torque, spring-loaded clutches are more accurate than friction-style slip clutches. Oil shear brakes resist high temperatures for better accuracy under cycling, but low inertia is paramount to quick-response stopping accuracy.

Tolerance stack

Sending motion over any distance reduces accuracy, especially when transmitting through various linkages. "The power transmission chain, particularly when powered by a rotary motor connected to couplings, gearboxes, belts, and screws can have significant negative impact on system accuracy, even if there is a sensor on the load downstream," says Curt Wilson, engineering and research V.P. at Delta Tau Data Systems, Inc., Chatsworth, Calif. Like a game of telephone, each time power transmission signals are handed off, error is introduced. The same goes for electronic components. "Minimizing low, mV-level signal paths by moving signal conditioning close to signal sources can significantly increase measurement quality," says Davis of sensors detecting accuracy. "In fact, modern component sizes have made this particular technique readily available, since reliable circuits can now be built in extremely small footprints." In the same way, mechanical linkages degrade performance less when they're small and close to main system action, with handoffs kept to a minimum. "For example, to reduce tolerance stack, some drives integrate a ballscrew, carriage, and pillow-block bearing supports in one rigid unit," says Clint Hayes of Bosch Rexroth Corp., Hoffman Estates, Ill.