Modern Machine Shop

DEC 2018

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Page 39 of 115

CNC TECH TALK MMS DECEMBER 2018 38 Processes and Programming Understanding CNC Machine Accuracy and Repeatability MIKE LYNCH | COLUMNIST Properly evaluating machine tool capability requires understanding how both user and the builder can influence precision. I define a CNC machine tool's accuracy as how precisely its axes can follow intended paths to commanded end points while under load. I define its repeatability as how precisely it can duplicate commanded motions (again, under load) during multiple cycles throughout the day. These are definitions for dynamic accuracy and repeatability. They likely vary from your machine builder's specifications. Builder speci- fications commonly indicate static accuracy and repeatability; that is, the machine is not in cycle performing machining operations when related measurements are taken. In fairness to machine builders, dynamic accuracy and repeatability vary with the amount of stress exerted on machine components. The greater the stress, the more difficult it is to maintain accuracy and repeatability. This makes it impossible for machine builders to provide, much less guarantee, dynamic accuracy and repeatability specifications. There are simply too many variables. That said, machine builders should be able to establish whether their machine can achieve accuracy/repeatability requirements for your particular application. They should be willing to guarantee as much if you ask them to do so prior to purchasing a new machine tool. Certain accuracy-related factors are beyond a CNC user's control once a machine is installed. These include: • The machine's construction. It must be able to perform the most powerful machining operations in your application without excessive deflection of its support components. • The feedback system. Linear scales directly monitor the position of the moving component for an axis. Unlike rotary encoders, they are not highly dependent upon the integrity of axis system com- ponents (way systems, ballscrews and couplers). Other accuracy-related factors are the responsi- bility of the machine user. These include: • Machine tool calibration. Machine builders initially calibrate pitch error and backlash com- pensations, but if accuracy is to be maintained, end users must repeat these calibrations at regular intervals during a machine's life. • Environment. Machine tools must be placed in a stable working environment that minimizes ambient temperature and humidity variations. Ensuring that a machine installation can pro- vide adequate dynamic accuracy for your appli- cation—and keeping it properly maintained—is but half the issue of producing consistent, accept- able components. You must also confirm that the machine can accurately repeat from the first workpiece to the last—hour after hour, day after day—even as machine components warm up after idle periods. An important repeatability-related issue linked to machine design is thermal variation of moving components. Primary concerns are the machine's spindle and way systems because they have the biggest impact on machined surfaces. As these components warm, they grow. As they cool, they shrink. This makes it difficult—maybe impossi- ble—to hold size on critical, tight-tolerance sur- faces during the machine warm-up period. Machine builders go to great lengths to min- imize thermal changes in machine components (cooling the spindle and/or way systems, for instance). Additionally, they incorporate design methods that minimize the repeatability impact of thermal variation. With CNC turning centers, for

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