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 could duplicate commanded motions (again, under load) during multiple cycles through the day.
They're definitions for dynamic accuracy and repeatability. They likely vary from your machine builder's specifications. Builder specifications commonly indicate static accuracy and repeatability; that is, the equipment is not in cycle performing machining operations when related measurements are taken.Hit on positioning accuracy and repeatability of cnc machine to explore more about our services and sites. Hope you ll like our more services.
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In fairness to machine builders, dynamic accuracy and repeatability vary with the quantity of stress exerted on machine components. The greater the strain, the more challenging it is to keep accuracy and repeatability. This helps it be impossible for machine builders to provide, not as guarantee, dynamic accuracy and repeatability specifications. You can find simply too many variables.
Having said that, machine builders should manage to establish whether their machine can perform accuracy/repeatability requirements for your particular application. They must be prepared to guarantee just as much if you inquire further to do this just before purchasing a brand new machine tool.
Certain accuracy-related factors are beyond a CNC user's control once a device is installed. These generally include:
• The machine's construction. It must manage to perform probably the most powerful machining operations in your application without excessive deflection of its support components.
• The feedback system. Linear scales directly monitor the positioning of the moving component for an axis. Unlike rotary encoders, they're not highly dependent upon the integrity of axis system components (way systems, ballscrews and couplers).
Other accuracy-related factors would be the responsibility of the equipment user. These generally include:
• Machine tool calibration. Machine builders initially calibrate pitch error and backlash compensations, but when accuracy is to be maintained, clients must repeat these calibrations at regular intervals throughout a machine's life.
• Environment. Machine tools must certanly be put into a well balanced working environment that minimizes ambient temperature and humidity variations.
Ensuring that the machine installation can offer adequate dynamic accuracy for your application—and keeping it properly maintained—is but half the issue of producing consistent, acceptable components. You must also confirm that the equipment can accurately repeat from the very first workpiece to the last—hour after hour, day after day—even while machine components warm up after idle periods.
A significant repeatability-related issue associated with machine design is thermal variation of moving components. Primary concerns would be the machine's spindle and way systems because they have the largest effect on machined surfaces. As these components warm, they grow. As they cool, they shrink. This helps it be difficult—maybe impossible—to put on size on critical, tight-tolerance surfaces during the equipment warm-up period.
Machine builders visit great lengths to minimize 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 instance, the headstock might be perpendicular to the bed. Since it warms, only the height of the cutting tool's edge changes. This minimizes the quantity of machined diameter variation from part to part as the equipment warms up.
When purchasing any new CNC machine, you should know how the builder deals with thermal variation. More to the point, you need to confirm that machined-surface variations due to thermal growth during warm-up will not exceed tolerances. Otherwise, you might maintain for a productivity-wasting surprise when you discover your new machine must run for a warm-up period before it can be used in production.
Some of the very severe repeatability issues have nothing regarding machine design. Instead, they're influenced by the machine's application. Variations of any kind—during a creation run or from one time employment is run to the next—can impact repeatability. Issues that differ from cycle to cycle may cause the requirement for a time-consuming adjustment. If the variation is very good enough, it may result in scrap.
What are examples of variations?
Samples of variations during a creation run include:
• Tool wear. As cutting edges wear, machined surfaces will vary. External surfaces grow while internal surfaces shrink
.
• Dull tool replacement. When dull cutting tools are replaced, careful attention is necessary to ensure cutting edge(s) do not vary from their predetermined position(s).
From one time employment is run to the next include:
• Workholding setup. Many factors affect workpiece stability (placement/alignment of the workholding device, clamp location and force applied, and program zero assignment, for instance).
• Cutting tool assembly, measurement and offset entry. Component and assembly variations result in rigidity variations that may result in machining issues.
• Machine condition. Variations due to mishaps and the neglect of preventive maintenance can lead to sizing difficulties with jobs which have run successfully in the past.
