Accuracy is Everything

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 that likely vary from your machine builder’s specifications that commonly indicate static accuracy and repeatability.

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.

Machine Builders’ Influence

Machine builders should be able to establish whether their machine can achieve accuracy/repeatability requirements for your particular application. 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 components (way systems, ball screws and couplers).

Machine users’ responsibility Other accuracy-related factors are the responsibility of the machine user. These include:

Machine tool calibration: Machine builders initially calibrate pitch error and backlash compensations, 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.

Coping with Thermal Changes

An important repeatability-related issue linked to machine design is thermal variation of moving components. As these components warm, they grow. As they cool, they shrink. This makes it difficult—maybe impossible—to hold size on critical, tight-tolerance surfaces during the machine warm-up period.

Machine builders go to 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.

When purchasing any new CNC machine, you must confirm that machined-surface variations caused by thermal growth during warm-up will not exceed tolerances. Otherwise, you could be in for a productivity-wasting surprise when you discover that your new machine must run for a warm-up period before it can be used in production.

Ensuring Repeatability

Variations of any kind—during a production run or from one time a job is run to the next—can impact repeatability. If the variation is great enough, it could result in scrap.

Examples of variations during a production 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, extreme caution is required to ensure that cutting edge(s) do not vary from their predetermined position(s).
Workholding set-up : Many factors affect workpiece stability – placement of the workholding device, clamp location and force applied, program zero assignment etc.
Cutting tool assembly, measurement and offset entry : Component and assembly variations result in rigidity variations that can lead to machining issues.
Machine condition : Variations caused by mishaps and the neglect of preventive maintenance can result in sizing problems with jobs that have run successfully in the past.

Dynamic accuracy and repeatability vary with the amount of stress exerted on machine components. This makes it impossible for machine builders to provide, much less guarantee, dynamic accuracy and repeatability specifications.