by Drew Koppelmann

Evaluating Machine Tools

It is difficult and costly to cut good parts using bad machine tools. Unfortunately, many machinists spend several hours each day tweaking machines and setups, reprogramming CNC controllers and "creeping up" on a dimension. If the machines worked right in the first place, that time could be spent more profitably, producing parts.

Machine tool evaluation&emdash;also known as calibration or characterization&emdash;is a process that identifies and quantifies inaccuracies in machines by measuring their performance with specialized gaging instruments. The process is a valuable approach to ensuring quality in the metalworking trades. Statistical process control may reveal that something is wrong with the process, but it often fails to indicate precisely what. Machine tool evaluation can identify the problem and suggest means to solve it. It puts the "process control" in SPC.

Machine tools can go wrong in dozens of ways. Every axis of motion has a number of potential inaccuracies of position and orientation. A three-axis machine has 21 such "degrees of freedom." Each linear axis may exhibit errors of pitch, yaw and roll. Each may have errors of straightness, and of squareness relative to the other two axes. Each is subject to errors of linear positioning. The machine's spindle is also subject to axial and radial errors.

The ability to efficiently maintain dimensional, relational, geometric and even surface finish tolerances depends on the machine tool being in a good state of adjustment. Every machine inaccuracy can show up in production as a failure to meet a specification. Even the best operator has difficulty isolating problems on a "seat of the pants" basis with so many sources from which to choose.

Happily, there are gaging instruments to measure the sources of potential error, and a standard to instruct and guide users in their application. The ANSI/ASME B5.54 standard, titled "Methods for Performance Evaluation of Computer Numerically Controlled Machining Centers," is equally applicable to non-CNC machines. It defines the types of errors to which machine tools are subject and describes how to test for them.

There is room here to include only the briefest listing of the equipment involved. Electronic levels measure pitch and roll in a horizontal axis, pitch and yaw in a vertical axis, and squareness between horizontal and vertical axes. Laser interferometers measure straightness, squareness and linear positioning accuracy. Telescoping ball bars test overall contouring performance. Spindle analyzers test machine spindles for axial and radial accuracy.

While the evaluation "tool box" is complex, worthwhile evaluation programs can be started economically, using just some of this equipment. The potential benefits are significant and extend beyond merely identifying sources of machining problems.

No matter how precisely a machine tool has been manufactured or rebuilt, when installed, it must be straightened, squared, trued and adjusted. Thus, on-site calibration is essential. Even if suppliers do the installation and calibrate the machine to their satisfaction, purchasers should carefully monitor the process and may perform their own tests, as a condition of acceptance.

Evaluation results can lead to straightforward corrective action. For example, should an electronic level test reveal pitch or roll in a horizontal axis, the solution may be a simple adjustment of the machine leveling feet. The test can then be rerun and the adjustment confirmed or repeated.

As with SPC, a program of periodic machine tool evaluation can catch problems before they occur. By monitoring machines annually or semiannually, production supervisors may see when equipment is coming due for an adjustment. By knowing the current capabilities of every machine, they can assign work on the basis of required accuracy. Machines that are losing calibration can be relegated to less precise work, while the most accurate machines can be reserved for tight-tolerance jobs.

All machine tools have limitations of accuracy, beyond which it is not possible to improve through adjustment. Likewise, accuracy tends to vary because the machine's ways, lead screws and other components may have been machined more or less accurately in some areas than in others. Thus, while evaluation can be used to ensure that a machine meets minimum accuracy specifications at all points, it can also identify certain areas where the machine performs at its best. By setting up work in these "areas of optimum accuracy," it may be possible to exceed the machine's stated accuracy specifications.

About the author

As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance to companies in a wide range of industries. He may be reached by fax at (401) 784-3246.