Forty characteristics on one housing
A gearbox housing has forty toleranced characteristics on its drawing: bore diameters, distances between bores, positions of bolt holes, flatness of the mating face. Measuring them by hand with callipers, height gauges and a surface plate would take an experienced inspector most of a shift, and some of them, such as the true position of a hole pattern, are hard to measure manually at all. A coordinate measuring machine does the whole housing in minutes, the same way every time.
What a CMM is
A coordinate measuring machine, CMM, is a machine that determines the coordinates of points on a part’s surface in a three-dimensional coordinate system. In some workplaces it is simply called the measuring machine, abbreviated MM.
Most CMMs have three axes at right angles to each other, X, Y and Z, with precise scales. A probe on the moving arm either touches the surface (tactile probing) or measures it optically without contact. Common designs are bridge machines for small and medium parts, gantry machines for large parts and portable arms for use beside the line.
From points to a result
A CMM does not measure a diameter directly. It measures points, and the software calculates the characteristic from them:
- Alignment. The part’s coordinate system is set up from datum features on the part, as defined on the drawing.
- Probing. The program moves the probe to the planned points. To measure a bore, it might take eight points around the circumference at two depths.
- Evaluation. The software fits a geometric element to the points, for example a circle or cylinder, and calculates the characteristic: diameter, position, flatness, perpendicularity.
- Comparison. Each characteristic is compared with its tolerance limits and reported as within or outside.
Worked example (illustrative values). A bore is specified as Ø 22.000 +0.021/0 mm and its position as 0.05 mm relative to datums A, B and C. The CMM fits a cylinder to sixteen probed points, giving a diameter of 22.012 mm and a centre that deviates 0.012 mm in X and 0.009 mm in Y from the nominal position. The position deviation is 2 × √(0.012² + 0.009²) = 2 × 0.015 = 0.030 mm, within the 0.05 mm tolerance zone.
What affects the result
The performance of a CMM is specified and verified according to the ISO 10360 series, for example the maximum permissible error for length measurement. But the machine specification is only part of the picture. The uncertainty of an actual measurement also depends on:
- Temperature. Dimensions are defined at 20 °C (ISO 1). A steel part measured warm from the machine can be several micrometres larger than at 20 °C.
- Probing strategy. Too few points, or points in the wrong places, can miss form errors or give an unstable fit.
- Fixturing. A part that is clamped too hard or rests unevenly can be distorted during measurement.
- The program. Alignment choices and evaluation methods (for example least-squares versus minimum-zone fitting) can give different results from the same points.
- Cleanliness and probe condition.
For this reason, CMM measurements should also be evaluated with measurement system analysis. A Gage R&R study on a CMM often shows small repeatability, while the fixturing and the operator’s loading of the part contribute the reproducibility.
Making the results useful
A CMM produces a lot of data. A single report can contain hundreds of values. That data becomes far more useful when each value is linked to the context it came from: article, drawing revision, order, serial number, machine and operator. With that link, results can be plotted over time, compared across machines or cavities, and traced back if a customer asks about a particular part. That context is what annotations add.
Common file formats help. AQDEF, a format defined in the automotive industry, and DMIS output allow CMM results to be read into other systems without retyping.
Common misunderstandings
- “The CMM is always right.” It is a measuring system like any other, with its own uncertainty.
- “Accepted on the CMM means accepted.” Only if the program measures what the drawing actually specifies.
- “More points is always better.” More points improve the fit but cost time. The strategy should match the characteristic and its tolerance.
In short
A CMM turns probed points into characteristics and compares them with the tolerance, quickly and consistently. Trust it as far as the machine, program, fixture and temperature allow, and link its results to the parts they describe.
