--- title: "Coordinate measuring machine and probes" subject: "Metrology" chapter: 14 key: "semester_3/metrology:coordinate-measuring-machine-cmm" source: https://www.bctengineer.com/q/semester_3/metrology/coordinate-measuring-machine-cmm site: BCT Engineering --- # Coordinate measuring machine and probes Metrology · Chapter 14: Measuring machines - Asked in: year not given, 8 marks - Video: [The Basics of Coordinate Measuring Machines (CMM): CMM Types And Limitations](https://www.youtube.com/watch?v=Hb3qcj5RI1s) — RAPID DIRECT - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/coordinate-measuring-machine-cmm - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-14 ## Question With a neat sketch describe the construction and working of a coordinate measuring machine (CMM). Explain its types, probes and advantages. ## Short answer A CMM has a granite table and a bridge/ram carrying a probe that moves on air bearigs along X, Y, Z axes with linear encoders. Touch-trigger, scanning, or optical probes capture surface coordinates. Software fits geometric elements. Types include moving bridge, fixed bridge, cantilever. Avantages: high accuracy, speed, versatity for complex parts, automatic inspection. ## Answer A **CMM** measures the position of points on the surface of a workpiece along three axes ($X$, $Y$, $Z$) with a probe, and from these points computes dimensions, form, location and orientation. ```text Z-ram | X-bridge (moves in X) ______|___________ | | | | probe(+) | |______|___________| work on granite table <-- Y --> ``` #### Construction - A rigid granite or cast iron table for the work, with a bridge or gantry that carries a ram with the probe. - Three mutually perpendicular axes moving on air bearings, each with a measuring scale (linear encoder, glass scale or laser, resolution 0.1 to 1 $\mu$m). - A probe head, a controller and a computer with software to calculate points, circles, planes and so on. #### Types of CMM Moving bridge, fixed bridge, cantilever, column and horizontal arm. They are operated manually, by motor drive with a joystick, or by computer numerical control (CNC). #### Probes 1. **Touch-trigger probe:** a ball tip on a spring-loaded stylus that sends a signal when it touches the surface (most common). 2. **Analogue (scanning) probe:** gives a continuous output for scanning a surface. 3. **Non-contact probe:** an optical or laser probe for soft or delicate parts. 4. **Hard (fixed) probe:** moved by hand, with the operator recording each point. #### Working The machine is calibrated with a reference sphere to find the effective probe radius. Points are taken on each feature; the software fits a circle, plane, cylinder, etc. and compares the result with the CAD data. #### Advantages Universal measurement of complex components, high accuracy and speed, less set-up time and no need for special gauges, and automatic recording and statistical analysis; it is suitable for inspecting cams, gears, engine blocks and dies. Its limitations are the high cost, the need for a controlled environment, and the risk that contact probes deform soft surfaces. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a CMM do? A. Measures the positions of points on the surface in X, Y, Z with a probe and computes dimensions, form and location B. Measures only diameters with a micrometer head C. Compares the part with a gauge block by touch D. Projects the shadow of the part on a screen Answer: A. Points are collected in three axes and software computes features like circles and planes. (Key point: Principle) 2. Which probe is the most common and how does it work? A. Analogue probe: gives continuous output during scanning B. Non-contact laser probe used for all parts C. Touch-trigger probe: spring-loaded ball stylus sends a signal on contact D. Hard probe: operator reads a dial Answer: C. The touch-trigger probe is the most common; analogue scanning probes give continuous output, optical probes suit delicate parts. (Key point: Probe types) 3. Why is a CMM calibrated with a reference sphere? A. To set the table temperature B. To find the effective probe radius (tip) C. To level the granite table D. To find the pitch of the scale Answer: B. Probe radius compensation depends on calibration against the reference sphere before points are taken. (Key point: Calibration) 4. Which is a limitation of a CMM? A. It needs special gauges for each part B. It cannot measure complex components C. It has low speed and accuracy D. High cost and need for a controlled environment Answer: D. Advantages are universality, accuracy, speed and no special gauges; limitations are cost, environment and soft surfaces. 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