CMM Operation: Basics of Coordinate Measuring Machines in a Quality Lab
See what a coordinate measuring machine does, who runs it, what a lab day looks like, the five stages of a job, and what can damage the machine or hurt you.
Table of contents
A coordinate measuring machine (CMM) checks a part by touching it at many points and working out its real shape in three dimensions. Where a caliper gives you one distance, a CMM can check every dimension and GD&T callout on a drawing in one run. It is the tool that settles arguments about whether a part is good, so people who run one carry real responsibility.
What You Will Be Able to Do
- Explain what a CMM does and who runs it.
- Tell the main machine types apart.
- Describe a normal day in a quality lab.
- Name the five stages of a CMM job in order.
- Say what will damage the machine and what can hurt you.
What a CMM Does
A probe with a small ball tip, usually ruby, moves along three axes and touches the part. Each touch records an X, Y and Z coordinate, often to within microns. A micron is a millionth of a meter, or about 0.00004 inch. Software then fits shapes to those points, such as a plane, circle or cylinder, and compares them to the drawing or CAD model. Common software packages include PC-DMIS and Calypso.
Shops use CMMs to approve first articles, check parts during a production run, and verify GD&T callouts such as position, profile and perpendicularity that hand tools cannot check well.
Several jobs use one:
| Role | What they do |
|---|---|
| CMM operator | Loads parts, runs programs, flags problems |
| Quality inspector or technician | Runs the CMM along with other gauges, and signs off on parts |
| CMM programmer | Writes and proves inspection programs |
| Metrologist | Owns measurement methods, calibration and accuracy |
Machine Types
| Type | Strengths | Typical use |
|---|---|---|
| Bridge | Rigid and accurate, part sits on a granite table | The standard quality-lab machine |
| Cantilever | Open on three sides, easy to load | Smaller parts, shop-floor use |
| Gantry | Huge measuring volume | Very large parts |
| Horizontal arm | Open access to big, flat-sided items | Car bodies, large panels |
| Portable arm | Carried to the work, jointed like an arm | Large parts, checks at the machine |
Some systems do not touch the part at all. Optical and video machines suit small, delicate or flexible parts. Laser scanners and structured-light systems capture a whole surface as a cloud of points, which works well on freeform shapes and reverse engineering. Most new operators start on a touch-probe bridge machine.
A Day in the Lab
- Check the room. The lab should be near 20 degrees C (68 degrees F), which is the standard temperature for measurement. Look at the climate monitor. A lab that swings through the day gives drifting results.
- Start the machine. Follow your lab's startup steps. Clean the table, and look over the probe and stylus.
- Qualify the probe. Run the routine on the reference sphere.
- Work the queue. Production samples and first articles usually come first. Look at what is waiting, and at which parts are still warming or cooling to room temperature.
- Run jobs. Load, run, review and record.
- Handle problems. A part that fails gets checked again and flagged to the right person.
- Close out. File reports, park the machine in a safe position, and leave the table clear.
Temperature takes up more of your day than you might expect. A part that comes straight off a hot process has to soak, meaning it sits in the lab until it reaches room temperature. That is usually several hours, often about 4, with less for small parts. Follow your lab's rule. A hot part measures large, and the machine has no way of telling you why.
The Five Stages of a Job
Each stage takes real skill, and they always run in this order.
- Mount: fix the part to the table with clamps or a fixture, firmly enough that it cannot move and lightly enough that it does not bend.
- Qualify: calibrate the probe on the reference sphere, so the machine knows the tip's true size and location.
- Align: build the part's coordinate system, usually with 3 points on the primary datum, 2 on the secondary and 1 on the tertiary, so the machine knows how the part is sitting.
- Run: start the inspection program, stepping slowly through it the first time, and watch it work.
- Review: read the report's measured values and pass or fail results, and act on anything out of tolerance or close to it.
Two ideas make that repeatable across operators and shifts: the datum order on the print, and the 3-2-1 alignment. Learn those first.
What Will Damage the Machine
A CMM is a delicate, expensive instrument. Most damage comes from a crash, when the probe or stylus hits something it should not.
- Jog slowly with a clear view of the path, especially near the part.
- Never start a new or changed program at full speed.
- Park the head safely before changing a stylus.
- Clamp every part. A loose part can shift mid-run, and the probe will hit it.
- Remember clamps and fixtures when you plan a path. They are as solid as the part.
- Keep the granite table and the air bearing surfaces clean, and never use the table as a workbench.
- Set parts down gently. A dropped part can chip the granite.
- Clean coolant, chips and burrs off a part before it enters the lab.
If you crash the machine, stop and tell someone right away. A hit can bend a stylus or shift the probe, and every reading after that is suspect until it is checked and requalified. Nobody expects a new operator to never crash. Hiding one is the mistake that costs you.
What Can Hurt You
A lab feels safer than a shop floor, and the hazards are less obvious.
Warning: Keep your hands, head and body out of the working area while the machine is moving. It can start moving under program control without warning and can pinch you against the part or table. Know where the emergency stop is before you run anything.
- Heavy parts. Use a hoist or get help. Do not lift a heavy casting onto a table alone, and watch your fingers when you set it down.
- Sharp edges. Burrs on unfinished parts cut hands. Wear gloves when you handle them.
- Cleaning fluids. Isopropyl alcohol and similar cleaners are flammable. Read the label and Safety Data Sheet, and use small amounts.
- Compressed air. Many machines ride on air bearings fed by shop air. Service work is for trained technicians, with the machine locked out and the air bled down.
- Spills. Wipe up coolant or cleaner on the floor right away.
Check Your Understanding
1. A long part straight off the grinder is warm, and the machinist wants a quick reading. What do you do?
Explain that a warm part measures wrong. Let it soak by the lab's rule, then measure it.
2. You have to reach in to reposition a clamp, and the program is paused. Is that safe?
Do not reach in while a program is active. Stop the program according to your lab's procedure first, and move the probe clear. A paused machine can still move.
3. The stylus taps a clamp at low speed. There seems to be no damage. What next?
Stop and tell your lead. Check the stylus, and requalify before you trust any more readings. A small hit can still shift the tip.
4. Why do the stages run mount, qualify, align, run and review, and not align first?
The alignment is built using the probe, so the probe must be qualified first. The part must also be fixed in place, or the alignment will not hold.
Key Takeaways
- A CMM touches a part at many points, and software turns those points into measured features.
- Bridge machines are the lab standard. Other types suit very large parts, portable work or non-contact checks.
- Every job follows the same five stages: mount, qualify, align, run, review.
- Temperature matters, so let hot parts soak by the lab's rule.
- Stay out of the working area when the machine is moving, and report every crash.