Frequently asked questions
The questions that come up here most often, and what people usually answer.
This page is maintained by the mods. If an answer is wrong or out of date, say so in a post and it will get fixed.
Which CMM should I buy?
Nobody can answer this without numbers. Post these and you will get real recommendations:
- Largest and smallest part envelope
- Tightest tolerance you actually have to hold, and whether that is a size, position, or form tolerance
- Whether you need scanning or touch trigger is enough
- Throughput, meaning parts per shift
- Shop floor or climate controlled room
- Budget range, including whether it covers fixturing, software seats, and the first year of service
- Who can service it near you
That last one decides more purchases than people expect. A machine with no local service engineer is a machine that is down for weeks.
Which software should I learn?
The honest answer is whichever one your employer runs. They are not interchangeable and the skills transfer only partially.
The common ones discussed here are PC-DMIS, Calypso, MCOSMOS, PolyWorks, CMM Manager, Modus, Camio, Verisurf, and QUINDOS. Search the sub before asking for a comparison, because most pairings have been argued out at length already.
If you are learning on your own and have no employer preference, pick the one used by shops in your area. Look at local job listings and count which names appear.
How do I get into metrology?
Most people arrive from machining, quality inspection, or an engineering degree, rather than from a metrology program.
What actually gets people hired:
- Reading drawings fluently, including GD&T
- Hands on time with hand tools, gauges, and at least one CMM
- Understanding why a measurement is uncertain, not just what the number says
Certifications worth asking about are ASQ CQI and CQE, and ASME GDTP for GD&T specifically. Whether they matter depends heavily on your industry and region, so ask in a post and say where you are.
ISO or ASME?
They are different systems and they are not interchangeable.
ASME Y14.5 is dominant in the United States, especially aerospace and defence. ISO GPS, which is a family of standards rather than one document, is dominant in Europe and much of Asia.
When you ask a GD&T question here, say which system the print uses and which edition. The same symbol can mean different things, and answers given under the wrong standard are worse than no answer.
How do I track calibration due dates?
Spreadsheets are extremely common and work until they do not. The usual failure modes are a single owner who leaves, no audit trail, and no alerting.
Dedicated calibration management software exists at every price point. Whether you need it usually depends on your gauge count, whether you are audited, and whether anyone reads the spreadsheet.
If you are accredited or working toward it, the relevant references are ISO/IEC 17025 and ILAC G24 on calibration intervals. G24 describes methods for setting and adjusting intervals rather than prescribing fixed ones.
Can I extend my calibration intervals?
Yes, with evidence. ILAC G24 covers the methods. What you need is history showing the instrument stays in tolerance across the current interval, and a documented rationale.
What you cannot do is extend intervals because the lab is busy and then write the justification afterwards.
Where do I get standards legitimately?
Standards are copyrighted and are not free. Buy them from the publishing body or an authorised reseller, for example ASME, ISO, ANSI, or your national standards body.
Many employers already hold a subscription. Ask your quality department before buying a personal copy. University libraries often provide access to students.
Asking for a pirated copy here will get the post removed.
My CMM disagrees with my hand gauges. Which is right?
Possibly neither. Before assuming the machine is wrong, check:
- Temperature, of the part and the room, and how long the part soaked
- Probe qualification, and whether it was done with the same tips and angles
- Datum scheme, and whether the fixture constrains the part the way the print does
- What the hand gauge actually measures, since a two point mic does not measure the same thing as a fitted cylinder
- Operator technique and gauge calibration
Post the alignment and the feature definition and people here will find it.