When difficult part features need to be qualified a coordinate measuring machine (CMM) can be the answer for an awkward problem. If those features are external threads, there are a number of options available to obtain measurements besides a CMM. However, if internal threads are involved, a CMM is often chosen for the job but may be physically limited due to the size of the probe needed to get into smaller threads, but is still useful despite this limitation.

The cost of large fixed limit gages often inspires manufacturers to use their CMM to save this expense until the customer rejects parts using his fixed limit gages. Sadly, all is not sweetness and light when conflicts arise between fixed limit gaging and CMM measurements. It becomes a battle of the hardware where a machine costing say, over thirty thousand dollars, is up against a not-so-flashy-looking plug or ring gage worth a few hundred dollars.

It is hard to convince someone who has spent the big bucks that their digital wonder is not capable of doing the job they thought it capable of. Unfortunately, the reason why this may be so requires knowledge of metrology and how a CMM actually measures things—in other words, you have to get technical. As my readers are often reminded, I’m a simple guy, so I’ll try to keep the explanation simple as well.

The problems arise because we’re trying to compare apples and oranges. Each method is good at what it does but they are not doing the same thing. The CMM will take a number of measurements, usually pitch diameter established by two or more points of contact. Fixed limit gages inspect the thread through a functional check so if the thread is not round, they could reject it while the CMM may not even know this condition exists. Similarly, if there is significant pitch error in the component, the CMM operator may not even check for it but it can change the functional size of the thread and prevent acceptance by a fixed limit gage. 

Even if the CMM operator did check linear pitch of the thread, he would have to adjust the measured pitch diameter to arrive at something close to the functional size. One other element that can cause hair pulling and gnashing of teeth is the thread form. Once again, the CMM is unlikely to detect any problems in this regard but significant variations can cause a thread gage to reject the thread. The minor diameter of external threads or major diameter of internal threads will escape scrutiny by the CMM but significant deviations can be detected by the lowly plug or ring gage.

I haven’t bothered to bring in the CMM ball contact probe diameters into the discussion because they are usually not the recommended diameters or ‘best size’ for threads. Engineers can provide corrections to compensate for this situation but they are most reliable if you are dealing with a thread form that is perfect—you know, like the ones you make all the time. 

When the pitch diameter of external threads are measured, the thread wires used are free to locate in the thread form. When internal threads are involved and direct measurement by CMM is the process, the ball contact has to be positioned carefully to ensure it is accurately placed for the reading on one side of the thread. The process is repeated on the opposite side of the thread after the probe is positioned one half a thread pitch from the first measurement. The ball contacts on these probes are not free to float so the entire head of the machine has to be positioned which requires a technician with skill and patience. 

Fixed limit gages have some other benefits over the CMM: one being speed of use while another is low skill levels. Why, even I can use them quite effectively. Yes, they don’t give you numbers like a CMM does but then, as you can see from the foregoing, you have to know what those numbers actually mean.

If you remember one point from this discussion it is that both methods for evaluating a threaded component have their good and not so good points. Sometimes the simple plug or ring gage is all you need to do the job and, on balance, the most reliable choice.