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What's the best way to precisely measure intricate mechanical parts?

For example, notice the tiny differences between the pitch in these two threads, and the tiny variations in external width of the cylinder: enter image description here I've been using a micrometer and a vernier caliper. enter image description hereenter image description here The micrometer is capable of more precise measurements, but it's no good at getting into tight spots. The jaws of the verniers have a sharp, bevelled edge that can fit into some tight spots, but it's less accurate, and hard to know if the jaws are making firm contact to the desired surface, or catching an edge or another surface (like the outer diameter of a thread, for instance).

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    $\begingroup$ The answer really depends on how much money you have and just how precisely you need the answer to be. $\endgroup$
    – Carl Witthoft
    Commented Nov 27, 2017 at 20:19

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A relatively inexpensive and quite useful device would be a "pocket optical comparator".

comparator

comparator device

They come with Several etched glass plates with features of size that you optically/visually compare to your part. I'm happy to explain more if need be.

Furthermore, you can also use a two-part rapid set silicone mold material to take molded impressions of your features and then slice up the molds and use them with the comparator. This can be cumbersome in a production environment.

https://www.google.com/search?q=fowler+pocket+optical+comparator&safe=off&client=safari&hl=en-us&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjHq8nZuOLXAhUE4SYKHUwmCqUQ_AUIEygC&biw=375&bih=559

Also, depending of the size of the features you're measuring, you could get a set of Gage Pins and then you can use them to measure a feature width. For example, the width of a groove.

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  • $\begingroup$ Alright! Now we're on the right track. I used to be an automotive & mechanical (workshop) tools salesman. I've never even heard of one of these. I'll price one up. Got any more ideas? $\endgroup$
    – voices
    Commented Nov 29, 2017 at 11:14
  • $\begingroup$ I added that you can use gage pins to measure small features that might be difficult to measure with calipers. $\endgroup$
    – GisMofx
    Commented Dec 1, 2017 at 2:55
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Usually threads are done by some standard. You can use calipers to measure and than find closest in standard, if your product is from eu, its easy you need eu standards like this ISO 68-1:1998 or maybe BS pipe

So the answer would be to measure as accurate as possible but also use literature. Mechanical engineering handbooks like Kraut are great to have at hand to check each machine component. If your measurements are for quality control and you do them all the time in same range, thread etalons would be great. You can make them using some precise technology and give them to measuring institution or some accredited lab that have 3d optical measurements and if they are accurate use them instead of calipers for quality control. Optical 3d measuring device is also great piece that is used when high accuracy is needed. Also some special measuring devices are available Micrometer or wires. Depending on your industry needs you should upgrade your measuring equipment.

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    $\begingroup$ Might be worth mentioning thread gauges, assuming the OP is only working with standard ISO -listed part sizes. $\endgroup$
    – Carl Witthoft
    Commented Nov 27, 2017 at 18:42
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In the oil industry where it is critical to get the many hundreds of different threads correct; they use working thread gauges ( precision hardened steel ) that are traceable to master gauges , which are traceable to grand master gauges. The carbide tools used to cut these threads are generally verified on optical comparators. A machine shop would not encounter these type threads, but that is how they are measured. The answer to measuring any threads is usually a set of gauges, as has been recommended .

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