Micrometer: best practices with plastic parts

Hello from a new member. I work in a small factory and have recently been given a new task of measuring a variety of small molded parts with a micrometer. I have plenty of experience using a micrometer with metal parts, but not with plastic, and we seem to have some disagreement among the staff about how it should be correctly done.

A standard micrometer has a ratchet on the handle so that when you close it it will "click" onto the piece, the click-ratchet ensuring that the same pressure is applied every time. (This way, a heavy handed person won't squeeze too tight and a light handed person won't squeeze too soft -- all users will end up in theory with the same measurement.)

At least that's how it works with a solid metal part. Some people I work with think that with plastic parts, which have a tiny bit of give to them (ABS plastic), you shouldn't use the click-ratchet, but should just close the micrometer until you can "feel" it touch the part. I do understand what they are saying, but it seems to me this is guaranteed to introduce a lot of variation into the measurements, with different people getting different readings according to how light or heavy their touch is.

Is there a standard "best practice" for measuring small plastic parts with a micrometer? Or maybe a special attachment to address this issue? (As I said, we are a very small company and there aren't a lot of technical people around -- we're mostly self-taught.)

Pointers or links will be welcome. And if the admins think this belongs in a separate quality-related forum I'll be glad to move it.

Many thanks.

"PeterP"

Know what you are saying! I used to work with HDPE and LDPE closures and "Mics" and calipers were tricky propositions ( especially LDPE). We would grab the part softly and open the instrument, the point where the part fell though was the reading.TPE and TPR can also be be a pain.
QA used to accuse me of " making parts in tolerance with heavy handed Mic and caliper manipulation@

KOM

brent
Talk about gage R and R!
Vision based systems can help if money is available. T

Any chance you can use a drop gauge instead? (Google images if you're unfamiliar). This would mostly be applicable to parts with large flat sections, so it depends on what you're trying to measure. I actually use a micrometer to measure plastic parts and it is true that the amount of torque makes a much bigger difference here than when you're measuring metals, but it all depends on what level of precision you need and the tolerance stack. e.g. If the tolerance is +/- 0.002" and micrometer torque creates a variation of 0.0005", then as long as you meet +/- 0.0015" you still know you're in spec. Considering Brent's comment about "making parts in tolerance", it's best if you don't know the spec. when you make the measurement to eliminate bias, write down the measurement and then compare it to the spec, but you might already have all the numbers memorized.

I've seen the open until it drops for small soft parts. That company did change to using an optical comparator and the issue went away. You can find them used to help keep the price down.
Rick.

Many thanks to all for these helpful replies. At least I know I'm not alone!

I like the open-until-it-drops idea, and we may experiment with that. I also like the idea of not knowing the specs when you measure so you will be unbiased, but realistically I doubt we will be able to implement that. I'll look into other instrumentation possibilities also.

Many thanks again.

If your part geometry allows perhaps you could make a set-up with a dial indicator & height gage on a surface plate. You can zero the set-up with a gage block then check your parts. The pressure will be consistant (consistency & repeatability are critical for accurate mesurements) and most dial indicators don't exert enought pressure to deform plastic. Also depending on the style of the indicator there are replaceable tips, a wide flat tip will distribute pressure rather than causing part disortion.