Measuring Tubes with Calipers

Sounds like your calipers aren't straight and you may need new ones.

I think the answer to this really depends on what point the calipers have been zeroed.

Whatever the point of contact is they meet when you zero them, is where you should measure your part.

The jaws should be parallel along the entire length of mating surfaces.

You should have calipers calibrated.

bwire I have two pair of calipers, one pair purchased in 1970 & a digital pair purchased in 1987. Both have seen very heavy use and are as accurate today as the day purchased. Neither pair have a provision for adjusting, just quality workmanship & materials and always handled as they should be. I almost forgot my third pair, they are made of plastic and have a company logo on them for they were given away by the box full at a company picnic and believe it or not they are accurate and have no adjustment. J.Conway

The flat part is used when measuring the outside of a cylinder, the sharp edges when measuring the inside, of a tube for example.....

the last shop I worked in as Quality Manager. We calibrated the majority of our measuring equip in house. We were ISO 9000 compliant. It takes a little work but is not that tough once you get a good plan in place.

And the response are correct. The nibs (ID measurements) and the jaws (OD measurements) should be calibrated. If the deviation from the flats of the jaws to the tips of the jaws is present as you suggest then the calipers should be sent out.

a few things:

1) an inexperienced tech may be applying to much pressure to the thumb wheel/ head of the tool. Practice against a NIST gage block/ring gage to get the right feel throughout the tool.

2) the use of micrometers may be desirable here.

3)measure at 3 points. roundness issues (ovality)

.Regarding calibrating a caliper using the long flat surfaces against your gauge block is correct. One thing to note is cleanliness of the faces when doing this. also after you zero the caliper say it is a 6" one you should check it with a 3" block to prove it is correct with a reasonable travel.You will note the block measuring faces have length & breadth, using the long face on the full length of the jaws should give the correct reading,then using the smaller breadth measure at the top & then the bottom of the jaws this should prove the jaws are parallel.

I like to setup on something similar to the test piece if possible. I'd use a gage pin of the nominal size to be measured if available to get "the feel" of it.

Gage blocks, balls, pins and rings or a combination there of can be used as a nominal reference for almost any geometry.

I always understood that the proper tool to measure a circular object was with a micrometer.

My quality control manual requires I calibrate or check my measuring instruments once a year. On calipers, micrometers etc I have a set of gauge blocks and I do this myself. While calipers are good for ID/OD measurements I prefer micrometers for wall measurements of tubes or pipes. You should use the micrometers with a round anvil only to get an accurate measurement for walls of tubes/pipes. I invested In a UTT measuring instrument for wall thickness measurements a few years back and it is calibrated every time I use it via the same gauge blocks. I use this UTT instrument allot and it is very handy to check new tubes/pipes where you can see in the end of the item as well as for checking tubes/pipes that are in use in the field.