Adjusting a micrometre?

[bollie7]

There will be a small hole in the barrel of the mic at the end of the barrel where it is attached to the frame. This is what you use the pin spanner for. If after cleaning, they are still out, you adjust the barrel by turning it with the pin spanner until the zero line on the barrel lines up with the zero line on the thimble. This can be quite fiddly to get right. Before you start adjusting it might be worth rechecking the mics at various times of the day to see if the reading stays the same. I'm thinking temperature variations here.

bollie7

 

[Tony Wells]

Many people are not fans of the ratchet micrometers. How many clicks, they ask. Temperature does affect readings somewhat, but the idea is that as long as the standard and micrometer are the same temperature, they should agree. But know this: Micrometer standards can be off size. The standard practice is to have them calibrated and marked actual size. It is a nice idea to trust them from the supplier, but that's optimistic.

You say you checked the standard with your verniers. How do you know your verniers are on? That's kind of backwards. You should be checking your verniers against the standard.

 

[bollie7]

Mayhem
If your mics are still off, just adjust them yourself. As I said it can be a bit fiddly but its still not hard to do. Just zero the mics against the std and then turn the barrel until the mics read zero. You will soon get them reading right.

bollie7

 

[Tony Wells]

Finding a discrepancy between readings at the low end and high end is not at all uncommon. Iin fact, very very few micrometers read accurately throughout the entire range. It's part of the specifications. It's lead error. Specification commonly is 0.000040, and most used micrometers are within 0.0001, but with some wear, all micrometers should be checked at different points in their range. And, check at odd values, those that don't stop the thimble at a X.XXX0 all the time. You need to see if the spindle is square and the face parallel, so check at 0.105, 0.210, 0.315, 0.420, 0.500, 0.605, 0.710, 0.810, 0.920, and 1.000 for Imperial micrometers. Same idea for Metric.

Optical flats are required also for a real calibration.

 

[pdentrem]

Certification (sp?) of mics is always an issue. When checking mics at small error like what you are getting is even worst! It is likely just heat from your hand.

We use digital mics at work and I regularly make metal ribbon that calls for .0021" +/- .0002" and last week a customer wanted .0005" +/- .0003". Now this is somewhat hard to do and to measure .0005" beside a 540 F hot mill is just a problem. The mics are checked in a controlled environment, temperature and humidity etc, and put back into the shop and used where there is NO environmental controls at all. We clean the anvils just about every time we pick it up. We check 0 every time we take a measurement. Heat is a big issue obviously.

I had to scrap one mic because it would read different if held in the right hand vs the left hand! It would also read differently if held vertically vs horizontaly or any other position.

BTW Digital mics can be used to find out if you have a bad ground as well. At times an machine operator will come to me a report that the mic is measuring wrong at the machine but OK at the bench. The first time this happened it was a mystery of what was happening but we figured it out. Just touching the mic to the machine without grounding it caused it to pick up a charge and the reading would start changing without moving the spindle. Reground the machine and problem gone. Just a tip if you see this.