accuracy of silver steel

[VicParticipant @vic]

Would Linear shafts be good enough?

**LINK**

[SillyOldDufferModerator @sillyoldduffer]

Trust nothing when taking high precision/high accuracy measurements!

Below, say, 0.005mm /0.0001" it's difficult to measure length without taking special precautions. It's close to tool-room and metrology lab territory involving clean rooms, temperature control, calibrated equipment, optical methods, multiple measurements, trained operatives and the like.

How exactly did you take the measurements lfoggy? If done in an ordinary environment with basic skills it's possible you're detecting errors in your equipment or technique as well as, or even instead of, problems with the silver steel.

Not intended as a criticism – I'm particularly conscious of human error and rate myself below average when using tools. Others are far more competent!

Dave

[David George 1Participant @davidgeorge1]

I used to operate a centerless grinder about 40 years ago in the manufacture of hydraulic cylinder shafts and inspection would have rejected anything with more than a couple of tenths out of round as when they were chromed and then reground you could not guarantee the chrome thickness.

David

[lfoggyParticipant @lfoggy]

Thanks for all useful replies. Didn't know about h8 and H8 but it makes sense.

My measurements were all taken with an ordinary Mitutoyo digital micrometer. This seems to give remarkably consistent results to within a few microns. This is the micrometer measuring a setting standard at 20C. With care it measures to within 0.002mm of 25mm fairly consistently.

The part is required to help a friend restore an old Reichert microtome. It is in fact just a replacement for a damaged shaft so nothing special really but it does need to match the original which seems to be a few microns under 10mm. I fear that my box of random bits of silver steel may not be the best place to look….

[Neil WyattModerator @neilwyatt]

Posted by lfoggy on 03/07/2018 16:18:11:

Thanks for all useful replies. Didn't know about h8 and H8 but it makes sense.

My measurements were all taken with an ordinary Mitutoyo digital micrometer. This seems to give remarkably consistent results to within a few microns. This is the micrometer measuring a setting standard at 20C. With care it measures to within 0.002mm of 25mm fairly consistently.

The part is required to help a friend restore an old Reichert microtome. It is in fact just a replacement for a damaged shaft so nothing special really but it does need to match the original which seems to be a few microns under 10mm. I fear that my box of random bits of silver steel may not be the best place to look….

Well that's the sort of approach needed to get that sort of accuracy.

I found a Mitutoyo publication on the web 'Quick Guide to Precision Measuring Instruments' and it's quite illuminating. A 325mm micrometer will show errors between 0.0 and 5.5 microns depending on how and which way up you mount it, for example. A 200mm standard bar will expand 20 microns when held in a warm palm for 10 minutes, also test bars and micrometers don't expand at the same rates with temperature, and a common source of inaccuracy is the test equipment and object being measured not being at the same temperature.

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