# Calibrating inside micrometer



## Sshire (Oct 4, 2011)

Just got a Starrett 700 (0.2-1.2) inside micrometer on eBay. Beautiful condition. Great looking classic Starrett velvet lined case. 
Somewhere in its life it lost the calibration disk. I closed the jaws and measured them with an outside mike for which i have calibration rods Reading was 0.02. 
Without a calibration ring or a ring gauge (gauge ring?) is there a better method for calibration. I did set and lock the outside mike to 0.5 and 1.0. Then measured the opening with the inside mike. Seems fine. 
Am I being too compulsive? The other new acquisition is a Starrett 0 to 1" outside digital mike which reads to half a ten thou. Been running around the house reading everything. Too much fun. 
Thanks
Stan


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## Artie (Oct 5, 2011)

Hi Stan, in my world, calibrating and inside mike was always done with an already calibrated outside mike...just as you have done.
I love Starret gear, but mostly use Mitutoyo because of cost... doesnt mean Im not looking... ;D

Cheers Rob


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## Blogwitch (Oct 5, 2011)

Stan,

What you are doing is the correct thing if the setting ring is missing.

I have some very expensive tiny Mitutoyo bore gauges, 1mm to 7mm, and they tell you to calibrate them exactly as you have done.

So are you being compulsive? the answer is defintiely NO, you are doing it perfectly correctly.



John


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## Sshire (Oct 5, 2011)

Thanks for the responses. 
I've done well assembling a set of Starrett mikes (1" - 4") on eBay for a bit less than 1 new one. There was only one other bidder on the model 700. 
The 0-1" electronic digital was a birthday gift and it is quite nice. 
My concept re: calibration is this. If my measuring tools are spot on then any errors are due to my relative inexperience at machining. If the measuring tools are a bit off, then I'm compounding the error ( unless the machining measures a bit over and the mike reads a bit under, but that too much given to chance). 
Thanks for confirming my logic in using an outside mike to calibrate an inside one.
Best
Stan


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## mgbrv8 (Oct 5, 2011)

I also use my height gauge when I need to set it to a larger size then I have a mic for ;D

Dave


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## mklotz (Oct 5, 2011)

How about wringing a stack of Jo blocks together, leaving the two endmost blocks "sticking out" a bit so the jaws of the mike could be placed between them?

I've never tried it but it seems like it might work.

(IIRC, gage block accessory sets have endpieces that could be attached to a stack to perform just this sort of function.)


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## Artie (Oct 9, 2011)

mklotz  said:
			
		

> How about wringing a stack of Jo blocks together, leaving the two endmost blocks "sticking out" a bit so the jaws of the mike could be placed between them?
> 
> I've never tried it but it seems like it might work.
> 
> (IIRC, gage block accessory sets have endpieces that could be attached to a stack to perform just this sort of function.)



Merv, Ive got no idea what you have just said... am I missing something????


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## GailInNM (Oct 9, 2011)

For quick and dirty checking up to 1 inch ID, ball bearings are ground to quite close tolerances on the ID. For the lowest grade of bearing, ABEC1, the ID tolerance is worst case is +0/-.0004 inch and on ABEC3 and above the tolerance is +0/-.0002. 

A little awkward to use as the inner race rotates as you are trying to get the ID micrometer jaws seated, but if you can spare a bearing, rinse the oil or grease out and put a drop or two of Locktite bearing mount grade, or glue, in it to lock the races together and dedicate the bearing to your calibration box with the micrometer.

Gail in NM


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## Tin Falcon (Oct 9, 2011)

> Merv, Ive got no idea what you have just said... am I missing something



Believe it or not I understood what Marv said perfectly. Jo blocks is slang for gauge blocks . They were original manufactured by the Johansson Company. The finish is such that they can be stuck /wrung together with instrument oil. So if you build up blocks to equal say .500 then put additional blocks at 90 degrees to the first ones you have a near exact .500 reference this can in theory be done for just about any measurement within the limit of the gauge block set. 
A 81 piece set of genuine Johansson Blocks , Starrett Webber or Mitutoyo can be well over $1000 these sets also often have useful accessories. 
On the other hand enco often has shopfox brand with traceable certificate for IIRC about $150 new. no accessories. 
Like any other tool know what you are shopping for and shop around. 
A bit of history of Mr. C.E. Johansson Here
Tin


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## steamer (Oct 9, 2011)

Tin and Marv are correct....a full set has the end blocks of varying shapes and sizes, even a base and a scriber blade for layout work.  Once you get to the "high" end of the spectrum....lab quality....it's just blocks.

Actually I've been using a set like this to set a dial bore gage to a nominal dimension....like 9" ...for the past two weeks. The same stack is then used without the end blocks to set a "snap gage" caliper gage to measure the mating shaft. There by assuring a minimum of error between the two measurements.

Treat the blocks with extreme care....and don't drop them....the accumulated tolerance of small nicks can be a big problem if they are not treated well. Don't handle them too much either as your warm hands with change the dimension significantly.

I personally have an import set, certainly not up to lab quality....but a great reference in my shop.

Wish my set had all the accessories though....it would be very much more useful

Dave


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## Sshire (Oct 9, 2011)

When Marv suggested using the Jo blocks, I went down to the shop and tried it with my 'import' Jo blocks. Perfect solution. 
Of course, that led to a search about C. E. Johansson. There is a long out-of-print book, "The Master of Measurement" which I managed to locate at A.B.E. Books (IMO the best source of used and out of print books - I got my Machinery's Handbook there). It is from a bookseller in Germany. Didn't cost much more than a good pair of plug and bottom taps . 
Tin's reference is excellent. Fascinating guy. Much of what we use today must have equally interesting stories. Who sat down and thought, "geez, this .100 scale just isn't precise enough. Hey! If I took a really, really fine thread and then engraved some numbers around the barrel......."
Best
Stan


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## Sshire (Oct 9, 2011)

Couldn't resist. John Louis Palmer. 1848


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## Sshire (Oct 9, 2011)

OK, now I'm really done. 
http://www.mitutoyo.com/pdf/History_of_Micro.pdf
Stan


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## mklotz (Oct 9, 2011)

Something else to remember when calibrating micrometers...

If you're going to check a micrometer for accuracy, it's important to use a set of gage blocks that cause the spindle to seat at different orientations so drunken thread errors will be noticed. The preferred set for inch micrometers is: 0.105, 0.210, 0.315, 0.420, 0.500, 0.605, 0.710, 0.815, 0.920, 1.000. For metric micrometers the preferred set is: 3.1, 6.5, 9.7, 12.5, 15.8, 19.0, 21.9, 25.0.


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