1.142": Micrometer screw gauge (B&W)

Micrometer screw gauges are used “to measure the thickness or diameter of small objects with high accuracy. They consist of a frame, anvil, spindle, sleeve, thimble, ratchet stop, and locking clamp. Additionally, micrometers can measure in both metric and imperial units and are available in different types, such as outside micrometers, inside micrometers, and depth micrometers.”
—“How to Use a Micrometer Screw Gauge: A Guide for Beginners”

“Precision instrument”? New ones cost a couple hundred US$. If you need a few different sizes, you can purchase sets that start at about US$850 and go up to several thousand dollars! But I digress.

This “outside micrometer” belonged to Clare’s dad (I’m pretty sure). It’s calibrated for imperial units for objects that are between one- and two-inches thick, or across. (I have been reminded of this each time I’ve searched it out to measure things that are smaller than one-inch.) Difficult to photograph with its shiny and curved surface; diffused lighting would have been better.

Never having used a micrometer myself, I had to search for — and learn — how to read a measurement, which was a significant motivator for this post. I wanted to figure out how to read this thing.

Skipping ahead to the answer— In the photo, the gap has a measurement of
     1.000" + 0.100" + 0.025" + 0.017" = 1.142",
so approximately 1-9/64".

Yes, there are four components to the measure that have to be added together! I pieced together the method from “How to Use a Micrometer Screw Gauge: A Guide for Beginners” and a Starrett User Manual (the manufacturer).

Details from Physics Calculations:
“Using the Sleeve Scale
The sleeve scale is the stationary outer part of the micrometer that contains markings in millimeters or inches. The sleeve scale is divided into 10 or 20 equal parts [10 here], each representing a specific measurement unit [1/10" here].

Using the Thimble Scale
“The thimble scale is the rotating inner part of the micrometer that contains markings in millimeters or inches. The thimble scale is divided into 25 or 50 equal parts [25 here], each representing a fraction of a measurement unit [1/1000" here].

Adding the Reading
“To get an accurate measurement, you need to add the readings from the sleeve and thimble scales. The sleeve scale reading is obtained from the first line that coincides with the thimble scale. The thimble scale reading is obtained from the line that coincides with the axial line of the sleeve.”

This 25-part thimble scale (rotating handle at the right edge of photo) goes around 4 times to change the sleeve scale by 0.1", so each time around is 1/10" ÷ 4 = 1/40" = 0.025" and each of the 25 marks is 0.025" ÷ 25 = 0.001" or one-thousandth of an inch.

More?
» L.S. Starrett Co. very much still in the micrometer business, in Athol, Massachusetts!
    • Company history
    • Current version of this micrometer: 436.1XRL-2: $234
» Micrometer at Wikipedia
    • Image at Wikipedia (compare to post!)

[ IMG_4199_12x8tm :: 60mm :: f/32 :: +1 :: foldio :: tripod ]

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