S.A.E. (English, Imperial) Measuring

S.A.E. (English, Imperial) Measuring

STANDARD UNIT: 1 inch







In precision measuring each inch is divided into:
A. Thousandths: .001=1/1,000
B. Half-Thousandths: .0005=5/10,000
C. Tenth-Thousandths: .0001=1/10,000

Thousandths are o.k. for measuring spark plug gaps, distributor air-gaps, most end floats, etc., but not accurate enough for piston/cylinder clearance, crankshaft journal sizes or bearing fits. Here we need to measure more closely. Good S.A.E. micrometers will measure to the tenth-thousandth of an inch (1/10 of one thousandth of an inch). In the following example we will be using a .0001" micrometer.

Micrometer Sleeve (Fixed Scale)
First divided into tenths (.1, .2, etc.)







Then each tenth is divided into four quarters or 25 thousandths of one inch (.025, .050, .075)

Micrometer Thimble (Rotating Scale)

The thimble which rotates past the sleeve is divided as follows:







Each major line = one thousandth of one inch (.001)
Each minor line = on half-thousandth of one inch (.0005)

The thimble moves .025 inch for each revolution and therefore id divided into 25 major and minor lines, or graduations.

Vernier Scale (Fixed Scale)
The sleeve contains a vernier scale divided into ten-thousandths to achieve the final reading. The vernier scale is only found on micrometers accurate to .0001".







Read the line which most closely aligns with a thimble scale division. This value is then added to the total to achieve the final reading.







Using The Micrometer
Using the clutch, close the micrometer until the spindle touches the anvil or standard (1"-2", 2"-3", 3"-4", etc.) DO NOT FORCE. If the "0" on the spindle does not align exactly with the "0" graduation on the thimble, calibrate the micrometer. If using a standard, make sure that it is square with the spindle and anvil. Both micrometer and standard should be at room temperature.

Open the micrometer and place on part to be measured. Use the clutch when closing while gently rocking the mike to square the jaws. A micrometer is not a C-Clamp.