The Micrometer

The Micrometer

History
In the 18th century the industrial revolution was in full swing. New industrial and agricultural machines were invented and patented every year.

One thing that hindered precision manufacture of these machines was a accurate method of measurement. In 1772 James Watt - who also invented the steam engine - invented a screw device that moved a pair of jaws in and out (downt). When a part made contact with those jaws two disk-like dials would show the measurement.

The Frenchman, Jean Palmer in 1848 designed and patented the the portable one-handed micrometer now used throughout the world (up). Brown and Sharpe, American manufacturers of clocks, watches, jewelry, and silverware, saw this device at the Parisian Exposition in 1867. Ten years later, they obtained the patent rights from Palmer to produce the tool.

Patents for the micrometer have long run out and now many manufactures worldwide are producing the device in a wide range of prices. It is possible to place the least expensive with a costly mike side by side and it would be difficult to visually tell them apart. More than likely the least expensive micrometer will measure accuracy within .001 and better. For the casual user this may be adequate, for the professional the instrument must have reliability and longevity.

 The Parts of the Micrometer

It will be important to memorize the parts of the micrometer so that the remainder of this topic makes sense (and other topics too).

As shown, there are some differences in design and nomenclature between micrometers brands.
It is common to hear a sleeve called a barrel and it is also common to see two different types of lock nuts.
The micrometer below does not have a ratchet screw as does the micrometer on the right.







The micometer is often called a "mic" which is pronounced "mike"

Spindle

The heart of the micrometer is the spindle.
The spindle is the part of the micrometer that makes contact with the part being measured (along with the anvil). The spindle has a portion which is threaded at 40 threads per inch. This threaded portion is mated to an identical inside thread on the frame.
By rotating the spindle one turn it moves closer or further from the anvil at an exact rate of .025 per revolution (40/1000). The thimble is attached to the spindle and rotates with it.The spindle should be made from tool steel for longevity and the thread should be ground for accuracy. Inexpensive spindles may be machined from more economical steel which is easier to machine and keeps production cost down. Through constant use these spindle threads will become worn and inconsistent and will no longer be able to maintain accuracy throughout the one inch range.
Again there is a difference in design and nomenclature. The thimble shown makes reference to "friction" thimble which- as will be shown later - is used the same way as a "ratchet screw" (previous page). Also note the "carbide tip". It is usual to have carbide attached to the tip of the spindle and the anvil.

Thimble

The thimble is permanently attached to the spindle which fixes the two as a single assembly. Therefore it is important to recognize that rotating the thimble also rotates the spindle and thereby moves the assembly in and out in relation to the frame (the frame is threaded like a nut to fit the spindle).
The thimble can be a solid piece with a knurl gripping surface or in two parts with a friction thimble that slips when the mike is clamped lightly between the anvil and spindle. Some micrometers, in place of a friction thimble, have a rachet on the end of the thimble that also slips when clamped lightly.

Frame

It is best to discuss the frame as assembly of different parts.

The anvil is permanently press fit into the frame.

The lock is assembled into the frame.

The Barrel and barrel nut are permanently assembled together and then fitted to the frame.

The spindle assembly can now be assembled (threaded) into the frame assembly and the resulting product is a micometer.

Adjusting

Micrometers will need adjusting from time to time. The anvil and spindle faces wear down and the fit between  the barrel nut and the spindle requires adjustment. Before taking a measurement the tips of the micrometer should be cleaned by clamping lightly on a clean sheet  of paper and pulling the paper from the tips. This removes all particles of dirt and dust.Check for accuracy by closing the spindle gently against the anvil and note if the zero line on the thimble  coincides with the zero on the spindle. Use the ratchet on the end of the micrometer to apply the closing  pressure.Now look at the alignment up close. When the micrometer tips are closed the "0" line on the thimble should  be  lined up with the reading line on the barrel.. This micrometer is off by quite a bit (it looks like maybe .0005 inch). An adjusting spanner wrench like the one on the left probably came in the box with your micrometer. You will need it to adjust the error.