Micrometer

How it works

The spindle is part of a ground screw that turns in a fixed nut. Brown & Sharpe’s booklet gives the principle as a screw free to move in a fixed nut, where the backward movement of the screw opens a gap for the work and the size is read from graduations. The thimble is attached to the spindle, so turning the thimble moves the spindle toward or away from the anvil.

On an inch micrometer the screw has 40 threads per inch, so one turn of the thimble moves the spindle 0.025 inch. The sleeve line is divided into 40 parts for those turns, and the beveled edge of the thimble is divided into 25 parts of 0.001 inch each. A metric micrometer has a pitch of 0.5 mm and a thimble of 50 parts of 0.01 mm. The reading is the visible sleeve divisions added to the thimble line that coincides with the sleeve’s reading line.

Anatomy

Frame: The C-shaped body that carries the anvil and the sleeve.
Anvil and spindle: The two measuring faces; the work is placed between them.
Sleeve (barrel): The fixed graduated tube. Its graduations lie in a line parallel to the axis of the screw and conform to the pitch.
Thimble: The rotating graduated sleeve attached to the spindle.
Ratchet stop: A slipping drive on the end of the thimble that limits the pressure on the work. Brown & Sharpe says to turn it only far enough to cause two clicks.
Lock: A lock nut or clamp ring that holds the spindle at a setting for repeated measurements; Brown & Sharpe says it originated its clamp ring.
Vernier (on fine-reading models): Ten divisions on the sleeve equal nine on the thimble, so a micrometer can read to 0.0001 inch.

Variants

Outside micrometer: The common form, in one-inch steps of range.
Vernier micrometer: Adds the ten-thousandths vernier to the sleeve. Starrett’s metric version reads 0.002 mm.
Inside micrometer: Measures inside diameters with extension rods and collars; Starrett says to rock it slightly to feel the true diameter.
Depth micrometer: A ground base combined with a micrometer head and rods of one-inch increments.
Micrometer head: The screw, sleeve and thimble as a unit for building into other instruments; Starrett listed micrometer heads in its 1902 catalog.
Pocket sheet metal gauge: Brown & Sharpe’s first form of the tool in 1867, for sheet metal.
Bench micrometer: A screw-measuring machine on a stand, such as the one Henry Maudslay kept as a court of appeal in his shops.

Specifications

Screw pitch, inch · threads per inch: 40One turn of the thimble moves the spindle 0.025 inch.
Screw pitch, metric · mm: 0.5One turn moves the spindle 0.5 mm.
Thimble graduations · divisions per turn: 25 (inch) or 50 (metric)Each division is 0.001 inch or 0.01 mm.
Vernier resolution · in: 0.0001Starrett’s ten-thousandths micrometer; the Starrett metric vernier micrometer reads 0.002 mm.
Standard reference temperature · °C: 20NIST gives 20 °C (68 °F) as the standard temperature for dimensional measurements.

Made to

ISO 3611
ASME B89.1.13
DIN 863-1

Safety and care

Care

Brown & Sharpe’s booklet says to take the micrometer apart only when absolutely necessary, because dirt or grit in the screw is injurious to the threads, and warns that in a grinding room emery can find its way into the screw and rapidly reduce accuracy.
To oil, use a very light oil in small quantity, such as sewing machine or gun oil. Clean the threads with naphtha first and let the tool dry before applying oil.
Before storing for a long time, clean the tool, coat it with oil to prevent rust, and wrap it in oil paper to exclude dust and dampness.
Brown & Sharpe recommends sending a tool to the factory for repair, where it can be adjusted by experts under good conditions.

Tips

Let the ratchet limit the pressure
Brown & Sharpe says the ratchet stop suits a series of measurements because each object is measured at the same pressure, and that the stop need only be moved far enough to produce two clicks. Measuring faces brought up too forcefully give an incorrect reading, especially on a ten-thousandths micrometer.1
Read the sleeve like change from a ten-dollar bill
Starrett suggests counting the numbered sleeve figures as dollars, the vertical sleeve lines as quarters and the thimble divisions as cents, then adding the change and putting a decimal point in place of the dollar sign. On its example the sleeve shows 1, three more lines are visible, and thimble line 3 coincides, giving 0.178 inch.1
Read ten-thousandths from the vernier on the sleeve
First take the thousandths reading, then find which of the vernier lines on the sleeve lines up with a thimble line and add that many ten-thousandths. Starrett’s example reads 0.2507 inch where vernier line 7 coincides.1 2
Check zero with a sheet of paper between the faces
Brown & Sharpe says to clean the anvil and spindle by closing them on a clean sheet of paper and drawing the paper out, which the pressure should just permit, then to test the zero reading. A zero error is corrected by the adjusting screw that moves the anvil, turning it a small amount at a time.1
Keep the heat of the hand out of the reading
Brown & Sharpe warns that the heat of the hand, the cutter or the work makes a momentary difference in the reading, and recommends leaving the tool somewhere unaffected by heat until it is needed. NIST’s Gage Block Handbook gives 20 °C (68 °F) as the standard temperature for dimensional measurement.1 2
Take out screw wear with the tapered nut
Brown & Sharpe describes removing play in the screw by removing the thimble and spindle and turning the slotted tapered nut with the supplied wrench; because the threaded part of the barrel is split, a very slight turn snugs the screw. The thimble is then rolled between the palms to confirm the threads run smoothly.1
Rock an inside micrometer to find the true diameter
Starrett says to hold the assembled inside micrometer across the diameter with one end touching, screw the thimble until the other end lightly contacts the work, and rock the gage slightly in different directions to feel the true diameter before reading.1

History

Earliest documented

about 1639 (screw micrometer, England); 7 September 1848 (hand-held micrometer caliper patent, France)

England (Gascoigne, as reported by Mitutoyo); Paris (Palmer)

Mitutoyo’s history reports that William Gascoigne in the UK made the first screw-operated micrometer in about 1639. The Science Museum Group dates Henry Maudslay’s bench micrometer to about 1805. The hand-held form is documented by the French patent of Jean Laurent Palmer, 7 September 1848, patent no. 7518 according to the Science Museum Group, for a screw micrometer caliper with a circular vernier.1 2 3

The micrometer applies a screw to measurement and is far older than the instrument that bears its name. Mitutoyo’s history reports a screw micrometer by William Gascoigne in about 1639, and the Science Museum Group holds a bench micrometer by Henry Maudslay of about 1805, which the museum says was considered the ultimate court of appeal for work in his shops.

Palmer’s contribution was to put the screw, the scales and the caliper frame into one hand-held tool. The Linda Hall Library historian William Ashworth writes that the micrometer idea was 180 years old and calipers were far older, but that no one had combined the two into a single hand-held tool. Palmer’s workshop in Paris produced drawn wire and seamless tubes, and according to the same account he apparently needed an instrument to measure wire diameter and the thickness of sheet and tube walls. His 1848 patent was for a screw with a circular vernier, and the Science Museum Group’s record gives its pitch as 1 mm with a thimble divided into 20 parts.

Brown & Sharpe’s account of its own adoption is specific. Its booklet says that earlier in 1867 an American brass manufacturer had a shipment returned because it was out of gauge, and that of the three U.S. standard gauges then in use no two agreed. The manufacturer sent a sketch of a measuring tool that the firm found too difficult to read. After Brown and Sharpe visited the Paris Exposition, they introduced the Pocket Sheet Metal Gauge in the fall of 1867, using Palmer’s divisions and adding compensation for wear. The Science Museum Group describes the same event, with the firm copying Palmer’s principle to make a sheet plate gauge for which there was demand.

After 1867 the instrument grew by feature. Brown & Sharpe says it originated the clamp ring, and its booklet shows the ratchet stop, the split barrel with tapered nut for taking up wear, and the ten-thousandths vernier. By 1934 the company said it made over 400 different models. In the 1902 catalog Starrett already listed rings, ratchets, metric models, inside and depth micrometers and heads. Linda Hall’s account says that in 170 years of refinements the basic design had stayed unchanged.

Mitutoyo’s history describes the micrometer as a caliper that uses the mechanical magnification of the inclined plane of a screw thread to give resolution down to the micrometer level, and notes that it is not considered a caliper in the same sense as the sliding types even though its formal name was micrometer caliper. The history says the title of ISO 3611:1978 was revised from Micrometer calipers for external measurement to Micrometers for external measurement, and that the French still call the instrument a Palmer.

Timeline

about 1639
Gascoigne in the UK builds a screw-operated micrometer, which Mitutoyo’s history calls the world’s first.1
about 1805
Henry Maudslay makes a bench micrometer, nicknamed “The Lord Chancellor” because it was considered the ultimate court of appeal for work in his shops; the surviving small example may be a model or an unfinished smaller version.1
7 September 1848
Jean Laurent Palmer is granted French patent no. 7518 for a micrometer caliper with a screw and circular vernier. The Science Museum Group describes the replica as having a screw of 1 mm pitch and a thimble divided into 20 parts, reading to 0.05 mm over a range of 0 to 20 mm.1 2
1867
At the Paris Exposition, J. R. Brown and Lucian Sharpe of Providence see Palmer’s “Système Palmer” micrometer. According to Brown & Sharpe’s own booklet, on returning in the fall they introduce a “Pocket Sheet Metal Gauge” adopting Palmer’s divisions and adding compensation for wear of the screw and measuring surfaces. The Science Museum Group says they bought one and took it home.1 2 3
1868
The Brown & Sharpe Mfg. Co. is incorporated, according to the company’s 1904 catalog.1
1877
Brown & Sharpe’s booklet shows a “Micrometer Caliper of 1877.”1
1878
Brown & Sharpe makes its first one-inch micrometer caliper in metric form, reading 0 to 25 mm with a scale reading to 0.02 mm, according to the Science Museum Group record.1
1884
Thomas Humpage makes and designs a 3-inch to 6-inch workshop micrometer reading to a thousandth of an inch, now in the Science Museum Group.1
1885
Brown & Sharpe’s booklet shows an “Improved Micrometer Caliper of 1885.”1
about 1902
Starrett’s Catalogue No. 17, dated 1902 by the Internet Archive, lists micrometers with finger ring, ratchet stop, metric micrometers, micrometer heads, paper-gauge micrometers, inside micrometers and a patent micrometer depth gauge.1
1934
Brown & Sharpe’s booklet, whose printing code reads 10-34 and which the Internet Archive dates 1934, says the company produces a line of over 400 different micrometer calipers.1
1975
Starrett’s Bulletin 1203 documents inch and metric micrometers with vernier, depth micrometers and inside micrometers, among them a vernier micrometer reading 0.002 mm.1

Where the sources disagree

Who first made a hand-held screw micrometer?

Jean Laurent Palmer, by his French patent of 7 September 1848. The Science Museum Group says Brown & Sharpe’s copy of its principle was the origin of the modern hand-held micrometer gauge, and the Linda Hall Library says no one had combined screw and caliper into one hand-held tool before him.1 2
Science Museum Group (object record); Linda Hall Library (Ashworth)
William Gascoigne made the first screw-operated micrometer in the UK about 1639.1
Mitutoyo’s history
Brown & Sharpe made a micrometer by adopting the Palmer system in 1867 for sheet metal.1
Brown & Sharpe’s booklet

How did Brown & Sharpe come by Palmer’s micrometer?

Brown and Sharpe were impressed by the tool at the 1867 Paris Exposition and, on their return, introduced a Pocket Sheet Metal Gauge.1
Brown & Sharpe’s own booklet
They bought one at the exposition and brought it back to the United States.1
Science Museum Group
Brown & Sharpe took Palmer’s micrometer to the United States.1
Mitutoyo’s timeline

Gaps in the record (7)

Palmer’s French patent text, and a Palmer instrument record from the Conservatoire national des arts et métiers, have not been located; his patent number comes from the Science Museum Group person record.
Gascoigne’s micrometer is known here only through Mitutoyo’s history; Gascoigne’s own record has not been read, and what his instrument measured has not been confirmed.
Brown & Sharpe’s and the Science Museum Group’s given name for Sharpe is spelled differently (Lucian and Lucien); this entry follows the spelling in the company’s own booklet.
The Linda Hall account is by a historian of science but is a short secondary article; it relies for Palmer’s biography on collector discussion that has not been read.
No sourced safety guidance was located.
No independent test of micrometer accuracy or of any buying criterion was located, so buying is null.
Standards: the slugs listed are candidates to be resolved; Mitutoyo names ISO 3611 in its history, the others are assumed.