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Outside Micrometers
Inside Micrometers
Micrometer Heads
Depth Micrometers
There are probably more variations in configurations for outside micrometers than any
other single device. The basis of operation is pretty much the same. A frame holds the anvil (fixed surface) and the screw assembly (also fixed to the frame). A spindle is attached to the thimble which is
threaded onto the screw assembly. As the thimble is rotated, the spindle advances toward the anvil. One surface of the piece to be measured is held in contact with the anvil
(the fixed surface at one side of the frame). The thimble is then rotated by hand to advance the spindle to the other side of the piece to be measured. When contact is
made, the outside dimension of the piece can be determined by interpreting the scale of the micrometer.
In the case of inch mics, each full rotation of the thimble advances the spindle EXACTLY
.025”. And, the barrel is divided into .025” divisions while the thimble has 25 divisions on it (each representing an advance of .001” of the spindle). This allows interpretation
(or resolution) of units down to .001”. Some models have an additional vernier scale on the barrel with 10 divisions which allows interpretation of readings down to .0001”. This
addresses the mechanical instruments. Of course the digital and electronic digital versions make the interpretation much easier.
The costs of mics vary from around $50 to $500. This depends on the type selected as well as the manufacturer. One thing to keep in mind is the service you will be expecting
from the instrument. If you will be demanding long term accurate service, you should find out who is going to calibrate and repair your mics. Ask them for a recommendation of
brand. Most calibration and repair sites will be able to service the name brands but get the particulars sorted out before your purchase. This does not mean that other brands
are useless and untrustworthy, just that they may not be easily or quickly returned to useful service if they have problems.
The care and maintenance of micrometers is typical of what you might expect from other precision instruments. Clean pieces to be measured before you check them. Wipe the
mics clean with a lint free cloth. Store them in a protected area (preferably in their own case). NEVER lubricate the threads as this will affect the accuracy. If the barrel
becomes difficult to turn, there is probably some dirt between the spindle and frame or in the threads - or they may have been dropped. A competent calibration/repair company
should get involved at this point - don’t try to do it yourself unless you have been specifically trained. NEVER store the mics with the anvil and spindle faces in contact.
Do not scribe or vibra-pen your name/identification on the frame - believe it or not, this can effect the accuracy - rather mark the barrel if you must mark something. If the
thimble is a friction or ratchet type (used to achieve a consistent measuring force) - ALWAYS use them rather than relying on your own “touch”.
Be aware that the mic measurement are in the thousandths of an inch or in some cases ten-thousandths of an inch. Your hair is about 6 thousandths of an inch in diameter - so
VERY SMALL chips, machining oil, and temperature can have a dramatic effect on what you interpret the outside dimension to be.
Calibration of mics should always be done by a qualified source. If you must maintain traceability to national standards (NIST) there is a web site that lists companies that are
certified by A2LA to calibrate. By advancing the spindle to the anvil you can check for proper 0. If your mics have ranges other than 0 to 1 inch / 0 to 25 mm you can place the
reference standard that came with the mics between the anvil and spindle as well. Just keep in mind that there are many ways for the mic to wear which could generate errors in
readings and calibration is the only way to determine if the micrometer is still yielding accurate readings.
Outside Micrometers
The travel of the micrometer screw is limited to about 1 inch. Therefore, measuring
dimensions larger that one inch requires larger frames. While some very large frames can be custom made, about the largest "standard" size is 24 inches. Since there are so
many configurations available, a cost/feature comparison is one way of communicating the variety and uses.
The standard 1" mic with flat spindle and anvil and .001" divisions costs around $70 for
brand names and about $25 for generic.
Adding carbide faces, .0001" resolution and either friction or ratchet stops (for improved consistency) adds between $20 and $30.
Add a mechanical "odometer" type readout (to keep from reading the .001" divisions on the barrel) for another $50 and your up to about $150.
Compare this to an electronic version with inch/metric conversion and electronic communication for about $180. If you are generally measuring small pieces, it's a good
idea to get a mic stand for the electronic mics. First of all, if you drop one, it probably won't bounce back and secondly, they are quite bulky compared to the standard mic.
And trying to hold the mic in one hand while getting a precision measurement is a real challenge. The $20 for the stand is a good investment.
A set of OD mics to allow measurements from 0 to 6" will run about $400. The larger sizes are quite pricey with a 20 to 24" set running around $800. Mics larger than 1"
generally come with a standard to allow setting of the "0" reading (for example 2" on a 2 to 3" mic). If you don't get one, you need to figure out how you are going to verify the mic
setting as wear occurs. The standards can be periodically calibrated traceable to NIST but setting to the length standard is not the only consideration in calibration. There are a
host of ways that mics can become worn, including face wear on spindle and anvil, thread wear in a constantly used range, out of parallelism and flatness of the anvil and
spindle. So if you are using the mics for production work and particularly part size validation, they need to be calibrated by a certified (A2LA or equivalent) lab. The cost of
calibration with certification are not too steep, perhaps $15-30 per mic and this is very inexpensive compared to costs of inaccurate measurements.
Other Configurations include: Multi-Anvil (great for measuring steps and distances from edge of holes). These run about $150. Deep Throat (taking readings up to 6" from the
edge of thin plate or sheet stock) for about $200. Disc Mics (large flat discs on the anvil and spindle for getting into small recesses and on resilient sheet stock) for about $200.
Point Mics (for getting into small grooves) for around $150. Screw Thread mic at about $100. Spherical Mics (half sphere instead of a flat anvil, excellent for measuring wall
thickness of thin wall tubing) with an approx cost of $125
Inside Micrometers
If you need to measure inside dimensions, an inside mic can do the trick. The standard
micrometer screw is used, but there are a series of precision bars than when coupled with the screw head cover a large size range. The spindle and anvil have slightly curved
surfaces to allow measuring inside diameters. As the screw is advanced, it increases the length of the device and readings may be determined when both the anvil and
spindle faces are in contact with the work piece. The Typical screw heads are 1/2" travel and 1" travel. If you are checking sizes less that 4" you will need the 1/2" travel
screw head type because of the overall length of the 1" screw head. A 1/2" travel set to handle 1 1/2" to 8" runs around $175.
A special Inside Mic is made for checking inside dimensions from .200 to 1.200". Caliper type jaws are fitted to a micrometer screw assembly, with an appearance
somewhat like outside micrometers. As the thimble is rotated, the jaws move apart, contacting the internal surfaces being measured. These run approximately $175 for standard or $400 for electronic
Micrometer Head
The micrometer head assembly that is used on outside mics is available as a separate
unit. These are great when building a special gage or when you need to provide very precise movements of lightweight parts or components. They can be used for both
measuring and positioning down to the closest .0001". They are available with both rotating and non-rotating spindles. 1/2" travel heads run about $50, 1" travel about $100
and a special 2" travel with non-rotating spindle for about $700. Of course the standard mic divisions, mechanical counter and digital options are available.
Mount one on each member of a cross slide and you can measure/position in two axis. Add a vertical slide and you're up to a 3 axis measuring positioning system.
Depth Micrometer
If the depth of holes or heights of steps is in order, the Depth Mic is a good choice.
Using the micrometer screw and a series of rods the range of measurement can be provided from 0 to 6". With the 1" screw limit, the rods must be changed, depending on
the depth to be measured. The base (which serves as the reference surface for this type of mic) is available in several lengths. This feature is important because on the one
hand the largest possible flat surface is most desirable, but in some measuring environments restrictions may prevent a very wide base. Name brand standard 0-6"
measuring range will run around $170 while a digital electronic version would be around $400.
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