Brunson 190 Guide

Model 190 and 160 Family of
Optical Micrometers
Instrument
Company
Maintenance and
Adjustment Manual

Helping the World Measure
Since 1927
www.brunson.us
8000 E. 23rd Street
Kansas City, MO 64129
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(877.632.7873)
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Fax: [phone]
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Copyright 2011 Brunson Instrument Company

Table of Contents
Thank you for purchasing a Brunson Optical Micrometer.
Remember that our customer support does not stop after
shipment of a product—we are here to help you with any
measurement challenges that you may have.
1 Introduction
2 Backlash
2 Zero Centering
3 Range Accuracy
6 Model 160 Dual-Axis Micrometer Calibration
8 Adapter for 2.250” Barrels
10 Exploded Parts Diagram
11 Parts List

Model 190 Single-Axis Micrometer
Model 160 Dual-Axis Micrometer
Front Micrometer
Rear Micrometer

1Maintenance and Adjustment Manual
This manual applies to all the different versions of the
Brunson Instrument Company Model 190 single-axis
and Model 160 dual-axis optical micrometers. For
the Model 160, simply perform all Model 190 procedures on the
front micrometer (closest to the cover glass) first, then repeat them
for the rear micrometer. See page 6 for calibrations details on the
M160.
In order to be used or calibrated, all of the various optical microme-
ters we manufacture and sell must be directly mounted on any of
our telescope-based instruments (with the exception of our align-
ment scopes, which require an adapter). In this manual, we refer to
the instrument on which you mount the 190 or 160 as the "Test In-
strument".
Normally, the optical micrometer will need no maintenance beyond
cleaning the cover glass on its front. However, if the micrometer is
used in environments with heavy airborne dirt and grime, it should
be sent back to the factory for complete disassembly, cleaning and
recalibration.
Because there are so many traps in the pursuit of accuracy, the tests
and adjustments in this manual should be made only by people
with proper training and experience in the use and calibration of
optical measurement equipment. For the same reason, it is very im-
portant that your test results be repeatable. If you can't get the
same results every time you repeat a test, the results are invalid. In
that case, it is very likely that something is loose or broken some-
where - either in the instrument or in the test setup. You must find
and solve that problem before you can accurately calibrate the opti-
cal micrometer. It is also important to follow the sequence of cali-
bration checks and adjustments as it is written because the accuracy
of each adjustment depends upon the correctness of the one preced-
ing it. If you have a question about a procedure or instrument in
this or any other optical tooling manual, please call the factory for
over-the-phone assistance.
The principles of calibra-
tion apply to both the sin-
gle and dual axis mi-
crometers. Essentially,
the M160 consists of two
M190’s mounted perpen-
dicular to each other.
See page 6 for calibration
details on the M160.
Model 190 and 160 Family of Optical Micrometers
Introduction
CAUTION!
Do not over tighten the clamping screw when mounting the
micrometer on your Test Instrument. Over tightening can
cause the objective lens in the Test Instrument to flex
slightly, possibly damaging the instrument and certainly un-
dermining its validity as a calibration reference.

Model 190 and Model 160 Family of Optical Micrometers2
In this test, you will be checking backlash in the sec-
tor gear. To begin this test, the micrometer can be ori-
ented to measure either horizontal or vertical dis-
tances. Because correction of backlash requires complete disassem-
bly of the micrometer, it should not be attempted in the field. If you
can't repeat your readings with less than one half of one minor
graduation of backlash in either direction, the micrometer cannot be
calibrated accurately. Return it to the factory for repair.
1. With the micrometer mounted on the Test Instrument, set the
micrometer drum to zero. Place an optical tooling scale (or a simi-
lar bifilar target) about three feet away from the Test Instrument.
2. While looking through the Test Instrument, use the tangent
screws to bring the reticle image into register on the target. Turn
the micrometer drum away from zero at least ten minor gradua-
tions, then turn it back until the reticle image is again in register on
the target. Check the micrometer reading.
Your reading from this direction should be zero, or the error should
be less than one half of one minor graduation on the micrometer
drum.
3. Repeat step two, this time turning the micrometer drum away in
the other direction.
Your reading from this side should also be zero, or less than one
half of one minor graduation in error. Repeat the entire backlash
check to confirm the validity of your results. If backlash is not
within these tolerances, return the micrometer to the factory for re-
pair.
In this test you will prove that when the micrometer
reading is zero, the micrometer does not displace the
line of sight. Mechanically, you will be checking and
correcting the perpendicularity (with the micrometer at zero) of the
plano-parallel window to the optical axis of the Test Instrument.
To begin this test, the micrometer can be oriented to measure either
vertical or horizontal distances. In step 4 below, be careful not to
move the Test Instrument while rotating the micrometer. If the Test
Instrument is moved during the test, your results will be unrepeat-
able. Again, repeatable readings are essential to the validity of cali-
bration test results.
1. With the micrometer mounted, place a bifilar target about three
feet away from the Test Instrument and at least nominally perpen-
dicular to its line of sight.
2. Loosen the micrometer clamping screw just enough to be able to
Backlash
Zero-Centering
Be careful not to disturb
the Test Instrument once
the target is sighted.

3Maintenance and Adjustment Manual
rotate the micrometer smoothly about the end of the Test Instru-
ment barrel. If your Test Instrument barrel has an indexing pin,
slide the micrometer forward slightly so you can move it easily past
the pin.
3. Set the micrometer drum to zero, then use the tangent screws to
register the measuring reticle line (the line that moves when you
turn the micrometer drum) precisely on the target. The other reticle
line should be approximately on target.
4. Rotate the micrometer 180oabout the Test Instrument barrel. If
necessary, turn the micrometer drum to bring the measuring reticle
line back onto the target. Record the micrometer reading.
5. Repeat this entire test to confirm the validity of your results,
then leave the micrometer drum in position showing the error and
remove the micrometer from the Test Instrument.
The error indicated on the micrometer drum is twice the actual
amount of error. If the actual error (one half the error indicated on
the micrometer) is not less than one half of one minor graduation,
correct it this way:
1. Rotate the micrometer drum halfway back toward zero from its
position at the end of step 5, above.
2. Hold the drum to prevent it moving and loosen the three small
screws located near the edge of the top of the micrometer
drum. The graduated ring will then rotate freely between the top
and bottom pieces of the micrometer drum.
3. Without moving the rest of the drum assembly, carefully rotate
the graduated ring the remainder of the distance back to zero.
4. Retighten the three screws and repeat the entire zero centering
test to confirm the effects of your adjustment.
Continue the cycle of checking and adjusting until the actual devia-
tion from zero (half the error indicated on the micrometer) is less
than one half of one minor graduation on the micrometer drum.
In this test, you will be checking the axial position
(range symmetry) and operating radius (range
length) of the sector pin. Because changing the sector
pin geometry affects backlash and zero centering, this calibration
can become very complicated. If you make a correction to either
dimension of the micrometer range, you must recheck backlash and
recalibrate zero centering, in that order, before you can see the re-
sults of your adjustment to the range. As always, and particularly
in this case, repeat the test to confirm the validity of your results
Because zero centering af-
fects range accuracy
slightly, you should re-
check the range after any
adjustment to zero center-
ing.
Range Accuracy

Model 190 and Model 160 Family of Optical Micrometers4
before you make an adjustment.
For this test, select a reference target with graduations correspond-
ing to the micrometer range on either side of zero. If, for example,
the micrometer has a range of plus or minus .100", the target should
have graduations .100" apart (though these need not be the only
graduations on the target). Place the target about three feet from
the Test Instrument and at least nominally perpendicular to its opti-
cal axis. To begin this test, the micrometer can be oriented to meas-
ure either vertical or horizontal distances.
1. With the micrometer mounted on the Test Instrument, set the
micrometer drum to zero. While looking through the Test Instru-
ment, use the tangent screws to bring the reticle image into register
on one of the target graduations corresponding to the micrometer
range.
2. Turn the micrometer drum to move the measuring reticle line
(the line that moves when you turn the drum) to the next target
graduation corresponding to the micrometer range. With the reticle
line precisely registered on the second target graduation, the mi-
crometer reading should exactly correspond to the interval between
the target graduations. If the two numbers do not correspond, re-
cord the difference and repeat the test to confirm the validity of
your error observation.
3. Repeat steps one and two, only this time turn the micrometer
drum the other direction to check the other end of the micrometer
range.
The micrometer range should be symmetrical (same reading in ei-
ther direction) and its length accurate, with less than one half of one
minor drum graduation of error in any dimension. If error is
found, follow the next sequence of adjustments carefully, in the or-
der it is written.
When range error is greater at one end than it is at the other, the
micrometer drum (and thus the sector pin) is not centered over the
optical axis of the micrometer. You must correct this asymmetry of
the range before you can check range length.
1. Notice which end of the range shows longer error, then move the
drum assembly a little to one side or the other (see Figure 1) using
the small Allen screws in the sides of the micrometer body. You
may have to loosen the front or back Allen screw a bit to allow the
drum assembly to move.
2. Recheck backlash and recalibrate zero centering (in that order)
after each adjustment, then recheck range symmetry.
Continue the cycle of checking and adjusting until range error is
within one half of one minor graduation of being equal on both
Once you have made the
range even on both sides
and corrected zero center-
ing, range length should
be relatively easy. How-
ever, it can be difficult to
change range length with-
out putting range symme-
try off a little. Try walk-
ing them in slowly, ad-
justing each axis a little at
a time.
Range adjustments are
very delicate. About a
quarter turn of the screws
is enough to change sym-
metry by half a minor
graduation.

5Maintenance and Adjustment Manual
sides, and backlash and zero centering are within the tolerances
stated in those sections above. When all these conditions are satis-
factory, continue with the steps below.
When the range is symmetrical, but too long or too short, the ra-
dius of travel of the sector pin is incorrect. Correct sector radius
this way:
1. Notice whether the micrometer reads shorter or longer than the
reference target graduations, then move the drum assembly a little
to the front or back (see Figure 1) using the small Allen screws in
the front and back of the micrometer body. You may have to loosen
the side screws a bit to allow the drum assembly to move.
2. After each movement of the drum assembly, recheck backlash
and zero centering (in that order). Make any necessary adjust-
ments, then recheck range symmetry. Correct range symmetry
(checking backlash and zero centering again) before you recheck
range length.
Continue the correction cycle until the range error on both sides of
zero is less than one half of one minor drum graduation, with back-
lash and zero centering also within the tolerances specified in their
respective procedures above.
Figure 1

Model 190 and Model 160 Family of Optical Micrometers6
The principles of calibration for the Model 190 single-
axis micrometer also apply to the Model 160 dual-axis
micrometer. Essentially, the M160 consists of two
M190’s mounted perpendicular to each other. There-
fore it is a simple matter to perform all previously de-
scribed calibration procedures on the front micrometer first, then
repeat them for the rear micrometer.
Two additional checks are required on the Model 160. Proper se-
quence of calibration is detailed below.
Initially set the rear micrometer to zero in preparation for calibrat-
ing the front micrometer.
Front Micrometer Calibration
1. Backlash (see page 2)
2. Zero-Centering (see page 2)
3. Range Accuracy (see page 3)
Note: When all parameters on the front micrometer are set, align
the graduated drum to zero to prepare for calibration of the
rear micrometer.
The front micrometer is
closest to the cover glass
and the rear micrometer is
nearest the objective end of
the telescope.
M160 Dual-Axis Micrometer
Calibration
Front Micrometer
Rear Micrometer