Transcript Measuring Instruments - Brent Payne | Engineering

Measuring Instruments
Metrology

science of weights and measures
Discrimination

degree to which a measuring
instrument divides a basic unit of
length
10 to one rule for
discrimination
a measuring instrument should
discriminate 10 times finer than the
smallest unit that it will be used to
measure
 instrument lands between two lines do not use to measure

Parallax error

the apparent shift of an object when
viewed from different angles
Calibration

Process by which a measuring
instrument is compared to a known
standard

The international prototype kilogram is a platinum iridium
alloy, created in 1879, that is the standard by which all the
world’s weight measurements are calibrated. It is lock in a
Paris vault and is only brought out on very rare occasions

The length of a meter is defined by the distance light travels
in a vacuum in 1/299,792,458 seconds
Handle precision measuring
instruments with care
Use of precision measuring
instruments
keep calibrated
 when possible, take readings while
the instrument is in contact with the
work piece
 on cylindrical work pieces, check
twice - 90 degrees apart
 best temperature is 68 degrees F.

Direct reading instruments
1. Steel Rules
2. Calipers (Can be Vernier,
Dial, or Digital Reading)
Outside
 Inside
 Depth

3. Micrometers (Can be
Vernier, Dial, or Digital
Reading)

Outside
 frame,
Inside
 Depth

anvil, spindle, sleeve, thimble
Comparison measuring
instruments
comparing to a known standard
 represents a known distance
 used in conjunction with an
instrument that has the capability of
showing measurement
 sometimes referred to as transfer
measurement
 direct reading instruments should be
used whenever possible

Common types
1. spring calipers
 2. telescoping gages
 3. small hole gages
 4. adjustable parallels
 5. radius gages
 6. thickness gages
 7. planer gages (similar to adjustable
parallels)

8. Squares

a. machinists combination square set
 protractor
 centering
head
b. solid beam square
 c. cylindrical square

9. Indicators
balanced vs. continuous readings
 mounting must be rigid

 magnetic
bases
 flexible link holding arms
 height gage applications (test)
2 types of indicators

a. dial
 some
can be used to make linear
measurement
 discriminations range from .00005 to
.001”
 ranges are from .003 to 2.0”

b. test
 used
for comparisons only (no direct
measurements)
 discrimination approx .0005”
 range of about .030”
Angle Terminology
acute angles = < 90 degrees
 right angles = 90 degrees
 obtuse angles = > degrees
 full circle = 360 degrees
 1 degree = 60 minutes of an arc
 1 minute = 60 seconds of an arc
 radian = metric unit of angular
measurement

Angle measuring
instruments (not for
measuring angularity)
1. Plate protractor

discrimination of 1 degree
2. Bevel protractor

part of the machinists combination
set - discrimination of 1 degree
3. Universal bevel
protractor
includes a vernier scale
 measure to accurately to within 5
minutes of a degree (remember there
are 60 minutes in one degree

4. Sine bar

5” and 10” center spacing of
cylinders
To find angles with the sine
bar:
elevate the sine bar with adjustable
parallels until the indicator reads
zero at each end of the workpiece
 parallel is removed and measured
with a micrometer
 transpose the sine bar elevation
formula and solve for the angle
 sin of the angle = elevation / sin bar
length (5 or 10”)

Gage blocks

permit comparison between working
measurement and instruments of
mfg.
Uses:
calibration of measuring instruments
 establishment of precise angles (with
sine bars)
 positioning of machine tools and
cutting tools
 used to set snap gages

Common set has 81 to 88
blocks
Range in thickness from
.050 to 4.0”
3 Grades
grade 1 = lab
 grade 2 = inspection
 grade 3 = shop

Grade on tolerance = +/.000002”
Tolerance on a stack of 30
gage blocks assembled to
represent 20”?
Use as few combinations as
possible when stacking
Care
avoid unnecessary contact with
hands
 clean with proper solvents
 handle and lay on lint free cloth
 don’t leave blocks rung for extended
periods of time
 spray with a preservative

Wringing gage blocks
Main cause of gage block
wear is wringing poorly
cleaned blocks
Checking with optical flats
Wear blocks
used in applications where direct
contact is made
 usually .050” or .100”

Accessories
scribes
 bases
 screw sets

Other machines
Optical comparator
 Coordinate measuring machine
(CMM)
