1. Choosing a Measuring instrument
Basic Rules
Choose instrument in keeping with the tolerance and
dimensions to be measured.
Use an instrument that gives a direct reading wherever
possible.
Don’t use damaged or worn instruments.
Instrument resolution is not always the accuracy to which it
can be used.
Inbuilt accuracy's often exceed resolution.
The larger the size being measured, the more difficult it will be to attain a
high standard of accuracy.
2. Instrument
Or Measurement
Type of
Measurement Range
Value of smallest
Graduation
(Resolution)
Suggested
Reliability
Steel Rule Direct 150 – 600mm
6 - 24 in
.5mm
.02in
±.5mm
±.02in
Depth Gauge Direct 150mm
6in
.5mm
.02in
±.5mm
±.02in
Calipers Transfer 150mm
6in
none ±.5mm
±.02in
Vernier Calipers Direct 600mm
24in
.01mm
.001in
±.05mm
±.002in
Vernier Depth Gauge Direct 300mm
12in
.01mm
.001in
±.05mm
±.002in
Vernier Height Gauge Direct 600mm
24in
.01mm
.001in
±.05mm
±.002in
Micrometers Direct 0-25mm
0-1in
.01mm
.001in
±.01mm
±.001in
Direct 25-50mm
1-2in
.01mm
.001in
±.01mm
±.001in
Inside Micrometers Direct 2.5-130mm
0.1-5.2in
.01mm
.001in
±.01mm
±.001in
Depth Micrometers Direct 0-300mm
0-12in
.01mm
.001in
±.01mm
±.001in
Slip Gauges End Standard Up to 100mm
Up to 4in
.001mm
.0001in
.0005mm
.00001in
Choosing a Measuring instrument
3. Checking Surface Texture
Roughness average index
The Roughness average (Ra) index is an accepted method
of measuring surface finish.
The table shows a series of surface textures which can be
produced by various manufacturing processes.
4. Checking Surface Texture
Using Comparison Plates
Comparison plates are a range of blocks machined to
various calibrated surface textures, each texture measured in
microns (µm) and micro-inches (µin).
5. Visually ...
Clean work piece and appropriate comparison plate.
Compare the workpiece and plate from many different angles.
The workpiece finish should be at least as good as, if not better.
Checking Surface Texture
6. ... or by Touch
Draw your fingernail lightly over the comparison plate.
Repeat the operation on the workpiece and note any differences.
The workpiece finish should be usually better than the plate.
Checking Surface Texture
8. With an Instrument
Pickup
Stylus
Measurement
Length
Surface to be
measured
The Pickup is pulled across the surface for a pre set distance.
A diamond tip stylus measures the roughness of the surface.
The result in Ra (Roughness average) is shown on the display.
Checking Surface Texture
9. With an Instrument
Ra result taken on a Surtronic instrument
Checking Surface Texture
10. What is Ra?
Mathematically, Ra is the arithmetic average value of the
profile departure from the mean line, within a sampling length.
A method of visualising how Ra is derived is as follows:
Graph A: A mean line X-X is fitted to the measurement data.
Checking Surface Texture
11. What is Ra?
Graph B: The portions of the profile within the sampling length
“l” and below the mean line are then inverted and placed above
the line.
Graph C: Ra is the mean height of the profile above the
original mean line.
Ra
Checking Surface Texture
12. Ra has its Limitations
All the above profiles have different form, but the Ra value
is the same. Other parameters are used to calculate these
differences.
Ra
Ra
Ra
Ra
Checking Surface Texture
