How to match threads for nuts and bolts 1

From the workshop
Fasteners
How to match threads for nuts and bolts
FIGURE 1 Screw thread terms
by
Josh Giumelli and Ben White,
KONDININ GROUP
T
he task of matching an unknown thread
on a nut or bolt can be made easier with
the help of a few simple tools.
Most threaded components on farm
machinery are Unified National Coarse
(UNC), Unified National Fine (UNF) and
metric in both fine and coarse thread varieties.
Other less common threads include British
Standard Whitworth (BSW), British Standard
Fine (BSF), American National Fine (ANF) and
American National Coarse (ANC).
Pipes and fittings generally have their own
specified thread types, including British
Standard Pipe Thread (BSPT) and National
Pipe Thread (NPT).
Often a machine’s country of origin can
provide a lead as to the thread types on fasteners
used. For example, machinery made in
Pitch
(metric)
Threads
per inch
(imperial)
Thread
angle
Diameter
Source: Kondinin Group.
Germany will be fitted with metric thread
fasteners while North American machinery will
feature fasteners with UNF or UNC threads.
Imperial threads such as UNF, UNC, BSW,
BSF, ANF, ANC, NPT and BSPT are
categorised by their diameter in inches and
threads per inch (TPI). As the name suggests,
TPI is recognised by the number of threads
along an inch length of the bolt shank.
Fine bolts will have a higher TPI than coarse
threaded bolts.
Metric fasteners, including metric coarse
and metric fine, are specified according to their
diameter in millimetres and thread pitch.
Thread pitch is a measure of the distance
between two successive threads in millimetres
(see Figure 1).
For both metric and imperial fasteners,
the size of the head on the fastener does
not always reflect the thread diameter on
the bolt.
1
To identify the fastener type, look for markings
on the bolt heads. These do not generally
indicate the thread type but can indicate the
mechanical properties and construction materials.
The fastener head pictured left is marked with lines
indicating its imperial style, while the fastener on
the right is marked with ‘8.8’ indicating it is a metric
grade 8 bolt. For information on bolt grades and
strength, refer to Farming Ahead No.95, page 26.
2
Before determining the thread type, clean the
fastener using a wire brush or wire brush wheel.
Excess dirt and grease in the threads can make the
threads feel mismatched.
TABLE 1 Imperial thread key
3
A thread gauge is a useful tool for identifying the
thread type but a different gauge is required for
each thread series. For example, the gauge
pictured is suited to British Standard Whitworth
(BSW) threads. A thread gauge costs about $30.
Threads per inch
Size (inch)
1/8
1/4
5/16
3/8
7/16
1/2
9/16
5/8
3/4
7/8
1
11/8
11/4
13/8
11/2
13/4
2
18
Unified
National
Coarse
(UNC)
Unified
National
Fine
(UNF)
British
Standard
Whitworth
(BSW)
British
Standard
Fine
(BSF)
British
Standard
National Pipe
Thread
(BSPT)
(NPT)
—
20
18
16
14
13
12
11
10
9
8
7
7
6
6
5
4.5
—
28
24
24
20
20
18
18
16
14
12
12
12
12
12
—
—
—
20
18
16
14
12
12
11
10
9
8
7
7
—
6
—
—
—
26
22
20
18
16
16
14
12
11
10
9
9
8
8
—
—
28
19
—
19
—
14
—
—
14
—
11
—
11
—
11
—
11
27
18
—
18
-—
14
—
—
14
—
11.5
—
11.5
—
11.5
—
11.5
TABLE 2 Metric thread key
Pitch
Size (mm)
5
Metric coarse
0.8
Metric fine
—
6
1.0
—
7
1.0
—
8
1.25
1.0
10
1.5
1.25
12
1.75
1.25
14
2.0
1.5
16
2.0
1.5
18
2.5
—
20
2.5
1.5
22
2.5
—
24
3.0
2.0
27
3.0
—
30
3.5
2.0
FA R M I N G A H E A D
No. 140
September 2003
Fasteners
From the workshop
7
4
Working through the thread profiles on the thread
gauges, visually select the blades on each gauge
closest to the fastener in question.
For metric fasteners, measuring the distance
between threads or pitch without vernier calipers
can be difficult. A simpler method is to measure
the length of 10 threads and divide by 10 to
determine the distance between each thread
accurately. The length of the 10 threads on the
fastener pictured measures 20mm, indicating it has
a pitch of 2.0mm. The bolt diameter can further
define between metric coarse and metric fine
series. As the fastener has a diameter of 16mm,
this categorises the fastener as a M16 x 2.0 pitch
metric coarse thread. Refer to Table 2 for further
classifications of metric fine and metric coarse
threads. Note that a metric fine thread of 2.0mm
pitch would have a diameter of 24mm.
11
To identify thread types quickly, a board of
common nuts and bolts can be assembled and
hung in the workshop.
5
The correct thread profile on the thread gauge
should perfectly match the fastener profile.
Note that for Unified National Coarse (UNC) and
British Standard Whitworth (BSW), thread sizes are
very similar. Take care to distinguish between the
thread types. Often the threads per inch (TPI)
of UNC and BSW are the same for a given bolt
diameter (see Table 1). UNC has a thread angle
of 60 degrees while BSW has a thread angle of 55°.
8
For difficult to reach internal threads, a piece of
Blu-Tack or putty can be used to gain a negative
impression of the thread profile.
12
A selection of Unified National Coarse (UNC),
Unified National Fine (UNF), metric coarse and
metric fine nuts and bolts will cover most fasteners
encountered on-farm. Ensure the board is clearly
labelled with the type of thread, bolt diameter,
thread pitch or threads per inch (TPI). The
fasteners pictured were assembled for about $30.
9
Clean the internal thread before pressing the
Blu-Tack into the threaded wall of the fastener.
6
Another method of determining imperial sized
threads is to count the number of threads along an
inch (25.4mm) section of the fastener shank.
Using a steel ruler or calipers as pictured, measure
one inch of thread and count the number of
complete ridges along the section. This method is
suited to the coarser threads. Fine threads can be
difficult to see with the naked eye.
FA R M I N G A H E A D
No. 140
September 2003
10
Remove the Blu-Tack and follow the same thread
recognition processes carried out for external
threads as pictured.
13
To use the fastener board, simply test fit the nut
onto the bolts on the board. Alternatively, the nuts
can be removed from the board to test bolts for fit.
19