# How to determine the thread size of your main lathe... Page 1

```How to determine the thread size of your main lathe spindle
Why do you have to know the thread size of your spindle?
There are many lathes on the market, all with different thread sizes on the main spindle. To obtain suitable accessories (e.g. faceplates, chucks or adapters), you need to
know the thread of the main spindle.
Required measuring tools
A metric precision calliper with 1/10 accuracy or a calliper with an inch gauge.
Before you measure the thread, check the manufacturer‘s guide or operating instructions for information on the thread. You may need to ask the manufacturer, in which
case it is helpful to know the model and year of manufacture of the lathe.
If you can‘t find any information about the thread size, measure it as described below.
Figure 1
1. Measuring the external diameter of the thread
You need to establish whether it is a metric or inch thread. An inch is about 25.4 mm,
therefore a 25.0 mm thread would be metric (metric-thread sizes see table 1). It is
very important that you measure carefully and read off the measurement accurately.
External thread diameter see figure 3, measurement A.
Typical metric threads are:M18, M20, M24, M25, M30, M33, M40. If you read off
diameters with decimal places, it is an inch thread. For the mm – inch conversion,
see table 2.
2. Measuring the thread pitch
In the case of a metric thread, this is the distance between two neighbouring crests
(see figure 2). Thread pitch see figure 3, measurement B.
In the case of an inch thread, you check the number of crests per inch, i.e. per
25.4 mm. This is referred to as TPI (Threads Per Inch).
Figure 2
Figure 3
3. Measuring the collar
On some machines, the collar of the spindle may have a bigger diameter than the
thread itself. Therefore also measure the collar (see figure 3, measurement C).
4. Identifying the direction of the thread
In the case of an RH (right-hand) thread, the thread flanks ascend to the right when
seen from the top; in the case of an LH (left-hand) thread, they ascend to the left.
There are also RH/LH threads, i.e. threads with two different ridges.
To make sure you order the right parts, the following information is useful:
Manufacturer / Brand________________________________ Thread pitch_______________________________________
Model, if possible year of construction ________________ Collar diameter_____________________________________
External diameter of the thread_______________________ Thread direction____________________________________
Thread form or flank angle of the ridge
- UNC, UNF/NF and metric threads (also referred to as ISO threads) have a 60° angle
- Whitworth, BSW, BSF threads have a 55° angle.
Description e.g.: M33 x 3,5 means:
Diameter 33 mm, Pitch 3,5 mm.
UNC (Unified National Coarse Thread)
Description e.g.: 1’’ - 8 UNC means:
Diameter 1’’ - 8 Threads per 1 inch thread length.
UNF / NF (Unified National Fine Thread)
Description e.g.: 3/4’’ - 16 UNF means:
Diameter 3/4’’ - 16 Threads per 1 inch thread length.
BSW (British Standard Whitworth Coarse Thread) or Withworth
Description e.g.: 3/4’’ - 10 BWS means:
Diameter 3/4’’ - 10 Threads per 1 inch thread length.
BSF (British Standard Fine Thread)
Description e.g.: 1’’ - 10 BSF means:
Diameter 1’’ - 10 Threads per 1 inch thread length.
DICTUM GmbH · Donaustr. 51 · 94526 Metten · Germany
www.more-than-tools.de · [email protected]
Page 1
How to determine the thread size of your main lathe spindle
Ø in mm
Distance in mm
from one crest to
the next
18
2.5
20
2
20
1.5
M18 x 2.5
M20 x 2
M20 x 1.5 e.g. Elektra Beckum
24
3
M24 x 3
25
2
M25 x 2
30
1.5
M30 x 1.5
30
3.5
M30 x 3.5
33
3.5
M33 x 3.5
33
3.5
M33 x 3.5
40
6
M40 x 6 e.g. Sweden
TPI or Pitch in mm
Ø in mm
Ø in Inches
Decimal
Ø in Inches
Number of Distance in mm
Threads per from one crest to
Inch (TPI)
the next
12.700
0.500
1/2“
20
1.27
1/2“ x 20TPI
19.050
0.750
3/4“
16
1.59
3/4“ x 16TPI UNF
19.050
0.750
3/4“
14
1.81
3/4“ x 14TPI BSP
19.050
0.750
3/4“
10
2.54
3/4“ x 10TPI BSW
22.225
0.875
7/8“
14
1.81
7/8“ x 14TPI NF
25.400
1.000
1“
12
2.12
1“ x 12TPI
25.400
1.000
1“
10
2.54
1“ x 10TPI BSF
25.400
1.000
1“
8
3.18
1“ x 8TPI Dual Thread LH/RH
25.400
1.000
1“
8
3.18
1“ x 8TPI UNC
28.575
1.125
1 1/8“
12
2.12
1 1/8“ x 12TPI UNF
28.575
1.125
1 1/8“
7
3.62
1 1/8“ x 7TPI BSW
28.575
1.125
1 1/8“
7
3.62
1 1/8“ x 7TPI UNC
31.750
1.250
1 1/4“
8
3.18
1 1/4“ x 8TPI UNS
38.100
1.500
1 1/2“
6
4.23
1 1/2“ x 6TPI BSW RH/LH
38.100
1.500
1 1/2“
8
3.18
1 1/2“ x 8TPI NF
DICTUM GmbH · Donaustr. 51 · 94526 Metten · Germany
www.more-than-tools.de · [email protected]
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