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Ref. ISO 1502      ISO general-purpose metric screw threads
- Gauges and gauging

Introduction :
 The purpose of this information is to explain possible causes of the problems which can arise when only thread plug and ring gauges 
are used for manufacturing and inspecting threads - especially if the finished product (thread) is close to the max. or min. tolerance limits. 
The ideal fit between a screw and a nut is probably best obtained with a combination of inspection (using a GO solid gauge) and measurement, 
as the ideal fit should neither be to loose or too tight. Thread pitch diameter tolerances are often much larger than most realise.

N.B. It is recommended that ISO 1502 be read as it contains more details and tables.

EXTERNAL THREAD
d  = Major diameter

d2 =
Pitch diameter

d1 = Minor diameter
INTERNAL THREAD
D  = Major diameter

D2 =
Pitch diameter

D1 = Minor diameter

P = Pitch

a
  =
Thread angle 60O

Tolerance classes for metric threads

If not specified then the pitch diameter tolerancen for a 
standard
metric nut is 6H and for a standard metric screw is 6g

 

Max
d2
skrue

  

 

Min
d2

skrue

Max
D2
mψtrik

 

 

Min
D2

mψtrik

Td2  is the screw pitch diameter tolerance
TD2
is the nut pitch diameter tolerance 
D2 / d2
  is the nominal pitch diameter 
es  
is the distance from nominal d2 
El
   is the distance from nominal D2 
ZR
  is the distance from the middle of the tolerance limit area 
     TR and the thread's max limit 
ZPL 
is the distance from the middle of the tolerance limit
      TPL and the thread's min limit 
TR 
is the manufacturing tolerance for GO and NG ring gauges 

TPL
  is the manufacturing tolerance for GO and NG plug gauges

 

Tolerances and values (mm)

 

Permissable wear on the pitchdiameter of plug gauges and rings (mm)

Td2 / TD2 (mm)

TR

TPL

ZR

ZPL

Td2 / TD2 (mm)

WGO

WNG

above

up to

above

up to

GO ring GO plug NG ring NG plug

-
50
80

50
80
125

8
10
14

6
7
9

-4
-2
2

0
2
6

-
50
80

50
80
125

10
12
16

8
9,5
12,5

7
9
12

6
7,5
9,5

125
200

200
315

18
23

11
14

8
12

12
16

125
200

200
315

21
25,5

17,5
21

15
19,5

11,5
15

315
500

500
670

30
38

18
22

20
28

24
32

315
500

500
670

33
41

27
33

25
31

19
23

 

Pitch diameter tolerances for GO and NO-GO thread ring and plug gauges

Thread rings 

Thread plug gauges

New GO ring    d2 – es – ZR ±  TR/2
New NO-GO ring    d2 – es – Td2  - TR/2
± TR/2

New GO plug    D2 + El + ZPL ±  TPL/2
New NO-GO plug    D2 + El + TD2  + TPL
± TPL/2

Used GO ring    d2 – es – ZR + WGO
Used NO-GO ring    d2 – es – ZR + WNG

used GO plug    D2 – El – ZPL + WGO
Used NO-GO plug    D2 + El – ZPL + WNG

 

The calculation examples are pitch diameter tolerance limits for a standard M36x4-6g screw and M36x4-6H nut
plus a set of GO/NO-GO M36x4-6g thread rings for a screw and a set of GO/NO-GO M36x4-6H thread plug gauges for a nut.

  Nominal pitch diameter (d2 / D2) for a M36x4 thread is 36,000 – 2,598 = 33,402

M36x4-6g screw  
d2 max = 33,402 – 0,060 =33,342
d2 min = 33,402 – 0,284 = 33,118

M36x4-6H nut  
D2 max = 33,402 + 0,300 = 33,702
D2 min = 33,402 – 0,000 = 33,402

d2 tolerance = 33,342 – 33,118 = 0,224 ( Td2 )

 

D2 tolerance = 33,702 – 33,402 = 0,300 ( TD2 )

 

Calculations for thread ring and plug gauge pitch diameter tolerances

    Max Min  

New GO ring d2 – es – ZR ±  TR/2
Used GO ring d2 – es – ZR + WGO

New NO-GO ring d2 – es – Td2  - TR/2
± TR/2

Used NO-GO ring d2 – es – ZR + WNG

New GO dorn D2 + El + ZPL ±  TPL/2
Used GO dorn D2 – El – ZPL + WGO

New NO-GO dorn D2 + El + TD2  + TPL
± TPL/2

Used NO-GO dorn D2 + El – ZPL + W
NG

33,402 – 0,060 – 0,012 ± 0,0115
33,402 – 0,060 – 0,012
± 0,0255
33,402 – 0,060 – 0,224 – 0,0115 
± 0,0115
33,402 – 0,060 – 0,224 – 0,012 
± 0,0195

33,402 + 0 + 0,016 ± 0,007
33,402 + 0 + 0,016
± 0,021
33,402 + 0 + 0,300 + 0,007
± 0,007
33,402 + 0 + 0,300 + 0,007 – 0,015

33,3415
33,3555
33,1180
33,1255

 33,4250
33,4250
33,7160

33,7160

33,3185
33,3185
33,0950
33,0950

 33,4110
33,3970
33,7020
33,6940

 

Tolerances on threaded components and thread gauges

M36x4-6g
screw
d2 tolerances

M36x4-6g
thread ring
d2 tolerances

 

M36x4-6H
nut
D2 tolerances

M36x4-6H
thread plug
D2 tolerances

max 33,342
min 33,118

GO max 33,3555 / min 33,3185
NO-GO max 33,1255 / min 33,0950

Max 33,702
Min 33,402

NO-GO max 33,7160 / min 33,6940
GO max 33,4250 / min 33,3970

Tolerances on a GO thread ring is 33,3555 – 33,3185 = 0,0370
Tolerances on a NO-GO thread ring is 33,3555 – 33,3185 = 0,0305

Tolerances on a GO thread plug gauge is 33,7160 – 33,6940 = 0,0220
Tolerances on a NO-GO thread pluggauge  is 33,4250 – 33,0970 = 0,0328

Thread manufacturing close to either the max or min tolerance limit will, when two different sets of gauges for manufacturing and inspection are used
(even if these two sets of gauges are within tolerance) as well as machine repeatability, probably give situations where the components could be rejected as defect. 
Many operators regard a "tight" fit between gauge and component as desirable.
This approach gives increased wear on the gauge and the need for frequent callibration.

By combining thread measurement  (for example with FMS accessories and thread inserts for external and internal pitch diameter measurement) with
 thread inspection  (using  GO thread plug and ring gauges) will the conditions for manufacturing a correct fit between screw and nut be met.
For example a M36x4-6H/6g fit should have a miimumn pitch diameter fit of 33,402 – 33,342 = 0,060 and a  maximum pitch diameterfit of 33,702 – 33,118 = 0,584. 
The ideal fit (slack) between the screw and nut is  0,584 – 0,060 / 2 = 0,262 mm

There are several possibilities for thread measurement and inspection. The most advangeous
(and inexpensive) is probably the combination of a "GO" thread plug or ring gauge for correct profile and pitch and measurement of the thread pitch diameter for correct positioning within tolerance limits.