ISO1502

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

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ISO1502

Max d2

skrue

Min d2

skrue

Max D2

møtrik

Min D2

møtrik

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

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ISO1502

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

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ISO1502

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 gaugesM36x4-6g 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

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ISO1502

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.

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