Measure Thread Pitch Diameter – External & Internal
Thread diameter denominations
d= major diameter
d2 = pitch diameter
d1 = minor diameter
D = major diameter
D2 = pitch diameter
D1 = minor diameter
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External pitch diameter
Thread ring gauges (Go NoGo) do not measure a thread. They inspect. Go and No-Go simply determine if the thread is within most specifications or not. Furthermore thread ring gauges do not determine if the thread major diameter is below tolerance and/or the thread minor diameter is below tolerance.
Probably the most common method of measuring an external thread pitch diameter is by using 3 wires. Loose wires are inexpensive but require both patience and skill plus a calculation. Wires that attach to micrometers are expensive and also require a calculation.
Thread wires that attach to micrometers.
N.B. There are 3 “standard” micrometer spindle diameters (6.35mm/1/4”, 8mm/5/16” and 6.5mm) so the correct wires must be bought.
FMS thread inserts have the advantage, after zeroing them on the digital caliper, is that what you see after measuring is the thread pitch diameter. No calculation necessary. Furthermore, with standard FMS external thread inserts 4 pairs cover the pitch range of 0.5 – 8mm / 48 – 3 TPI for threads with flank angles between 55º – 80º.
FMS thread inserts type 21B for pitches
from 2 – 4mm / 13 – 6 TPI
Internal pitch diameter
Thread plug gauges (Go NoGo) do not measure a thread. They inspect. Go and No-Go simply determine if the thread is within most specifications or not. Furthermore thread plug gauges do not determine if the thread minor diameter is below tolerance and/or the thread major diameter above tolerance.
As with external FMS thread inserts 4 pairs of internal cover the pitch range of 0.5 – 8mm / 48 – 3 TPI for threads with flank angles between 55º – 80º. There are though 2 types of FMS internal thread inserts. Type 22 for small diameter threads (from 6mm/1/4”) and type 23 for larger diameter threads (from 35mm/1½” and over). Type 22 can also measure large diameter threads but cost more than type 23.
Threads with “small” flank angles (Trapezoid 30º and ACME 29º) and more unusual threads such as Buttress require a pair of FMS thread inserts (external or internal) for each pitch.
FMS thread inserts
types 21, 22 & 23
These 3 types are for measuring pitch diameter on threads with a flank angle between 55º – 80º.
21 for external and 22 & 23 for internal.
e.g. M (metric), UN (UNC, UNF, UNEF), G (Whitworth profile straight threads), NPS, Pg, BSW & BSF.
M & UN threads having a J on the denomination means that the radius on the external thread minor diameter are slightly larger than “standard”.
FMS thread inserts
types 24 & 25
These 2 types are for measuring pitch diameter on tapered threads.
24 for external and 25 for internal.
e.g. R, Rc (Whitworth profile tapered threads) & NPT. Whitworth has a 55º flank angle and NPT a 60º flank angle.
One pair of FMS thread inserts type 24A can measure the pitch diameter of external tapered threads (R & NPT). 25A can measure the pitch diameter of internal tapered threads (Rc & NPT).
Both 24A & 25A measure these threads with pitches from 19 TPI – 8 TPI.
FMS thread inserts
types 26, 27 & 2
These 3 types are similar to 21, 22 & 23 but for measuring pitch diameter on threads other than those with a flank angle between 55º – 80º.
26 for external and 27 & 28 for internal.
e.g. Metric Trapezoidal (Tr), ACME, Stub Acme, Rd & Buttress.
N.B. To measure internal thread pitch diameter also requires a FMS calibration plate as a reference. FMS thread inserts 24A (external taper) also require the use of a calibration plate.
FMS calibration plates
types 30 & 34
These can be supplied with a calibration attest from an authorized calibration facility if required.
Advantages of measuring
threads rather than
Solid thread gauges inspect and don’t measure.
1 When machining small quantities of threaded items the necessity for gauges can be expensive. Unusual or non standard thread gauges often require a lengthy delivery time.
2 When machining large numbers or mass production measuring pitch diameter means that starting (set-up) near the middle of the pitch diameter tolerance should give less inspection.
3 For those companies that require proof of calibration of their gauges means that when measuring pitch diameter of the threads to be inspected gauge wear can be expected to be significantly less and thus longer calibration intervals.
4 Solid thread gauges have manufacturing and wear tolerances so if your customer uses gauges to inspect you could risk having tour delivery rejected if the pitch diameter is very near the upper or lower pitch diameter tolerance limit.
“Interesting” (?) facts
1 On the most common types of threads (M & UN) the pitch diameter tolerance on internal threads is approximately 30% more than on the same external thread.
2 Whitworth profile (55º) pipe threads have only 4 pitches. 28, 19, 14 & 11 TPI.
3 60º flank angle pipe threads (NP) have only 5 pitches. 27, 18, 14, 11½ & 8 TPI.
4 Rd (Knuckle) threads have only 4 pitches. 10, 8, 6 & 4 TPI.
5 Pg (Steel conduit thread) (80º) only has 3 pitches. 20. 18 &16 TPI.
Measure other than
a thread pitch diameter
To measure an external thread OD (d) then a micrometer or caliper can be used.
To measure an internal thread ID (D1) then a caliper is often enough. Preferably digital (or dial) as they are more accurate than vernier.
A thread pitch gauge is useful to inspect thread pitch.