While measuring Threads: External Thread Profile

Threaded surfaces are helical surfaces generated by a determine profile, whose plane  contains the axis and describes a cylindrical helical path around this axis. There are two types of threads: External (Screws) & Internal (Nuts)

1.- Thread defining elements:

• Outside or nominal diameter. (OD=DN=D)
• Inner or background diameter. (ID=DB=D1)
• Pitch diameter or medium. (Pd=DM=D2)
• Flank angle. (a)
• Pitch. (P)


Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.
UN form screw threads: The UN thread is intended for general purpose fastening operations.
UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.
UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.
UNEF (Unified National Extra Fine): Used when finer pitches than UNF are needed (Ex: Short engagement length).

Gewinde nach ausländischen Normen
Photo Credits: EMUGE FRANKEN Poster

2. – Control of external threads:

2.1- Outside diameter measurement (nominal) with analog or digital micrometer screw.

2.2. Angle Measurement
2.2.1. Method of the two rods.                                                             2.2.2. Profile Projector.

2.3. Thread pitch.

• The difficulty for the pitch measurement in the profile projector is the poorly defined thread flanks.
• To fix the projector reticle It is flush with the baseline of bevelled blades, better defined.

2.4. Measuring pitch diameter
2.5. -Measurement of the pitch diameter.
2.5.1. –Analog Contacts cone & V-shaped Eµm

2.5.2. –Digital Contacts cone& V-shaped Eµm

2.5.3. –Three-Wire Method to Measure Thread Profiles

The pitch diameter (Pd) of a threaded rod can be measured directly with specialized thread micrometers.
By using three wires of the same known diameter, the thread pitch can be measured with a standard micrometer.
These formulas are only useful with 60° single-start threads.
Let’s start by saying that is a little tricky handling three wires together with a micrometer all at the same time.
A micrometer support can be very helpful. There are several tricks of the trade that can make it easier, but learning to make the measurement without “accessories” can ultimately be faster and more accurate.

IMG_20151215_132314

Here’s the process for taking a measurement. M.
2.5.3.1.- Insert your EµM into the EµM support to easily handle the rod + calibrated wires+ EµM srew within your hands.
2.5.3.2.- Based on the pitch diameter Pd of the thread you are measuring, use the table below to select the proper set of thread calibrated wires.
2.5.3.3.- Adjust your EµM to about 0.010″ larger than you expected measurement is to be.
2.5.3.4.- Put two wires in adjacent V’s on the bottom of the part. Use the fixed anvil of the micrometer to hold them in place.
2.5.3.5.- Now on the top of the part, slip the third wire into a V treaded porofile under the movable anvil of the EµM.
2.5.3.6.- Take your EµM measurement reading. This process sounds to be harder than it is. Your reading will be more accurate after your third try. Some people use grease on the threads, rubber bands or modeling clay over the ends of the calibrated wires. Any of these tricks will take longer than the method above, and they all can affect the accuracy of the measurement.

Calculating the Pitch Diameter
Now that you have your measurement, it’s a simple process to find the pitch diameter Pd.
Find the Constant for the thread pitch you are measuring from the tabela below.
Subtract the constant from the measured value.
Here’s the formula: Pd = M – k
Pd is the pitch diameter you are trying to find
M is the measurement you took
k is the Constant value from the chart below.

For instance: 13 ½” UNS – 4 2A calculation with 3.632 mm calibrated wires and a brand new eµM reading 345,915 mm.

Pd = M – k; Pd = 345,915 – 5,397 = 340,518 or
Pd = 13’6187″– 0’2125” = 13’4062”
Pd = 340,518 mm or 13’4062”

 

A 13 ½” UNS–4 2A class thread should be between 13’41622″ and 13’40722″, but it is not surprising that threaded rod is somewhat small. This can take into account the bath protection if it´s planned for outside.

 Theads per Inch, Wire Sizes, k
48 0,46 0,913384
44 0,46 0,871728
40 0,46 0,82169
36 0,46 0,760476
32 0,61 1,141476
28 0,61 1,043178
27 0,61 1,013968
24 0,74 1,293368
20 0,74 1,10998
18 0,81 1,216406
16 1 1,673098
14 1 1,476756
13 1,1 1,736852
12 1,4 2,357882
11,5 1,4 2,278126
11 1,4 2,191258
10 1,4 1,99136
9 1,6 2,356358
8 1,8 2,73685
7,5 2,1 3,239262
7 2,1 3,029712
6 2,34 3,344164
5,5 2,74 4,230116
5 3,05 4,744466
4,5 3,23 4,78917
4 3,63 5,397246
3,5 4,7 7,812024
3 4,7 6,764528

How Does It Work?

In theory, you are measuring with wires of a known diameter that contact the threaded part on the pitch line. As with most things in life, actual practice involves compromises.

There are three formulas for calculating appropriate wire sizes:
Smallest wire diameter = 0.56 × Pitch; Largest wire diameter = 0.90 × Pitch; Diameter for pitch-line contact = 0.57735 × Pitch; If you do the math, you will find that all the suggested wires in the table above are between the smallest and largest values.

The thread pitch for an American National Standard Unified (ASME B1.1 2003) 60° thread is: Pd = M + 0.86603P – 3d

Notice that if we know the pitch diameter (Pd) and the wire diameter (d), then for any particular pitch, the formula reduces to: Pd = M – k, where k is a constant for a particular pitch and wire size combination. These are the constants given in the table above.
Take a look at the first formula for a moment. The value 0.86603 is a constant because we are only considering 60° threads. It encapsulates some trigonometry involving the thread angle. The 3d term is the interesting one. It highlights the fact that your thread measuring calibrated wires must be the correct size. Any error in the wires is magnified three times. There is more to consider than wire diameter. Bent, distorted, or wires will also affect the measurement. (Rubber bands, anyone?)

Metric Threads under B1.13M 2005 Standard are also 60° threads so these formulas work just as well with them.

The following table shows the wire sizes and constants for Metric Threads.

Pitch, Wire Size, k
0,8 0,61 1,1360
1 0,74 1,3438
1,25 0,81 1,3559
1,5 1,02 1,5325
1,75 1,02 1,6969
2 1,14 1,7490
2,5 1,6 2,6355
3 1,8 2,8883
3,5 2,06 3,1411
4 2,34 3,5463
4,5 2,74 4,3325
5 3,05 4,8139
5,5 3,22 4,9142
6 3,63 5,7004
8 4,7 7,1688

Note that these constants assume you are working in millimeters, not inches.

2.5. – Direct measurement of the pitch diameter.
2.5.1. – 3D CMM         2.5.2.- 3 wires method           2.5.3.- Profile Projector

To measure pitch diameter flush the reticle stroke “A” and moving the table in the direction of the arrow to the mark with the line “B” blade.

2.6. Complete control in the multi-thread measuring instrument
PhotoCredit: LMM: Laboratorio de Metrología y Metrotécnia, ETSIIM

PhotoSource: LMM: Laboratorio de Metrología y Metrotécnia, ETSIIM

Sources: MITUTOYO, STARRET, BROWN & SHARPE, EMUGE FRANKEN & TESA Flyers

TÍTULO: Curso de Metrología Dimensional.
AUTOR: Carro de Vicente Portela.
EDITORIAL: E.T.S.I.I. Madrid, 1978. pp 169 y siguientes.

TÍTULOS: Consejos de metrología de la A.E.C.C. (varios) AUTOR: Comité de Metrología de la A.E.C.C. Madrid. EDITORIAL: Asociación Española de Control de Calidad.

TÍTULO: Clasificación de instrumentos de metrología dimensional. AUTOR: Ministerio de Industria, Comercio y Turismo. Dirección General de Política Tecnológica.
EDITORIAL: Sección Publicaciones Ingenieros Industriales. Madrid, 1992.

TÍTULO: Metrologia básica
AUTOR: Manrique, E., Casanova, A.
EDITORIAL: Edebé

EGA is registered as  14.302 Engineer at C.O.I.T.I.Madrid

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