Manufacturing Process - Screw Threads

1

• Thread is used to fasten two parts and to transmit force or motion. In case of thread work, the

object is to ensure mechanical strength of the screw thread being governed by the amount of flank

contact.

• DESCRIPTIONS

o What is it?

A spiral ridge on a rod (screw or bolt), a spiral

groove in a hole (nut).

o What causes it?

Formed into the screw or bolt.

o What is the effect?

Screw or bolt can be rotated into or out of an object.

o What changes it?

Can be various sizes and shapes, can be external

(screw) or internal (nut), can be left hand or right

hand.

2

• There are many different thread forms in use today. The forms most widely used for power

transmission screw threads are illustrated at Figure below.

• The most common screw thread form is the one with a symmetrical V-Profile. The included angle

is 600. This form is prevalent in the Unified Screw Thread (UN, UNC, UNF, UNRC, UNRF) form as

well as the ISO/Metric thread.

Various types of screw threads

3

Various types of screw threads

• The advantage of symmetrical threads:

o They are easier to manufacture and inspect compared to non-symmetrical threads. These

are typically used in general purpose fasteners.

• Other symmetrical threads are the Whitworth, and the Acme.

o The Acme thread form has a stronger thread which allows for use in translational

applications such as those involving moving heavy machine loads as found on machine

tools.

o Previously square threads with parallel sides were used for the same applications. The

square thread form, while strong, is harder to manufacture. It also cannot be compensated

for wear unlike an Acme thread.

• Great care must be taken as many forms are almost identical. The Acme form (290 included angle)

is only 10 different from the ISO Metric Trapezoidal form (300 included angle).

The Withworth Thread

4

• Many thread forms such as Unified, Metric ISO and Acme are subject to published standards

while others, including Ballscrew and Worm threads are not defined in detail by any standards

organizations.

• The Whitworth thread has an included angle between flank of 550. The Unified, Metric ISO with

600 angle and Whitworth threads are intended for use as standard nuts, bolts and pipe work.

The Unified Screw Threads (UN and UNF) The ISO Metric Screw Threads

Six basic elements in identifying screw threads:

1. Thread Pitcho The thread pitch is the axial distance from one thread

groove to the next.o The thread pitch can be measured with a steel rule or ao The thread pitch can be measured with a steel rule, or a

calliper or comparator can be used.o The pitch can be determined by dividing the count into the

length.o In the example shown in Figure on the right, there are 5

pitches in 1 in. so the thread pitch is .200 in.

2. Hand Of The Threado The hand of the thread can be easily determined by visual

inspectioninspection.o Most threads are right hand and Left hand threads are

common on manual drives where clockwise handlerotation raises, tightens, extends, or creates motion awayfrom the operator.

5

Six basic elements in identifying screw threads:

3. Major Diametero Major diameter is the diameter of an imaginary co-axial cylinder which touches the crests of an

external thread or the roots of an internal thread.o Major diameter is an external /outer diameter of bolt or nut. The major diameter can be measuredj j

with a micrometer, calliper or steel rule. Major diameters are generally the first numbers found inthread designations. Care must be taken to measure the major diameter on a section of the screwthread that is not worn.

4. Minor Diametero Minor diameter is the diameter of an

imaginary co-axial cylinder touching theroots of an external thread or the rootsof an internal thread. It simply calls asinternal diameter.

o The minor diameter can be determinedo The minor diameter can be determinedby direct measurement on an opticalcomparator or by measuring the depthof the thread with a depth micrometerand subtracting twice the measureddepth of thread from the major diameter.

Six basic elements in identifying screw threads:

5. Pitch Diametero The pitch diameter is the diameter at which the thread

tooth and the thread space are equal.o To accurately measure the pitch diameter requires any p q

optical comparator or thread wires.o The optical comparator is the easiest to use as the

measurement can be directly made and nomathematics are necessary.

o The disadvantage to the optical method is that thescrew must be physically removed from the machineand taken to the comparator.

6. Angles of thread o The angle of thread is known from the name of theo The angle of thread is known from the name of the

thread.o All National form and Unified threads have a 600

angle.o Acme and Worm threads have a 290 angleo Whitworth threads have a 550 angle.

6

There are three common standards used in identifying thread:

1. Metric standards

o A metric screw thread is specified by how far, in millimetres, it advances in one turn of the

screw.

o For example, if one turn of a filter brings it 1 mm closer to the lens then it is called "M1.0".

o It is the distance from one peak of the thread to the next one. This number is referred to as "The

Pitch".

o The most popular metric threads are: M.5, M.75, M.9, M1.0, M 1.25. The usual filter thread in

medium size filters is M.75.

2. U.S.A (inch) standards

3. British (inch) standards

o English threads are specified by how many peaks there are in one inch of the length.

o They are specified as "Threads per inch" written "TPI" The diameter is specified in "thousandths

of an inch“.

o The most popular English threads are: 48 TPI, 40, 36, 32, 30, and 24 TPI.

DESIGNATION EXAMPLES OF VARIOUS THREADS

ORIGIN COARSE THREAD FINE THREAD EXTRA FINE THREAD

METRIC M 8 M 8 X 1.0 M 8 X 0.75

U S A 3/8 - 16 UNC 3/8 - 24 UNF 3/8 - 32 UNEFU.S.A. 3/8 16 UNC 3/8 24 UNF 3/8 32 UNEF

BRITISH 1/2 BSW 1/2 BSF NONE

ORIGIN MINIATURE THREAD PIPE TAPERED THREAD PIPE PARALLEL THREAD

METRIC M 1.0 M 12 X 1.5T (TAPER) M12 X 1.5

U.S.A. 1.0 UNM 1/8 - 27 NPT 1/8 - 27 NPS

BRITISH 10 BA 1/2 BSPT 1/2 BSPF EXAMPLEThere are two kinds of ISO metric threads: coarse and fine.o Fine metric threads are designated by the letter M followed by the nominal major diameter of the

thread and the pitch in millimeters.For example M10 x 1.0 indicates that the major diameter of the thread is 10mm and the pitch is1.0mm.

o Coarse metric threads haven’t indicated the pitch value.For example stating that a thread is M10 indicates a coarse thread series is specified ofdiameter 10mm (giving the thread a pitch of 1.5mm).

7

Unified National thread and identification of

thread

ISO metric thread and identification of

thread

British Thread (British Standard Whitworth) -- Coarse Pitch

NominalSizeW

ThreadFormT

MajorDiameter

mmPitchmm

Threadsperinch

PitchDiameter

mm

MinorDiameterMale Thd.

ThreadHeight

H1

TapDrill

DiameterWw Type mmd=D p inch

tpimm

d2=D2Male Thd.

d3 H1 Diametermm

1/16" BSW 1.587 0.423 60 1.315 1.050 0.270 1.153/32" BSW 2.381 0.529 48 2.041 1.703 0.338 1.901/8" BSW 3.175 0.635 40 2.768 2.362 0.406 2.50

5/32" BSW 3.969 0.793 32 3.459 2.952 0.507 3.203/16" BSW 4.762 1.058 24 4.084 3.407 0.677 3.707/32" BSW 5.556 1.058 24 4.878 4.201 0.677 4.501/4" BSW 6.350 1.270 20 5.537 4.724 0.813 5.10

5/16" BSW 7.938 1.411 18 7.034 6.131 0.904 6.505/16 BSW 7.938 1.411 18 7.034 6.131 0.904 6.503/8" BSW 9.525 1.588 16 8.509 7.492 1.017 7.90

7/16" BSW 11.113 1.814 14 9.951 8.789 1.162 9.201/2" BSW 12.700 2.117 12 11.345 9.990 1.355 10.405/8" BSW 15.876 2.309 11 14.397 12.918 1.479 13.403/4" BSW 19.051 2.540 10 17.424 15.798 1.627 16.257/8" BSW 22.226 2.822 9 20.419 18.611 1.807 19.251" BSW 25.400 3.175 8 23.368 21.335 2.033 22.00

8

Nominaldiameter

d = DPitch

P

RootRadius

R

pitchdiameterd2=D2

minor diameterd3 D1

thread heighth3 H1

drilldiameter

M1.0x0.2 0.20 0.029 0.870 0.755 0.783 0.123 0.108 0.80M1.1x0.2 0.20 0.029 0.970 0.855 0.883 0.123 0.108 0.90M1.2x0.2 0.20 0.029 1.070 0.955 0.983 0.123 0.108 1.00M1.4z0.2 0.20 0.029 1.270 1.155 1.183 0.123 0.108 1.20M1.6x0.2 0.20 0.029 1.470 1.355 1.383 0.123 0.108 1.40M1.8x0.2 0.20 0.029 1.670 1.555 1.583 0.123 0.108 1.60M2x0.25 0.25 0.036 1.838 1.693 1.729 0.153 0.135 1.75

M2.2x0.25 0.25 0.036 2.038 1.893 1.929 0.153 0.135 1.95M2.5x0.35 0.35 0.051 2.273 2.071 2.121 0.215 0.189 2.10M3x0.35 0.35 0.051 2.773 2.571 2.621 0.215 0.189 2.60

M3.5x0.35 0.35 0.051 3.273 3.071 3.121 0.215 0.189 3.10M4x0.5 0.50 0.072 3.675 3.387 3.459 0.307 0.271 3.50

M4.5x0.5 0.50 0.072 4.175 3.887 3.959 0.307 0.271 4.00M5x0.5 0.50 0.072 4.675 4.387 4.459 0.307 0.271 4.50

M5.5x0.5 0.50 0.072 5.175 4.887 4.959 0.307 0.271 5.00M6x0 75 0 75 0 108 5 513 5 080 5 188 0 460 0 406 5 20M6x0.75 0.75 0.108 5.513 5.080 5.188 0.460 0.406 5.20M7x0.75 0.75 0.108 6.513 6.080 6.188 0.460 0.406 6.20M8x0.75 0.75 0.108 7.513 7.080 7.188 0.460 0.406 7.20M8x1.0 1.00 0.144 7.350 6.773 6.917 0.613 0.541 7.00

M9x0.75 0.75 0.108 8.513 8.080 8.188 0.460 0.406 8.20M9x 1 1.00 0.144 8.350 7.773 7.917 0.613 0.541 8.00

M10x0.75 0.75 0.108 9.513 9.080 9.188 0.460 0.406 9.20M10x1 1.00 0.144 9.350 8.773 8.917 0.613 0.541 9.00

M10x1.25 1.25 0.180 9.188 8.466 8.647 0.767 0.677 8.80

Nominaldiameter

d = DPitch

P

RootRadius

R

pitchdiameterd2=D2

minor diameterd3 D1

thread heighth3 H1

drilldiameter

M11x0.75 0.75 0.108 10.513 10.080 10.188 0.460 0.406 10.20M11x1 1.00 0.144 10.350 9.773 9.917 0.613 0.541 10.00M12x1 1.00 0.144 11.350 10.773 10.917 0.613 0.541 11.00

M12x1.25 1.25 0.180 11.188 10.466 10.647 0.767 0.677 10.80M12x1.5 1.50 0.217 11.026 10.160 10.376 0.920 0.812 10.50M14x1.0 1.00 0.144 13.350 12.773 12.917 0.613 0.541 13.00M14x1.25 1.25 0.180 13.188 12.466 12.647 0.767 0.677 12.80M14x1.5 1.50 0.217 13.026 12.160 12.376 0.920 0.812 12.50M15x1 1.00 0.144 14.350 13.773 13.917 0.613 0.541 14.00

M15x1.5 1.50 0.217 14.026 13.160 13.376 0.920 0.812 13.50M16x1 1.00 0.144 15.350 14.773 14.917 0.613 0.541 15.00

M16x1.5 1.50 0.217 15.026 14.160 14.376 0.920 0.812 14.50M17x1.0 1.00 0.144 16.350 15.773 15.917 0.613 0.541 16.00M17x1.5 1.50 0.217 16.026 15.160 15.376 0.920 0.812 15.50M18x1.0 1.00 0.144 17.350 16.773 16.917 0.613 0.541 17.00M18x1 5 1 50 0 217 17 026 16 160 16 376 0 920 0 812 16 50M18x1.5 1.50 0.217 17.026 16.160 16.376 0.920 0.812 16.50M18x2.0 2.00 0.289 16.701 15.546 15.835 1.227 1.083 16.00M20x1.0 1.00 0.144 19.350 18.773 18.917 0.613 0.541 19.00M20x1.5 1.50 0.217 19.026 18.160 18.376 0.920 0.812 18.50M20x2.0 2.00 0.289 18.701 17.546 17.835 1.227 1.083 18.00M22x1.0 1.00 0.144 21.350 20.773 20.917 0.613 0.541 21.00M22x1.5 1.50 0.217 21.026 20.160 20.376 0.920 0.812 20.50M22x2.0 2.00 0.289 20.701 19.546 19.835 1.227 1.083 20.00M24x1.0 1.00 0.144 23.350 22.773 22.917 0.613 0.541 23.00M24x1.5 1.50 0.217 23.026 22.160 22.376 0.920 0.812 22.50

9

Errors in screw threads can arise during it’s manufacture or storage.

These errors may be minor, major, pitch diameter and thread form.

• Error In Major And Minor Diameters

These errors will result due to interference with the mating thread and strain in the joint and more

force is required for fitting. If this errors is present, it will lead to rapid wear and weakening of the

screw thread.

• Angle Errors

Any errors in angle of thread results in interference between bolt and nut. These errors increase

the virtual effective diameter of a bolt and decrease that of nut. This will cause a progressive

tightening and interference on assembly.

• Error in pitch

The threads are generated by a point cutting tool. The errors in pitch will result total length of

thread engaged will be either too small or to great.

• Tolerance of screw threads is complicated by the complex geometric nature of the screw thread

form.

• Clearances must be applied to the basic profile of the threads in order that a bolt thread can be

screwed into a nut thread.

• A tolerance class is made up of two parts, a tolerance grade and a tolerance position.

• Tolerance grades are represented by numbers, lower the number the smaller the tolerance.

• There are:

o 5 tolerance grades (grades 4 to 8) available for the minor diameter of the nut thread.

o 3 tolerance grades (grades 4,6 and 8) for the major diameter of the bolt thread.

o 5 tolerance grades (grades 4 to 8) for the pitch diameter tolerance of the nut thread.

o 7 tolerance grades (grades 3 to 9) for the pitch diameter tolerance of the bolt thread.

• A full designation for a metric thread includes information not only on the thread diameter and pitch

but also a designation for the thread tolerance class.

• For example a thread designated as M12 x 1 - 5g6g indicates:

• the thread has a nominal diameter of 12mm and a pitch of 1mm.

• 5g indicates the tolerance class for the pitch diameter

• 6g is the tolerance class for the major diameter.

10

• Tolerance positions are indicated by letters, upper case letters for nut threads and lower case

letters for bolt threads.

• The tolerance position is the distance of the tolerance from the basic size of the thread profile.

o For nut threads

There are two tolerance positions,

H with a zero fundamental deviation (distance of the tolerance position from the

basic size) and

G with a positive fundamental deviation.

As an example, for Metric system, M8 x 1 – 6H.

o For bolt threads

There are four tolerance positions,

h has a zero fundamental deviation and

e, f, and g negative fundamental deviations.

A positive fundamental deviation indicates that the size for the thread element will be

larger than the basic size.

A negative fundamental deviation indicates that the size for the thread element

will be smaller than the basic size.

There are number of method in measuring thread. However, it will dependon working environment.• The normal method of checking threads in production is by the use of

Screw Plug gauge or Screw Ring gauge.o Screw plug gauges are generally supplied with GO and NO GO

members to check maximum and minimum material conditions.

Screw plug gauge

o Screw ring gauges are supplied as individual GO and NO GOgauges.

• Most combinations of pitch and thread form can be manufactured asrequired.

• Screw thread roller caliper gauges are used to gauge external screw threads and can be manufactured to gauge all standard thread forms.

• However, there is limitation of thread roller caliper gauge such as:o Gauge is presented in only one axial plane at a time o Not recommended for thin wall products o Not recommended for pitch diameter tolerances of less than

0 050mm

Screw ring gauge

0.050mm.

Tapered screw plug gauge

Tapered screw ring gaugeScrew thread calliper roller gauge

11

Profile Projector (Optical Comparator)

• Used for measuring objects by enlarging their projected profile on a

screen.

• It is capable to measure:

o Thread Angle

o Pitch length

o Major diameter and minor diameter (depending on the size of

screw).

Measuring Microscope

• Featuring high precision measurement by magnifying minute objects

from low to ultra high magnifications.

• The microscope is capable to measure the elements of thread as profile

projector.

• The thread angle can be measured by using specific reticle.

• SCREW THREAD MICROMETER

Measures the minor diameter of external threads.

12

Three Wire SystemIt is essential in using this method that the micrometers used be accurate and the measuring faces flat and parallel; that the wires used be hardened. The surfaces must be properly finished and the set of wires the same diameter within .00003 inches if measurement within 0001 inch is desired

3-wire unit

within .0001 inch is desired.

In the table of thread elements below is a column headed “Diameter of Best-Size Wires.” This column lists the size of the wire, of a given pitch, that touches the thread exactly on the pitch diameter when placed in between two threads.

Adjacent is a diagram showing the method of applying the wires for the measurement of a thread. Below will be found the best size wires to use for the various pitches and the constant to subtract from the micrometer reading to obtainand the constant to subtract from the micrometer reading to obtain the pitch diameter.

The wire sizes and constants given are for use with a thread which has an included angle of 60°.

P = Pitch DiameterD = Wire DiameterW = Measurement Over Wires

Three Wire System To calculate the “over wire measurement” when the “best-size” wires are available, refer to the table of thread elements. To the basic pitch diameter, add the constant for the “best-size” wire.

Example:

3-wire unit

Example:The basic pitch diameter of a 3/8"-16 thread = .3344The constant for the “best-size” wire = .0541The measurement over wires = .3885

To calculate the “over wire measurement” when the “best-size” wires are not available, refer to the table of thread elements. Subtract from the pitch diameter, the single height V-thread and to the result add three times the diameter of the available wire.

Example:Th i h di f 3/8" 16 3344The pitch diameter of a 3/8"-16 screw = .3344 Minus the single height V-thread = .0541

.2803 Plus three times .040" (available wire) = .1200 The measurement over wires = .4003

P = Pitch DiameterD = Wire DiameterW = Measurement Over Wires

13

Table of Thread Elements

ThreadsPer Inch Pitch

Diameter of"Best Size"

Wires

Constant for"Best Size"

Wires

Single* HeightSymmetricalThread Form

0.64952P

Width of Flat onCrest and Root

NC and NF

Single HeightV -Thread

140 .007143 .0041239 .006186 .004640 .0009 .0061861201009690

.008333

.010000

.010417

.011111

.0048112

.0057735

.0060141

.0064150

.007217

.008660

.009021

.009623

.005412

.006495

.006766

.007217

.0010

.0013

.0013

.0014

.007217

.008660

.009021

.009623 8072646056

.012500

.013888

.015625

.016667

.017857

.0072168

.0080182

.0090210

.0096225

.0103097

.010825

.012027

.013531

.014434

.015464

.008119

.009021

.010149

.010826

.011598

.0016

.0017

.0020

.0021

.0022

.010825

.012027

.013531

.014434

.015464 50484440

.020000

.020833

.022727025000

.0115470

.0120279

.01312140144337

.017320

.018041

.019682021650

.012990

.013531

.014762016238

.0025

.0026

.00280031

.017320

.018041

.01968202165040

36 .025000.027777

.0144337

.0160370 .021650.024055

.016238

.018042 .0031.0035

.021650

.024055 3230282726

.031250

.033333

.035714

.037037

.038462

.0180421

.0192448

.0206194

.0213833

.0222057

.027063

.028867

.030929

.032074

.033308

.020297

.021650

.023197

.024056

.024982

.0039

.0042

.0045

.0046

.0048

.027063

.028867

.030929

.032074

.033308 2422201816

.041666

.045454

.050000

.055555

.062500

.0240558

.0262428

.0288675

.0320746

.0360843

.036083

.039364

.043301

.048112

.054126

.027063

.029523

.032476

.036084

.040595

.0052

.0057

.0062

.0069

.0078

.036083

.039364

.043301

.048112

.054126

Table of Thread Elements

ThreadsPer Inch Pitch

Diameter of"Best Size"

Wires

Constant for"Best Size"

Wires

Single* HeightSymmetricalThread Form

0.64952P

Width of Flat onCrest and Root

NC and NF

Single HeightV -Thread

14 .071428 .0412389 .061858 .046394 .0089 .0618581312

11 1/211

.076923

.083333

.086956

.090909

.0444114

.0481123

.0502040

.0524863

.066617

.072168

.075306

.078729

.049963

.054126

.056480

.059047

.0096

.0104

.0108

.0114

.066617

.072168

.075306

.078729 1098

7 1/27

.100000

.111111

.125000

.133333

.142857

.0577350

.0641499

.0721687

.0769800

.0824784

.086602

.096224

.108253

.115467

.123717

.064952

.072168

.081190

.086602

.092788

.0125

.0139

.0156

.0167

.0179

.086602

.096224

.108253

.115467

.123717 6

5 1/25

4 1/2

.166666

.181818

.200000222222

.0962246

.1049726

.11547001282998

.144336

.157458

.173205192449

.108253

.118094

.129904144338

.0208

.0227

.02500278

.144336

.157458

.1732051924494 1/2

4 .222222.250000

.1282998

.1443375 .192449.216506

.144338

.162380 .0278.0312

.192449

.216506

*To calculate the pitch diameter of Unified and American National Form Threads, subtracts the figures found in this column from the Basic Outside Diameter. Use the “Three Wire Method” to convert pitch diameter calculations into Over Wire Measurements. NOTE: Symmetrical thread height equivalent to the basic height, h, of the original American National Form.