50699244 screw thread measurement

7/28/2019 50699244 Screw Thread Measurement

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Screw thread- definition

a screw thread is the helical ridgeproduced by forming a continuous helicalgroove of uniform section on the externalor internal surface of a cylinder or a cone.

A screw thread formed on a cylinder isknown as straight or parallel screw thread,

while the one formed on a cone is knownas tapered threads.


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Types of thread

External thread: a thread formed on outside of a workpiece is known as external thread. Example: on bolts orstuds etc.

Internal thread: a thread formed on inside of a workpiece is known as internal thread. Example: on a nut orfemale screw gauge.

Multiple-start screw thread: forming two produces thisor more helical grooves equally spaced and similarlyformed in an axial section on a cylinder. This givesquick traverse without sacrificing core length.


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Screw thread - use

Screw threads are used:

To hold parts together-act as fastner (ex: V-threads)

To transmit motion & power (Square, Acme threads)


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Screw Thread terminology

PitchCrest

Root

Flank

ThreadAngle

Pitch line

Axis of thread

Axial thickness

Addendum

Dedendum

Flankangle

Major dia Pitch dia Minor dia

EXTERNAL THREAD TERMINOLOGY


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Screw Thread terminology Pitch: The distance from a point on a screw thread to a

corresponding point on the next thread measured parallel tothe axis.

Lead The distance a screw thread advances in one turn. For asingle start threads, lead=pitch,

For double start, lead=2xpitch, & so on.

Thread Form: The cross section of thread cut by a planecontaining the axis.

Major Diameter: This is the diameter of an imaginarycylinder, co-axial with the screw, which just touches the crestsof an external thread or roots of an internal threads. It isalso called as Nominaldiameter


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Minor diameter: This is the diameter of an imaginarycylinder, co-axial with the screw which just touches theroots of an external thread or the crest of an internalthread. This is also referred to as root or corediameter.

Effective diameter or Pitch diameter: It is thediameter of an imaginary cylinder coaxial with the axis ofthe thread and intersects the flanks of the thread suchthat width of the threads & width of spaces between

threads are equal. Flank: It is the Thread surface that connects crest with

root.

Depth of thread: It is the distance between crest and

root measured perpendicular to axis of screw.


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Screw Thread terminologyAngle of thread: Included angle between sides of

thread measured in axial plane. Helix angle: Angle that thread makes with plane

perpendicular to thread axis.

Flank angle: It is half the included angle of thethread.

Addendum: It is the distance between the crest and

the pitch line measured perpendicular to axis of thescrew.

Dedendum: It is the distance between the pitch line

& the root measured perpendicular to axis of the

screw.


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Errors in Thread

In case of plain shafts and holes, there is only onedimension, which has to be considered, and errors onthis dimension if exceed the permissible tolerance,

will justify the rejection of the part.

While in case of screw threads there are at leastsix important elements, which require consideration,and error in any one of these can cause rejection ofthe thread.


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Errors in Thread

The errors in screw thread may arise duringthe manufacturing or storage of threads. Theerrors either may cause in following sixmain elements in the thread.

1) Major diameter error2) Minor diameter error

3) Effective diameter error4) Pitch error5) Flank angles error

6) Crest and root error


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Errors in Thread

1) Major diameter error:It may cause reduction in the flank contact andinterference with the matching threads.

2) Minor diameter error:It may cause interference, reduction of flankcontact.

3) Effective diameter error:If the effective diameter is small the threadswill be thin on the external screw and thickon an internal screw.


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Errors in Thread

4) Pitch errors:If error in pitch, the total length of threadengaged will be either too high or too small.

The various pitch errors may classified into1. Progressive error.

2. Periodic error.3. Drunken error.4. Irregular error


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Errors in Thread

4) Pitch errors: reason

A point cutting tool generates Generally screw

threads. In this case, for pitch to be correct, theratio of linear velocity of tool and angularvelocity of work must be correct. This ratio mustbe maintained constant; otherwise pitch errors

will occur. If there is any error in the pitch thetotal length of thread engaged would be eithertoo great or too small, the total pitch in overalllength of the thread being called the cumulative

pitch error.


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Errors in Thread

1) Progressive error:The pitch of the thread is uniform but is longer or shorterits nominal value and this is called progressive error.

Causes of progressive error:1. In correct linear and angular velocity ratio.2. In correct gear train and lead screw.3. Saddle fault.4. Variation in length due to hardening


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Errors in Thread

2. Periodic errorThese error repeats itself at regular intervals alongthe thread.

Causes of periodic error:1. Un uniform tool work velocity ratio.2. Teeth error in gears.3. Lead screw error.4. Eccentric mounting of the gears.


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Errors in Thread

Length of thread

Cumulative

pitcherror

Length of thread

Cumulativepitcherror

ERRATIC ERRORSPROGRESSIVE ERROR

Fig (c) Fig (d)


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Errors in Thread3) Drunken error:

Drunken errors are repeated once per turn of the thread in aDrunken thread.In Drunken thread the pitch measured parallel to the threadaxis. If the thread is not cut to the true helix the drunken

thread error will form


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Errors in Thread

Drunken error: this is the one having erratic pitch, in whichthe advance of the helix is irregular in one complete revolutionof the thread. Thread drunkenness is a particular case of a

periodic pitch error recurring at intervals of one pitch. In sucha thread, the pitch measured parallel to the thread axis willalways be correct, the error being that the thread is not cut tothe true helix. If the screw thread be regarded as an inclinedplane wound around the cylinder and if the thread beunwound from the cylinder, (that is development of the threadbe taken) then the drunkenness can be visualized. The helix

will be a curve in the case of drunken thread and not a straightline as shown in the figure


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Errors in Thread

4) Irregular errors:It is vary irregular manner along the length ofthe thread.

Irregular error causes:1. Machine fault.2. Non-uniformity in the material.

3. Cutting action is not correct.4. Machining disturbances


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Effect of Errors in Thread

Increase the effective diameter of the boltand decreases the diameter of nut.

The functional diameter of the nut will beless.

Reduce the clearance.

Increase the interference between matingthreads.


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MEASUREMENT OF VARIOUS ELEMENTS OF

THREAD

To find out the accuracy of a screw thread itwill be necessary to measure the following:

1) Major diameter.2) Minor diameter.

3) Effective or Pitch diameter.4) Pitch5) Thread angle and form


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Measurement of major diameter

The instruments which are used to find the majordiameter are byOrdinary micrometer

Bench micrometer

Bench micrometer


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Ordinary micrometer: The ordinary micrometer is quite suitable for measuringthe external major diameter.

It is first adjusted for appropriate cylindrical size (S)having the same diameter (approximately).This process is

known as gauge setting .

After taking this reading R the micrometer is set on themajor diameter of the thread, and the new reading is R2


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Measurement by Bench micrometer:

Clamp

FiducialIndicator

MeasuringAnvils Holding centres

Micrometer head

Supports

BENCH MICROMETER


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Measurement by Bench micrometer:For getting the greater accuracy the bench micrometer is used for

measuring the major diameter.

In this process the variation in measuring Pressure, pitch errors arebeing neglected.

The fiducial indicator is used to ensure all the measurements are madeat same pressure.

The instrument has a micrometer head with a vernier scale to read theaccuracy of 0.002mm. Calibrated setting cylinder having the same

diameter as the major diameter of the thread to be measured is used assetting standard.

After setting the standard, the setting cylinder is held between theanvils and the reading is taken


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Then the cylinder is replaced by the threaded work piece and the newreading is taken


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Holding centre

Measuring anvil

Holding centre

Measuring anvil

StandardCy

lind

er

Screw

Thread

Measurement of Major diameter


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Measurement of the major diameter of an

Internal thread:

The major diameter is usually measured by threadcomparator fitted with ball-ended styli.

First the Instrument is set for a cylindrical referencehaving the same diameter of major diameter of internalthread and the reading is taken.

Then the floating head is retracted to engage the tips ofthe styli at the root of spring under pressure.

For that the new reading is taken,


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Measurement of the major diameter of an

Internal thread:

An indirect approach of measuring internaldia is obtained by obtaining the cast of theThread. The main art thus lies in obtaining aperfect cast.


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Measurement of Minor diameterThe minor diameter is measured by a comparativemethod byusing floating carriage diametermeasuring machine and small V pieces which makecontact with the root of the thread.

These V pieces are made in several sizes, havingsuitable radii at the edges.

V pieces are made of hardened steel.

The floating carriage diameter-measuring machineis a bench micrometer mounted on a

carriage.


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Measurement of Minor diameter


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The threaded work piece is mounted between the centers ofthe instrument and the.V pieces are placed on each side of the

work piece and then the reading is noted.

After taking this reading the work piece is then replaced by astandard reference cylindrical setting gauge.

Measurement of Minor diameter


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Measurement of Minor diameter of

Internal threads:

The Minor diameter of Internal threads

are measured by1. Using taper parallels2. Using Rollers.


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Internal threads:1. Using taper parallels: For diameters less than 200mm the use of Taper parallelsand micrometer is very common.The taper parallels are pairs of wedges having reduced and

parallel outer edges.The diameter across their outer edges can be changed bysliding them over each other.


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Internal threads:Using rollers:

For more than 200mm diameter this method is used.Precision rollers are inserted inside the thread and

proper slip gauge is inserted between the rollers.

The minor diameter is then the length of slip gaugesplus twice the diameter of roller.


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Measurement of effective diameter

Effective diameter measurement iscarried out by following methods.

1. One wire,2. two wires, or

3. three wires method.4. Micrometer method.


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Two wire method:

The effective diameter can not be measured directlybut can be calculated from the measurements made.

Wires of exactly known diameters are chosen such thatthey contact the flanks at their straight portions.

If the size of the wire is such it contacts the flanks atthe pitch line, it is called the best size of wire whichcan be determined by geometry of screw thread.

The screw thread is mounted between the centers &wires are placed in the grooves and reading M is taken.

Then the effective diameter E =T+P

where T =M-2d, & P is a value which depends on

diameter of wire, pitch & angle of the screw thread.


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Two wire method:

M

M-Dimension over the wire


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Two wire method:


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Two wire method:


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Two wire method:


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Two wire method:P

AP=OP-OA


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Three Wire method

The three-wire method is the accurate method.

In this method three wires of equal and precisediameter are placed in the groves at opposite sides ofthe screw.

In this one wire on one side and two on the otherside are used. The wires either may held in hand or

hung from a stand.

This method ensures the alignment of micrometeranvil faces parallel to the thread axis.


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Three Wire method


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Three Wire method

This method is more accurate than two wire

method as it ensures alignment of micrometerfaces parallel to the thread axis.

Here, three wires of exactly known diameters are

used, one on one side & the two on the other side.The wires may be held in hand or hung from astand.

From the fig, M=diameter over the wires

E= effective diameter (to be found)

d= diameter of wires, h=height of wire centerabove the pitch line, r=radius of wire, H=depth of

thread, D=major diameter of the thread.


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Three Wire method

E M

H

A

BC

D

P

h

EM

Dia 'd'

E


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Three Wire method

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Three Wire method

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BEST WIRE SIZE

P

A

P/2

P/4

Pitch line

BEST SIZE OF WIRE

B


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BEST WIRE SIZE

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Pitch measurement

The most commonly used methods formeasuring the pitch are

1. Pitch measuring machine2. Tool makers microscope3. Screw pitch gauge


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Tool makers microscope:


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Lamp

Hollow base

Collimator lens

Base

Column

Eye pieceOptical head

Mirror

work table

with carriage


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1. Worktable is placed on the base of the base of the instrument.2. The optical head is mounted on a vertical column it can be movedup and down.3. Work piece is mounted on a glass plate.4. A light source provides horizontal beam of light which is reflected

from a mirror by 90 degree upwards towards the table.5. Image of the outline of contour of the work piece passes throughthe objective of the optical head.6. The image is projected by a system of three prisms to a groundglass screen.7. The measurements are made by means of cross lines engraved onthe ground glass screen.8. The screen can be rotated through 3 60.9. Different types of graduated screens and eyepieces are used

50699244 screw thread measurement

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