50699244 screw thread measurement
TRANSCRIPT
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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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Screw Thread terminology
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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Screw Thread terminology
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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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Measurement by Bench micrometer:
Then the cylinder is replaced by the threaded work piece and the newreading is taken
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Measurement by Bench micrometer:
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Measurement by Bench micrometer:
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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Measurement of Minor diameter of
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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Measurement of Minor diameter of
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
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A
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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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Tool makers microscope:
Lamp
Hollow base
Collimator lens
Base
Column
Eye pieceOptical head
Mirror
work table
with carriage
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Tool makers microscope:
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Tool makers microscope:
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