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SCREW THREAD MEASUREMENT
Definition –
It is defined as a helical ridge which is formed by a continuous helical groove of uniform cross section on the external or internal surface of the cylinder or cone. The threads formed on cylinder are known as straight threads and the threads formed on a cone or a fulcrum of a cone are known as tapered threads. The screw threads are applied to many devices for various purpose
- To hold parts together as in the case of fastening
- To transmit power
- To control movement as in micrometer
- To increase the effect of applied effort as in auto jack
- To convey material as in the case of fastening
Classification of threads
The threads may be classified as follow,
- According to the surface on which the threads are cut: a. External Thread: Threads formed on the outside of the work piece body are known as external threads. Example: Bolts and Studs etc. b. Internal Thread: The threads formed on inside of the work piece body are known as Internal Threads. Example: Nuts
- According to the direction of rotation of the thread a. Right hand or left hand thread: The Thread is placed in such a way that its longitudinal axis is normal to the observer and the thread is rotated in clockwise direction, if it is moving away from the observer, then it is a right hand thread; if it is moving towards the observer, then it is left hand thread.
LEFT: LEFT-HAND THREADS ON A LEFT PEDAL. RIGHT: RIGHT-HAND
THREADS ON A RIGHT PEDAL.
- According to the number of Starts a. Single start threads – A single start thread screw is one in which there is a movement of one thread for one complete turn round the screw or bolt. b. Multi start threads – A Multi start thread screw is one in which there is a movement of more than one thread for one complete turn round the screw or bolt.
threaded screw the lead is two times the pitch; on a triple headed screw the lead is three times the pitch, etc.
Specification of a Screw Thread
To specify a screw thread the following points are given due consideration:
- Shape or Form of thread
- Pitch
- Size (Diameter)
- Length
- Number of Starts
- Material
- Direction of Thread
- Internal or External Threads
Forms / Types of Thread
Many different shapes of threads are employed but they can be divided into following three main classes.
- V- threads
- Square threads
- Modification of Both The various screw thread forms are discussed in detail below:
- BSW thread – it is most widely used thread section in the V thread. The thread angle is 55 degree. This thread is employed in general machine construction, where condition favors the use of bolts, screws and other thread piece where quick and easy assembly of parts is required.
- BSF thread – this thread is same as BSW but the pitch for any given diameter is smaller than the BSW. So in this bolt is having a fine threads and these are stronger than BSW. So it is used where weight is important factor like design of aircraft and automobile work, plant machine tools etc.
- BSP thread – These thread also have fine pitches and it is used for gas water and steam work.
- BA thread – These threads has rounded V form. These are used for small screw used in clocks etc. and thread angle is 47.5 degree.
- Metric thread – these are in metric units. They employed 60 degree angle. They are used in motor car practices.
- Sellers thread – these are known as US thread and these are use in engineering work such as for bolt, screw etc. thread angle is 60 degree
- Square thread – this thread has its faces nominal to the axis of the screw. There is less friction and less wear and most commonly used for transmission of power, clamps, lathe, jacks etc. They are less stronger than V thread and are more expensive to cut on a lathe.
- ACME thread – it is modification of square thread having an angle of 29 degree. It is stronger than the square thread and is more easily cut, milled or ground. It is commonly used on lathe.
- Knuckle thread- this is also a modification of square thread. The cross section of the thread is semi circle. The radius is one quarter of pitch and and the depth is half the pitch. It is used in railway carriage couplers.
1) Progressive error The pitch of the thread is uniform but is longer or shorter its nominal value and this is called progressive. Causes of progressive error:
- Incorrect linear and angular velocity ratio.
- In correct gear train and lead screw.
- Saddle fault.
- Variation in length due to hardening.
2) Periodic error These are repeats itself at regular intervals along the thread Causes of periodic error :
- Un uniform tool work velocity ratio.
- Teeth error in gears.
- Lead screw error.
- Eccentric mounting of the gears.
3)Drunken error Drunken errors are repeated once per turn of the thread in a drunken thread. In Drunken thread the pitch measured parallel to the thread axis. If the thread is not cut to the true helix the drunken thread error will form.
Fig 3.5 Drunken Error
4) Irregular errors It is vary irregular manner along the length of the thread. Irregular error causes :
- Machine fault.
- Non-uniformity in the material.
- Cutting action is not correct.
- Machining disturbances.
Effect of pitch errors · Increase the effective diameter of the bolt and decreases the diameter of nut. · The functional diameter of the nut will be less. · Reduce the clearance. · Increase the interference between mating threads.
Measurement of various elements of Thread
To find out the accuracy of a screw thread it will be necessary to measure the following:
- Major diameter.
- Minor diameter.
- Effective or Pitch diameter.
- Pitch
- Thread angle and form GEAR MEASUREMENTS
Definition – A gear is a wheel provided with teeth which mesh with the
teeth on another wheel, or on to a rack, so as to give a positive transmission of motion from one component to another. It is most commonly used for changing power speed ratio. It is mainly used to change in torque and speed of driving shaft and driven shaft.
For measuring the individual gear parameters and their errors, it is necessary to know the errors of the gear cutting machine so that the gear production can be controlled and the machine can be reset. Individual measurement is also important for the inspection of the high quality gears.
Principle
It is work on the basic principle that energy is neither be created or destroyed; it can be converted into one form to another. We know that power is the function of speed and torque or we can say that power is product of torque (Force in rotary motion) and speed (P = TV) of the shaft. So when we connect a small gear on driving shaft and a larger gear on driven shaft its speed decreases of driven shaft per unit rotation of driving shaft. Because the power is conservative so according to this the torque of driven shaft increase according to the ratio of driving gear to driven gear or according to ratio of driving velocity to driven velocity. So by use of various sizes of gears we can obtain many combination of torque and speed of driven member.
Types of Gear – Gears can be classified in various types according to
construction of teeth, Use, the direction of motion transfer etc. but basically it is classified according to design of teeth.
- Worm Gear – worm gear connect two non parallel, non intersecting shafts which are usually at right angles. One of the gear is called the worm. It is essential part of the screw, meshing with the teeth or gear wheel, called the worm wheel. The gear ratio is the ratio of number of teeth on the wheel to the number of threads on the worm. It is smooth and quite.
- Rack and Pinion – his gear is used in steering system of automobile. In this type of gear teeth are cut on a straight rectilinear geometry know as rack and one spur gear known as pinion. This is used to transmit rotary motion to linear motion. It is seen as the infinite radius driven gear.
Methods of making gears
Most gears are produced by some machining process. Accurate machine work is essential for high speed, long wearing, quite operating gears. The various methods are used for producing gears.
- Casting
a. Sand casting b. Die casting c. Precision and investment casting
- Stamping
- Machining a. Formed tooth process i. Form cutter in milling machine ii. Form cutter in broaching machine iii. Form cutter in shaper b. Template process c. Cutter generating process i. Cutter gear in shaper ii. Hobbing iii. Rotary cutter iv. Reciprocating cutters simulating a rack
- Powder metallurgy
- Extruding
- Rolling
- Grinding
- Plastic moulding
Gear tooth terminology
Most of the term used in connection with gears teeth are explained below –
known as clearance. The circle passes from the top of the tooth in meshing condition is known as clearance angle. Total depth- The sum of the addendum and dedendum of a gear is known as total depth. It is the distance between addendum circle to the dedendum circle measure along radial direction. Working depth- The distance between addendum circle to the clearance circle measured along radial direction is known as working depth of the gear. Tooth thickness- Distance of the tooth measured along the circumference of the pitch circle is known as tooth thickness. Tooth space- Distance between the two adjacent tooth measured along the circumference of the pitch circle is known as the tooth space. Backlash- It is the difference between the tooth thickness and the tooth space. It prevents jamming of the gears in meshing condition. Profile- It is the curved formed by the face and flank is known as profile of the tooth. Gear tooth are generally have cycloidal or involute profile. Path of contact- The curved traced by the point of contact of two teeth form beginning to the end of engagement is known as path of contact. Arc of contact- It is the curve traced by the pitch point form the beginning to the end of engagement is known as arc of contact. Arc of approach- The portion of the path of contact from beginning of engagement to the pitch point is known as arc of approach. Arc of recess- The portion of the path of contact form pitch point to the end of the engagement is known as arc of recess.
Gear tooth Vernier caliper
It is widely used to measure the tooth thickness. As the tooth thickness varies from top to the bottom any instrument for measuring on a single tooth must (i) measure the tooth thickness at a specified position on the tooth (ii) fix that position at which the measurement is taken.
The gear tooth Vernier caliper is similar to the ordinary Vernier caliper but having a second beam at right angle to the main beam. For measuring the chordal thickness, the tongue of the gear tooth Vernier is set so that the jaw will touch the flanks at the pitch circle. The thickness of the tooth is then measured on the main scale of the instrument.
Gear Errors
The various possible types of errors on gears are given below:
a. Adjacent pitch error
b. Cumulative pitch error
c. Profile error
d. The tooth to tooth composite error
e. Total composite error
f. The tooth thickness error
g. Cyclic error
h. Periodic error i. Run out
j. Radial run out
7. Waviness : - Surface irregularities which are of greater spacing
than roughness.
8. Roughness height : - Rated as the arithmetical average
deviation.
9. Roughness width : - Distance parallel to the normal surface
between successive peaks.
10. Mean line of profile : - Line dividing the effective profile
such that within the sampling length.
11. Centre line of profile : - Line dividing the effectiveness
profile.
Analysis of surface finish
The analyses of surface finish being carried out by
1. The average roughness method
2. Peak to valley height method
1. Average roughness measurement
The assessment of average roughness is carried out by
a Centre line average (CLA).
b Root mean square (RMS)
c Ten point method
a. C.L.A. method
The surface roughness is measured as the average deviation
from the nominal surface.
b. R.M.S. method
The roughness is measured as the average deviation from the
nominal surface. Let, h1,h2, ... are the heights of the ordinates
and L is the sampling length
c. Ten point height method
The average difference between five highest peaks and five
lowest valleys of surface is taken and irregularities are calculated
by
2. Peak to valley height method
Peak to valley height measures the maximum depth of the
surface irregularities over a given sample length and largest value
of the depth is accepted for the measurement.
Here, = Maximum peak to valley height in one sampling lengths.
R = Maximum peak to valley height
· Touch Inspection
It is used when surface roughness is very high and in this method
the fingertip is moved along the surface at a speed of 25mm/
second and the irregularities as up to 0.0125mm can be detected.
· Visual Inspection
In this method the surface is inspected by naked eye and this
measurement is limited to rough surfaces.
· Microscopic Inspection
In this method finished surface is placed under the microscopic
and compared with the surface under inspection. The light beam
also used to check the finished surface by projecting the light
about 60° to the work.
· Scratch Inspection:
The materials like lead, plastics rubbed on surface are inspected
by this method. The impression of this scratches on the surface
produced is then visualized.
· Micro-Interferometer
Optical flat is placed on the surface to be inspected and
illuminated by a monochromatic source of light.
· Surface Photographs
Magnified photographs of the surface are taken with different
types of illumination. The defects like irregularities are appear as
dark spots and flat portion of the surface appears as bright.
· Reflected light Intensity
A beam of light is projected on the surface to be inspected and
the light intensity variation on the surface is measured by a
photocell and this measured value is calibrated
· Wallace surface Dynamometer :
It consists of a pendulum in which the testing shoes are clamped
to a bearing surface and a pre determined spring pressure can be
applied and then, The pendulum is lifted to its initial starting
position and allowed to swing over the surface to be tested.
2. Direct instrument measurements
Direct methods enable to determine a numerical value of the
surface finish of any surface. These methods are quantitative
analysis methods and the output is used to operate recording or
indicating instrument. Direct Instruments are operated by
electrical principles. These instruments are classified into two
types according to the operating principle. In this is operated by
carrier-modulating principle and the other is operated by voltage-
generating principle, and in the both types the output is
amplified.
Some of the direct measurement instruments are
1. Stylus probe instruments.
2. Tomlinson surface meter.
3. Profilometer.
4. Taylor-Hobson Talysurf
INTERCHANGEABILITY
It is the principle employed to mating parts or components. The
parts are picked at random, complying with the stipulated
specifications and functional requirements of the assembly. When
only a few assemblies are to be made, the correct fits between
parts are made by controlling the sizes while machining the parts,
by matching them with their mating parts. The actual sizes of the
parts may vary from assembly to assembly to such an extent that
a given part can fit only in its own assembly. Such a method of
manufacture takes more time and will therefore increase the cost.
There will also be problems when parts are needed to be
replaced. Modern production is based on the concept of
interchangeability. When one component assembles properly with
any mating component, both being chosen at random, then this is
interchangeable manufacture. It is the uniformity of size of the
components produced which ensures interchangeability.
The advantages of interchangeability are as
follows:
