Autocollimator

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Autocollimators are used to detect small misalignments. If a parallel beam of light is projected from the collimating lens and if a plane reflector R is set up normal to the direction of the beam, light will be reflected back along the same path and light will be brought back to focus exactly at the position of the light source

 

If the reflector is tilted through a small angle (Theta) the parallel beam will be reflected through twice the angle (2?) and would be brought to focus in the same plane as the light source but to one side of it. The image will not coincide but there will be a distance equal to focal length times the angle of reflection (2f??) where f is the focal length of the collimating lens.

The distance between the reflector and the lens has no effect on the separation between the source and the image. For high sensitivity, a long focal length is required. Although the distance of the reflector does not effect the reading, if it is moved too far back, reflected rays will miss the lens completely and no image will be formed.

Element by Element Inspection of Gear: Laborious, useful in error analysis, can’t be used by the gear manufacturers.

  • Tooth thickness measurement
  • Base pitch measurement

Composite Error Checking: Used in Gear Inspection by conducting only one test

  • Parkinson Gear Tester

Thread Inspection - Terminology

  • Pitch - It is the distance measured parallel to the axis between the corresponding points on adjacent surfaces in the same axial plane.
  • Lead - It is the axial distance advanced by the thread in one revolution. Lead is equal to the same multiples of pitches as the number of starts.
  • Included angle - It is the angle between the flanks of the thread measured in an axial plane.
  • Pitch diameter - It is the diameter of tee imaginary coaxial cylinder which intersects the surface of the thread in such a manner that the intercept on the generator of the cylinder is equal to half the pitch.
  • Major Diameter - It is the diameter of imaginary coaxial cylinder that just touches the crests
  • Minor Diameter - It is the diameter of imaginary coaxial cylinder that just touches the roots

Measurement of Major Diameter

Bench micrometer is normally used for this measurement. It has measuring anvils so that the thread can be held between them. It has fiducial indicator so that the required pressure can be applied for all the measurements.

Measurement of Minor Diameter

Bench micrometer can not be used for Minor Diameter. So, another one, Floating Carriage micrometer is normally used for this measurement. In this instrument, the thread is held between the centres so that the influence of helix angle can be nullified in this arrangement.

Measurement of Pitch (effective) Diameter

Again Floating Carriage micrometer is normally used for this measurement. Here, the prisms used in the measurement of minor diameter are replaced by the steel wires whose size is chosen such that they pitch at the effective diameter.

Two wire Method

The two wires used should be identical in diameter and should pitch properly between the flanks. Reading of the floating carriage micrometer is taken over these two wires so that the effective diameter can be calculated using the formula developed.

Diameter of the best wires

If the wires used make contact exactly at the pitch diameter, then such wire is called the best wire. The diameter of such wire can be estimated from the formula. Compression & Rake correction.

Measurement of flank angle

Flank angle may be measured using Tool Maker’s Microscope with a goniometric head. This consists of glass screen with datum lines which can be rotated through 3600. The thread is mounted on centers and illuminated from below. The microscope is mounted above the thread in such a way it can be swiveled to be in line with the thread helix and avoid interference of the image.

The datum lines in the microscope head are set to zero and the table is rotated until the crests of the thread coincide with the horizontal datum and the angle is measured.

Measurement of Major Diameter

The major diameter of internal thread is normally measured using horizontal comparator fitted with ball end styli of radius less than the root radius of the thread to be measured. When the thread is mounted on a comparator, it aligns itself with the axis of the machine and measurement would be done along the helix of the thread as indicated above.

Measurement of Internal Threads

  • Measurement of Effective Diameter: The effective diameter of internal thread is measured using the horizontal comparator using ball ended styli of the best size wire.
  • Measurement of Flank angle: The semi cast of the thread form may be made using dental plaster and measurement is obtained as done for external thread.

Thread inspection can also be carried out using gauges designed according to the Taylors Principles

Limit gauges for Internal threads

  • Full form Go Gauge
  • Truncated Effective Diameter NOGO gauge
  • Minor Diameter NOGO gauge

Limit gauges for External threads

  • This is usually carried out using caliper type gauges
  • Full form Go Gauge
  • Truncated Effective Diameter NOGO gauge
  • Major Diameter NOGO gauge
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