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35.6.11 The Angle Dekkor

• Measures surface flatness using collimated light, and a moving lens to focus the beam as it travels across the test surface.

35.7 MEASURING APARATUS

35.7.1 Reference Planes

•Very flat surfaces are needed when setting up height or angle measurements. This is because the measuring instruments are moved across the surface, and if the height varies, accuracy will suffer.

•Typical plates are made from cast iron, or granite, and are from a few inches per side, and up. A typical plate might be 2 feet by 2 feet.

35.7.1.1 - Granite Surface Plates

•The surfaces are finished by rotary lapping machines.

•When done the flatness of the surfaces are inspected for flatness. This is done with auto-collima- tors or laser alignment equipment followed by geometrical analysis oncomputer.

•The general advantages of these plates over cast iron are,

-durability

-closer tolerances

-lower cost

-lower thermal expansion

-quality

-non-rusting

-burrs do not occur, but chipping does

-ease of use

-non-magnetic

-less glare

-no oil is required, thus dust does not stick

-less wringing

-inserts are often provided for clamping

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35.7.1.2 - Cast Iron Surface Plates

• Whitworth’s three plate method of manufacture is outlined below. This method is particularly desirable because the flatness is self generating.

Plate B

Plate C

Plate A

Three plates are shown (with exaggerated curves in the surface). These plates will be hand scraped in alternate combinations to reduce the surface curvature. As the process continues, the plates will become flatter.

Plate A

Plate B

Step 1:

plates A and B are scraped.

Plate C

Plate A

Step 2:

Plate ‘C’ is scraped to match ‘A’

Plate B

Step 3:

The process is repeated by scraping ‘B’ and ‘C’.

This reveals errors, and reduces error.

Plate C

***NOTE: Plate ‘A’ is the master plate