How do you read a light band?
Straight, parallel, equally spaced light bands indicate true flatness. As the workpiece becomes less flat (concave or convex) the bands will show more curvature. In all cases the basis of comparisons is an imaginary line tangent to the band and parallel to the line of contact between the workpiece and the optical flat.
What is a light band measurement?
The light bands are made up of a bright and dark fringe. Combined, these correspond to the wavelength of the monochromatic light which in the case of a Sodium light source is equal to 589nm.
What is a lightband?
A method of measuring seal face flatness. One helium light band equals 0.0000116 inches or 0.3 microns. Seal faces are normally lapped to within three helium light bands of flatness. (
How thick is a light band?
One helium light band equals 0.0000116 inches, or HLB * . 0000116 = IN.
How do we measure the flatness of a specimen?
The easiest and best way to test the flatness of a flat lapped or polished surface is with an optical flat. Such surfaces are found on micrometers, measuring machines, gage blocks, snap gages, ring seals, valve seats and precision flat lapped parts.
What is the best tool to measure flatness?
Using a height gage to evaluate flatness In application, one way to physically measure flatness is to use a height gage, as we can see in Figure 2. To use the height gage correctly, the part to be measured is first placed upon 3 columns with adjustable heights.
Does surface finish control flatness?
Flatness is a macro-level measurement while surface finish is a micro-level measurement. Flatness may have a large impact on fit and function, while surface finish is more typically function than fit.
What is meant by optical flat?
An optical flat is a precisely polished flat surface, used as a reference against which the flatness of an unknown surface may be compared. Edmund Optics offers both single surface and dual surface optical flats in either ZERODUR® or Fused Silica.
What is difference between flatness and roughness?
Flatness is the ratio of the measure of the surface divided by its projection, and roughness the average of the square of the mean curvature of the surface per unit area.
How do I know if I have surface flatness?
Using a Dial Gauge Set the dial gauge so that its measuring part comes into contact with the measurement surface. Move the target so that the measurement surface is evenly measured, and read the dial gauge values. The largest deviation value is the flatness.
What instruments are used to measure flatness?
Flatness Gauge A flatness gage is a dial indicator type instrument used to measure lap plate flatness. It consists of a gauge body with two contact feet at one end and one vertically adjustable foot at the opposite end.
How do I know what surface finish I have?
Surface finish can be measured in a number of ways, including direct measurement, non-contact, comparison, or in-process methods. Direct measurement, or “contact,” methods use a stylus, like the needle of a record player, to physically measure surface finish by creating a profile of its roughness parameters.
What are light bands and how are they formed?
They are an interference pattern created by the reflection of light between two surfaces. When using a monochromatic light source it is possible to use the phenomenon to calculate the flatness of a component, but the surface of the component must be reflective in order for the light bands to appear.
What do the bands on an optical flat mean?
The closer the flat to the work the wider the bands will be. Many fine bands indicate that dust or dirt is between the flat and the work making correct interpretation difficult. The line, or point, of contact referred to in the illustrations is the line, or point, at which the air wedge between the optical flat and the work is thinnest.
What are the light bands of a sodium light source?
The light bands are made up of a bright and dark fringe. Combined, these correspond to the wavelength of the monochromatic light which in the case of a Sodium light source is equal to 589nm. When checking parts for flatness, it is only the dark bands that are counted, so as this is half the total fringe, each dark band equals 294nm or 0.00029mm.