1.7.1 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.
126.96.36.199 - 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-collimators or laser alignment equipment followed by geometrical analysis oncomputer.
• The general advantages of these plates over cast iron are,
- closer tolerances
- lower cost
- lower thermal expansion
- burrs do not occur, but chipping does
- ease of use
- less glare
- no oil is required, thus dust does not stick
- less wringing
- inserts are often provided for clamping
188.8.131.52 - 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.
• Squares use known angles as a measurement reference. Generally a square is used to measure 90 degree angles (i.e., square corners)
• The basic types are,
- Combination Set - This has a sliding blade and is used for layout.
- Standard Square - There are three grades: 1. Reference, 2. Inspection, 3. Workshop
- Toolmakers Square
- Cylindrical Square
- Direct Reading Type
• The advantages of the Toolmakers, and cylindrical squares are,
1. There is a line of contact between the part and the square.
2. More resistant to damage.
3. Can be checked by rotation.
• Standard Squares can be checked for errors using a reversal test. In this test an angle plate is placed on a reference plane, and a standard square is placed against the angle plate. A dial indicator is run along the square from one end to the other, and the drop/rise is measured. The square is now rotated so that the other side is now measured. The drop/rise in height can be used to calculate the angles of both the square, and the angle plate.
184.108.40.206 - Coordinate Measureing Machines
• generally measure x-y-z coordinates using touch probes
• these measurements can be made by positioning the probe by hand, or automatically in more expensive machines.
• reasonable accuracies are 5 micro in. or 1 micro metre.
• The method these machines work on is measurement of the position of the probe using linear position sensors. These are based on moire fringe patterns (also used in other systems).
220.127.116.11 - Practice Problems
1. If Moire fringes are 1/4” in height and an array of 4 photocells are used to pickup the light patterns, how far apart would 50 micro in. wide lines have to be if the two patterns were at 45° angles?
2. Given that four measurements (x, y, z) were taken on a sphere, develop an expression to estimate a radius for the sphere, and an average error.
AM:1.7.3 Coordinate Measuring Machines (CMM)
• Automated machines used for inspection of parts
• Variety of methods for determining point locations in space,
- touch sensors,
- laser grids,
- video cameras
- can automate inspection process
- less prone to careless errors
- allows direct feedback into computer system
- fixturing is critical
- requires a very good tolerance model