16. Comparators
• Accuracies commonly below 1/10 thousandth of an inch
• These instruments try to reduce the friction that is such a problem for the dial indicators
• There are four common principles used to design these instruments,
mechanical
pneumatic
electrical
optical
• comparators have very limited ranges of motion, but very high sensitivities (and therefore accuracies). As a result the comparators are often calibrated against standards such as gauge blocks.
• The basic requirements of these instruments are,
rigidity of the design
linear magnification within the operation range
coarse and fine offset adjustments
16.1 Mechanical Comparators
• The Johansson Mikrokator used a twisted strip with a pointer attached. as the plunger is depressed, it causes the strip to stretch. As the twisted strip is stretched, it changes the angle of the pointer, and thus the indicated deflection.
• The Sigma Mechanical Comparator uses a partially wrapped band wrapped about a driving drum to turn a pointer needle.
16.2 Mechanical and Optical Comparators
• The Eden-Rolt Reed system uses a pointer attached to the end of two reeds. One reed is pushed by a plunger, while the other is fixed. As one reed moves relative to the other, the pointer that they are commonly attached to will deflect.
16.3 Optical Comparators
• These devices use a plunger to rotate a mirror. A light beam is reflected off that mirror, and simply by the virtue of distance, the small rotation of the mirror can be converted to a significant translation with little friction.
16.4 Pneumatic Comparators
• Flow type
the float height is essentially proportional to the air that escapes from the gauge head
master gages are used to find calibration points on the scales
the input pressure is regulated to allow magnification adjustment
a pressure bleed off valve allows changes to the base level for offset
The pressure is similar to that shown in the graph below,
• The Soloflex Back Pressure System uses an orifice with the venturi effect to measure air flow. If the gas is not moving, the pressure on both sides of the orifice will be equal. If the flow is moving quickly, the air pressure on the downstream side of the orifice will be at a lower pressure.
• A Differential Back Pressure system uses a split flow channel, one flow goes to the gauge head, the other goes to a zero offset valve. A meter measures the difference in pressures, and thus gives the differences in pressure.
16.5 Level Gages
16.5.1 Clinometer
16.5.2 The Brooke’s Level Comparator
16.5.3 The Angle Dekkor
• Measures surface flatness using collimated light, and a moving lens to focus the beam as it travels across the test surface.
16.6 Measuring Apparatus
16.6.1 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.
16.6.1.1 - 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-collimators or laser alignment equipment followed by geometrical analysis on computer.
• The general advantages of these plates over cast iron are,
durability
lower thermal expansion
quality
burrs do not occur, but chipping does
ease of use
no oil is required, thus dust does not stick
inserts are often provided for clamping
16.6.1.2 - 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.
16.6.2 Squares
• 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.
