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Surface Plates and Micrometers

To make a precise dimensional measurement, there must be a reference plane or starting point. The ideal plane for dimensional measurement should be perfectly flat. Since a perfectly flat reference plane does not exist, a compromise in the form of a surface plate is commonly used. Surface plates are customarily used with accessories like: a toolmaker's flat, angles, parallels, V blocks and cylindrical gage block stacks. Dimensional measurements are taken from the plate up since the plate is the reference surface.

Surface plates must possess the following important characteristics:

  • Sufficient strength And rigidity to support the test piece

  • Sufficient and known accuracy for the measurements required


Micrometers or “mics,” are commonly’ used hand-held measuring devices. Micrometers may be purchased with frame sizes from 0.5" to 48". Normally, the spindle gap and design permits a 1" reading span. Thus, a 2" micrometer would allow readings from 1" to 2". Most common “mics” have an accuracy of 0.001 of an inch. With the addition of a vernier scale, an accuracy of 0.0001 of an inch can be obtained. Improvements in micrometers have led to “super-micrometers” which, with laser attachments and when used in temperature and humidity-controlled rooms, are able to make linear measurements to one millionth of an inch. Micrometers consist of a basic C frame with the part measurement occurring between _a fixed anvil and a moveable spindle. Measurement readings on a traditional micrometer are made at the barrel and thimble interface. Micrometers may make inside, outside, depth or thread measurements based upon the customization desired. The two primary scales for reading a micrometer are the sleeve scale and the thimble scale. Most micrometers have a 1" “throat.” All conventional micrometers have 40 markings on the barrel consisting of 0.025" each. The 0.100", 0.200", 0.300", etc. markings are highlighted.The thimble is graduated into 25 markings of 0.001 " each Thus, one full revolution of the thimble represents 0.025".

Ring Gages

Ring gages are used to check external cylindrical dimensions, and may also be used to check tapered, straight, or threaded dimensions. A pair of rings with hardened bushings are generally used. One bushing has a hole of the minimum tolerance and the other has a hole of the maximum tolerance. Frequently, a pair of ring gages are inserted in a single steel plate for convenience and act as go/no-go gages. Ring gages have the disadvantage of accepting out of round work and taper if the largest diameter is within tolerance. A thread ring gage is used to check male threads. The go ring must enter onto the full length of the threads and the no-go must not exceed three full turns onto the thread to be acceptable. The no-go thread ring can be identified by a groove cut into the outside diameter.

Plug Gages

Plug gages are generally go/no-go gages, and are used to check internal dimensions. The average plug gage is a hardened and precision ground cylinder about an inch long. The go/no-go set is usually held in a hexagonal holder with the go plug on one end and the no-go plug on the other end. To make it more readily distinguishable, the no-go plug is generally made shorter. The thread plug gage is designed exactly as the plug gage but instead of a smooth cylinder at each end, the ends are threaded. One end is the go member and the other end is the no-go member. If the go member enters the female threads the required length and the no-go does not enter more than three complete revolutions, the threads are deemed acceptable.

Dial Indicators

Dial indicators are mechanical instruments for measuring distance variations. Most dial indicators amplify a contact point reading by use of an internal gear train mechanism. The vertical or horizontal displacement of a spindle with a removable contact tip is transferred to a dial face. The measurement is identified via use of an indicating hand. Commonly available indicators have discriminations (smallest graduations) from 0.00002" to 0.001" with a wide assortment of measuring ranges. The proper dial must be selected for the length measurement and required discrimination.

Pneumatic Gages and Interferometry

There are two general types of pneumatic amplification gages in use. One type is actuated by varying air pressure and the other by varying air velocity at constant pressure. Depending upon the amplification and the scale, measurements can be read to millionths of an inch. In the pressure type gage, filtered compressed air divides and flows into opposite sections of a differential pressure meter. Any change in pressure caused by the variation in the sizes of the work pieces being measured is detected by the differential pressure meter. In the flow type of air gage, the velocity of air varies directly with the clearance between the gaging head and the surface being measured.

The greatest possible accuracy and precision are achieved by using light waves as a basis for measurement. A measurement is accomplished by the interaction of light waves that are 180° out of phase. This phenomenon is known as interference. Interference occurs when two or more beams of monochromatic light of the same wave length are reunited after traveling paths of different lengths. When the light waves pass from a glass medium to an air medium above the surface of the object, a 180° phase change takes place. The reflected light from the surface of the test object “interferes” with the light waves of incidence and cancels them out. Irregularities are evidenced by alternate dark and light bands.

Laser Designed Gaging

The use of lasers have been prevalent when the intent of inspection is a very accurate non-contact measurement. The laser beam is transmitted from one side of the gage to a receiver on the opposite side of the gage. Measurement takes place when the beam is broken by an object and the receiver denotes the dimension of the interference to the laser beam.The laser has many uses in gaging. Automated inspection, fixed gaging, and laser micrometers are just a few examples of the many uses of the laser.

Coordinate Measuring Machines (CMM)

Coordinate measuring machines are used to verify work piece dimensions using computer controlled measurements which are taken on three mutually perpendicular axes. Work pieces are placed on a surface plate and a probe is manoeuvred to various contact points to send an electronic signal back to the computer that is recording the measurements. CMMs can be driven by the computer to measure complex work pieces and perform automated inspection of complex shapes.