Using Precision Gage Blocks

TM 9-243 USING PRECISION GAGE BLOCKS - Continued 4 Shift gage blocks so that their sides are in line. Any combination of gage blocks may be stacked together in this manner. The combination will be as solid as a single block. NOTE The adhesive force that binds two gage blocks together is a combination of molecular attraction and the suction cup action due to the film of oil or moisture on the surfaces wrung together. Separate gage blocks by sliding them apart, using the same movement as when wringing them together. 2 Shift the blocks. If the blocks are clean, they will CAUTION begin to take hold. Do not leave blocks wrung together for 3 Slide the two blocks together, using a slight pres- long periods of time since surfaces in sure and a rotary motion. contact will tend to corrode. FACTORS TO CONSIDER WHEN USING GAGE BLOCKS Ordinary changes in temperature have a sizable effect When using gage blocks consider the source of error on measurements made with precision gage blocks. resulting from temperature. Metals other than iron and The standard measuring temperature is 68°F, which is steel (such as aluminum) have a much different coeffi- just a little lower than the average temperature in most cient of linear expansion which will result in a difference shops. Since the room temperature affects the work as between the room temperature measurement and the well as the block, the expansion in the work will be standard measuring temperature measurement. Care- matched in most cases by a similar expansion in the less handling of gage blocks may produce an error of block. The coefficient of linear expansion of several several millionths of an inch and this error increases metals and blocks is listed below: proportionally with the dimension of the block. Material Steel Iron Phosphor bronze Aluminum Copper Gage blocks Millionths of an inch 5.5 to 7.2 per degree F 5.5 to 6.7 9.3 12.8 9.4 6.36 to 7.0 The temperature of the work may be either lower or higher than the room temperature as a result of a machining operation and this difference may be suffi- cient to cause a sizable error. Theoretically, the measuring pressure should increase proportionally with the area of contact. For practical purposes, it is better to use a standard measuring pres- sure. The most commonly used pressure is 1/2 to 2 pounds. Gage blocks are used in the layout and in checking the accuracy of tools, dies, and fixtures. They are also used in machine setups and in checking parts in process of manufacture and finished parts. Gage blocks are commonly used in setting adjustable instruments and indicating gages and verifying inspec- tion gages. Gage blocks are used to verify the accuracy and wear of ring and snap gages and many other special-purpose gages. The classification of blocks depends largely on the accuracy required. Typical clas- sification is shown on the following page. Handle blocks only when they must be moved and hold them between the tips of your fingers so that the area of contact is small. Hold them for short periods of time only. NOTE Avoid conducting body heat into the block by careless handling. Body heat may raise the temperature of the block, caus- ing a serious error in a measurement, par- ticularly if a long stack of blocks is being handled. 15-8