TM 9-243USING PRECISION GAGE BLOCKS - Continued4Shift gage blocks so that their sides are in line. Anycombination of gage blocks may be stackedtogether in this manner. The combination will be assolid as a single block.NOTEThe adhesive force that binds two gageblocks together is a combination ofmolecular attraction and the suction cupaction due to the film of oil or moisture onthe surfaces wrung together.Separate gage blocks by sliding themapart, using the same movement as whenwringing them together.2Shift the blocks. If the blocks are clean, they willCAUTIONbegin to take hold.Do not leave blocks wrung together for3Slide the two blocks together, using a slight pres-long periods of time since surfaces insure and a rotary motion.contact will tend to corrode.FACTORS TO CONSIDER WHEN USING GAGE BLOCKSOrdinary changes in temperature have a sizable effectWhen using gage blocks consider the source of erroron measurements made with precision gage blocks.resulting from temperature. Metals other than iron andThe standard measuring temperature is 68°F, which issteel (such as aluminum) have a much different coeffi-just a little lower than the average temperature in mostcient of linear expansion which will result in a differenceshops. Since the room temperature affects the work asbetween the room temperature measurement and thewell as the block, the expansion in the work will bestandard measuring temperature measurement. Care-matched in most cases by a similar expansion in theless handling of gage blocks may produce an error ofblock. The coefficient of linear expansion of severalseveral millionths of an inch and this error increasesmetals and blocks is listed below:proportionally with the dimension of the block.MaterialSteelIronPhosphor bronzeAluminumCopperGage blocksMillionths of an inch5.5 to 7.2 per degree F5.5 to 6.79.312.89.46.36 to 7.0The temperature of the work may be either lower orhigher than the room temperature as a result of amachining operation and this difference may be suffi-cient to cause a sizable error.Theoretically, the measuring pressure should increaseproportionally with the area of contact. For practicalpurposes, it is better to use a standard measuring pres-sure. The most commonly used pressure is 1/2 to 2pounds.Gage blocks are used in the layout and in checking theaccuracy of tools, dies, and fixtures. They are also usedin machine setups and in checking parts in process ofmanufacture and finished parts.Gage blocks are commonly used in setting adjustableinstruments and indicating gages and verifying inspec-tion gages. Gage blocks are used to verify the accuracyand wear of ring and snap gages and many otherspecial-purpose gages. The classification of blocksdepends largely on the accuracy required. Typical clas-sification is shown on the following page.Handle blocks only when they must be moved and holdthem between the tips of your fingers so that the area ofcontact is small. Hold them for short periods of time only.NOTEAvoid conducting body heat into the blockby careless handling. Body heat mayraise the temperature of the block, caus-ing a serious error in a measurement, par-ticularly if a long stack of blocks is beinghandled.15-8
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