E-33EDenition of the MeterThe 17th General Conference of Weights and Measures in 1983 decided on a new denition of the meter unit as the length of the path traveled by light in a vacuum during a time interval of 1/299792458 of a second. The gauge block is the practical realization of this unit and as such is used widely throughout industry.Selection, Preparation and Assembly of a Gauge Block StackSelect gauge blocks to be combined to make up the size required for the stack.(1)Take the following things into account when selecting gauge blocks.a.Use the minimum number of blocks whenever possible.b.Select thick gauge blocks whenever possible.c.Select the size from the one that has the least signicant digit required, and then work back through the more signicant digits.(2)Clean the gauge blocks with an appropriate cleaning agent.(3)Check the measuring faces for burrs by using an optical at as follows:a.Wipe each measuring face clean.b.Gently place the optical at on the gauge block measuring face.c.Lightly slide the optical at until interference fringes appear. Judgment 1: If no interference fringes appear, it is assumed that there is a large burr or contaminant on the measuring face.d.Lightly press the optical at to check that the interference fringes disappear. Judgment 2: If the interference fringes disappear, no burr exists on the measuring face. Judgment 3: If some interference fringes remain locally while the at is gently moved to and fro, a burr exists on the measuring face. If the fringes move along with the optical at, there is a burr on the optical at. e.To remove burrs, follow the directions on page E-30.(4)Apply a very small amount of oil to the measuring face and spread it evenly across the face. (Wipe the face until the oil lm is almost removed.) Grease, spindle oil, vaseline, etc., are commonly used.(5)Gently overlay the faces of the gauge blocks to be wrung together.There are three methods to use (a, b and c as shown below) according to the size of blocks being wrung: 2370153423mm14mm237015123769023mm14mm1237623mm1237690Gauge BlocksQuick Guide to Precision Measuring Instrumentsa. Wringing thick gauge blocksb. Wringing a thick gauge block to a thin gauge blockc. Wringing thin gauge blocksCross the gauge blocks at 90˚ in the middle of the measuring faces.Overlap one side of a thin gauge block on one side of a thick gauge block.To prevent thin gauge blocks from bending, rst wring a thin gauge block onto a thick gauge block.Rotate the gauge blocks while applying slight force to them. You will get a sense of wringing by sliding the blocks.Slide the thin gauge block while pressing the entire overlapped area to align the measuring faces with each other.Then, wring the other thin gauge block onto the rst thin gauge block.Apply an optical at to the surface of one thin gauge block to check the wringing state.Wipe the exposed measuring face (s) and continue building up the stack, in the same manner as above, until complete. Finally, remove the thick gauge block from the stack.Irregularinterference fringesAlign the measuring faces with each other.Thermal Stabilization TimeThe following gure shows the degree of dimensional change when handling a 100 mm steel gauge block with bare hands.Time ngers are releasedThe gauge block is held with ve ngers.The gauge block is held with three ngers.512345678910203040506070Lapse of time (minutes)Elongation (µm)Figure 1Figure 2

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