page_title: Gauge block
section_title: Wringing
hierarchical_section_title: Gauge block / Description / Wringing
caption_attribution_description: English: Photo of 36 Johansson gauge blocks wrung together and held horizontally, from a 1907 toolmaking book. The blocks are held together solely by the adhesion of their extremely flat surfaces, which is so strong that it easily supports their weight. Caption of photo: "Thirty-six gages wrung together and held horizontally". Alterations to image: removed caption, increased brightness.
context_page_description: Gauge blocks are a system for producing precision lengths. The individual gauge block is a metal or ceramic block that has been precision ground and lapped to a specific thickness. Gauge blocks come in sets of blocks with a range of standard lengths. In use, the blocks are stacked to make up a desired length.
  An important feature of gauge blocks is that they can be joined together with very little dimensional uncertainty. The blocks are joined by a sliding process called wringing, which causes their ultra-flat surfaces to cling together. A small number of gauge blocks can be used to create accurate lengths within a wide range. By using 3 blocks at a time taken from a set of 30 blocks, one may create any of the 1000 lengths from 3.000 to 3.999 mm in 0.001 mm steps. Gauge blocks were invented in 1896 by Swedish machinist Carl Edvard Johansson. They are used as a reference for the calibration of measuring equipment used in machine shops, such as micrometers, sine bars, calipers, and dial indicators. Gauge blocks are the main means of length standardization used by industry.
context_section_description: Wringing is the process of sliding two blocks together so that their faces bond. Because of their ultraflat surfaces, when wrung, gauge blocks adhere to each other tightly. Properly wrung blocks may withstand a 300 N (67 lbf) pull. While the exact mechanism that causes wringing is unknown, it is believed to be a combination of:
  Air pressure applies pressure between the blocks because the air is squeezed out of the joint
  Surface tension from oil and water vapor that is present between the blocks
  Molecular attraction that occurs when two very flat surfaces are brought into contact; this force causes gauge blocks to adhere even without surface lubricants, and in a vacuum
  It is believed that the last two sources are the most significant.
  There is no magnetism involved, although to a user the clinging together of the blocks feels a bit like weak refrigerator magnets sticking together. Unlike magnets, however, the cling only lasts while the blocks are completely joined—the blocks do not attract each other across any visible gap, as magnets would.
  The process of wringing involves four steps:
  Wiping a clean gauge block across an oiled pad (see the accessories section).
  Wiping any extra oil off the gauge block using a dry pad (see the accessories section).
  The block is then slid perpendicularly across the other block while applying moderate pressure until they form a cruciform.
  Finally, the block is rotated until it is inline with the other block.
  After use the blocks are re-oiled or greased to protect against corrosion. 
  The ability for a given gauge block to wring is called wringability; it is officially defined as "the ability of two surfaces to adhere tightly to each other in the absence of external means." The minimum conditions for wringability are a surface finish of 1 microinch (0.025 μm) AA or better, and a flatness of at least 5 μin (0.13 μm).
  There is a formal test to measure wringability. First, the block is prepared for wringing using the standard process. The block is then slid across a 2 in (51 mm) reference grade (1 μin (0.025 μm) flatness) quartz optical flat while applying moderate pressure. Then, the bottom of the gauge block is observed (through the optical flat) for oil or color. For Federal Grades 0.5, 1, and 2 and ISO grades K, 00, and 0 no oil or color should be visible under the gauge block. For Federal Grade 3 and ISO grades 1 and 2, no more than 20% of the surface area should show oil or color. Note that this test is hard to perform on gauge blocks thinner than 0.1 in (2.5 mm) because they tend not to be flat in the relaxed state.
caption: 36 Johansson gauge blocks wrung together easily support their own weight.
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