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Parallels are rectangular blocks of metal, commonly made from tool steel, stainless steel or cast iron, which have 2,〔 4 or 6 faces ground or lapped to a precise surface finish. They are used when machining with a mill, drill or any other machining operation that requires work to be held in a vise or with clamps - to keep work parallel or raised evenly such as in a milling vise to give adequate height for the cutting tool/spindle to pass over. == Description == Parallels come in pairs of two, which are machined to be the same dimensions their corresponding faces. They come in a variety of thicknesses and size, allowing them to be stacked up or to support a workpiece which doesn't have a flat profile. Parallels commonly have a series of holes drilled on the 'front' face - allowing them to be used to position a workpiece or secured using t-slot clamps, and a countersink on each side to remove any sharp edges. Generally, workshop parallels have 4 faces that are machined and ground - the front, back and sides, although some do have the ends with a smooth surface. The surface of a parallel can often tell how it was manufactured, with a 'grain' showing that it was ground - and a smooth or mirrored finish showing it has been lapped. Parallels that have a good surface tolerance can be lightly bonded together by sliding or rotating two parallels together, and the smooth surfaces allows a temporary molecular-attraction to take place - this is known as Wringing and is also found with gauge blocks. == Manufacturing == There are two main grades of surface tolerance:〔Engineering Parallels - Metric/Imperial Units (BS 906-1,906-2)〕 * Grade A - Used for close-tolerance work, such as inspection or precision machining * Grade B - Lower surface tolerances, and are used for lower-precision machining Parallels are first machined to rough dimensions, leaving a few millimeters to allow the rest to be ground. Parallels that only have 2 or 4 precision faces will often have the tool-marks from the machining on the non-ground sides. They are then paired and placed in a grinding machine, and each face is ground until the overall dimensions are correct - they are paired during this stage so that even if the dimensions are not correct, they are still parallel to each other. Then, the individual finishes are applied, from drilling to machining a chamfer along the edges to remove any burrs or sharpened edges. They may also be lapped to achieve a mirror smooth surface. Most parallels are also hardened.〔.〕 Parallels are manufactured to either imperial or metric dimensions, and are often sold in a set, with several pairs of different sizes.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parallels (engineering)」の詳細全文を読む スポンサード リンク
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