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A machinist's level is a precise leveling tool designed to level the various machines used within a machine shop. From lathes to milling machines, the machinist's level is used to provide an absolute level setting of the machine's chassis, frame rails and cutting table. The bubble level indicator inside of a machinist's level is graduated in order to provide the exact level position in a much more accurate manner than a typical carpenter's level. The body of the machinist's version of the level is usually made from cast iron and has a machined finish on the level's contact surfaces.
In order to produce a perfectly machined component from any machine shop tooling, the machine must be perfectly level prior to using it. The machinist's level is used to ensure that the machinery is set up exactly level when it is installed in the shop. Just a slight variation in a lathe's frame can lead to drastic differences when turning a lengthy product. On tools such as a milling machine, the milling table must not only be set up in an exact position by using a machinist's level; the milling arbor must also be set in perfect alignment with the table.
Failure to set up any milling machine by using a machinist's level can result in finished mill work that is unevenly thick at differing locations on the workpiece. The same can be found in surface grinders, planers and automatic wood sanders. The machinist's level is the most accurate method of assembling and installing machinist tools in the most level position. Most users of the machinist's level recommend leaving the level sit, untouched, for three to four minutes after placing it on the surface of the product being leveled. This allows the bubble to sit absolutely still and gives the most accurate level possible.
On large surfaces that have a propensity for unevenness, it is common for the level to be placed at several locations and in many directions across the surface. This placement checks for levelness of the entire surface. Unlike a carpenter who might use a very long carpenter's level to check a board or framework of a building for levelness, the machinist must take care to check the entire surface against any machining flaws in the surface being examined. A long level may simply span a low spot, creating the illusion of a level surface when there is a actually a low spot.