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How to Read Machine Shop Blueprints

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In a machine shop, blueprints are used to make parts on manual and CNC machines of varying types. The machinist must know how to read them properly and must also know which tools to use to take the raw material to a completed part. Measuring tools must be kept calibrated to make sure that all dimensions are measured properly and that the tolerances are followed with absolute accuracy. Once you are able to read blueprints, formulating the processes and operations needed becomes much easier.

Analyze the dimensions that dictate the finished size of the part or parts on the blueprint. Note the outer dimensions of the part on the print to get a good idea of the size of the raw material that will be needed. Cut the raw material slightly larger than what is noted on the print, enough to get a good finish, but not too much as to waste time cutting away material.

Note the universal tolerances for the dimensions not specified individually. These numbers are located in the main information box on the blueprint. For machined parts, the tolerances can be as small as .001 of an inch for important dimensions and as large as .05 of an inch for other dimensions that are inconsequential. The universal tolerances are used for any dimension that is not noted with its own tolerance range.

Note the blueprint markings for all the screw threads. The screw threads will be individually marked and you must use the drawing to determine if the screw is internal or external. External threads will be drawn with no material around the threads while internal threads will be within the material itself. The size and pitch of the threads to be cut will be the two most important aspects of the thread designation.

Read and understand the importance of any tolerance designations on individual dimensions. Through-holes and areas where parts must fit together often have their own tolerance markings. These markings are usually noted just under the dimension itself. These supersede the universal tolerances in the information box of the blueprints. Often they are non-standard such as +.0005/-.001 of an inch. This means that the particular dimension can be no more than .0005 or less than .001 of an inch away from the dimension noted.

Determine the finish quality of the various inside and outside dimensions when noted on the blueprint. The finish dimension must be completed to a particular finish when noted with a check mark. The check mark will also have a number that will help you determine the maximum roughness of the finish. Lower numbers mean smoother finishes while large numbers will allow you to use roughing tools to save time.


Christian Mullen is a graduate from the University of Central Florida with a bachelor's degree in finance. He has written content articles online since 2009, specializing in financial topics. A professional musician, Mullen also has expert knowledge of the music industry and all of its facets.

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