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How to Read a CNC Blueprint

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One of the most important aspects of a machinist's career is the ability to read blueprints and calculate the necessary operations to be performed on a CNC (Computer Numerical Control) machine. CNC machines use the information on the blueprint to cut and shape various types of materials, including metals, wood and plastic. With experience and a bit of trial and error, you can quickly read a blueprint to input the necessary parameters in the machine to produce an acceptable final product.

Inspect the blueprint for overall tolerance values, which are usually located in the lower right-hand corner. These numbers are used for dimensions that are not clearly marked with their own tolerances numbers. Tolerances are the amount of deviation that is acceptable for final dimensions after the part has been machined in the CNC mill or lathe.

Look at the largest dimensions, which are usually related to the basic shape of the part to be machined. In many cases, you must cut these dimensions first to complete the other operations. Note any other numbers next to these dimensions. Some blueprints have tolerances that are separate from the universal tolerance numbers noted on the blueprint. They will be have pluses or minuses of any combination next to them.

Read the designations for the fillets or rounds on a CNC blueprint. They will designate the radius necessary for any if indicated. Fillets are inside parts, while rounds are outside dimensions. With CNC mills, you will need to use an end mill of that exact radius or smaller for either a fillet or round corner. These designations do not have tolerances in most cases.

Look at depths and diameters of holes on a CNC blueprint. Their location is noted using a particular zero-zero point. This is what all the dimensions on the blueprint are based. Hole diameter and depths will be noted unless it is a through hole, which goes completely through the part.

Check thread designations for tapping operations. Relief for the threads will be noted on the blueprint as well. There are standardized designations for threads. When calculating the tooling need to cut thread on a CNC blueprint, cross-reference the drill size need and the tap you will need to use. There are taps for blind holes and those for through holes. The thread tolerance will also be noted directly on the blueprint and is different from the universal tolerances on the blueprint.

About the Author

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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