How to Read Blueprints for Metal and Technical Drawings

Blueprints show you what to cut and where.

You must be able to read blueprints if you are to read technical drawings for a machine shop. Your ability to read these technical drawings will help you plan out what you will need to do as far as the operations necessary to complete the part. The shape and angles, as well as the tolerances and surface finish designations, will help you choose the right tooling for the job and ensure that the part will be correct and useful for the vendor when it is completed.

Instructions

1. Check the noted outer dimensions on the blueprint to determine the raw material size you will use to manufacture the part. The outer dimensions will be noted on the outside of the drawing toward the edges. For raw material, add at least 1/4 inch to the dimension as you will have a hard time cutting all of the raw material to the exact size using a cut-off or shop saw.

2. Note the main tolerances for the dimensions on the print. The main tolerances will be noted in the information box. Note which tolerance amounts correspond with which noted dimensions. It may be + or - .01 for .x, .005 for .xx and .002 for .xxx. The smaller the dimension, the tighter the tolerance. If a dimension is noted as .056, the final dimension can be no smaller than .054 or larger than .058.

3. Inspect the drawing to find any other tolerance numbers that are not universal to the drawing. Technical drawings may contain dimensions that need to be held tight within a very small deviation. Many holes must be within .001 of an inch, and other dimensions will have a split tolerance that may indicate +.000/-.002. This dimension can be no bigger than the number noted, but it can be .002 larger, which is important to the overall operation of the part.

4. Check for any thread designations on the print. Threads may be drawn very crudely with the appropriate thread dimensions or they may be drawn more realistically; either way, you must follow the thread specifics. Check to see if the threads are external or internal as that will change the type of tool you will need to cut them. External threads will require a die while internal threads will usually require a tap of some kind. Either can also be cut with a threading tool on a CNC milling machine.

5. Check the various dimensions for surface markings; the surface markings will have a check mark next to them. There may be a universal surface designation that will be located in the information box or individual dimensions may be marked. Lower numbers signify smoother finishes, so an 8 would be smoother and shinier than a 32. The surface finish will be dictated by the tool you use as finishing end mills are capable of a mirror-like finish if run at the proper speed and feed.

References

• Shaw Corporate; Chapter 4: Machine Drawing

Resources

• Read this Article in Spanish