How to Read Machine Drawings
- 1). Check the drawing for an information box. This box will give you important information about the machine drawing including the overall or universal tolerances for all of the dimensions. The tolerance amount indicates how much you can be off on a dimension and still have the part be considered workable. The tolerances may be displayed as .xx = +/- .005 and .xxx = +/- .002, which would indicate a larger tolerance amount for larger dimensions.
- 2). Check the information box about the finish required on the part. The information box will have a check mark with a number next to it to specify a surface finish requirement for all dimensions that are not marked on the part itself. Use the universal surface finish designation unless the surface is otherwise marked. A lower surface finish number denotes a smoother finish overall.
- 3). Check the drawing for information regarding the outside dimensions of the part. The large numbers on the outside of the part will allow you to determine the overall size for the raw material. Use these numbers to calculate the size of the raw material that must be cut for the production run. Add roughly 1/4-inch to each dimension to assure that the piece is to size after it has been machined.
- 4). Check the tolerances of the individual dimensions. Although there are universal tolerances, there may be individual dimension tolerances that would be right below the dimension in question. Universal tolerances are plus or minus the same number, while these may deviate from that format and include a +.000/-.003, which would mean the final dimension can be no larger than stated, but can be less than stated by .003 of an inch maximum.
- 5). Check the thread designations of the drawing to prepare your tools for machining. Determine if the thread is an external or internal type. Most external types will be cut with a die or carbide insert threading lathe tool while internal threads will be performed with a tap or milling machine threading tool. Taps are quicker, but result in more tap breakage. If it is an ultra-fine thread consider using a threading tool.
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