Machine Shop : Mechanical Drawings and Blueprints
A mechanical drawing, made with special instruments and tools, gives a true representation of an object to be made, including its shape, size, description, material to be used, and method of manufacture.
A blueprint is an exact duplicate of a mechanical drawing. These are the most economical and satisfactory working drawings in use. They do not soil easily and are comparatively easy to read. Blueprint paper is a good grade of white paper coated with a chemical solution. making it greenish yellow. A blueprint is made by placing a tracing of a mechanical drawing on a sheet of blueprint paper and exposing it to light. During exposure, the light penetrates where there are no lines or printing on the tracing but does not penetrate where there are lines or printing. The print is then washed in water, which changes the exposed chemical to a dark blue and washes the chemical off where lines and printing prevented exposure. In other words. the process leaves white lines on dark blue background.
Working From Drawings
Detail prints usually show only the individual part or piece that must be produced. They show two or more orthographic(straight-on) views of the object, and in special cases. they may show an isometric projection. without dimension lines, near the upper right corner. An isometric projection shows how the part will look when made. Each drawing or blueprint carries a number, located in the upper left-hand corner and in the title box in the lower right-hand corner of the print. The title box also shows the part name, the scale used, the pattern number, the material required, the assembly or sub-assembly print number to which the part belongs, the job order number, the quantity and date of the order and the names or initials of the persons who drew, checked, and approved the drawings. Accurate and satisfactory fabrication of a part described on a drawing depends upon the following:
Correctly reading the drawing and closely observing all data on the drawing.
Selecting the correct tools and instruments for laying out the job.
Use the baseline or reference line method of locating the dimensional points during layout, thereby avoiding cumulative errors.
Strictly observing tolerances and allowances.
Accurate gaging and measuring of work throughout the fabricating process.
Giving due consideration when measuring for expansion of the work piece by heat generated by the cutting operations. This is especially important when checking dimensions during operations, if work is being machined to close tolerances.
Limits of Accuracy
Work must be performed within the limits of accuracy specified on the drawing. A clear understanding of tolerance and allowance will help you avoid making small, but potentially large errors. These terms may seem closely related but each has a very precise meaning and application. The paragraphs below point out the meanings of these terms and the importance of observing the distinctions between them.
Working to the absolute or exact basic dimension is impractical and unnecessary in most instances: therefore, the designer calculates, in addition to the basic dimensions, an allowable variation. The amount of variation or limit of error permissible is indicated on the drawing as plus or minus (+ ) a given amount. such as + 0.005 or + 1/64. The difference between the allowable minimum and the allowable maximum dimension is tolerance. When tolerances are not actually specified on a drawing, fairly concrete assumptions can be made concerning the accuracy expected, by using the following principles: For dimensions which end in a fraction of an inch, such as 1/8, 1/16, 1/32, 1/64. consider the expected accuracy to be to the nearest 1/64 inch. When the dimension is given in decimal form the following applies: If a dimension is given as 2.000 inches, the accuracy expected is +0.005 inch: or if the dimension is given as 2.00 inches, the accuracy expected is +0,010 inch. The +0.005 is called in shop terms, “plus or minus five thousandths of an inch.” The + 0.010 is called “plus or minus ten thousandths of an inch.”
**Allowance Precautions **
Allowance is an intentional difference in dimensions of mating parts to provide the desired fit. A clearance allowance permits movement between mating parts when assembled. For example, when a hole with a 0.250-inch diameter is fitted with a shaft that has a 0.245-inch diameter, the clearance allowance is 0.005 inch. An interference allowance is the opposite of a clearance allowance. The difference in dimensions in this case provides a tight fit. Force is required when assembling parts which have an interference allowance. If a shaft with a 0.251 inch diameter is fitted in the hole identified in the preceding example, the difference between the dimensions will give an interference allowance of 0.001 inch. As the shaft is larger than the hole, force is necessary to assemble the parts.
Be sure you have the correct blueprint for the part to be made or repaired. You want the blueprint which has not only the correct title, but also the correct assembly number. Never take a measurement with a rule directly from the blueprint because the tracing from which the print was made may not have been copied from the original drawing perfectly and may contain scaling errors. Also, paper stretches and shrinks with changes in atmospheric conditions. Dimensions must be taken only from the figures shown on the dimension lines. Be very careful in handling all blueprints and working drawings.
When they are not in use, place them on a shelf, in a cabinet, or in a drawer. Return them to the blueprint file as soon as the job is done. Blueprints and working drawings are always valuable and often irreplaceable. Make it a point never to mutilate, destroy, or lose a blueprint.
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