The product design process starts with the concept models, proceeds to prototype models, and then on to production. The concept and prototype models are primarily made in the companies, model shop or machine shops where most of the individual components are fabricated by one or two toolmakers. The emphasis is to prove the concepts. The toolmakers work with the designer and are given some latitude in order to make the parts fit together.
The product is still being designed at this stage, so changes are frequent and the parts are altered to fit. Because of time constraints, the changes are drawn by freehand sketches and given to the toolmakers. When the models are completed and assembled, they go through extensive testing. More changes are made and incorporated. After testing is completed, drawings are updated to reflect the changes. Suppliers are selected and orders are issued to produce parts.
When parts are received for the first time and are assembled for production, it is frequently discovered that they do not fit. At this time, it can also be discovered that detailed tolerance analysis was not performed due to schedule pressures. Drawings made and released based on the concept and prototype models did not account for the manufacturing process variability.
8.3.2 Mechanical design and tolerance analysis
No manufacturing process can a make a part to exact dimensions. Hence maximum and minimum limits of dimensions (or tolerances) are specified with two goals in mind:
1. The limits must be set close enough to allow functioning of the assembled parts (including interchangeable parts).
2. The limits must be set as wide as functionally possible, because tighter limits call for expensive processes or process sequences.
Once the limits (or tolerances) are set by the designer, all parts or components are manufactured to those specified limits. Assembly of the parts causes tolerances to accumulate, which can adversely affect the functioning of the final product. In addition, tolerance accumulation can also occur, based on the method by which the parts are dimensioned. Tolerance accumulation that occurs in the assembly of parts is sometimes referred to as “tolerance stackup. ”
To make sure that parts successfully mate at subassembly or final assembly, and the products function per the design intent, an analysis is performed to uncover the existence of any interference. It is referred to as “tolerance analysis.” The following is a brief review of tolerance issues.
Tolerance (per ANSI Y14.5M) is the total amount by which a specific dimension is allowed to vary. Geometric tolerance is a general term applied to the category of tolerances used to control form, profile, 〇rj. entation, location, runout, etc. Tolerances are primarily of two types. tolerance of size and tolerance of form.
Tolerance of size is stated in two different ways:
1. Plus-or-minus tolerancing, which is further subdivided into bilateral and unilateral tolerancing. Bilateral tolerance is applied to both sides of a basic or nominal dimension. Examples are:
0.375 ± 0.010
0.375 + 0.005/-0.002
2. Limit dimensioning is a variation of the plus-or-minus system. It states actual size boundaries for a specific dimension. It will eliminate any calculation on the part of the manufacturer. Limit dimensioning is practiced in two ways: Linear or one next to another, and dimensions placed one above the other. Examples are:
0.625 - 0.635
When one dimension is placed above the other, the normal rule is to place the larger dimension above the smaller.
There are no rigid guidelines regarding tolerancing techniques. The choice depends on the style of the designer and very often both types of tolerancing methods (the plus-or-minus and limit dimensioning) are used in the same drawing.
Tolerance of form includes location of geometric features and geometric properties such as concentricity, runout, straightness, etc.