Dimensions, properties, or conditions may have some variation without significantly affecting functioning of systems, machines, structures, etc. A variation beyond the tolerance for example, a temperature that is too hot or too cold is said to be noncompliant, rejected, or exceeding the tolerance. A primary concern is to determine how wide the tolerances may be without affecting other factors or the outcome of a process. This can be by the use of scientific principles, engineering knowledge, and professional experience. Experimental investigation is very useful to investigate the effects of tolerances: Design of experiments , formal engineering evaluations, etc. A good set of engineering tolerances in a specification , by itself, does not imply that compliance with those tolerances will be achieved.
Engineering fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can move independently from each other, or are then temporarily or even permanently joined. Engineering fits are generally described as a "shaft and hole" but are not limited to just round components. Within each category are several codes to define the size limits of the hole or shaft - the combination of which determines the type of fit. A fit is usually selected at the design stage according to whether the mating parts need to be accurately located, free to slide or rotate, separated easily, or resist separation. Cost is also a major factor in selecting a fit, as more accurate fits will be more expensive to produce, and tighter fits will be more expensive to assemble. The International Organisation for Standardization system splits the three main categories into several individual fits based on the allowable limits for hole and shaft size.
If held to these tolerances, cutting tools, material stock, and gages are generally available throughout the world. The hole basis fits have four preferred hole tolerances H11, H9, H8, and H7 ; the shaft basis fits have four preferred shaft tolerances h11, h9, h7, and h6 as shown in Table Select the basic size from Table and one of the ten fits from Table and read or cut and paste limit dimensions and clearances interferences from Tables through or CD. Now also available on-line at the site shown below. Tolerance Zones for holes are shown in Fig. Loose running fit for wide commercial tolerances or allowances on external members.