Understanding tolerances and fits: ISO versus ANSI standards

March 2012 - In the global world of manufacturing, forming and fabrication, a U.S. worker must have knowledge of International Organization for Standardization tolerances and fits as well as those from the American National Standards Institute. Knowledge of one and not the other could result in problems producing parts.

The global market has been ruled by the metric system, and U.S. companies and employees are doing a great job working with it. But many times, as experience demonstrates, ISO tolerances require American firms to learn and handle additional subject matter. Automotive applications, such as diesel engines and many other modern manufacturing systems, apply ISO. However, there is still wide use of the standard inch fits under the ANSI tolerances.

The conversion decision
Fabricators always can make the decision to convert dimensions, but when it applies to tight measurements, some of the conversions fall through the cracks.

For example, a large tool and die shop receives all of the prints and details to manufacture progressive dies in metric units. Most of the journey people on the shop floor and the engineers feel more comfortable working in the imperial measurement system, so they convert the required metric dimensions into inches.

The problem becomes significant during the die run-off in the progressive presses. The stamped parts appear to be within specification, but the converted dimensions change the spring effect on the parts in such a way that none of the produced stampings can fit together with other parts according to the print.

The workers in this shop should have applied this rule of thumb: If the dimensions and specifications are in inches, they should work in inches. Likewise, if the dimensions are metric, they should work in metric units.

Let’s get specific: In manufacturing a workpiece, shaft or hole, it is very difficult to obtain the dimensions as specified in a drawing without applying tolerances. Tolerance equals T, which is the difference between limits. This is also true with precision modern CNC equipment, turning or milling.

The permissible variation is marked off by a maximum limit (dimension) and a minimum limit (dimension). The actual size of the workpiece must be within the limit. Instead of giving both limits, it is preferable to indicate the tolerance zone showing the maximum and minimum limits in relation to the nominal size.

It is also an advantage that the workpieces, which are assembled later, carry the same nominal size and can be identified easily as one of the group. The interchangeability of parts and the increased demand for manufactured products were driving the development of advanced production systems and techniques. Such efforts became the basis for mass production and low-cost manufacturing.

Standard and ISO symbols
Both standard inch fits and ISO fits use the basic hole system and basic shaft system. These are the letter symbols used in standard inch fits:

RC=Running and sliding fit

LC=Locational clearance fit

LT=Locational transition fit

LN=Locational interference fit

FN=Force or shrink fit

In the identification of ISO fits, note the fundamental deviations (position of tolerance zones) are designated by letters. The shafts are noted with small letters and the holes with capital letters. The size of tolerances depends on the quality number specified for the purpose of use and the nominal size. The basic shaft and hole systems are used to call out clearance fit, transition fit and press fit.

Here is a quick reference for the systems:

Nominal size N: The size indicated in the drawing according to which workpiece can be designated.

Maximum limit G: The maximum permissible size. It should not exceed the actual workpiece size.

Minimum limit K: The minimum permissible size. It should not be less.

Limits: Maximum limit and minimum limit are called the limits of size.

It is also important to know allowance above nominal size A is the difference between the nominal size and the maximum limit. Allowance below nominal size A is the difference between the nominal size and the minimum limit. Actual size I is the size of the actual workpiece obtained through the actual measurement. It should lie between the limits.

Knowing this information can help workers fabricate accurate workpieces to the specified tolerances, whether using the ISO or ANSI standards. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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