Recently, I’ve noticed that an increasing number of nondestructive examiners are unable to interpret standards and read fabrication and repair drawings. Additionally, when I’m assessing fabrication or repair operations in companies, I find that the inspection requirements are not fully understood.
We calibrate tools; we know the accuracy of equipment, but what about the people using the tools that verify the dimensional tolerances of the parts? How accurate are your workers?
Understanding Tolerance and AllowanceTolerance and allowance may seem closely related, but each has a precise meaning and application. A tolerance is easier to understand as the small picture in a large concept because it typically deals with one dimension such as one length of a boom section. The section length may have ±1/½ inch of its stated drawing length. An allowance may be authorized as a modifier on a drawing to allow the tolerance of a section length to accommodate the big picture of the crane boom length.
For example, a crane boom may be 100 feet long, have 15 section lengths with a tolerance of ½-inch, and an allowance may be stated on the drawing as a modifier to ensure the boom length is 100 feet ±¾ inch when the sections are assembled. The most critical dimension is the boom length because all of the calculations of the crane’s lifting capacity are based on the length of the boom. This means that if a dimension needs to be adjusted to ensure the critical dimension of the boom length is correct, it is possible to modify one or two of the 15 sections even if the sections are within tolerance to meet the critical big picture.
Product and Process RequirementsTolerance should fit product and process requirements. Although it is possible by use of sufficient time and care to work as closely to a given dimension as desired, it is impossible to manufacture to an exact size. Regardless of the accuracy displayed, it is always possible to choose a finer measuring method that can show discrepancies in the dimension.
Because working toward higher accuracies increases costs in terms of money, time and equipment, it is more practical and economical that dimensions should be permitted to vary within the widest limits for which they can still function properly. This variation is permitted by the use of tolerances added to dimensions in such a way that they indicate the permissible variation. Theoretically, the designer applies dimensional tolerances as wide as can be safely used.
In most instances, it is impractical and unnecessary to work to the absolute or exact basic dimension. 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 allowance minimum dimension is tolerance. For example:
• Basic dimension = 4 inches
• Long limit = 4 1/64 inches
• Short limit = 3 63/64 inches
• Tolerance = 1/32 inch
When tolerances are not actually specified on a drawing, the inspector can make a fairly concrete assumption concerning the accuracy that is expected by using the following principles:
For dimensions that end in the fractions of an inch, such as 1/8, 1/16, 1/32, 1/64, the expected accuracy is ±1/64.
When the dimension is given in decimal form the following applies:
If the dimension is given in three decimal places as in 3.000, the accuracy expected is ±0.0005 inch; or if the dimension is given in two decimal places as in 3.00, the accuracy expected is ±0.005 inch.
Understanding how to interpret standards and specifications can reduce the need to ask for engineering assistance when it is clearly defined on the drawing. If you understand how to read it, I recommend specific training in the understanding of drawings and geometric tolerance.
Editor's Note: For more on geometric dimensioning and tolerance, listen to Quality's Q-Cast podcast with Professor Ed Morse at www.qualitymag.com/q-cast.