Tolerances in Tube Bending

In general, tolerance is the stated limit of acceptable deviation from a nominal dimension called out on a drawing. It is the unintended difference from the nominal or ideal value that corresponds to the part. The use of tolerances helps to ensure that the final product is suitable for its end use, especially if the part is going into a larger assembly or must mate up with another piece.

Tolerances will vary on the product application and end use of the part. Normally found in the title block or listed on the print, these specifications may refer to any measurable value, and help the supplier or manufacturer understand what will be required to successfully produce the part.

Within the world of tube fabrication, tolerances are specified to accommodate differences or unique characteristics in raw materials and quality as well as other factors such as the equipment being used, the machine operator or the setup parameters.

Tolerances are typically specified before a job is quoted. If a tight tolerance is required, cost may increase due to the time required to achieve the desired range. If tolerances need to be amended to control costs, which is known as an allowance, or intentional deviation from the nominal dimension and tolerance, this must be approved by the originator.

If no tolerance is specified, the manufacturer will still try to stay close to the nominal value but there will normally be slight of deviation due to the reasons mentioned earlier. However, there should still be a tolerance established by the supplier to control production quality.

Is there a range in the world of tolerances? What is inside or outside tolerance?

Tolerances can range from microns, fractions of millimeters, to a quarter inch. It all depends on the industry and the scale of the project, as well as the material being used and the way the parts are designed. Anything inside the specified tolerance is considered acceptable, and helps to determine a good part, whereas anything outside of this range is considered a bad part and cannot be used.

How do you determine a tolerance and is there an industry standard?

Tolerance is typically determined during the design phase by a company’s engineering and product development groups. Before an engineering print is finalized and signed off on, numerous tests are done to determine allowable deviations. Known as tolerance stacking, these tests help to ensure that the numbers are accurate.

For example, the industry standard for bend angles is +/- one degree. Our internal or company standard is +/- 0.5 degrees. However, tighter tolerances to 0.2 degrees can be achieved based on the needs of the project and application.

How are tolerances measured, reported and verified?

Tolerances can be measured using digital scanning equipment, reading information from the print to complete a dimensional inspection as a part of a quality control process. Check fixtures are also oftentimes used to verify tolerances during production. With larger tolerances, a caliper or even a measuring tape can also be used.

In essence, tolerances guide the process, helping parts manufacturers and others understand what is required. They serve as guideposts and also help determine the scope and costs involved in a project.

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