METAL DIMENSION TOLERANCES EXPLAINED

2020-11-25

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What is a Tolerance Why Use Them?

A tolerance is a range of acceptable measurements outside of the nominal dimension. A tolerance set of tolerances is made to give the manufacturer of a metal product some measurement allowance in the production process. Tolerances are needed because metal forming processes have unpreventable sources of variation. These include changing tool die dimensions, chemistry variation, furnace temperature fluctuation, different cooling rates, to name only a handful. While these variations are typically small, they must be considered.

A tolerance can be beneficial to both the producer the purchaser. For instance, a purchaser may want a steel coil that is 1219mm wide with 0.3mm thick. Without a specified tolerance, this could mean the purchaser wants a metal sheet that is 1219.000000000mm wide with 0.300000000mm thick. This level of accuracy would generally be unachievable by the metal producer. If it was achievable, it would be very expensive take a very long time to complete because of the expertise expensive machinery required to get the metal sheet to the final size. Typically, a customer will be satisfied with a certain range on a metal sheet. A tolerance of 1219 +/- 3mm, 0.3 +/- 0.05 mm is much quicker to produce can be done at a lower cost. If the application can still be successful with a wider-ranging metal tolerance, then that is usually the best option.

Types of Tolerances

Tolerances can come in a variety of forms. Tolerances can be used to describe widths thicknesses like in the example above. Tolerances can also be used to define the boundaries on length, quantity requirements as well. Holes, slots, grooves, perforations are just a small sample of other metal geometries that can have tolerances placed on them.

The nominal is always the target of the tolerance, sometimes the tolerance may be asymmetrical in order to skew the final size in one direction towards the nominal dimension. Examples of asymmetrical tolerances:

Nominal is Minimum: 2 mm +0.05 / -0.00
Nominal is Maximum: 2 mm +0.00 / -0.05
Neither Minimum Maximum: 2 mm +0.025 / -0.025

Quantity tolerance (more less clause)

+/-10% of order quantity: 100 MT +10 MT / -10 MT
+/-5% of order quantity: 100 MT +5 MT / -5 MT
as customer's requirements.

Sheet Metal Tolerances: Hot-Rolled versus Cold-Rolled

Some metal forming processes allow for more precision than others. An example of this can be found with hot-rolled cold-rolled metal. Hot rolled steel starts as a large, rectangular length of metal, called a billet. The billet is first heated, then pre-processed, it is flattened into a large roll. It is then kept at a high temperature run through a series of rollers to reach its desired dimensions. Hot rolled steel is often the material of choice dimensional tolerances aren’t as important as overall material strength, a smooth surface isn’t a priority.

Cold rolled steel is essentially hot rolled steel that has been through additional processing. After hot rolled steel has cooled, it is re-rolled at room temperature to attain more precise dimensions more desirable surface characteristics.

Both types of steel sheet have tolerance ranges; however the range of tolerance on hot rolled sheet is much wider than the range found on cold rolled sheet. The range of the tolerance (minimum thickness to maximum thickness) in each gauge range decreases as the sheet material becomes thinner on its nominal ( mean) thickness.

Cold-Rolled Metal Applications: