Metal Dimension Tolerances
When ordering a metal, the customer typically asks for a specific shape and size. The size specified is known as the desired nominal dimension. In an absolute, perfect world, the metal that was ordered would be delivered to the customer in exactly that size to the nth degree. However, there are variations that are inherent to the metal forming and sizing processes that require a measurement designation known as a tolerance.
What is a Tolerance and Why Use Them?
A tolerance is a range of acceptable measurements outside of the nominal dimension. A tolerance or 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 and die dimensions, chemistry variation, furnace temperature fluctuation, and 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 and the purchaser. For instance, a purchaser may want a metal sheet that is 48 inches (1219mm) long by 96 inches (2438mm) wide. Without a specified tolerance, this could mean the purchaser wants a metal sheet that is 48.00000000 inches by 96.00000000 inches. This level of accuracy would generally be unachievable by the metal producer. If it was achievable, it would be very expensive and take a very long time to complete because of the expertise and 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 48.000 (1219mm) +/- 0.125 inches (3.17mm) by 96.000 +/- 0.125 inches is much quicker to produce and 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 lengths and widths like in the example above. Tolerances can also be used to define the boundaries on thickness requirements as well. Holes, slots, grooves, and perforations are just a small sample of other metal geometries that can have tolerances placed on them.
The range of a tolerance can also be large or small depending on the requirements of the application. Examples of relative large versus small tolerances:
- Large: 2 inches +/- 0.250 inch (6.35mm)
- Medium: 2 inches +/- 0.125 inch (3.17mm)
- Small: 2 inches +/- 0.0625 inch (1.59mm)
- Very Small: 2 inches +/- 0.006 inch (0.15mm)
The nominal is always the target of the tolerance, and 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 inches +0.005 / -0.000
- Nominal is Maximum: 2 inches +0.000 / -0.005
- Neither Minimum or Maximum: 2 inches +0.0625 / -0.1250
Sheet Metal Tolerances: Hot-Rolled (Black) versus Cold-Rolled (Bright)
Some metal forming processes allow for more precision than others. An example of this can be found with hot-rolled and cold-rolled metal. Hot rolled steel starts as a large, rectangular length of metal, called a billet. The billet is first heated, and then pre-processed, where it is flattened into a large roll. It is then kept at a high temperature and run through a series of rollers to reach its desired dimensions. Hot rolled steel is often the material of choice where dimensional tolerances aren’t as important as overall material strength, and where 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 and 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 (or mean) thickness.
Cold-Rolled (Bright) Metal Applications:
- Aerospace components
- Medical devices
- Automotive Engine Parts
Hot-Rolled (Black) Metal Applications:
- Metal Structures
- Railroad tracks
- Low-end furniture
- Other, less critical applications in a variety of industries
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