Many aluminium extrusion profiles must be manufactured to precise
standards. Questions like: How straight is straight enough? How flat is flat
enough? How uniform must a wall thickness be to be acceptable? are not
The specified, acceptable range of deviation from a given dimension
is known as tolerance. For many applications, in which an aluminium
extrusion will be part of an assembly of components, dimensional tolerances
Designers should be aware of the standard dimensional tolerances
to which extrusions are commercially produced. Tight tolerances can decrease
productivity, which leads to higher production costs. Judicious use of high
tolerances only where they are essential to the productivity of the profile will
help keep costs and deadlines in check.
Trying to achieve tolerances on non-critical dimensions is a major source of hidden costs. Frequently,
designers put too much emphasis on tolerances that do not affect the form, fit or function of the
final product. These non-critical dimensions can result in longer setups or repeat runs, which can
lead to costly, late or rush deliveries. Designers can decrease those costs by identifying only the most
critical product dimensions.
When the designer has defined the most critical product dimensions, the next step is to understand
which tolerances are achievable based on the specific manufacturing process. Tolerances are
affected by multiple factors, including press size, billet temperature, extrusion speed, die shape and
type, cooling time, and air temperature.
To understand tolerance expectations, it’s important to involve the aluminum extrusion experts in
the initial stages of design. It is important that the designers rely on extrusion professionals to
understand tolerance standards as well as how various factors, including aluminum temperature,
cooling time and the speed of extrusion – impact each part of the design.
Establishing the CPK value to be used is a critical element in determining the capability of dimensional
tolerances. Some CPK requirements will necessitate a capability study to determine the extent to
which the extrusion process can meet specified dimensions. Although this is an added cost it will
allow the extruder to understand process capability and repeatability. Process capability indices
measure the degree to which your process produces output that meets the customer’s specification.
Determining the accuracy and precision of your process will allow you to estimate the number of
failures that can be anticipated.
When tolerances are met, parts fit together well. They perform as intended, and do not require
unnecessary machining. Geometric dimensioning and tolerancing can be used to specify the shape of
extrusion on an engineering drawing. It is likened to a technical language, which has uniform
meaning to all; this can vastly improve communication in the cycle from design to manufacturing. It
more readily captures the design intent by providing designers and drafters better tools with which
to communicate their needs. It adds a new dimension to drawing skills in defining the part and its
features. When an engineer is concerned about fit and function geometric tolerancing is structured
to better control parts in a fit-and-function relationship.
Dimensioning a part for functionality without considering manufacturability often creates added cost
and frustration. Keeping the dimensioning format simple as will help keep costs down by reducing
excess machining operations, re-clamping and handling operations, while also reducing process
These tips offer an alternative to defaulting to block tolerances. By partnering with your aluminum
extrusion manufacturer early in the design process you can plan and design for both functional and
See how Presal Exstrusion can save you time and money.