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Aluminum Extrusion Dimensions and Tolerances Specification
Standard Dimensional Tolerances for Aluminum Extrusions
The industry's standard tolerances were developed by technical committees of The Aluminum Association and the American National Standards Institute, which took into account both the capabilities of extruders manufacturing and the needs of extrusion designers.
Industry Standards for Extrusions are published in National Standard Dimensional Tolerances for Aluminum Mill Products (ANSI H35.2) and Aluminum Standards and Data (ASD).
These standards feature specific dimensions and tolerances as well as formulas published in table format. The various mechanical tolerances are specified to dovetail the various degrees of difficulty an extruder faces in controlling different tolerance dimensions. Mechanical tolerances and manufacturing capabilities vary with cross-sectional size (as measured by the profile's fit within a circumscribing circle - see Extrusion Design Guide), and with the location or distance of each dimension of complex geometric shape. The aluminum alloy specified can influence certain tolerances, and are reflected in the standard tolerance tables.
Non-Standard Mechanical Tolerances
Close or loose dimensional tolerances relative to industry standards can be specified when necessary. To achieve tighter mechanical tolerances, the extruder may be required to make significant modifications to extrusion dies, experience slower extrusion rates, increased inspections, and sometimes a higher rejection rate.
Extra effort on the extruder does lead to higher end item costs to the customer. It is not uncommon to tight tolerance extrusion mechanical tolerances to be not be achievable, however an experienced extrusion manufacturer may be able to suggest a design changes that may help the designer achieve fit, form and functional requirements.
The extrusion purchaser and the extruder should fully understand on any special mechanical tolerances before a purchase order is accepted or issued.
In general, published standard tolerances for extrusions are easy to achieve, but can be very difficult for improperly designed extrusion profiles. Standard dimensional tolerances should always be specified when applicable.
Estimating Dimensional Tolerances by “Rules of Thumb”
Exact extrusion tolerances can be determined only by careful application of standard tolerance tables and consultation with the extruder. Often, however, it is not necessary or practical to determine exact dimensional tolerances when rough estimates may be adequate for initial product planning and design.
The following “Rules of Thumb” offer easy estimates of standard tolerances. However, it is emphasized that these “Rules of Thumb” approximations provide only rough estimates.
Diameter of Circumscribed Circle
|0.5 - 2.0||0.040||0.040||0.040|
|2 - 3||0.045||0.050||0.050|
|3 - 4||0.050||0.050||0.062|
|4 - 5||0.062||0.062||0.078|
|5 - 6||0.062||0.078||0.094|
|6 - 7||0.078||0.094||0.109|
|7 - 8||0.094||0.109||0.125|
|8 - 9||0.109||0.125||0.156|
|9 - 10||0.109||0.125||0.156|
|10 - 11||0.125||0.125||0.156|
|11 - 12||0.156||0.156||0.156|
|12 - 14||0.188||0.188||0.188|
|14 - 17||0.188||0.188||0.188|
|17 - 20||0.188||0.188||0.250|
|20 - 24||0.188||0.250||0.500|
Straightness = .0125 inches per ft (12 inches)
Twist = 0.5 degrees per ft
Flatness = .004 x width
Wall thickness = +/- 10%
All critical dimensions should be discussed between the purchaser and extruder to determine the most practical tolerances for each specific application.