Coordinate Metrology of Straight Aluminum Extrusions

Overview

If you are new to using aluminum extrusions in a product, consider using the datum scheme and measurement point layout below for tolerancing and 3D coordinate metrology of straight aluminum extrusions. This will enable coordinate point and point cloud measurements in a functional datum scheme as many applications for extrusions use them as beams between nodes of a structure. The measurement points shown will also enable easy measurement and evaluation of the manufacturer’s standard tolerances [Aluminum Association (AA) or European Standards (EN)] for items such as bow, twist and cross-section dimensions and angularity.

 

Tolerances at interfaces to mating parts can be directly measured relative to the functional datum scheme. Any interface or final product surface profile tolerance can be directly measured. The tolerances for these points would be based on functional requirements. They can be related back to AA or EN extrusion tolerances by a stack-up calculation for a feasibility assessment of the required extrusion tolerances to achieve the product requirements.

Simple Checks & Impact of Industry Standard Tolerances

Bow & Twist in Straight Aluminum Extrusions are similar to the deviations found in 2x4’s at the lumber yard. The allowable deviation for these is generally a function of the overall cross-section size, wall thickness and the length of the extrusion. The deviation from nominal of measurement point Surf_A-Mid-01 will be dominated by the amount of bow orthogonal to the XY plane since this surface is on the primary datum plane and this point is co-linear with Datum points A1 & A3. The deviation of Surf_A-End_2-02 will be dominated by the twist of the extrusion. These individual measurement points will give a simple and quick check of the bow & twist deviations of the extrusion. Realize that there can be bow in multiple directions on an extrusion. So, Point Surf_B-Mid-01 will be dominated by the bow orthogonal to the XZ plane since it is colinear between Datum Points B1 & B2 and is located close to Datum Plane A thus minimizing other 3D tolerance effects. Point Surf_A-Mid-02 will be affected by both twist and XY Plane bow. The bow on the A datum surface could be checked at any Y direction position with a secondary alignment. However, a quick check can be done by subtracting the bow inferred from point Surf_A-Mid-01 from Surf_A-Mid-02. The remaining deviation should be dominated by twist. It can be checked compared to the full twist from point Surf_A-End_2-02 after scaling by the half of the extrusion length. If this inferred twist matches ~50% of the twist inferred from point Surf_A-End_2-02 then the twist is likely continuous along the length. If it is not, then further investigation is warranted including implementing a secondary alignment and additional check points.

This methodology can be extended relate the other industry standard tolerances such as cross-section dimensions and angularity to the deviation relative to the functional datums. This information can be used for setting inspection limits & planning for feasible extrusions tolerances for your product.