I usually work with machine tolerances. Why are aluminum extrusion tolerances so large?
Machining generally controls dimensions on small areas. The extrusion process controls those cross-section dimensions on lengths as long as 150 and 200 feet. Other reasons for tolerance difference include extruding temperature, pressure variations, die wear, angularity, and type of alloy used. Even though these tolerances may be larger than machine tolerances, the extrusion process is much more cost-effective than machining when the application allows. Many times tolerances closer than those published are available from the extruder. Designers should consult with their extruder on what tolerances are available.
