What is T-Slot and other profiles

Aluminium T-slot and Fasteners extruded t slot aluminum As for strength and shape, the major advantage of this is the T shape, it resists bending very well, and just slightly less it resists torque extruded aluminum tslot accessories My question is - why use expensive extruded aluminum T-slot? I mean, for a CNC router it makes sense because you need sub-millimeter

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