In this photo you see three bars of extruded aluminum waiting to be sawed together. Notice the different bars marked with a straight line, a squiggly line and no line. Why? Because not all extrusions arrive with the same exact width and this can create a big problem when holding multiple parts in a vise. A narrow piece will be held with less force than a wide piece. If it’s a big enough difference (0.004″ or more) an endmill might easily throw the part out of the vise and potentially break a tool(s).
To ensure you’re holding same-sized parts, simply draw different lines or colors on each bar before sawing. One look at the markings on the saw cut pieces will tell you they came from the same bar. Load similarly marked bars together for disaster-free milling.