What is this picture and why is it representative of failure?
This picture shows my CNC mill in progress. Attached to the table are the supports and linear rails for the x axis. The part being held by the engine hoist (ominously foreshadowing my error) is the gantry. This component would ride the x axis’ rails to move back and forth along the table. The y axis linear rails can be seen attached to the gantry. The z axis and tool holder are not installed in this image.
The heavy steel is very rigid and does not rack even when being driven from only one side with a moderate load at the other. So why is this a failure? The key lies in the aforementioned ‘heavy’ steel. I had intended for the gantry to be installable by a single person. It turns out I am in fact capable of building a gantry so heavy I cannot lift it.
The gantry assembly will likely be installed and removed many times over the course of the mill’s construction. Even using the lift it is very difficult to line up all the parts which need to be attached to each other and damaging the surface of the rails is a very real danger.
So this is a failure of design. The designer (me) failed to take into account the weight of the component which would need to be installed compared to the available resources needed to install it (unfortunately also just me).
I already have the aluminum with which I intend to build the new gantry. Hopefully the new design will work out better though I am not thrilled at the prospect of drilling and tapping all of those holes over again.  

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