And thinking hypothetically, not necessarily to do with this build, With regards to twist to a gantry from a spindle, am I right in thinking that a rail front and back at the same height and connected round with suitable size material would cause no twist as the levering action from the spindle would be translated into an upwards force one side and a downwards the other??!!
I'm not sure if I follow, sounds like you want a saddle that wraps the gantry beam. Not good in my mind but that depends upon how much time you put into this. There are reasons why the overwhelming number of machines out there have their liners rails on one surface. One of the primary reasons is that it is easy to get the machining and thus rail alignment right. It would be incredibly difficult to do otherwise.
As for what the spindle will do it full depends upon what the spindle is doing at the time. The spindle can even pull the gantry down if it happens to cork screw a drill bit into something and jam. In Normal operation the forces depend upon the cutter used and the aggressiveness of the cut.
So with respect to the nail gun, the spindle would have a greater impact on the beam. It is up to you to determine how much of that impact is too much. Most guys in this forum are trying to achieve fairly good precision with their machines. In this case it sounds like you want to prefab walls for houses, as such the routing quality might not be important at all. I can't read minds here so I'm defaulting a machine design where higher quality is expected.
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