The design was determined by the available steel pipe and angle iron in my junk collection. The 6x6 was a piece I had saved when I built the chimney for my house about 16 years ago. The 3 ton chain hoist was $85 from Amazon and was worked it into a free-shipping order. The ten-foot treated 4x4s cost $12 apiece. Then there was about $5 worth of hardware and recipro saw blades. That adds up to $138.
The milling machine weighed around a ton as best I can guess. I'll look around for an exact weight. I figured I could build the gantry over a weekend. ONE MONTH LATER, I finished it while the mill sat on the trailer under a tarp. More on the mill after I clean it up and remove the extraneous junk bolted to it.
I was obviously concerned that the gantry wouldn't hold a ton given that I was doing gut engineering but, under load, there was no sign of bending or straining.