"Richard Smith" wrote in message
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I did a web-page about this concept
http://www.weldsmith.co.uk/skills/fe...-fold-stl.html


The company where I apprenticed as a machine designer in the 1970's
built custom automotive production test equipment weighing up to 5000
Lbs mostly out of relay racks they fabricated themselves from 12 gauge
(2.6mm) sheet steel. Their machinery was a Niagara press brake and a
Strippit punch press, and an old Polish welder whose TIG beads were as
smooth as his disposition was rough. They consistently achieved a
tolerance of 1/32" on the panel openings, so I could make my control
panels 19.00" or 24.00" wide without any custom fitting for width or
the squareness of the upper and lower corners.

We tapped the panel mounting holes #10-32 (~5mm) with a portable drill
freehand. I retired taps after ~1000 holes, and never broke one. Back
then the tapping fluid had carbon tetrachloride in it and worked very
well.

Naturally any large motors or refrigeration compressors were on
structural steel bases, but if the contents were all electrical the
row of bolted-together racks could be 20' long and still very rigid.
One such 5000# machine fell off a forklift during the customer's
morning break and landed face-down on the electrical control panels.
The frame was still in fine condition and was repainted and reused. I
salvaged a bucket of Variacs that had been broken in several different
ways and could be mixed and matched into nearly half as many usable
ones for my home projects.

5000 Lbs was the most we could manhandle out onto the flatbed truck
with pipe rollers and Johnson bars, due to lack of space to stand at
the front of the loaded truck bed. The overhead gantry hoist didn't go
out the loading dock.
https://www.pbs.org/wgbh/nova/pyrami...ry/blocks.html