Three are images of the hinges assembled in place and the fourth is a mechanical drawing of the hinge plate. That fourth drawing was made first, simply to work out the layout I needed.
The Hinge plates are made from 1/2" thick mild steel. The hinge pins are 3/4" diameter mild steel.
They were drilled and tapped 1/4-20 in one end. Each hinge pin is plug welded into the hinge plate from the back side, leaving the threaded hole to the outside. (Remember, there's a left and a right to this!)
You can probably see in the pictures that I bolted an oversize washer in place on the outboard end of each hinge pin so they couldn't come out of the hole if the stairs shifted.
The stationary angle plates are made from (I think) 1 1/2" X 1 1/2" X 1/4" thick angle iron. All bolts are 3/8-16.
I used 1/64" tolerances on all holes, except for the 3/4" hole in the Hinge Plate. There I drilled as close to 0.750 as I could get without reaming, so there'd be very little slop in the hole as the plug welding proceeded.
Don't forget that all your dimensions need to be planned so the top step has the same rise as the rest of the stairway.
