Ladder Fixture
To make the Ladder Fixture, I took a piece of aluminum and machined it to 1/2” by 1” by 4”. Actually it is .500” by 1.000” by 4.000”, plus or minus .005” in all three dimensions.                         The reason is that in case, at a later date, I have to add something, I just put a scale (6” rule) on it and know it is a nominal size. That way I don’t have to take anything apart or spot holes etc. There are four mounting holes for the Fixture.
The fixture can be held in a vise, which rotates for angular setup.
Or you can mount a work plate on the rotary table to do  the same thing.
I then machined a slot in the face of the 1.000” width that is a close fit for the side rail of the ladder I was making. The fixture can be made to accommodate any size rail and I therefore consider it a Universal Fixture. The slot in the Fixture is .210” wide and .072” deep. The slot is central to the 1.000” width, so the lands are .395”.
That is not critical to the use of the Fixture, just good toolroom procedure. 
Two angular recesses ( these were at 26 degrees) were cut across the .500” dimension to provide a mounting surface for the Plastic (“Lucite”, “Plexiglas”) blocks 5/16” by 11/16” by 3/4” ( actually .312” by .687” by .750”).
The plastic blocks are fastened in place by two flat head Allen screws. There is a drill bushing 3/16” by 3/4” by 1/16” (actually .188” OD by .750” by .063” ID ) pressed into each plastic block.
The drill bushings allow the locating (shot) pin to register the ladder side rail as it is being slotted for the steps that will be cemented into the side rails at assembly of the ladder. There are slots at the 26 degree angle to accomodate the shot pin and to clear the milling cutter as it goes across the side rail. The 26 degree angle can be changed to suit any ladder you will be making. 
The locating pin was made from a drill blank 1/16” by 1 9/16” with a .037” by 1/8” locator ground into the front of the  pin. This pin engages the slot cut into the side rail and maintains the spacing between the steps. In use, the Fixture is put in a vise with a rotating base, or screwed down to a workplate which is fastened to the rotary table. If you have neither, you can put the Fixture in a plain vise set at the angle you need. Or, you can clamp the Fixture down on to the milling table at the angle required. Once set up, the operation is straightforward. Use a short piece of scrap for the set up piece. Clamp the piece down, and cut the first slot. Move the piece in the slot until the locator pin engages the slot you just cut. The spacing is determined by the distance you have the cutter from the centerline of the locator pin. The .037” diameter of the front of the locator pin was arrived at by trying until I have a situation where the pin actually wedges on the bottom of the slot (in the side rail) and acts as a clamp. Since the Fixture slot is open, there is no limit to the length of the side rails you could slot and subsequently cut to the desired length of the ladder you are making. The reason for the two positions of the locator pins and bushings is because there are left and right side rails required for any ladder...This Fixture was developed to insure accurate and consistent results.