Construction of the stool involves some welding.
Step 1: The Pipe Frame
The dimensions of the pipe depend on the dimensions of the user. Think of a bicycle seat suspended in the air at a comfortable height. Then cut your pipe the right lengths to connect the seat with the ground.
Step 2: The Seat
I cut a piece of plywood to the shape I wanted and upholstered it with some mattress foam and vinyl material. The vinyl was folded over the foam and plywood and stapled to the bottom of the plywood.
The Plywood is mounted to the pipe frame with three brackets I made out of PVC pipe, heating the plastic to form the brackets. If you do that, make sure you have good ventilation and try not to burn the plastic. You could also buy metal brackets that do the same job.
Step 3: The Feet
If the feet caps wear out someday, they are easy to find and easy to replace.
Step 4: The Turntable Top
Loosening the hose clamp allows free turning movement for the turntable. Tightening the clamp keeps the top from turning when you want a more solid work surface.
The same sort of telescoping connection with a hose clamp is used below on the vertical support pipe, where it exits from the top of the front leg.
Step 5: The "T" Top
The basic idea can be demonstrated using PVC pipe and a "T" connector. The original "T" top was made out of 3/4" EMT pipe and fit loosely over the top of the vertical 1/2" support pipe.
A glass marble was glued to the top of the support pipe. It reduced friction from the "T" rubbing on the top of the pipe. The marble also creates less friction in the movement of the turntable top.
Given the loose fit of the "T" top on the support pipe, the top spins freely. It always conforms well to the angle of the drawing board and gives good support for the board.