can use existing equipment for design ideas, and modify to the size you want.
or.. improve the designs, as a lot out there are not ideal
Exactly, its what I did, got a mates multi gym, my benches and the attachements and hacked it all up, pulled parts of the gym set that were solid and dumped all the flimsy bits.
Steel was not an issue, at the time I worked for BHP, tubing of all shapes, sizes and finish was available by the ton.
I measured the room and bits I had kept, got onto autocad and drew up (what I thought) was the perfect home BB gym, then went to work on saw and mig. I modified bits, made bits, joined bits, fabricated stands, and racks c/w storage to perfectly fit the room. Than stripped it and my mate sprayed it white.
The result was awesome, solid as, perfect fit for the room and covered all exercises I was diong.
The sense of satifacion killed it, and I could never get enough of saying "you can't buy it anywhere, I built it" (well, a lot of it) when asked about it.
Stayed with the house when I sold it, realy regret not photographing it now.
