When making a new prototype folder, be it locking or slipjoint, I have found it to be an advantage to make a dummy liner out of a scrap piece of perspex.
The perspex allows me to watch the mechanism work in operation while holding every thing in place ,as two liners would.It protects me if a spring should snap or lift,a s has happened to me once on a layout block.
I use this method to tune my spring action, so the spring has an even and constant tension or "bend", reducing chance of weakness or breakage. It's also a good way to check for clearance between blade edge and spring/lockbar and maximise handle area when the blade is closed.
And sometimes its just cool to see them work.