A perspex model gives you the opportunity to refine where the blade and spring meet so you will get a good opening and closing action. Use perspex approximately the same thickness as your steel to give you a similar feel.
1. Scribe around the plastic pieces onto perspex, saw the pieces roughly to shape, then file towards the scribe lines, leaving a little extra around the tang of the blade and tip of the spring for fine tuning.
2. It it is really important that the holes line up exactly on the handle and spring, so drill one side first then use it as a drilling guide for the other side and spring.
3. At the same time drill these three holes in a piece of wood that is large enough to take your knife open; you will use this to get the blade and spring working together.
4. Put the spring into position on the board, put the blade in the open position on top, lining the angle of it up with the drawing underneath, then look for where the back square of the blade and the end of the spring need adjustment to fit.
5. File carefully and slowly to fit - this needs to be an exact, tight fit at this stage - any sloppiness here might cause problems later - if you take too much off anywhere it is better to cut a new piece from perspex and refit it than accept an error now that might magnify further down the line.
6. Put the blade into the closed position (don’t try to pivot it on the board yet or you will snap your spring) and mark where the kick and spring will meet - again file slowly and carefully until it is exactly right.
7. Put the blade into the half stop position and mark where the spring will meets it - this is the bare minimum length of tang - if it is any longer the spring will be raised at the half way stage - this might not be an issue for a knife for personal use but if it is for sale and will be displayed in this position for any length of time it might weaken the spring - again file slowly and carefully until it is exactly right.
8. Gently round the corners of the tang until you get the opening and closing action you require, making sure they do not cause the spring to be lifted clear of the back of the knife.
9. Finally, assemble it off the board with the handle sides - now you can refine the overall form looking at how the back of the blade flows into the handle shape and how it feels in the hand - be careful about taking too much off the back of the handle leaving the spring weak - again file slowly and carefully until it is how you want it - don’t be afraid to remake pieces that aren’t right, it’s far easier to do it in perspex than steel!
Here you can see knife designer Jeff Durber developing a model for his knife:
