Spring design

• The depth of the spring at the tip will depend on the thickness of the steel used and the stiffness required. Make it a little deeper than you think you need it - you can always take a little away if it’s too stiff. For 2.5mm - 3mm steel start with a 3mm spring.
• The middle pin is under constant stress so needs as much metal either side as the size of the hole. For a 2mm pin the spring needs to be 6mm deep at this point.
• The closer the middle hole is to the blade pivot the stiffer the spring, but the more stress the spring will be under at the middle pin - its weakest point. However, if the middle pin is too close to the end pin, leverage on the end pin will be increased which might weaken it long term. Start with the middle pin one third of the way along the spring.

Constructing: heat treating the spring

Heat treatment involves bringing the steel to an accurate temperature and holding it for a certain amount of time to make it less brittle. Most professional knife makers pay a specialist to do this for them.

Here Jeff Durber demonstrates a less accurate but more accessible method using a blow torch:

Fine tuning: adjust spring

The spring should lie flat in the handle when the blade is both open and closed, so with the blade open clean up the top of the handle at the bolster end, taking care to file the spring completely flat to the liner:

Then close the knife

If the spring sits lower than the bolster -

• take the blade out and remove a little material from the back square:

• put it back together; the spring should now be low when the blade is both open and closed
• with the blade open file the top of the knife flat again and it should now be flat when both open and closed

If the spring sits higher than the bolster -

• take the blade out and remove a little material from the kick and walk

• put it back together; the spring should now be flat both open and closed