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.