What is a slip joint? ... and other folding mechanisms.

Pen knives, pocket knives, jack knives, folding knives … there are many names for them and all have blades that fold inside the handle. The knives we have studied are ‘slip joint’ folding knives, but here are a variety of other mechanisms for holding the blade open. This is just a brief overview of what a slip joint is and some of the other common mechanisms.

Slip joint
A slip joint folding knife does not have a lock mechanism, but relies on a piece of steel along the spine of the handle to act as a spring and hold the blade open or shut - most Swiss Army knives are made in this way. These were also the most commonly made traditional Sheffield folding knife and have remained popular in the UK as they are legal to carry as long as the blade is less than 3” long.

Friction folder
The simplest folding knife of all has no mechanism to hold the blade open and has been found on archaeological excavations dating back at least x thousand years. The knife is simply kept open by holding the protrusion from the back of the blade in line with the handle.

Ferrule lock
The simplest locking mechanism on a knife is a twisting collar on the ferrule which stops the blade folding on the user’s fingers. The Opinel knife is the most common knife using this mechanism:

Back lock
A refinement of the slip joint where the spring along the back of the knife has a hook on it which engages in a notch in the back of the blade when it is opened. The knife is closed by pressing on the spring to raise the hook out of the notch.

Liner/frame lock
Part of the liner inside the knife, or the frame of the knife itself, is spring loaded to engage with the blade and hold it in place when open. The knife is closed by pushing the spring loaded piece out of the way.

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.

Blade design: tang

The tang is the part of the blade that is inside the knife. It’s precise dimensions will evolve as you develop your knife, but it will come out approximately square. There are two important considerations for its initial size:

• A short tang has less overlap between the blade and the spring, so the action will feel softer as the blade is folded BUT there is more likelihood of up and down wobble in the blade when open
• A deep tang will make the blade more stable in the handle, particularly if it is used for any prying action, BUT will offer more friction as it is turned.

Blade design: back square

The back square is the part of the end of the blade where the spring sits when the knife is open:

• When modelling in plastic for accuracy it is best to aim for an exact fit between the back square and spring, any slack here will result in up and down blade wobble when open. 
• When working in steel you might wish to gently round the corner of the back square. This allows some space between the two so the knife will still open accurately if dirt accumulates here. 
• Be aware that too much shaping here will lead the spring to be sat on just two points of the blade and just subtle changes to these will radically alter how the blade sits in the handle.
• In the closed position there is a tendency for the back square to protrude from the handle - this makes a sharp corner which will not be pleasant in the pocket. If possible the handle should be shaped to cover it although in some designs it is unavoidable:

Blade design: edge / kick

• Inside the knife the blade needs enough space so the sharpened edge will not hit the inside of the spring if it is pressed in past its natural resting position.
• You might wish to shape the underside of the tang into a distinctive ‘kick’ BUT leave this until working in steel. Be aware that too much shaping here will lead the blade in the closed position to be sat on two points and just subtle changes to these will radically alter how the blade sits in the handle.

Blade design: half stop

• The half stop slows the blade as it is shut, providing confidence to the user that their fingers won’t get trapped.
• If you will be selling your knives and wish to display them in the half open position, you might want to reduce the length of the tang so the spring is not raised at this point so is not left under pressure.
• Subtle changes to the shape of the corners greatly affect the opening and closing action. You might like to aim for a squarer corner at the top so it takes more pressure to go from open to half stop, then a rounder corner underneath so it takes less pressure to go from half stop to closed:

Constructing: nail nicks

This is one of those little jobs that makes all the difference to your knife if you can make it look tidy!

• you will need a 'slitting disc' that can be clamped in a pillar drill - they are quite brittle so Grace reinforces hers by gluing it to a perspex disc:

• mark out with masking tape the length you want your nail nick
• put a block underneath to rest your blade on, setting the height of the disc so it is the distance you want the nick to be from the edge of the blade
• then gently work the blade backwards and forwards making sure it remains in contact with the block - you can see Grace doing it here: