Slip-Joint Folder Designing By: Steve Culver

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1 Slip-Joint Folder Designing By: Steve Culver This tutorial shows how to design the joint mechanism of a single blade slip-joint folder, with a half stop. This method of designing a folding knife joint, is based on the size relationship between the height of the blade at the ricasso, the height of the blade tang and the height of the portion of the spring that functions against the blade's tang. This portion of the spring is called the "blade walk". The thickness of the blade and spring material do not change these dimensional relationships. It doesn't matter if the blade stock is 1/16 inch, or 1/4 inch thick. The dimensional relationships of the parts do not change. Designing a folding knife can begin with one of two available starting points. You can begin by designing the handle shape first and then make a blade that will fit inside it. Or, design the blade and make a handle to fit around it. Once your design work has reached the point of finding the height of the blade at the back of the ricasso, you have the information needed to design the knife s joint. This design principle applies to a single blade knife, as well as multi blade knives. All of the blades in a multi blade knife will have the same tang to blade-walk relationship, with the pivot pin location in the center of the tang. This designing method also works as a starting point for designing a folder that does not have a half stop, by simply changing the end of the tang to be rounded. A lock-back folder can be created from this tutorial, by adding a locking notch to the tang and locking tab to the blade-walk area of the back-spring. Also included in this tutorial are the steps taken to fit the blade and spring into the handle.

2 The size relationship between the blade, tang and spring: The illustration below is of a blade that is 1/2 inch (.500) in height. The combined height of the tang and the blade-walk, match the height of the blade at the back of the ricasso. The tang takes up two thirds of this measurement and the blade walk one third. This makes the blade walk one half the height of the blade tang. A tang that is 2/3 the height of the riccasso will give sufficient support to the blade. Note that the size of the tang also affects the lift on the backspring. The larger the tang, the greater the distance from the pivot pin to the corners of the tang. A larger tang will reduce the height and strength of the spring as well as increase the lift on the spring. Taken to extremes, a very large tang could lift the shortened height blade walk above the back of the knife. During the designing and first phases of building the knife, the tang is perfectly square. The bottom of the tang is level with the blade edge. The kick extends below the edge and the tang. The pivot pin is in the exact center of the square tang. This helps to ensure that the lift on the spring is the nearly the same amount as the blade is rotated and the spring goes over the corners of the tang.

3 Locating the pivot in the exact forward/aft center of the tang is very important. If the pivot is located closer to the rear of the tang, the entire blade will move towards the handle as the knife is opened. This will cause the end of the blade walk to extend past the tang, in the half-stop and closed positions. The lift on the spring will also be reduced. This knife can be made to function, but may necessitate lengthening the handle to accommodate the blade being positioned further into the handle in the closed position. The blade walk extending over the tang on the half-stop and closed positions is not esthetically pleasing and collectors will find it objectionable.

4 Locating the pivot closer to the front of the tang causes even more serious problems. In this situation, the blade walk will not reach past the tang and the bottom corner of the blade walk will tend to dig into the end of the tang. A large radius could be ground on the corner of the tang to move contact point behind the end of the blade walk. Rounding off the square corners of the tang actually will be done during the final fitting of the knife. However, creating a large radius on the corners of the tang will affect the knife's "walk and talk" and the blade will feel sluggish. More on the correct rounding of the tang corners in the fitting in section of this tutorial.

5 Building the Slip-joint Folder: Begin construction of a folder by cutting and grinding the blade, spring and liners to rough shape. Leave a little extra material on the top of the three parts for final fitting. The pivot hole in the blade and the three pin holes in the spring are drilled. The blade and spring are then heat-treated. Only drill the pivot hole and the hole for the top rear pin in the liners. The other pin holes will be drilled in the liners when the spring position is being set for pre-load.

6 The first step in fitting in the folder is to do the rough fit-up of the parts. Begin by sanding and finishing the spring notch (also called the run-up, or back square) in the blade as you want it in the finished knife. Also finish the bottom of the blade walk, where it contacts the tang and the kick. These two areas establish the position of the spring in the knife's open position. These two areas should not be sanded or changed later in any way. Assemble the blade, spring and one liner, using assembly pins. The tip of the blade walk section of the spring is now ground to bring the blade to the correct position with the knife open. Leave the point of the blade a little lower than you will want it to be in the finished knife. This is because the spring will flex when it is loaded and sometimes the tip of the blade will be slightly higher in the assembled and finished knife.

7 When grinding the end of the spring to position the blade, it is often possible to leave a small clearance area between the bottom of the spring and the spring notch in the blade. This clearance area is to allow a place for a small amount of dirt to accumulate. If this clearance cannot be made during the fitting of the blade, material could be removed from the top of the tang, at the front of the spring notch. Or, a small amount of material could be ground away from under the tip of the blade walk. There should be two contact points between the blade and blade walk; at the rear of the tang and also at the top of the spring notch, near the blade spine. Allow sufficient contact between the tip of the blade walk and the front of the spring notch to prevent the two parts from mushrooming. The next step is to fit the blade into the handle, in the closed position. The kick will be ground to let the blade into the handle. During this operation, the lower corner of the tang is also ground to lower the spring into the handle to match its position at close, to its position at open. Since the blade is not rotated a full 90 degrees from the open to closed positions, the square tang will not allow the spring to be lowered to the same level in the closed position as it is in the open position. Removing this material at the bottom of the tang will reduce the lift on the spring slightly during the rotation from half stop to the closed position. This is beneficial to the functioning of the knife, as it slightly reduces the amount of force necessary to open the knife from closed to half stop and also reduces the risk of the blade slamming closed forcefully enough to cause the blade's edge to hit the back-spacer portion of the spring.

8 The approximate amount of the kick that will removed to let the blade tip into the handle, can be seen in the sketch below. Grinding either the kick, or the lower corner of the tang, affects both the position of the blade tip and also the position of the spring. As the kick is ground to let the tip of the blade into the handle, the rotation of the tang will lower the spring into the handle. Grinding the lower corner of the tang will cause the blade tip to rise out of the handle and the spring to also lower into the handle. So it s a balancing act, grinding both the kick and the lower corner of the tang to position both the blade tip and the spring. Leave the spring a little high at the back of the knife. This provides some extra material for final fit-up. Next, the end of the tang will be ground to lower the spring to near the level it is at in the open position. As when adjusting the spring position at close, leave the spring a little high, providing some extra material for final fit-up.

9 After rough fittings of the parts, it is time to drill the pin holes in the liners for final assembly. Assemble the knife, using assembly pins through the blade pivot and the single hole through the spring and one of the liners. With the knife in the open position, mark the location of the bottom of the spring, near the tip. Remove the pin from the pivot hole and remove the blade. Lower the tip of the spring to provide spring loading and drill the remaining pin holes in the liners.

10 Assemble the knife. If the knife is too stiff in operation and it is necessary to lighten the action, you will grind to remove material from the spring between the blade kick and the center pin. A folder is usually stiffer in operation after final assembly, so lighten the spring a little more than you will want it to be in the finished knife.

11 When the knife is assembled, the loaded spring will be flexed from the pressure against the tang. This will probably change all of the positions of the blade compared to where they were during the rough fitting in. The tip of the blade may be higher than what it was after rough fitting. There isn't anything that you can do about that now.. The flexed spring can cause the blade to be more open than it should be at half-stop. To correct this, the lower corner of the tang is ground to let the blade rotate to a more closed position. In grinding the lower corner of the tang back, it may increase the unequal distance between the pivot pin and corners of the tang slightly.

12 As the tang is being ground to adjust the position of the blade at half-stop, the end of the tang should be ground to create two contact points. Having two contact points prevents the blade from rocking at half-stop. At this time, the corners of the tang should be slightly radiused. Rounding of the corners is done to remove the sharp square corners of the tang. It also moves the tang contact point back from the tip of the spring. Rounding the corners of the tang should done very carefully. The square corners of the tang must only be rounded slightly. In the sketch below, the rounding of the corners is exaggerated for illustration purposes. If the corners are rounded over too much, the blade feels sluggish when opening and closing the blade. As the user is turning the blade, the blade walk is sliding over the rounded corner with no change being made to the spring lift. The user must manually rotate the blade during the time that the blade walk is sliding over the rounded corner. With no change being made to the spring lift, the spring is not assisting with the movement of the blade and the user gets no feedback from the spring's affecting the movement of the blade. The larger the rounded corner, the longer the dwell time that the blade walk is sliding over the corner and the more degrees of blade rotation that the user has to do manually.

13 As the position of the blade at half-stop is being adjusted, the position of the spring is also being finalized to match its position when the knife is open. Final fitting of the blade into the handle in the closed position is adjusted next. Due to flexing of the spring, the blade tip will likely need to be set back into the handle again. The kick is ground to let the blade in and the lower corner of the tang is ground to position the spring the same as at open and half-stop. Again, it is best to create two contact point against the spring. The tang can be radiused along the bottom to provide the two points.

14 The drawings below show what the blade to spring fit-up should look like when the knife is finished.