How to Make a Slip Joint Pocket KnifeNow it's time to heat treat the blade and spring. Bring each part up to a cherry red until it no longer will stick to a magnet. Heat the part just a little longer and quench it in oil. It looks like I brought the spring up to a white heat in the first photo, but that was just a lighting trick played by the camera. Wipe the parts off good after they cool. They will be black with scale.
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I use the 1x30 grinder to clean up the edges of the spring, and to clean the bevels and edges of the blade. Flat sand the sides of the spring and the tang to remove the scale. I don't use a grinder on these areas because it's easy to take off a little more than you intended. Keep in mind that these areas have to stay perfectly flat in order for the knife's action to work correctly. (Now, don't that look better?)
The spring of the knife is not really a spring at this point. It's really just a hardened piece of steel. Now is the time to actually make it into a spring. This is done by slowly heating the end of the spring with a torch. The metal will start to turn colors as you heat it. It will start off by turning to a straw yellow, followed by a brownish color, then purple, bright blue, dull light blue, back to gray, and so on and so fourth. The spring needs to be heated to the bright blue color and then quenched in water to stop the temper. The bright blue color should run uniformly from the end of the spring to just past the center hole. It's important to go past the hole so that the spring will not be brittle and week at its center. It's real easy to overheat the spring. If this happens, the spring needs to go through the complete heat treating process again. This happened to me in the second picture. The third picture shows how it came out after going back through the heat treating, sanding, and tempering process. It's hard to see the blue color in the photo, but you should be able to see that the color is uniform from the tip to the center.
Now pin the blade and spring back on the board. Make sure that the back of the spring still lines up with the line that you made earlier. If it does not, just draw another line. Remove the blade and pivot the spring about two millimeters from the line. (See the arrows in the second photo.) Mark the wood where the back hole of the spring is now sitting. Drill this hole with a 3/32 inch bit and pin the spring to the board with both pins as seen in the second photo. This will create the load on the spring when the blade is pinned into place. Work the blade into place and put the pin in. It will be tight, but the spring should bend slightly allowing the blade and pin to be inserted. Try to get the pins deep into the wood to remove as much play as possible. If all has been done right, you will now be able to open and close the knife. The knife should snap open and shut. The spring will feel stronger when the knife is completed. The wood tends to slow the action down.
Close the blade and check to see if the tip of the blade needs to be lowered toward the spring. The tip of the blade will be raised a little when the spring under its load. I needed to bring the tip if this blade down a bit, so I ground a little from the tang at point C in the second photo. This brought the tip down as seen in the third photo. Be careful not to remove too much because you don't want the edge of the blade to be touching the spring.
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