Knife Design and Philosophy
ForwardI wanted to share a little more about my knives. I strive to build a functional tool capable of hard use without compromise as the art component is integrated. I have and will continue to reinvest in my business and craft to obtain the required tooling and experience to meet a high level of precision in all areas of the build.
In the following sections, my goal is to provide a resource for some increased detail into the construction, material choices, and build processes I use for my knives. This will be a living document, and I hope to add more subjects as time permits.
Bushing-Washer Pivot Joint
In my opinion, the blade folding action and the strength of the pivot joint is the heart of any folding knife. Utilizing the bushing-washer pivot system that is designed to be mechanically fastened with the proper torque (and resultant joint preload) in an integral frame provides for a very robust folding joint. The surface area available to react to and absorb the cutting forces is maximized with this design. The schematic below (Fig. 1) shows the bushing–washer system
(probably most recognized from the Chris Reeve Sebenza design).
I have utilized this design configuration in all my folding knife models, while only the materials and precision have evolved over the years. The thrust washers have remained phosphor bronze and have only varied in thickness. Similarly, the bushing was made from phosphor bronze on my earlier knives. The pivot and screw were made from 303 stainless in my very early models. As I understood more about the physical properties of the materials I was using, I began to iterate on the design choices I made. As of 2016 the bushing has evolved to a hardened stainless-steel construction with machined grease grooves. The pivot and screw have improved to a hardened stainless, and then to 6-4 Titanium, which I feel is slightly better; even though the material strengths are similar, additional corrosion resistance is gained.
There are several reasons why I feel this is the best pivot design. The core reason is the structural rigidity and integrity of the joint while under cutting loads. Another significant factor is that this design, when executed with high precision, ensures the open blade position and lock geometry is dead repeatable and does not change. This allows the integral frame lock to be precisely fit to the blade with no future relative movement that will most likely cause lock stick and wear. The pivot and screw are torqued providing a clamping force (preload) to the overall joint. A preloaded joint is significantly better at withstanding the loads generated by use than a joint without torque / preload. A non-torqued joint will gap under use and transfer greater loads to the individual components while allowing relative movement of the joint components. For a deeper and more technical explanation of this I recommend searching ‘bolted joint preload’ on the internet. A final advantage to the torqued pivot is that the preload is the screw’s primary locking feature, where the pivot remains tight without the need for Loctite and eliminating the risk of restricting blade opening action.
Utilizing this design with true parts and extremely tight tolerances, allows the blade and washers to completely fill the gaps in the joint (with lubrication clearance only). This leaves the pivot area much less susceptible to collecting debris than other pivot system designs and reduces the number of times the knife will need to be cleaned over its life.
It always felt wrong to me to build a knife that needed to be “tuned” or “adjusted” by the user. A takedown tool is provided with each knife strictly for the purpose of “servicing”, as required. The tolerances are designed into this joint, are repeatable and do not require any adjustment. When executed properly (this is key), this pivot system allows the owner to service their knife and have the same smooth blade pivot action, position, and lockup every time by simply torqueing the pivot screw (approximately 1/16 th of a turn past snug for non-clocked pivot/screw builds).
While I feel this is the optimum pivot configuration, this design is unforgiving and challenging for the maker. Very tight tolerances for size, squareness, roundness, parallelism, and concentricity for the pivot, screw, bushing, blade, washers, and handle are required to produce a centered blade with smooth action, and without play. The bushing, for example, dictates the blade position relative to the lock-bar. If all other parts are proper and the bushing is not concentric, the lockup geometry and operation will be affected each time the knife is serviced depending on the random rotational position of the bushing. If the bushing is not square, the blade will not be centered, it will bind as it rotates, and it will need to have play in it to compensate (Fig. 2 below is an exaggerated example of this). Similar deficiencies occur if any of the other pivot joint parts previously mentioned are not made to extremely tight tolerances.
Achieving the required level of precision drives the build processes, the required shop equipment, and the need for many custom engineered fixtures. Creating a hole in a handle or blade for instance, may often be done with a drill on a manual mill (Bridgeport) or drill press, and is then brought to final size with a reamer. While reamers are sized to the ten-thousandth of an inch (0.000X in.), a reamer will only follow the drilled hole which is not necessarily square. A drill and a drill press / Bridgeport are great for removing material, but they are not at all designed to make a precision hole due to the accuracy limitation of the machine and cutting tool deflection. Utilizing a CNC Mill or jig borer to machine / bore the part to establish a straight (square) hole, and in many cases bringing the hole to final size by lapping, is required to meet the tightest tolerances. The bushing, pivot and screw are all made from raw stock using high accuracy machines and processes to ensure the parts are square, concentric and to size.
To utilize this pivot system on an integral handle is a whole other level of difficulty. It is imperative that the two internal (handle) sides be dead flat and parallel. The inherent residual stresses that remain in the titanium handle after cutting the blade slot significantly affect the pivot area geometry. These stresses can vary from build to build as they vary with each bar of titanium. Countless hours were invested over the years in developing engineered tooling and processes to overcome this obstacle that is unique to a solid integral handle. In the early years I seriously considering going to a two-piece handle, which would take minimal time on a surface grinder to achieve flat and parallel, but I’m glad that I stuck with it; the increased rigidity of an integral handle is significant. What was not an option was to utilize an alternative pivot system (without the structural integrity) that would have masked the need for almost all the high tolerance requirements.