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Folding Knives

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Folders

Folders go back as far as Roman times and much later the Vikings carried clasp knives. Few folders have survived from the Middle ages and those that have are basic and utilitarian.
In the early seventeenth century, knives using a 360 degree Barry Woods type opening method with puzzle locking mechanisms became popular. Some of the mechanisms are very intricate. Some even had combination locks.
The puzzle knife was quickly superseded by the back sprung slip-joint familiar to us today. Folding travel sets were popular and included a main knife, the fork, and sometimes a spoon.


Around 1784 folding fruit knives came into fashion and were frequently included in travel sets. The fruit knife is distinguishable from a regular folder by its silver blade which was resistant to fruit acid.
The folding penknife evolved from fixed blade penknives around the early 1700's. Such knives were used to trim quills for writing. As well as the 11/2 " blade they also had a peculiar spike at the end of the handle which was used to split the quill, if required. The "penknife" outlived the quill to become the term most commonly applied to a large variety of folders right up to the present day.
Folders are inherently problematic as they require the blade to be in three states and to behave differently in each.

The first state is the closed position during which the blade must be sufficiently secured in the handle so as not to open inadvertently.

The second state is when the blade is in motion and must be loose enough to move easily from the closed to the open position or visa versa.

The third state is in the open position during which the blade should behave effectively like a fixed blade. Most folders lock in this position for safety reasons.

Obviously these three states are counterproductive to each other. Most folder makers solve the dilemma with a single pivot pin and detente, which is by no means as secure as the long tang and multiple pins of a fixed-blade. The balisong is one of the more secure designs because the handle is secured by three pins once the handle is locked open. There are also a few long-tang folders which approach the solidity of a fixed blade in the open position.
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There are as many different ways of making folders as there are folding mechanisms and all benefit from an emphasis on precision. A good fixed-blade maker should have the skills to turn out a credible folder. It may entail him some time to arrive at a suitable pattern but he has all the skills. Care must be taken in particular with the mechanism itself which will probably require testing and adjustment. Folders have more parts and an accurate pattern goes a long way toward ensuring they all fit together well. Folder making allows less leeway for mistakes and the open and closed positions must be a factor in the design for a unitary feel to the finished knife.

Lock Types

The Clasp Knife.

A folding knife without a back-spring. The Opinel is a clasp-knife. The two folders on Lynn's page are clasp knives.

The Slip-joint.

The Slip-joint is the conceptual basis for most folder designs and the place where most makers cut their teeth in folder making. A back-spring is used to encourage the blade to remain open but it is not actually locked.

The Ring-lock.

The ring-lock was developed by Joseph Opinel in the 1890's. A lock such as that used in the Opinel is basically a slip-joint without the back-spring. It has instead a rotating slip ring to lock the blade in the open position. In the 1990's the design was updated by Cold Steel in a knife called the "Twistmaster".

The Lock-back.

The Buck 110 is the definitive lock-back and the knife which popularized this type of mechanism. The design features a notched tang and mating pawl at the end of a rocking back spring.

The Walker or linerlock.

A locking liner, which is basically a split liner bearing on the heel of the tang, used to lock the blade in the open position. Both tang end and liner must be carefully ramped to ensure trouble free use. A good one will have very smooth action. Usually a tiny ball bearing detente will be used to retain the blade in the closed position.

The Sebenza lock.

A variation on the locking liner idea. A hollowed out section of the scale protrudes into the handle cavity to lock the blade open.

The Balisong.

The handle splits in two independent halves each pinned to the tang. A third pin is sandwiched between the two when the knife is locked open.

The Wood lock.

Barry Wood's design features a central pin around which the blade and two handle scales pivot independently. A second pin, anchored to one of the scales, protrudes into a slot in the tang and locks the blade in position.

The Swinglock.

A long-tang folder designed by Francis Boyd which is probably the strongest folder ever made. One pivot pin, one locking pin and a long tang ensure excellent reliability.

The Block lock.

A central square hole in the tang is registered with a block on the center pin which is laterally spring loaded.

StrutnCut

Partial open assist with a manual lock.

The Rolling lock

Have no idea how this one works even though I handled one once.

Compact Knife Types

Folders

The most common form of compact knife. The pivot works like a door hinge.

The Barrel knife

When the knife is in use the sheath attaches to the blade to become the handle.
The Blade trader Blades can be swopped. The "Stanley Knife" is a common example of a blade trader with the added advantage of retractiblity.

Autos

These incorporate a spring loaded open assist blade which is button activated and auto locked in the open position. Most adopt the folder configuration.

Retractibles

The blade is retracted into the handle using a thumb stud running in a slot in the handle.

Blade Patterns

The Spear-point is the most commonly used pattern and gets its other name "Pen" because it was once used to trim quills for writing.
The blank was clipped with a chisel during forging to create the point. Hence the name. The Bowie commonly carries a clip point.
The Spey blade was a favorite of cowboys though what they were used for does not bear thinking about....
The Sheepfoot is commonly used to cut rope by Sailors. A mallet is sometimes used on the back of the blade to drive the edge through.
The Hawkbill is a useful shape for slipping and grafting plants.
The Warncliff can scribe lines or make delicate precise draw cuts in wood.

Limits and fits.

An important factor in production folders is the effect of tolerance since each fit does not have the benefit of hand tuning. No part can be made exactly to blueprint so engineers factor tolerance into all their calculations. In this way any part in a production run will mate with any other part without requiring further fitting adjustments. Especially in the case of small part sizes it is often difficult to accurately measure the diameters involved never mind keeping them within tolerance. Measuring the inside diameter of small holes is particularly difficult. The hole can be finished with a reamer to help with accurate sizing.
Many makers get around the tolerance issue by making the pivot using a system of bolts, bushings and shims which facilitates adjustability and tuning. All folders involve moving parts and surfaces working against each other that eventually wear or deform from use or abuse. A bolt together type of construction generally allows the folder to be completely dismantled for cleaning, lubrication, inspection and adjustment.
The pivot is both a journal and a thrust bearing. The blade is a rotating beam on a shaft, the pivot pin, which is usually anchored in each side of the handle. Some blades bear directly on the pin but ideally the incorporation of a bushing of some bearing material or other is used. The use of steel on steel leads to spalling so bushings are generally made of brass or bronze. Liners which serve the function of thrust bearings are frequently made of brass to avoid spalling when the pivot is under axial load.
Bearing materials must support the load imposed on them. They should have a low coefficient of friction, usually achieved by a thin film of oil between surfaces. The reciprocal nature of life as a folding knife pivot bearing is conducive to lubrication loss at the moment of change in direction. Bearings which always move in the same direction are much easier to lubricate. So bearing materials should be naturally slippy relative to the material they bear on and have a tendency to retain the wetting effect of the lubricant.
Brasses are classified as "alpha", "alpha-beta", and "beta", depending on their copper/zinc content.
Bronzes are copper/tin alloys. They are harder than pure copper and make excellent journals. The phosphor bronzes are particularly wear resistant.
Sintered materials can make good bearings because their porosity facilitates oil impregnation.
PTFE or polytetrafleurethelene is a type of plastic sometimes used as a bearing material because it is naturally slippery and can be used dry. It is subject to plastic deformation and can only be used in thin sections.
Nylon also makes a good dry bearing material.
Depending on application one of three fits will be used to secure a pin.

Interference :- where the pin is larger than the hole. This is used in situations where the pin must not move relative to the housing.

Transition :- where the pin fits the hole but requires a push to get it in. This is handy for location dowels and allows dis-assembly as in auto crankcase mating surfaces.

Clearance :- where the pin is smaller than the hole. This allows a blade for instance to pivot around a center.

Setting pins is related to riveting because the hole is tapered and the pin is "upset" into the hole. This ensures an interference fit. In other instances the pin is domed with a light hammer so that the dome is left proud of the surface. A snap set is the traditional tool for finishing pins this way. Each part of the set has a little domed depression in the business end. The dome forms the head of the rivet or pin. To set pins and rivet parts correctly you should use a "snap set" or the little gismo described by Bob Engnath on the pins page.

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A couple of books on the bookshelf will be helpful.

In particular the little book by Ben Kelley. Jr is a basic grounding on slip-joints and a good starter.

Also the book on how to make
folding knives which shows three different ways of doing it.


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