Timber Frame Lesson 2 : Rules to Learn

In Mort’s last lesson he learned the most basic of timber frame joints; the Mortise and Tenon. But wait we got ahead of ourselves a bit since before any dog can learn his tricks (or any timber framer can cut their joinery) they must learn the rules first. So to draw the analogy out a bit, a puppy must learn the rules of the house such as; when and where to do their business, where they can and perhaps more importantly can’t sleep, and/or where they get their food (ie not off of our plates but in their bowl). Before the tricks can be taught, the rules have to be established so that all members of the houshold can get along and do as expected. In the timber frame world, the rules dictate how timbers will join to one-another and how the framer or joiner can layout and accomplish this. Rough sawn timbers are an unruly and untamed lot in the yard. Timbers come in varying dimensions; e.g. an 8 x 8 can vary plus or minus a 1/4 of an inch in the best of circumstances. Additionally timbers¬†have troublesome attributes such as crown, bow, sweep and twist.

If you are unfamilar with the some of the timber terminology … well we will cover them in detail in future lessons so why not subscribe now to receive future lessons and updates.

In timber framing there are three basic rules; Scribe Rule, Square Rule, and Mill Rule. Each one has its advantages and disadvantages and while we employ each as needed or appropriate we on a whole use Square Rule. We choose to use Square Rule joinery since we feel it provides a great deal of accuracy, provides a hand crafted aeshetic and allows us to produce frame in a timely and efficient manner. A fourth rule, often termed Snap Line joinery is often mentioned but it shares so many aspects with Square Rule joinery it more easily classified as a subset of Square Rule. It could be argued that Mill Rule is also a subset of Square Rule but since the former neglects reductions while the latter relies entirely upon reductions the differentiation becomes important. (Okay maybe not for you but it is for us, especially when we are training novices like our boy Mort). Each rule set has its own subtleties and intricacies but an overview is help so here goes…..

Scribe Rule Joinery – Each timber is custom mated to its adjoining timber and vice-versa. Exact dimensions as well as localized charateristics are transfered (copied or scribed) onto each other. The process requires laying entire frame assembles (ie bents or wall sections) in a shop or yard. Timbers are an exactly match and positioned and typically require specialized tagging/marking systems to track their future assembly. Scribes, Plumb bobs and specialized tansfer tools are often used to layout the joinery.

Square Rule Joinery – Every rough timber is considered out to have a smaller, ideal timber inside. Joinery is cut to match the idealized timber inside. For example a 8″ x 8″ is considered to be a 7-1/2″ by 7-1/2″ regardless of the actual dimensions. Within this system many timbers become interchangeable and timber can be cut without having to layout whole assemblies. Lines or reference faces/planes are used to define the edges of the ideal timber inside. Layout is from these references and reductions employed to make the joinery match the ideal timber inside.

Mill Rule Joinery – Timbers are jointed and planed to true and consistent dimensions. Layout can be done from any face and timbers may be interchangeable. To produce dimensional timbers, large industrial milling machines are required. Neither reductions or assembly is required.

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