Mill Rule and Modern Layout
Mill rule treats milled or planed timbers as truly square and lays out from the timber itself; with CNC joinery cutters and CAD, it's how most commercial frames are made today.
- 3 min
- 5 steps
- 3 questions
- Lesson 103 of 111
In this lesson
- What mill rule assumes
- Mill rule plus checking
- Modern layout tools
- Choosing a system
- What to take from this
Picking up where you left off.
What mill rule assumes
Mill rule is the simplest system: it assumes each timber is actually the size and squareness the drawing says. Joints are laid out directly from the timber’s faces and the drawing’s dimensions, with no imaginary inner timber and no housings to reduce the timber at joints. Any face can serve as a reference, because all faces are assumed to be true 1.
That assumption is reasonable for timbers that have been planed four sides to consistent dimensions, kiln-dried or glulam timbers, and timbers checked carefully before layout. With rough, green, or variable timbers it fails: a timber 1/4 inch oversize or slightly out of square puts that error straight into every joint.
Quick check
Mill rule assumes the timber is accurate. With planed, square timbers, joints can be laid out directly without housings to an imaginary inner timber.
Mill rule plus checking
In practice, mill-rule framers:
- Plane timbers to a consistent section, or select timbers that are close to true.
- Measure each timber and note variations.
- Still pick reference faces for consistency and lay out from them.
- Use housings selectively where a precise bearing surface is needed, for example at beam seats.
Modern layout tools
Commercial timber framing now leans on technology 2:
- CAD/CAM software designed for timber framing models the whole frame in 3-D and generates shop drawings and timber lists automatically. North House’s small-frame drawings were made in one such program, Dietrich’s 3.
- CNC joinery machines (Hundegger is the best-known maker) cut mortises, tenons, housings, and peg holes from the model in minutes per timber, to very tight tolerances, on planed timbers. They make mill rule exact.
- Layout aids: lasers, digital angle gauges, and story poles speed and check hand layout.
Many frames today are a hybrid: CNC-cut main joinery, with hand-cut braces, decorative work, and fitting.
Choosing a system
| System | Timbers | Speed | Fits |
|---|---|---|---|
| Scribe rule | Irregular, hewn, natural | Slow | Each joint to its partner |
| Square rule | Rough sawn, reasonably straight | Moderate | Interchangeable by type |
| Mill rule | Planed, true, or CNC | Fast | Interchangeable, if timbers are accurate |
For a first small frame of rough-sawn green pine, square rule is the usual choice: forgiving of rough timber, fast enough by hand, and what most courses and books teach. Mill rule makes sense if you can plane timbers to consistent size. Scribe rule is for natural and hewn timbers, and for log work.
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Quick check
Mill rule needs accurate planed timbers; scribe rule suits hewn and natural timbers.
What to take from this
Mill rule assumes timbers are exactly their nominal size and square, so joints are laid out directly with no housings. It works with planed or very accurate timbers and is the basis of CNC-cut commercial frames designed in timber-framing CAD software. For rough green timber by hand, square rule is usually the better choice.
Practice
If a timber is a quarter inch oversize or out of square, mill rule transfers that error to the joints. That’s what square rule’s housings avoid.
Lesson complete
Nice work.
Sources for this lesson
- 1Jack A. Sobon, Roger Schroeder. Timber Frame Construction: All About Post-and-Beam Building. Storey Publishing. 2012. verifiedClassic reference (first published 1984) on timber selection, tools, joinery, frame types, assembly, and raising. Cited at: Layout.
- 2Will Beemer. Learn to Timber Frame: Craftsmanship, Simplicity, Timeless Beauty. Storey Publishing. 2016. verifiedHeartwood School's step-by-step guide from timber sourcing to a finished 12×16 frame, including layout, joinery, and raising. Cited at: Modern Methods.
- 3Peter Henrikson. Build Your Own Timber Frame: 10×12, 10×16, and 13.5×13.5 Sauna Frames. North House Folk School. 2008. verifiedOverview drawings of North House's stock small frames, drawn by timber framer Peter Henrikson.