I posted this on the creek by request altho I don't think they were looking for the long winded version. LOL
LAYING OUT A SCARF JOINT or the evolution of a Scarf Joint.
1. To lay out a Scarf Joint in timber framing, you must first determine the use of the timber to be scarfed. What are the stresses on the piece, is it going to be Compression or Pushing (the ends pushing toward the center), Extension or Pulling (the ends pulling against each other), Is it going to be Load Bearing (supporting weight from above over its entire span), Is it going to be Supported from below ( a sill or joist).
2. A type of Scarf designed to withstand the load limits for the use must be deteremined. There are many types and styles of Scarfs.
3. Once the style of Scarf is determined. Orientation of the Grain in the piece must be determined, Grain orientated up and down as in figure A. below will support more weight and has very little flex under load. If the Grain is orientated cross ways the piece will have more spring and give to it.
4. The orientation of the Grain also affects the strength of the Joint. As shown in figure B. below crossways grain orientation makes the keyed or locking feature of the joint prone to shearing along the grain.
5. A birds mouth cut as shown in figure C. below gives resistance to spreading or seperating as shown by the Red Arrow.
6. Figure D. below shows what is known as a Stopped (the birds mouth cuts), Splayed (the mating surfaces cut at an angle), Keyed (the tapered locking wedges that pull the joint together and limit it from pulling apart), Pegged (doweled to give extra sideways support), Scarf (lap) joint.
7. The generally accepted rules pertaining to the cutting of this Joint are, the Birdsmouth cuts should be no deeper than 1/4 the depth of the beam, the Length of the scarf should be a Minimum of 2 1/2 times the depth of the beam, the key should be at least square in size and of hard wood. The dashed green lines are put in to show how the angle of the cut is obtained. The cuts are generally made wide of the splayed lines and pared to the line using a Slick and a square for squareness as fit is an important part of the joint.
8. The Wedge is generaly sawn from a single square of hardwood a bit longer than the width of the beam and the sawn faces are placed together and then driven in tight with a mallet. The Sawn faces have a locking effect to keep the wedges from slipping apart.
9. Tools I usually use are a Square, Ruler, 2 Bevel Squares, Crosscut Saw, Rip Saw, Wide Chisel, Pencil or marking knife.
10. Scarfs that support a load from above are usually placed over upright supports posts from below to give added support and an extra spreader support is placed underneath as shown in figure F.
11. Figure G. shows a simple Half Lap or Bladed and Pegged Scarf used for the main Sills that rest on the foundation.
12. Figure H. shows a Tabled (flat surfaces parallel to the top and bottom), Mortised (the small mortised locking ends), Keyed, (the locking wedges), Scarf. Keyed Scarfs may be doweled or not according to the application.
(note), Scarf Joints are named after the features used to make them.
I hope this hasn't been a too long winded explanation.
The Japanese Wood Workers have very elaborate Joints which are extremely strong but they get overly complicated just for the sake of the Visual Beauty of the joint and to show off their Skill at Woodworking.
As most of you know the first time you cut a joint it is nerve wracking and time consuming, but after many times it gets easier and your skill improves.
