Create finger, box and bridle joints (Introduction)

Dear Golem,

Our mission is to help you create a so-called finger/ box or bridle joint using modern technology in a cheap yet reliable way. A finger joint is a type of wood-joint as shown below. The fingers have to fit so tight that the two pieces of wood become one.

Why would you need a finger joint or box joint ?

There are several reasons:

  • The box joint is more decorative compared to a bolt/ screw.
  • If you bolt/ screw wood without glue, you get peak forces near the bolt/ screw. After some load cycles your connection breaks. For this reason you need glue.
  • Glue can only handle shear force. For this reason you need either bolts/ screws or finger joint flanges that handle the tension forces that always exist near glue connection edges.
  • Even if you bolt/ screw your wood connection with glue, you need more surface area for your joint because there will still be peak forces near the bolt/screw. For a given load, a well designed finger/ box joint will always be lower in mass than a bolted joint.
  • A bolt/ screw has a different thermal/ moisture expansion than the wood. For this reason after a few seasons, the connection gets loose. A finger joint has the same thermal/ moisture expansion (given that you use the same wood in the joint) and thus will suffer less from this phenomenon.

To prevent tension forces in the glue, the box joint/ finger joint has to be as tight as possible. Note that if the fit is too tight, it will be almost impossible to hammer the joint together.

goodlife_logoCreating tight fit wood joints is difficult because:
– A very small misfit results in loss of strength of the wood-joint
– A very small misfit can also result in the pieces of wood not fitting together at all.
– Saw/ mill dimensions vary due to wear and tear
– Saw/ mill reacts different to softer/ harder types of wood.

Due to these difficulties, a finger joint is mostly made using jigs or specialized machinery and a hand-operated mill. Also so-called “stacked dado blade sets” (See picture below) are used to create what is then sometimes called a “box-joint”.

stacked dadoo

Now if the finger joint teeth become bigger, it becomes more difficult to make a tight wood-joint. And now the joint is called a so-called “bridle joint” or “open tenon” joint. See picture below

open_tenon

We present the Jointmaster concept that is both cheap, easy and usable on a variety of machines (mill, saw) where the saw blade can be left in place to saw/ mill finger joint, box joint or bridle joint. An example video is shown below using a table saw setup with Android phone.

Using the Jointmaster software you can dimension the box joint dimensions up to 0.01 mm. The stepmotor can reach higher accuracy but at a certain point your sawing jig is not stiff enough/ you run into backlash problems. Note that the saw is always cutting at the same place. Thus you can very effectively remove sawdust.

 

Please read-on in the next post.

 

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