I have heard that three piece suits are making a comeback but I am down on three piece arm bows. Yes I know they can work but I have a short grain crack problem. I recently graduated from the Windsor Institute Sack Back Class. The Sack Back
chair has a bent arm bow which not only is strong but is also adjustable after it dries!
One of the myths that was dispelled by King Dunbar, was that bent wood has spring back. “False!” said his Highness. When we tie the freshly bent arm with string to keep it in the correct shape, the string is taught. When we get the arm three days later in the drying room you will notice that the string has slack in it. This is all part of the principle that wood contracts as it dries. Note to self: don’t over bend.
The chair in the pics has a three piece arm bow, and yes I cracked the short grain on both sides of the arm putting it over the spindles. This leads me to a second revelation, gaps between the spindle and the hole in the arm are good! I have the Lee Valley set of tools that produce different sized tennons. I drilled the holes with a spade bit that I had filed to be a bit undersized which results in tight
hole and no gap in the seat. So a half inch tennon was placed in a slightly less than half inch hole.
However, this fit is not so good for the arm. Where the spindles fit through the arm there should be enough of a gap to have a loose fit. This allows a wedge to tighten up the spindle in the hole and secure it for life. When there is not a
loose fit, like in my chair, you get tremendous stress on the arm when fitting it onto the spindles. His Highness also explained that a round tennon in a round mortis is the second worst joint in woodworking next to an end grain to end grain but joint. A wedge makes this joint work because it adds a mechanical means to keep it together. When your tennon is exactly the same size as the hole there is also no room for any difference in tennon hole placement, which is only a problem if you make slight errors in drilling angles. I mean when drilling by sight lines and using a bevel gauge what could possibly go wrong????? Ok, so I put glue in the cracked arm bow and clamped it and am hoping for the best.
Will let you know how the chair holds up. In the future I am going to find a way to steam bend. Will write about that and I highly recommend getting to this class. It was great.