Friday, February 14, 2014

Bending the Dorset Drum Frames

A mostly bent frame and the
original drum patterns printed
underneath it.
I hope my dislike for bending wood comes across clearly in this blog.  I'm really bad at it.  You might have wondered why I've been so quiet about that Dorset drum project that I started a few weeks back.  Well, its because I've been having a terrible time making progress with bending the wood frame. When I talk about things going poorly in the workshop its usually after I've made some sort of forward progress and have a few learned lessons to report.  While things are actually going badly, I focus on other things. Like tropical fish or snowshoeing.  Mercifully, I've finally made some headway with the frames and I should be able to finish them up fairly quickly and get this blog back on track.

Heat and then slowly
bend over my knee
I was hoping that bending green willow shoots would be so easy that I wouldn't have to pay attention to things like wood grain and growth rings and the cross-section of the wood.  But I was wrong.  After a bunch of trial and error, I finally worked out a system for bending the small drum hoops that gives me a good match for the size, shape, cross-section, and diameter of the original Dorset artifacts.  I'm still not certain that the Dorset drums found at Button Point were willow, but the willow is creating a good match so far.

I bend it to 180 degrees or so in one
session, soak the wood in snow or
water and then finish bending it to
270 degrees.
The best results came from very straight and fairly thick shoots, an inch or more in diameter at the base.  I split them down the middle and planed the centre of the shoots flat.  I removed just enough thickness from the inside of the shoots to remove the pith canal.  On the outside of the shoots, I removed the bark and tried to flatten the wood somewhat.  The trick with the outside of the shoot is to treat it like you are making a bow and avoid violating growth rings.  As you bend the wood the tension grows on the outside of the bend and the lamination between growth rings will want to crack and come apart.

If you look at the cross section through the original drum hoops, they are shaped kind of like a tall skinny salt box house - with a flat bottom and a peak on top. But like a saltbox, the peak has a long edge and a short edge.  The long edge is a long bevel inside the edge of the drum frame on the "top", where the skin is stretched.   I'm not sure whether the hoop was shaped to this cross section before or after bending, but right now my hunch is that it was a bit of both.  I'm finding it easier to bend thicker wood if I carve a long bevel on the top and bottom of the inside face.  I think I'll end up bending it with a top and bottom bevel and then planing off the bottom bevel to create the square edge on the bottom of the drum.   The inner bevels also seem to help avoid some of the compression folds or pinches that want to form on the inside of the curve as you bend the wood.  There are a couple compression folds in the original artifacts, so I'm not too worried about a few showing up in the reproductions, but I don't want them to become so acute that they harm the integrity of the drum.  These instruments are meant to be played.

The heat gun is clamped in a vice.
Its a little simpler and less prone to
scorching than an open flame or
To actually bend the wood, I'm using a combination of dry heat and steam-bending and boiling.  I suspect that if I had a pot big enough to boil the whole stick then I could just boil the wood and bend it around some sort of jig or frame in one go.  But I don't, so I'm using dry heat from a heat gun to incrementally bend the wood to at least 270 degrees, at which point the hoop will fit into a pot that I can boil on the stove and finish bending with boiling.  I found that it was safer to only apply heat from the heat gun to the inside of the bend as I went along.  The heat gun is strong enough to heat all the way through the wood to the outside surface, but if I applied the heat directly to the outside of the bend this outside surface would dry out and become prone to cracks and delamination.  Its a bit of an ordeal, but the occasional scorching from the dry heat is helping antique and harden the wood as I go, so I don't mind as long as its working. I had lots of trouble with this, but I have a system going now that seems to work.

The fish shaped hoop is done with
dry heat for now.  I'll boil it and clamp
it like the one clothes-pinned to the
pot lid.
I had tried boiling sections of the wood outside, but it is so cold here now that the part of the stick outside the pot would freeze and want to crack while I was bending the part inside the pot.   The willow also stays very flexible when only wet heat is used and wants to straighten itself out again, which makes clamping vital.  On the other hand, using dry heat the wood stays bent a little better as the moisture in the green wood is driven out and the new shape is locked into the wood. So far these frames seem to be holding up.  I have them bent a little smaller than they need to be to allow for some springback when the clamps come off.  The next steps will be working on the scarf joint to connect both ends of the hoop to each other and the groove that runs around the outside circumference of the hoop for the lashing to hold the skin on.

Photo Credits: Tim Rast

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