Thursday, March 4, 2010

More Shawms - long boring

Since I was making these shawms to be joined up as a single unit, the body pieces of wood were longer than in the first one, which had a midsection joint (see this old post).  The lathe bed was long enough, but I wanted to make some improvements to the long 6mm drill.

I had made a drill for lengthwise boring the first shawm by extending a 300mm auger with some steel rod.

This is the joint formed with some copper shim wrapped around the rod and soldered together.  It was strong enough, but required an oversized hole to enter the bore, which defeated quite a bit of the point of being able to make long bores.

This new joint was made by lapping the shafts for about 15mm and brazing them together (reclaimed my gas plant, ha ha), giving a substantially uniform joint (after grinding off the high spots).

 Here is the blank for one of the shawms, mounted in the lathe with the hollow centre.  The bed is long enough, but not too long. Notice that the drill guide and the workpiece are not far apart, and this probably contributed to the drilling problem to be described.

With my son Francis working the lathe's treadle again, I lead the drill into the end.  The boring started well enough, and the first 300mm was easy enough.  After that, things went a bit sour, with the drill seeming to present lots of friction and the shaft getting very warm.  I greased it with some candle wax, which made a bit of difference, but by no means not a complete solution.

Also, the drill became more difficult to withdraw to clean out dust, which rather suggested that there was a curve or kink in the bore, somehow.

Since parts of the bore needed opening up anyway, I ran in a 10mm reamer to widen the hole.  This reduced the drag on the 6mm rod for a while, but there was still this feeling of a curve or kink.  The bore should have been 560mm in length, but I gave up at about 450mm.

At this point, I wasn't sure that the workpiece was actually going to be usable, or how off-centre the bore had become.  I took a length of 3mm brazing rod, filed a flat on the end, and used it in an electric drill to extend the bore all the way through.  As you can see in the picture, the brazing rod has come through well off centre, but not so much to be unusable, so I carried on.

I finished the boring with an electric drill and some brute force. Sighting through the bore, there was a definite curve, deviating nearly 6mm from true.  Acoustically, I guessed/hoped that this would be tolerable, but the reaming would be interesting.

 Jumping ahead, here is the body and the bell being joined.  The body piece has been shaved down approximately with sideaxe and drawknife, centred on the eventual path of the bore.  I turned a tenon in the wide end to mate with a 30mm socket drilled in the end of the bell with a spade bit.

I tried holding these pieces together in a furniture cramp for gluing, but could not get the two pieces to sit squarely together reliably.  I could have made some supporting blocks or something, but this arrangement in the photo was easier to do, mostly. It  has the clamping force applied centrally, via the nylon cord running down through the bore to the weight. The force is only about 3kg-wt,  but it was enough.

I used a modern polyurethane glue (Gorilla Glue), which is plenty strong enough, with some space filling ability as well.


Let's leave it alone to set and come back in the morning...

No comments:

Post a Comment

Post a Comment