Full Chisel Blog

June 17, 2013

Blow Pipe Bellows

This has been on my list for a while and now I can cross it off.  It is based on a traditional design and is a typical two chambered bellows powered by a foot treadle.  I over thought the design and got retentive about vortices and valve placement, but was told not to worry about those things by a physicist.

I had some 1″+ thick pine, I used one piece of full thickness for the center board then had my apprentice re-saw the other board into two 1/2″ boards, one for each chamber.  Also had him hand plane the surfaces smooth [again worried about the smoothness and air currents].

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The leather for the bellows is oak tanned and quite thin, the pattern for the bellows needs to be offset to account for the fact that when it is open the moving boards are shorter than when closed, typical of bellows construction.

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I used the same leather for the main hinge and for the internal valves.  The center valve is inset in a rectangular shallow mortise to give more room in the upper chamber.  The entrance valve is mounted flush as the board is only 1/2″ thick.

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I used an old piece of flat spring steel that I fashioned into a spring to push the lower chamber back open after the foot pedal is pressed.  The weight of the lower board will also help open the chamber as the leather softens up a bit after use.  In my too-much-attention-to-detail, I inadvertently mortised the space for the spring on the wrong side, fortunately the center board was thick enough to accommodate the mortise on the correct side.

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I glued a small piece of leather on the hinge end to hold things together and eventually with both side leathers become the bellows hinges.

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The first side I glued and tacked on by myself, it was such a gluing frenzy that I ended up with glue in my beard.  I used Lee Valley Fish Glue because of its aggressive tack to glue the leather to the board.  All edges of the boards were smoothed and toothed with a toothing plane, glue applied to both leather and boards.

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The second side I had help from my apprentice and it went much more smoothly and after 226 tacks [total for both chambers] it was complete.  I then went on and made the base which consists of a board for the bottom and three uprights mortised and tenoned into the base and glued in place.  I then began on the treadle.

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Once again I was over-thinking the foot treadle design trying to come up with a mechanism that would push up on the bellows when the foot treadle was pressed.  I looked through 507 Mechanical Movements from Tools for Working Wood for inspiration.  One of those slap my forehead moments when I saw an illustration of a see-saw, teeter totter.  Push down on one side and the other goes up.

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A metal hinge simply would not work for the fulcrum of the treadle so I settled on leather held in place with round head screws and glue.  The end of the treadle that pushes against the bellows, I inserted a wooden wheel in a slot to reduce friction.

I hooked it up to a hose and blow pipe and it works as advertised.  Fun project now to find a buyer.

Stephen

 

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