Making Valves for Fuigo Box Bellows

Fuigo (鞴, Japanese box bellows) are among the most compact and efficient hand-powered bellows for forging work. With few complex or moving parts, they are easy to maintain and will provide years of service. Among the more technical points of construction are the four wooden flap valves, called ben (は弁), which control the direction and location of the airflow on each stroke. This article will focus on some of the more intricate and obscure details of their construction, based on research of antique fuigo.

Fuigo valves most often seen are the simple, flat, wooden style. These are generally quite adequate if mounted properly, and are usually the type found as quick replacements for antique auction sales. However, historical examples of fuigo valves range in complexity and the details discussed here will provide a starting point for improved function and longevity.

Functional requirements on the valves are that they open easily, close quickly, and seal tightly when closed. The ben style discussed here will include historical advancements that address each of these three issues in multiple ways including giving the valve a wedge shape, rounding the top edge, covering it in washi paper, and angling the hinge holes.

Wedge Shaped Profile

While the top of the valve is quite thin to allow the pivot point to remain small and centered just above the hinge holes, the bottom of the valve is about twice the thickness. This design detail reduces the overall weight of the valve to allow it to open easily, but places the mass where it is needed to quickly snap the valve shut at the end of a stroke.

The angled and blocked set-up for planing a wedge onto the end of a thin Nootka Cypress board. A relatively light wood such as a Cypress or Cedar serves well.

The wedge shaped fuigo valve. The faces should be be as flat and smooth as possible, the grain running parallel to the front face, and the bottom about twice the thickness of the top (eg. 6mm and 3mm).

Rounded Top Edge

At the point of contact along the top edge of the valve, there is a gently radiused lip which allows the valve to swing quite freely without greatly altering the tension of the string. The radius also effectively moves the point of contact lower in relation to the top edge, leaving more material above to reinforce the hinge holes. The angled hinge holes for the string should exit the face just below where the radius begins. Placed too far below the radius and the string tension would trap the valve shut on the intake but placed anywhere up onto the radiused area and the weight of the valve will cause it to hang slightly open when at rest (though this is less of an issue in certain dedicated use cases such as tatara or casting work).

The radius should be close to the thickness of the top of the valve and continue evenly all the way across.

Washi Paper Covering

Washi paper is wrapped from the back around the face and over the back again, fastened with sokui. This serves three main purposes, one to assist in providing a smooth surface to seal tightly against the planed wood of the fuigo, two to stabilize and prevent warping of the valve body which would break the seal, and three to strengthen the top which allows it to be thinner and the holes to be closer to the edge.

Strips of washi paper are cut to length and slightly wider than the valves.

Sokui is spread on the face and glued down, then the bottom flap is folded up and glued in place.

Sokui is spread over the back and the bottom flap and the top flap is folded over into place.

Extra sokui is spread over the washi along the front and back of the top edge to reinforce the area where the hinge holes will be drilled.

The excess washi is cut flush when the sokui is completely dry.

A thin area along the side edges is reinforced with extra sokui spread on top and allowed to dry overnight. (the goal is to seal just the edges but leave the paper surface soft where the seal is formed)

Angled Hinge Holes

The mounting hinge for traditional ben is a loop of linen string that goes through two holes in the valve and two corresponding holes in the body of the fuigo. The holes through the valve are placed quite close to the top and angled upwards so that the string exits very close to the point of contact for the top edge. Too far down the face of the valve and the string tension would trap the valve shut on the intake. Too far onto the top edge and the weight of the valve may cause it to hang slightly open when at rest. The holes should be drilled carefully and accurately, and not larger than necessary for the size of the mounting string.

A comparison of the face and back locations of the holes. They should angle slightly upwards from back to face. A small kiri is a helpful tool for making these holes. (note that the washi paper should be totally flat on the face, wrinkling indicates remaining moisture and longer drying time is needed to make crisp, clean holes)

If the face holes are located just below where the radius begins and the string tension is correct, the valve should sit flat and closed at rest.

A Note on String Tension

The tension of the string should be a loose enough to allow movement and spring, but tight enough that the holes remain aligned. If the string is too tight, the valve will not open very far on the intake. If the string is too loose and the valve drops too far, the top of the valve will not seal properly when closed due to the string holding open a small gap. Some trial and error will help pinpoint the correct balance for each valve.

A simple knot on the inside closes the loop. As it is tightened carefully and slowly the tension can be adjusted by pulling on either the loop or the tails.

A bit too tight, in this case the next tug should be on the loop rather than on the string tails. The valve should angle 20-30 degrees at wide open on the intake stroke (for a 7cm opening and a 2″ tuyere).

Tension right, ends trimmed. The valve is relaxed and almost hangs loose but does not drop so far that the hinge holes are below the corresponding holes in the fuigo body.

The view from the outside. The holes in the fuigo body are horizontal rather than angled but theoretically could be angled upwards towards the valve for more tension control.

Fuigo Valve Troubleshooting

Resting valve open at top	string too loose
Resting valve open at bottom	string too tight, hinge holes drilled too far onto the radius
Valve will not open enough on the intake stroke (heavy drag with opening <20 degrees)	string too tight, hinge holes drilled too far down the face
Valve will not snap closed quickly on the return stroke	valve too light
Valve bouncing at the start of the return stroke	valve too light
Valve leaking on the return stroke	face not flush/plane, valve warped, inside of fuigo not flush/plane
Piston lockup or heavy resistance	one set of valves installed on upstream side of fuigo body
Airflow on one stroke only (potentially dangerous condition when connected to a forge)	one of the manifold valves is not functioning (use will suck fire into the fuigo), or one of the exterior valves is not functioning
Reversed airflow (dangerous condition when connected to a forge)	both sets of valves installed on upstream side of fuigo body

See the fuigo archives: islandblacksmith.ca/tag/fuigo/