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                   by ALLEN INVERSIN
This Technical Bulletin shows an inexpensive way to make bellows,
the air pump traditionally used by blacksmiths to make fires
hotter.   The design here will supply enough air to make a fire that
is hot enough to melt steel.  It can be built entirely from locally
available scrap materials, with the possible exception of some nails
and a few pieces of wood.
The Bulletin also gives general guidelines for building a forge,
which is a blacksmith's special furnace.  Specific forge construction
plans are not included.  However, the Bulletin does include instructions
for using the forge and bellows.
The designs in this Bulletin were developed by Allen R. Inversin,
who worked with VITA in Papua New Guinea during the 1970's.   Mr.
Inversin, a graduate of the Massachusetts Institute of Technology
(MIT), previously worked with local technologies in Laos.
Please send test results, comments, suggestions,, and requests for
further information to:
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virginia 22209 USA
                Tel:   703/276-1800 * Fax:  703/243-1865
                                                    Revised August 1981
                                                    ISBN 0-86619-084-8
This Technical Bulletin is one of a series of
publications that offer do-it-yourself technology
information on a wide variety of subjects.
Technical Bulletins are idea generators, intended
not so much to provide a definitive answer as to
guide the user's thinking and planning.  Premises
are sound and testing results are provided, if
Users of the information are asked to send us their
evaluations and comments based on their experiences.
Results are incorporated into subsequent
editions, thus providing additional guidelines for
adaptation and use in a greater variety of conditions.
The bellows pictured here
is constructed from wood,
old inner tubes from auto
tires, lengths of steel
pipe, and a number of
nails and bolts.  The most
expensive tool required is
a drill.

46p01.gif (600x600)

Although built from inexpensive
materials, the
bellows is quite efficient.
It creates enough
air to make a fire that is
hot enough to melt steel.
It maintains air in a
reservoir so that the
operator must pump the
handle only once every
5-10 seconds.
None of the dimensions of
the bellows is critical.
Some of the dimensions of
other bellows may differ
because differently-sized inner tubes and wood may have been
used.   A few measurements are included in the plans.  These are
included only to give the builder a sense of the approximate
sizes of certain parts.
Carefully read the instructions and list of materials before
you begin construction.  Any questions or comments about the
construction or use of the forge and bellows should be sent to
*   A sheet(s) of wood that is at least 12mm thick.  Plywood is
   the most convenient kind, if available.  There should be
   enough wood to make four squares, each of which is no larger
   than 5cm X 5cm.   The exact size of the squares will depend
   upon the size of the inner tubes used.
*   Two car inner tubes that have been discarded by service stations.
   The tubes should have no major punctures.  Numbers are
   often printed on the inner tubes.   If possible, select inner
   tubes with numbers ending in -13 or -14.  Tubes with numbers
   ending in -12 or smaller might be too small to get enough air
   through the forge.   Tubes with numbers ending in -15 or larger
   are hard to work with.
*   A third, large car inner tube, or part of one.  This will be
   cut into pieces.
*   Four 10cm lengths of metal rod, 6mm in diameter.  Pieces of
   hardwood or bamboo whittled to form dowels of the same size
   may be substituted.
*   Strips of wood 4-5cm wide X 5-6m long X 1-2cm thick.
*   Steel pipe, 2cm in diameter X 50-60cm long.
*   Scraps of galvanized steel.
*   200 nails, 1-2cm long.
*   36 nails, 4cm long.
*   Two 8cm bolts with nuts and washers (optional).
*   Hammer
*   Saw (to cut wood)
*   Tin snips
*   Wood chisel
*   Tape measure or ruler
*   Drill and bit (to drill holes slightly larger than the rods
   being used)
*   Auger bit for 5cm diameter hole (optional)
*   Metal file (optional)
1.   Cut a piece from the center of each
    of the two car inner tubes.   The cut
    sides should be even.  Each piece
    should be wide enough (about 5cm) to
    include both the air valve and the
    thicker rubber around the valve.

46p04a.gif (393x393)

2.   Measure the cut sides of each piece by holding the opening
    as shown.  Stretch each opening by pulling firmly.  Measure
    (x)cm.  Each section has two open ends.  One end will be somewhat
    larger than the other.   You will have four measurements
    when you complete this step.

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3.   Cut four squares from the sheet of wood.  One square should
    be cut to fit each inner tube opening.  For an opening measuring  
    (x)cm, the piece of
    wood should be a square
    measuring (x)/2cm on each
    side.  For example, the
    square cut to fit a 10cm
    opening should be 5cm on
    each side.   Be sure to
    round off the corners so
    the rubber will not rip.
    The corners should be
    rounded as the arc of a
    circle whose center is 1cm
    inside the edge of the

46p04c.gif (437x437)

4.   Arrange the four boards.
    The boards that fit the
    larger openings in each
    inner tube should be placed
    next to each other.   Label
    the boards A, B, C, and D,
    as shown.  Also label the
    inner tube openings to correspond
    to the wood pieces
    that will fit into them

46p05a.gif (486x486)

5.   Draw a line through
    the center of board A,
    as shown, from top to
    bottom.  Center two
    holes close to each
    other on each side of
    board A.  These holes
    can either be chiseled
    squares or drilled

46p05b.gif (486x486)

6.   Cut two rectangular pieces of rubber from the third inner
    tube.  The pieces should be cut so that they lie fairly flat,
    not curled, on the board.   Each piece should be large enough
    to cover two of the holes in board A.   Fasten one piece of
    rubber at one end only over the first pair of holes on board
    A, using tacks or nails.   Position and fasten the other piece
    of rubber over the second pair of holes.

46p05c.gif (540x540)

7.   Draw a line from
    top to bottom
    through the center
    of board B,
    as shown.  Make
    three holes next
    to each other, as
    shown.  Each hole
    should be about
    the same size as
    those in board A.

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8.   Cut one piece of rubber to cover all three holes in board
    B.  Position and fasten in the same manner as in Step 6.

46p06b.gif (486x486)

9.   Draw a line from top to bottom through the center of board
    C, as done with boards A and B.   Make two or three holes in
    board C in about the same position as on board B.  Do not
    cover these holes.
    After finishing this step, boards A through D should look
    like this:

46p06c.gif (393x486)

10.   Make the horizontal support member E.   Use the long strip of
     wood.  Cut one piece that is 40cm longer than the largest
     wooden square.  It should extend about 20cm on each side of
     the largest wooden square (square B or C).

46p07a.gif (600x600)

11.   Cut a few pieces of wood to make a box around the rubber-covered
     holes of board B.   Support member E will form one
     side of the box.  These pieces of wood should all be the
     same width and height.   If possible, they should be planed
     to make them smooth so that the box will be airtight when
     completed.   Make sure the box is large enough so that the
     rubber valve can move up and down freely inside it.

46p07b.gif (540x540)

12.   Make a hole in support member E, as shown.  The steel pipe,
     which will carry air from the bellows to the forge, must
     fit tightly into this hole so that no air escapes.  Make the
     hole smaller than needed.   Then enlarge it as necessary.
     Nail (and glue, if possible) the box to board B, aligning
     the center lines of B and E.

46p08a.gif (486x486)

13.   Place board C over this box, matching the center lines of
     B, C, and E.   Make sure the holes in board C are positioned
     over the box nailed to board B.

46p08b.gif (540x540)

14.   Cut off four pieces, each
     about 3cm long, from the
     end of the remaining long
     strip of wood.   These
     pieces will make tabs to
     attach to squares A and
     D.  Drill a hole through
     the center of each piece
     So that the 6mm metal rod
     fits into the hole.   The
     rod may also be made from
     hardwood or sturdy

46p09a.gif (486x486)

15.   Place two tabs each near the center of boards A and D.  The
     distance between each pair is slightly more than the width
     of the wooden arms, F and G, which will fit between them.
     On board A, the tabs are placed opposite the side that has
     the rubber valve attached to it.   Nail the tabs to the

46p09b.gif (486x486)

16.   Cut a number of lengths of thin sheet metal that are a
     little narrower than the thickness of the square wooden
     board (for example, 10mm if the wood is 12mm thick).  Remove
     with a file any sharp edges along these strips.
17.   Stretch the correct inner tube opening over board B.  Center
     the strip of metal along the board over the inner tube.
     Hammer in short nails at 3cm intervals along the strip.  If
     the nails bend while going through the metal strip, sharpen
     a larger nail and use this as you go along to punch holes
     through the metal.

46p10a.gif (600x600)

18.   Fasten in the same manner the inner tube opening marked C
     over board C.
19.   Fasten in the same manner boards A and D to the appropriate
     inner tube openings.   Make sure that the tabs are facing out
     and are parallel to board E.
20.   Make the vertical support member H. Fasten the horizontal
     support member E to H with either nails or two bolts, as
     shown.  The pieces should
     be easily separated.   The
     length of H can be as short
     as shown below, or can be
     extended into the ground.
     The shorter length makes
     the bellows easier to
     move.  The longer length is
     sturdier.   Note the two
     square wooden spacers in
     the drawing below.   They
     should be thick enough so
     that there is room for arms
     F and G to move freely
     between the two vertical

46p10b.gif (600x600)

21.   Make handle F and
     upper arm G so
     that they appear
     as shown when the
     inner tubes are
     fully extended.
     These two arms
     should fit freely
     between the two
     vertical members
     of H and the two
     portions of the
     tabs.  Insert rods
     through the tabs
     and arm pivot
     points so that the
     arms move freely.

46p11a.gif (600x600)

The pivot points for arms F and G should be halfway between
the extended and compressed positions of the tabs, as shown

46p11b.gif (600x600)

A forge requires a cavity that contains the fire.  It permits
air to be blown in from underneath and does not burn or melt.
This cavity may most
easily be made from
clay.   Make a depression
in the middle of
the clay.   Place the
air pipe from the
bellows inside the
clay so the end of
the air pipe is in
the depression.  The
air will blow from
the bellows, through
the pipe, and into
the depression.

46p12a.gif (600x600)

Rocks or dirt may be used, as shown, if
clay is unavailable.

46p12b.gif (486x486)

This cavity of clay, or of dirt and rocks, is supported either
by a table or by a more solid structure.  The more solid
structure might be a wall made from cement, clay bricks, rocks,
or timber.  The wall is placed around a square area that is
filled with rocks and dirt.  The supporting structure may also
be built from wooden posts, as shown.

46p12c.gif (393x600)

The top surface of this support should be large enough to hold
not only the fire, but also the pieces of heated steel.  If the
surface of the support is too small, the metal being heated
must constantly be supported by one hand.  This is a nuisance.
The most comfortable height for the fire is level with your
hand when you are standing upright with your arm by your side.
Both the forge and bellows should be located under a shelter or
roof to protect them from bad weather.  Traditionally, a blacksmith
works indoors or in a dark area because it is easier to
observe the color of the steel when it is being heated.  The
color of the steel is very important during welding, hardening,
tempering, and working in general.  The color shows the steel
1.   Coal, coke, or charcoal(*) are often used as the fuel for a
    blacksmith's fire.   These burn much cleaner and with a
    higher temperature than ordinary wood.
2.   Make sure there is at least 3-4cm of charcoal between the
    pipe opening and the piece of metal being heated.  Otherwise,
    the air going through the pipe will not have a chance
    to get hot.   It will cool the
    metal rather than heating it.
    Also place some charcoal above
    the piece of metal.   This
    increases the heat of the

46p14a.gif (437x437)

    Charcoal burns fairly quickly.   Do not stoke the fire
    unnecessarily, such as between heatings.   Do not make the
    cavity larger than necessary.
3.   There is no need to pump the bellows quickly.  A full stroke
    every five to ten seconds should be sufficient.  If you want
    more air to go through the
    fire to make it burn hotter
    and faster, place weights
    on the upper inner tube.
    Rocks can be used.   The
    greater the weight, the
    more air is fed through the

46p14b.gif (486x486)

4.   The rubber inner tube nearest the fire must be protected
    from sparks, which can burn the rubber.  Some simple ways of
    doing this are:
    *  Put up a partition between the forge and bellows.
    *  Drape a piece of burlap or cloth over the bellows.
(*) See Making Charcoal:  The Retort Method, published by VITA.
    The metal pipe will get hot if the forge is used for
    several hours.   Don't let the upper inner tube rest against
    this pipe.
Theory of Operation
One of the most important parts of the bellows is the one-way
valve.   It lets air come in from one direction (a), but not the
other (b).

46p15a.gif (600x600)

When using the bellows, the upper tube is initially compressed
and empty.  The lower tube is extended and full of air (c).
Push the handle.  The air from the lower tube is forced up
through the upper valve into the forge (d).  Extra air fills the
upper tube (reservoir).
Compress all the air from the lower tube into the forge and
reservoir.   Then pull the lower tube down.   Air enters from below
to refill the tube.  The air still in the reservoir, compressed
by the weight of the upper board, continues to enter the forge

46p15b.gif (600x600)

The upper tube (reservoir) permits a constant flow of air
through the fire in the forge.  The bellows could be built
without the upper tube, but the air would then flow through the
fire only when the lower tube is compressed.  No air would flow
when this tube is expanded.  Uneven heat would be produced.

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