- a mandrel of the appropriate diameter (7 & 8 mm shown)
- a bottom spacer (same dia. as the mandrel)
- an upper plate
- a back spacer (the brass article)
- a well polished master propeller
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Then build the outer box for the first casting:
the
female mold,
and obtain a suitable base plate for mounting all
the hardware on.
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The base plate can be
reused for
more molds, provided it is strong, flat, and well waxed. This one shows that it was, several times. |
Slide the mandrel into the propeller, front side
up, then the bottom spacer onto the mandrel,
and the mandrel through the base plate in this
fashion:
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Yes, the prop is not
the same as
the one in the whole story, because I screwed up the original photo
–sorry. Anyway, this is also to show that a master prop can be readjusted if necessary before copying. This one in particular, had the hub reshaped and an aluminium front plate fitted to make things really nice and square, and to cancel usage marks. Propeller courtesy of Parent & Kelly (circa 1974) |
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Securely
fasten the
whole lot to the base plate by suitably packing the protruding stem of
the mandrel (with the bottom spacer, or anything else, as in this case)
and tightening
the 4 mm socket screw (you did tap the hole, in the mandrel, didn’t you?) Aligning the propeller
with the
marks you will have traced on the base plate will help afterwards,
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Once satisfied with all this, start packing up
the
back of your prop with plasticine,
taking care not to warp the blades upwards by
packing
too tight:
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Then scrape away the excess plasticine by running a 20 degree template all around the leading and trailing edges of both blades (unless you are making a single blade...) The tricky story will begin around the hub, when the template switches from a trailing edge, which is on the bottom, to a leading edge, which is on the top. The plasticine
will be
reduced to an extremely thin blade, which will then be the same thin
resin
blade on the male mold and I always thought it would break, therefore I
prefer to make the transition by packing up more plasticine and trying
to guess the correct separation line. |
In the end, you should come up with something like this:
| Once you are satisfied with the finish of
the plasticine,
and convinced that there are no undersquares, it is time to give a good
coat of your favorite release agent, including all of the base
plate.
And maybe another, after a while. Be extra careful at this point: the release agent will bring your precious master prop back to you, when all this mess is over! |
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Next, when the release
agent is
nice and dry, and polished if necessary, mount the upper aluminium plate on top of the mandrel cone with another 4mm socket head screw (you tapped that hole too, hopefully…). |
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To give
access to
the screw once everything is buried solid in resin, you’ll have to
install
a piece of tubing over the screw head, long enough to protrude over the
edge of the box.
Fasten the tube from the inside with a well packed blob of plasticine. |
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| After all this, you might as well fasten the box to the base plate with 4 wood screws driven through from the bottom of the base. A good idea is to pre-drill both pieces before starting with all the plasticine stuff. Don’t forget to mark the relative positions! To save resin I usually cyano some blocks of scrap wood inside the box, about 5 mm below the edge. Personally I don’t believe in making the box much shallower than about 50mm, as this will be the part of the mold in which most of the work will be done while making the props, therefore I like it to be rather solid and stable. Once ready for the first casting, the inside of your job will look like this: |
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The scalpel marks on
the walls
of the box are there to give a better grip to the resin.
Actually they are just useless: that damn’ stuff will grip to about anything! |
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While on
the outside,
it will be better to seal the joint
between box and base with some nice fillets of finger-smoothed plasticine. It is very unlikely that
the resin
will seep through, |
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