Welcome to the world, keel halves!

Okay, so it wasn't like giving birth.  But it's been a long time coming.  We really didn't know if it was going to be difficult, how it was going to be difficult, and how it could all go horribly wrong.  THAT'S just like kids, right?

Caution: Long post.  This is one of the few subjects about building boats that I have some prior and pretty intimate knowledge of.  Read at the risk of extreme (hopefully not terminal) boredom.

We needed to build the keel halves.  If you can find a big chunk of lead, you could conceiveably machine it.  Finding that chunk is hard, so you don't hear much about machining lead.  Mostly, since lead has a pretty low melting point, you hear about casting lead.  In bullets, in all kinds of non-load bearing weights, they're usually cast lead.  Since these bulbs/half bulbs will be bolted to a steel fin that will take the load of the keel bulbs, and the keel bulbs just have to keep themselves from falling apart, we'll cast them.  Lead is the metal of choice for us.  This is because the class requires it.  It's a good choice though, because it's cheap, it has a low melting point, and it's dense, so for a given weight, it can be small.  Metals have been cast in sand molds for a really, really long time.  I don't know how long, but really long.  I'll leave that up to Google and the curious.

Quick primer on sand casting...  Pretend you're at the beach.  Take a bucket.  That's the flask.  Fill it with sand.  That's the sand.  If you're at the beach, and you choose properly, the sand is a little wet, probably with water.  The water is the binder, and holds the sand in a shape, right?  Well, push your hand into the bucket and remove it.  Your hand is the pattern, and the hole left in the sand is the cavity.  If you poured jello into the cavity and stuck it into the refrigerator, you'd have a jello shaped hand in a few hours, right?  Well, the jello is the casting.  What is the refrigerator?  I don't know...  A refrigerator?  All analogies are partial.

Most sand castings are made in "green sand".  Green sand has a little bit of clay (the binder, like the water in the example) in it that helps keep its shape.  We don't have flasks that are used with green sand molds.  We could build them, but that's a bunch of time that we don't need to spend.  We're working with a real foundry (business that makes castings-they DO NOT MAKE METAL-they buy metal ingots of the right alloy and melt them for casting) that will make molds for us.  They are going to make us "hard sand molds" with a binder that "sets" and then you can remove the flasks.  The binder is an air-setting resin.  There are also other kinds- heat set, gas set (where you "soak" the sand in a gas like CO2 and that hardens the resin) and I'm sure some others that I don't know about.  Unfortunately, it being a business, we couldn't really walk around and snoop and take pictures and generally make a nuisance of ourselves while they're making molds...  They have a business to run.  Let's just assume that it happened.

To start at the beginning of this chapter, Hull 79 built the pattern for our keel bulb halves.  I have about 15 years of experience in building tooling for sand casting, so it only makes sense for him to learn the fun of wood chips flying off of a pretty large mass of spinning poplar.  You can see his chronicle of building the master here and gluing another bit to it and mounting it to a board here.

The upshot of all of this is these...

Drag side or bottom of mold cavity

Cope side or top of mold

The top (or cope) side of the mold has a couple of large holes.  These are used for pouring the material in and for venting gases and air out as the lead fills the cavity.  They also act as risers.  When a part cools, it shrinks, as discussed before.  A riser is excess material poured into the mold that can supply hot/liquid material to the part as it cools, so the part doesn't form a void as the material shrinks.  It's strange, but the physics works out that the top of the mold will FLOAT on the mold when it's full of lead.  If the top of the mold floats, then the lead isn't constrained any longer, and leaks all over the place.  (I believe that this is known as foreshadowing.)  To avoid this, the molds can be glued together with some magic glue that almost works (also foreshadowing).  The molds can be clamped together with normal bar clamps and this can help avoid the leaking problem too, as long as one remembers to clamp them together (more foreshadowing).

Chunk of wood and cardboard placed over holes in mold to keep stuff from falling in before filling.

When pouring mold 1, we didn't clamp anything, and the glue held and it worked great.  Pouring mold 2, the cope floated, the glue didn't hold and we had a leak.  No harm, no foul, but the casting had some extra flash, and it made a little bit of a mess.  All in all, crisis averted.  We did clamp the rest together when pouring, and had no further pouring problems.

In casting, you usually don't pour directly into the cavity of the part you want.  This is because the metal falls with some velocity from the ladle, and this causes the lead to "burn in" to the mold.  <sigh> Foreshadowing.

All metals shrink when pouring from liquid.  This isn't a big deal.  When you're pouring, you just keep pouring until the mold is full.  As it cools to the metal's freezing point, you just keep pouring to keep the mold full.  The problem is from the freezing point to the temperature that we live.  Once the material is solid, there's no way to keep the lead from shrinking, so the part winds up being a little smaller than the mold.  This also isn't a big deal.  The amount of shrink is known, and can be calculated.  Lead shrinks about 2.6% from its freezing point (roughly 370C) to room temperature.  If you make the pattern that much bigger than the desired part, the finished part will be the right size.  Of course, this is all dependent that your alloy is perfectly 100% what you think it is..  This is also foreshadowing.

As cast, after cutting off risers/gates

After cleanup

Once we knocked the castings out of the molds, we found two things...  

The first is why you have a gating system that is sacrificial.  Pouring directly into the mold cavity through a riser caused the material to lead to burn into the mold.  What happens is that the sand is eroded a little, mixes with the lead, and the mixture settles into the eroded area.  If there is a gating system, the lead "oozes" into the cavity and any burn in happens in the gating area and doesn't matter because the gates are cut off and recycled.  This non-homogeneous material is a mixture of sand and lead.  It can be filed/scraped/wire wheeled off down to clean material, but it's going to take some effort.

The second is that the lead we used wasn't of known origin.  Some of it was just in the pot that we borrowed and some of it was salvaged from a boat that was scrapped for some parts, but mostly scrapped.  I think that some of the lead had some other stuff in it (possibly, but not limited to, antimony, zinc, and about twenty other possible elements).  There shouldn't be any iron or some other things because it doesn't melt in solution with lead, so things like screws, nails, etc. just float and you skim them out before pouring.  We didn't have any problems with impurities this way, but the halves came out about 20mm too long.  It shrank in an unpredictable way.  Again, not a disaster, but a problem that requires effort to fix.  Fact is that we're tight on time to make the start line in October.  We didn't need something else to spend our time on.  But, it's where we are, so we'll fix it.

Our castings all came out in the 75kg range, and we'll make the shape correct relative to the (already built) keel fin.  Then we'll drill them using the keel fin as a template (using a magnetic drill to drill into the lead).  Turn the lead bulb half over, counterbore for bolt heads, then weigh.  Take off the appropriate amount of material to hit the target weight, and they'll be good to go.