Forging the Keel Bolts

Silicon bronze is an alloy of copper with about 3% silicon. It has excellent saltwater corrosion resistance, high strength and toughness, and develops an appealing green patina after exposure to the elements. Silicon bronze parts can be fabricated by casting (melting the bronze and pouring it into a mold) or by forging (hammering, bending, etc.) the material into the desired shape. When polished, silicon bronze looks quite a bit like gold.

I made quite a lot of my own silicon bronze hardware. The majority of the parts I made were investment cast (the “lost-wax” casting process). I hot-forged a few parts that have simple shapes, including the keel bolts.

Prior to my retirement in 2013, I was Professor of Physical Metallurgy in the Department of Materials Science and Engineering at MIT. One of my accomplishments there was to re-introduce blacksmithing to MIT in 1984. So I had access to MIT’s forge as well as my department’s foundry and machine shops. I also learned a lot from of some of my colleagues, and thereby acquired a pretty good idea what I should be doing. Still, there was a lot of “learning by doing.” This post will focus on how I made my keel bolts. I’ll describe how I cast some parts in a later post.

Silicon bronze rod with its diameter matching the bolt size is used as stock. In the photo below, the somewhat rusted dog-bone shaped item to the right is a heading tool. There are square holes of two different sizes (5/16″ and 3/8″ in this case) at the ends.

For this carriage bolt I used a piece of 5/16″ diameter bronze rod about 15″ long. The heading tool is to the right of the ruler. It’s forged to shape from a piece of mild steel,  then holes are hot-punched at each end, using a tapered square punch. The punched holes taper a bit, leaving the square hole a bit larger on the bottom side of the heading tool.  The taper makes it easier to insert and remove the bronze rod during the forging operations.

I used a coal-fired forge to heat the very end of the bronze rod to a medium red color. A fan delivers air up through the bottom of the fire and allows it to get very hot (indeed, hot enough to melt steel).

A forge is designed to produce a high temperature and to heat the piece rapidly. Air force up through the fire from the bottom makes for a high combustion temperature, and the moving hot combustion gasses transfer heat to the piece very effectively.

With the end of the rod heated, it is removed from the fire and struck along the rod’s axis with a hammer so as to upset or “mushroom” the end of the rod. This increases the diameter at the end of the rod, to the extent that this part will not pass through the square hole in the heading tool.

The first heat is used to “upset” the end of the rod: you hammer on its end, along the rod axis. This “mushrooms” or upsets the last 1/2″ or so of the rod, giving extra material from which the head of the bolt will be made.

The rod is dropped down through the heading tool leaving the upset end extending a few inches beyond the top of the heading tool, then the upset end is reheated in the forge.

After upsetting the end of the rod, I put the rod through the heading tool before heating a second time. This allows me to move quickly to the anvil and start forming the bolt head, without a delay from having to fiddle with getting the hot piece into the heading tool. I don’t heat the heading tool significantly while the piece is in the fire, but it does heat up some once you start forging hot bronze down into it!

The cold end of the rod is inserted into an oversize hole in the anvil (the “pritchel hole”) and the heading tool is allowed to rest on the face of the anvil. Then the heated, upset end of the rod is struck forcefully with the hammer several times. This serves to both drive some hot metal down through the square hole in the heading tool, and to begin to flatten out the head of the bolt.

The heading tool is at the left on the anvil, and the rod being upset passes through the anvil’s pritchel hole and hangs down toward the shop floor.

It takes me a few heats to finish forging the head of the bolt.

After the first heading operation, the bolt head is starting to take shape and the square “flats” under the head are beginning to form.

When all goes well, after a few heats the bolt head has a slight crown, it’s centered on the bolt shaft, and there’s a nice square at the top of the bolt’s shank, just under the bolt’s head.

It usually takes me at least two heats to fully form the bolt head over the heading tool.

After the forging is completed, the rod can be hammered out of the heading tool from the back side and cut off to give the required length of the bolt. I always did some filing of the square part of the shaft to clean it up and give it nice sharp corners. I usually cut the bolt’s threads using a lathe because it’s quick and I was fortunate to have access to one. I also cut some by hand, using a die of the appropriate size.

A completed 5/16″ x 8″ carriage bolt. When the bolt is installed and snugged down, the square at the top of the shaft gets drawn down into a round hole (in wood or lead) and this locks the shaft of the bolt so it won’t turn as the bolt’s nut is tightened.

Most of the carriage bolts I made were 5/16″ or 3/8″ diameter and fasten through Justine’s keel plank and floors. Eight 1/2″ bolts hold on her lead ballast keel. A few 5/16″ bolts simply fasten the stem and keel plank together.