Tuesday, August 14, 2012

Tech Tip Tuesday

One of the things you learn pretty quickly when you start building boats - and I mean any kind of boat - is that you must become a skilled tradesman at many different things.  The only thing that I have been unable to make on my own boats (other than weaving fiberglass or mixing epoxy and varnish...) is hardware.  Bronze hardware, that is.

I have long desired to brush up my casting skills.  When I was in high school, I was fortunate enough to go to a public high school that still had a "manual arts" program which included drafting, woodworking and metal working.  As a freshman, you could take a class that was structured so that you got an introduction to all three.  In the metal shop, you learned some sheet metal work, aluminum casting, lathe and milling skills along with some basic welding and brazing.  Pretty cool, really.  I must say, when the high school was renovated, they removed the metal and wood shops - probably due to cost and liability.  I think that was a huge mistake for kids who would go on to study engineering disciplines in college.  Anyway...

The WoodenBoat School in Brooklin, Maine offers some great classes - if you have any interest in boats, boating or related arts and crafts, you owe it to yourself to go there.  It's summer camp for adults.  One of the classes they offer is a bronze casting class that is taught by Sam Johnson.   Sam got his start in casting when he wanted to get a copy of a port light that he had on his boat.  He approached a foundry regarding a copy of the port light and was appalled at the price they wanted.   He taught himself how to cast bronze and replicated the parts himself - learning how cheap the original quote truly was in the process. 

I'd seen some of the results of Sam's classes when I was at WoodenBoat before, seen his demonstration at Mystic Seaport during the WoodenBoat Show and had heard good things from previous students of his.  Back in December, when the school's catalog came out, I made up my mind that this would be the year for me to work on those skills.

The class was a wonderful hands-on class where we were learning the skills to sand-cast bronze hardware.  For the uninitiated, sand casting is a process by which you use a master pattern to create a part-shaped void in special oil sand with risers and gates to allow you to pour molten metal into the void.  When the metal cools, you have your part.

On our first day, we started out learning about how the process works - from a very basic perspective.  We started with instruction regarding safety in the foundry.  It is imperative to wear leather shoes, long cotton pants, safety glasses, welding gloves and preferably, long-sleeved cotton shirts.  The temperatures required to pour bronze are around 2,200° Fahrenheit (just over 1,200° centigrade for those working in the metric system) - really, really hot.  The other important safety tip is to make sure that any metal or crucibles are both dry and hot before going into the furnaces - moisture at that temperature will flash to steam and can cause a steam explosion, splashing molten metal out of the crucible.  We then delved into the construction of home-made propane-fired furnaces, tongs, crucible holders, and other items that were used in the process.  With the exception of the petro-bond sand (Graded sand that is coated with clay and then oiled to make it stick together), crucibles (Ceramic flask in which the metal is melted) and a few special items, pretty much everything was hand made or readily available in local stores.

Sam then showed us how to make the mold for a simple part.  The part looks basically like a bronze tuning fork and is used in the molding process with an awl to gently tap the patterns, gates and risers out of the sand. 

The first thing you do is to place a small piece of plywood, just larger than the base of the flask (The two-part wooden frame used to hold the sand.) on your bench.  You then separate the cope (top of the flask) and the drag (bottom of the flask)  Putting the cope aside and then flipping the drag upside down on the board (the bottom must be facing you for the process to work), you then put the pattern (Wood, plaster or plastic copy of the part to be molded), or part to be molded, on the surface of the board along with a piece of wood to form the runner (A channel for the molten metal to flow inside the mold).    Placement can't be too close to the edge or too close together to avoid having the bronze push out through a sidewall or burn the wooden flasks that we were using, or having the runner bleed into the part. 

You then apply a parting compound - a powdery substance used as a mold release to let the patterns and runners be easily pulled from the mold.  It also keeps the sand from sticking to itself.  We used both Calcium Carbonate and Corn Starch depending on the conditions.  Sand was then sieved in using a riddle (a box with a fine screen on the bottom) to get good detail of the parts.  More sand was then shoveled in and rammed into place with a piece of steel bar.  The sand should be firmly packed, but not to hard to allow gasses to escape.  This helps to yield a good surface finish.  Once the drag was slightly over-filled, a stick was used to strike off the excess sand yielding a flat surface on the bottom of the drag. 

The drag was then flipped over using the plywood to support the sand.  The cope was installed to complete the flask.  A gate block (Basically a small runner to connect the part to the runner.) was then put on top of the runner and touching the top surface of the part to connect them.  More parting compound was applied and a riser (A tapered piece with a square cross-section used to feed molten metal from the outside world to the runner.  The riser has this shape to avoid swirling of the molten metal that would erode the sand, contaminating the part.) was held against the runner and more sand was riddled onto the parts and more sand shoveled and packed into place.  The sand was basically packed in until it reached the top of the cope.  A small area was scooped out adjacent to the riser with a melon-baller to provide a "pouring cup"  The "tuning fork" was then used to tap the riser loose and it was gently pulled straight out of the sand.  Loose sand was blown away and sharp, fine edges of sand were removed.  The cope was gently removed and set aside exposing the gate, runner and part.  An awl or gimlet was inserted into each part and tapped gently to loosen them in the sand and then they were removed.  Care was taken to blow out loose particles of sand and to keep sharp edges around the part.  This helps to yield a high-quality part without "flash" at the edge of the part. 

The cope was then re-installed on the drag and they were clamped together.  This is important as the metal is so dense, when you pour in the molten bronze, the cope can float off the drag spilling hot metal everywhere.  (Don't ask me how I know this...)  Supporting the assembled sand mold on a piece of plywood, Sam brought it over to the casting area.  Removing the top from the furnace, he used a scoop to remove slag (Metal oxides, sulfides and contaminants.) from the crucible.  Picking up the tongs, he pulled the crucible from the furnace and in one smooth motion, poured the bronze into the pouring cup and filled the mold.  After a few minutes, he broke of the pouring cup (to return to the crucible)  and after a few more minutes broke open the mold to reveal the new part, still attached to gate, runner and riser.  Some quick sawing and sanding yielded the finished product.

This simplifies the description of the process somewhat as there are some "tricks to the trade".  I won't reveal them all - you should go learn for yourself if you're interested!  Basically, the rest of the week built on that process - how to make patterns and how to get complex items out of the sand  - parts with undercuts, odd shapes and the like - the real challenge is how to get them out of the sand.  Also, the metal shrinks as it cools, so you have to deal with this when recreating parts that require close dimensions.

Here's some pictures of a more complex part - a jib ring - being made. (No, the person shown isn't me or Sam, he's a fellow student.)

Original Forged Iron Jib Ring

Drag with Part and Runner

Packed Cope with Riser and Pouring Cup

Prepared Mold Shown Open

 Furnace and Pouring Area

 Removing Slag

Hot Crucible! 

Pouring Bronze

Cracking the Mold Open

 Part with Gate, Runner and Riser

As I've noted, we got lots of practice casting and made quite a few parts.  Pulleys, a mermaid fid, a sounder and many others.  Some folks brought parts or patterns they wanted to reproduce.  One fellow brought a pattern for a bracket to hold a signaling canon that he'd made to his boat's rail.  He hadn't known enough about pattern making and the gate for the part was problematic.  Sam suggested a horn riser and described what it was and how it worked.  The student went and made it proceeded to successfully pour the part.  Sam later told him that it was something that he, himself hadn't tried, but had read about! 

One of the major reasons that I went was to try to make a set of pad-eyes to mount on the canoe decks.  Ultimately I want to create something that's a little bit fancier than this, but it was a good first try.  Here's the pattern:

In the top right is a core-box.  It's used to create a sand shape to fill the cavity left when the master pattern is removed from the sand.  The leaf-looking thing is the master pattern with a half-round shape that will be filled by the core.  The two smaller pieces are the bosses which attach to the back of the master pattern in the top half of the mold.  The bosses would later be tapped to allow for machine screws that would hold the part from underneath the deck.  When the part was removed from the mold, it looked like this:

After removing the runner and gates, I had this:

After more clean up and a bit of polishing, we had these - not looking bad, eh?

We spent a fair amount of time learning how to finish our parts as well - sanding, grinding, filing and pollishing the parts.  We also learned a bit about applying various patinas to the bronze. 

I'll end the post like the end of every day - with the pouring of ingots to empty the crucibles:


isculpt said...

I have been trying to find Sam Johnson to get some photographs of his sand casting process for my book on 3D technology in Fine Art and Craft. Do you know how to reach him? If I can't find him do you have photographs that I can share? THanks in advance,

Bridgette Mongeon

Canoez said...


Sam is the head of the Columbia River Maritime Museum in Astoria Oregon and can be reached there. If you can't reach him, I have some pictures that I'd be willing to share, but not many. Sorry for the delayed reply!