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Thread: Casting thin shell bronze blocks

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    Default Casting thin shell bronze blocks

    I've lately been trying to cast a block similar in pattern to the one shown below. These attempts have been notes in my catboat thread but the process will be ongoing for some time to come and it would be better if the efforts were set in their own thread, if only for continuity. I hope to fit more details of the process here.

    For anyone interested in the original posts they can be seen here...http://forum.woodenboat.com/showthre...atboat/page136

    To begin...the block shown below is made for 3/4" line. The blocks I need are all sized for 1/2" line with 2 1/4"sheaves. For comparison I've included a wooden block of the size needed.

    Some scaling down is needed...



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    Quote Originally Posted by Jim Ledger View Post
    I've lately been trying to cast a block similar in pattern to the one shown below. These attempts have been notes in my catboat thread but the process will be ongoing for some time to come and it would be better if the efforts were set in their own thread, if only for continuity. I hope to fit more details of the process here.

    For anyone interested in the original posts they can be seen here...http://forum.woodenboat.com/showthre...atboat/page136

    To begin...the block shown below is made for 3/4" line. The blocks I need are all sized for 1/2" line with 2 1/4"sheaves. For comparison I've included a wooden block of the size needed.

    Some scaling down is needed...




    Hi, Jim.

    Happy New Year to you and Tracey.

    Good to see this broken out as it’s own thread. I will be watching with much interest ( and not a little awe)

    Kevin


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    Default Re: Casting thin shell bronze blocks

    Thanks for your interest, Kevin. Go easy on the awe, you know how I can get when things go well, we all have our struggles.

    I should have taken more pictures in the early stages because I now find myself with little material to work with. Somehow we'll muddle through, though.

    Close examination of the block showed that it was cast in two halves, which were then mated together and bored through for two pins and the axle. This method makes for a much simpler casting operation than trying to cast the whole block as one piece. As it is the molding and casting are fairly straightforward, the pattern being essentially a flatbacked, one-piece pattern with plenty of draft all over. The thinness is what makes the pouring of these half-shells problematic. They must be poured hot so that the molten metal has time to fill the thin sections before freezing.

    This is the pattern I made. It's carved from Cherry, the inside carved while the plank was clamped to the bench, and then the outline cut and the outside carved. In this photo I had alreay cast four pieces, two successfully and two not. They were too thick, though, each one weighing a pound before any cleaning up, much too heavy. After much work carving, sanding, filling and painting we see the pattern being thinned out.


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    Default Re: Casting thin shell bronze blocks

    Jim, this pattern is very cool looking in its own right, right? About how thick (thin) will the bronze shell be finished? Engineering - light enough to do the job but no more??

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    Default Re: Casting thin shell bronze blocks

    (Got my popcorn; settling in.)

    Did you use a following board?

    Alex

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    Default Re: Casting thin shell bronze blocks

    Looks like the start of another great thread, Jim -- Thanks for all your efforts in following the casting process


    Rick

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by gregleetaylor View Post
    Jim, this pattern is very cool looking in its own right, right? About how thick (thin) will the bronze shell be finished? Engineering - light enough to do the job but no more??
    Greg, the pattern was very enjoyable to make, difficult to put down once I got started. I remember being taught that technique in shop class all those years ago...the Walnut candy dish. Different times.

    The thin part of the shell will finish out around a sixteenth of an inch, or thereabouts. I pinch the shell between two fingers to gauge the thickness, eyes closed while imagining the gaff and sail full of wind whipping over in a horrific jibe, "prolly go a bit thinner yet".


    Quote Originally Posted by Pitsligo View Post
    (Got my popcorn; settling in.)

    Did you use a following board?

    Alex

    The bottom board was pierced for the two protruding lugs, allowing the pattern to sit flat for molding. But I'm getting ahead of myself.




    Quote Originally Posted by hawkeye54 View Post
    Looks like the start of another great thread, Jim -- Thanks for all your efforts in following the casting process


    Rick

    This should be an interesting thread, Rick, I hope you enjoy it.


    Here's the start of the pattern. The outline was traced from a paper template, four holes drilled to establish some sort of depth and we're off.


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    Default Re: Casting thin shell bronze blocks

    While I appreciate you love a challenge Jim and like nothing more than banging your head against a wall, just wondering if you considered the wooden block route at all?

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    Default Re: Casting thin shell bronze blocks

    1/16". Wow, you have some major changes in thickness and some very thin areas. I think that you will need a very fluid alloy, and it may be a challenge to cast without cracking. If it does want to crack, it might be easier to cast it thicker on the outside and grind down.

    In my vast experience of having personally cast 2 bronze objects that didn't work out very well... take my advice with a grain of salt.
    https://maritime.org/doc/foundry/
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    Default Re: Casting thin shell bronze blocks

    Sounds awesome

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    Default Re: Casting thin shell bronze blocks

    Manganese bronze? My understanding is that it flows especially well.

    Alex

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by andrewpatrol View Post
    While I appreciate you love a challenge Jim and like nothing more than banging your head against a wall, just wondering if you considered the wooden block route at all?
    We all create our own pain, Andrew, it's what we manage to squeeze out of it that counts. But, yeah, I've considered wood blocks. They would have cast bronze tangs, Lignum Vitae shells, custom bronze roller sheaves, each one a unique little masterpiece.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by MN Dave View Post
    1/16". Wow, you have some major changes in thickness and some very thin areas. I think that you will need a very fluid alloy, and it may be a challenge to cast without cracking. If it does want to crack, it might be easier to cast it thicker on the outside and grind down.

    In my vast experience of having personally cast 2 bronze objects that didn't work out very well... take my advice with a grain of salt.
    https://maritime.org/doc/foundry/
    http://www.totalmateria.com/Article64.htm

    Thanks for the links, Dave. I have that Navy foundry manual on my shelf and find it very useful, if intimidating. As a beginner I find that the too much information barrier is never very far away. I'm more comfortable operating in a limited fact environment. Pour the thing and then look up what went wrong.


    Although there are differences in thickness in the shells they are rather small castings that freeze quickly, so the opportunity for stress is limited. Further, the transitions between the thicker sections and thinner are beautifully rounded and faired, which does away with one major cause of stress cracks, abrupt, angular changes from thick to thin.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Hreoaj View Post
    Sounds awesome
    Totally.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Pitsligo View Post
    Manganese bronze? My understanding is that it flows especially well.

    Alex
    Silicon bronze. It's the easiest pouring bronze, and at this point I need to not make things more difficult than they have to be.


    Here's the pattern, cut free from the plank, roughing out the outside of the shell...



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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Jim Ledger View Post
    We all create our own pain, Andrew, it's what we manage to squeeze out of it that counts. But, yeah, I've considered wood blocks. They would have cast bronze tangs, Lignum Vitae shells, custom bronze roller sheaves, each one a unique little masterpiece.
    Well said Jim. I have explained to some that my time sailing will not even approach time building. Some know why that doesn't matter. I wonder how we have the gifts we do. It is my great gratification to see yours, theirs and to add mine...

    Happy New Year!

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    Default Re: Casting thin shell bronze blocks

    I'm mucking around with some patterns for bronze frames for wood shelled blocks and bronze bushed sheaves at the moment, good fun.

    On the shell design the main problem of very thin castings seems to be freezing of the metal as it fills the void and with manganese bronze, shrinkage causing depressions in thicker sections like the axle housing in the centre of your shell. It might be worth allowing a bit of extra outside of that area to feed the thin shell, something that could be ground of easily.

    I love your carved pattern, well beyond my wood butchering skills!
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    Default Re: Casting thin shell bronze blocks

    Oops, I just saw the post above #15. I had assumed manganese bronze but have no experience of silicon bronze so have no idea of it's shrinkage.
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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by gregleetaylor View Post
    Well said Jim. I have explained to some that my time sailing will not even approach time building. Some know why that doesn't matter. I wonder how we have the gifts we do. It is my great gratification to see yours, theirs and to add mine...

    Happy New Year!
    Happy New Year to you, Greg, and thanks for the thoughts and kind words.



    Quote Originally Posted by PeterSibley View Post
    I'm mucking around with some patterns for bronze frames for wood shelled blocks and bronze bushed sheaves at the moment, good fun.

    On the shell design the main problem of very thin castings seems to be freezing of the metal as it fills the void and with manganese bronze, shrinkage causing depressions in thicker sections like the axle housing in the centre of your shell. It might be worth allowing a bit of extra outside of that area to feed the thin shell, something that could be ground of easily.

    I love your carved pattern, well beyond my wood butchering skills!

    Feel free to post your work here, Peter, it's always instructive and a pleasure to see.

    The thin parts of the shell is certainly the most difficult part of the casting to cast successfully. The small volume of metal spread out thin wants to freeze very quickly indeed

    Silicon bronze is a most forgiving metal to cast, so it's a good choice to work out the casting procedure, there being enough difficulties with the gating, molding and pouring.

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    Default Re: Casting thin shell bronze blocks

    This is the pattern in the bottom half of the flask ready for the molding sand. The circular pattern is for a runner, the channel which supplies the molten metal to the pattern





    Here is the casting setup. That's a homebuilt propane fired crucible furnace capable of melting about thirty pounds, although these pours will be about five pounds each. There's two flasks waiting. The large hole is the sprue, down which the metal is poured. The three smaller holes are risers, which vent the mold and provide fluid metal to the cooling casting. The pipe contraption is a pouring shank, which holds the crucible while pouring.



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    Default Re: Casting thin shell bronze blocks

    I've considered making that style of block but had thought to make the two sides together using a sodium silicate core for the opening . The two separate shells allows different thickness sheaves to be used though, probably a better idea..
    '' You ain't gonna learn what you don't want to know. ''
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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Jim Ledger View Post
    Thanks for the links, Dave. I have that Navy foundry manual on my shelf and find it very useful, if intimidating. As a beginner I find that the too much information barrier is never very far away. I'm more comfortable operating in a limited fact environment. Pour the thing and then look up what went wrong.


    Although there are differences in thickness in the shells they are rather small castings that freeze quickly, so the opportunity for stress is limited. Further, the transitions between the thicker sections and thinner are beautifully rounded and faired, which does away with one major cause of stress cracks, abrupt, angular changes from thick to thin.
    It has been my experience that after all the computerized solidification analysis, etc, that is how it works. I'm not saying that they don't save a few rounds of moving the gates and risers around on a really complex part. If you don't expect to eat the first few castings, you are a manager who would rather complain about cost and schedule than let the engineers get the job done.

    Untitled3.jpg
    Can't say why 872 is missing data. The text says it is now 873 (Everdur) and liquidus is 1510F. 94 Cu, 4 Si 1.1 Mn.
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    Default Re: Casting thin shell bronze blocks

    I've considered making that style of block but had thought to make the two sides together using a sodium silicate core for the opening . The two separate shells allows different thickness sheaves to be used though, probably a better idea..
    Casting the shells in two halves also allows you to cast a separate center divider and make up doubles or triples with the same cheeks.

    Alternately, lengthen the bosses on the insides of the cheeks to be the full width of a sheave, and you could use a simple plate of sheet bronze as the center divider on doubles. Mill up spacers the width of a sheave, and cut out more dividers, to create triples or quads. Grind the bosses short --half the width of a sheave-- for a single. I'm thinking in terms of production work, of course, but you can see why the originals were cast up as they were.

    Very much enjoying the thread, Jim.

    Alex

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    Default Re: Casting thin shell bronze blocks

    Blocks , treble and single.jpg Port Townsend Foundry blocks , illustrating Alex's description.
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    Default Re: Casting thin shell bronze blocks

    I really like those PT Foundry blocks. They're probably lighter than the polished shell you're making Jim.
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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by PeterSibley View Post
    I've considered making that style of block but had thought to make the two sides together using a sodium silicate core for the opening . The two separate shells allows different thickness sheaves to be used though, probably a better idea..
    The core would have to be a complicated and very precise thing, Peter, if the inside of the shells was to be hollowed out like these, not that it couldn't be done. Once cast there would be no way to clean up the insides.

    There are advantages to casting half shells aside from the ease of casting. As we'll see there can be a variety of attachments made to fit on either end of the block, parts to hold shackles in the correct orientation, attach beckets and gaff saddles, and so on. These parts will be in placed before the halves are pegged together and captured between the two halves.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Sailor View Post
    I really like those PT Foundry blocks. They're probably lighter than the polished shell you're making Jim.
    A bit too agricultural for me. Lack elegance.
    I'd also be worried about the rope chafing where it enters and leaves the swallow rubbing those hard edges.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by MN Dave View Post
    It has been my experience that after all the computerized solidification analysis, etc, that is how it works. I'm not saying that they don't save a few rounds of moving the gates and risers around on a really complex part. If you don't expect to eat the first few castings, you are a manager who would rather complain about cost and schedule than let the engineers get the job done.

    Untitled3.jpg
    Can't say why 872 is missing data. The text says it is now 873 (Everdur) and liquidus is 1510F. 94 Cu, 4 Si 1.1 Mn.


    The foundrymen probably have a few laughs and then use the computer analysis to light the burner.

    Seriously, though, at this backyard casting level the methods are so crude, the necessary experience so lacking and the conditions so variable that plain luck is the biggest variable.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Pitsligo View Post
    Casting the shells in two halves also allows you to cast a separate center divider and make up doubles or triples with the same cheeks.

    Very much enjoying the thread, Jim.

    Alex
    Thanks, Alex, glad you're enjoying this.

    The pattern for the center divider for the double blocks has yet to be made. It will simply be a pierced flat plate with lugs on either side to match up to the shells.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Sailor View Post
    I really like those PT Foundry blocks. They're probably lighter than the polished shell you're making Jim.

    Thanks for the picture, Peter. Beautiful work, as usual, from the Port Townsend Foundry.

    Dan, I wouldn't bet on it. The one I'm finishing up feels pretty light...and smooth!

    Jim

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by Peerie Maa View Post
    A bit too agricultural for me. Lack elegance.
    I'd also be worried about the rope chafing where it enters and leaves the swallow rubbing those hard edges.

    Thank you, Nick. You've saved me from having to run out and make a new pattern.


    Here's the result of the first two pours. It's fortunate that these two worked out because I had a string of failures following. As it turned out, though, the shells were too thick, making them too heavy. I could have ground off the excess but thought it best to thin the pattern to save doing the work multiple times.

    This might be a good time to introduce some foundry terms to those interested. The cone shaped bits are the sprues, the passage into which the metal is poured. The oval track is called the runner, which distributes the metal from the sprue. The four tabs on top of the runner are the ingates, which channel the flow into the mold cavity itself. The three chimneys are the risers, which act as vents to allow air to escape from the cavity and the fill with metal. The risers stay fluid until after the piece has frozen and provide fluid metal to the piece as it cools. Because the metal shrinks as it cools there must be a reservoir of fluid metal to replace the volume lost to shrinkage. If there is no riser the casting will feed off itself, leaving hollow spots.

    Last edited by Jim Ledger; 01-10-2018 at 09:45 AM.

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    Default Re: Casting thin shell bronze blocks

    Jim, I have zero casting experience, but couldn't you heat the sand / mould In an oven to help keep the metal fluid? Might help with the thin sections...?

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    Default Re: Casting thin shell bronze blocks

    You're just having too much fun there, Jim. Fun to watch too.

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    Default Re: Casting thin shell bronze blocks

    Quote Originally Posted by lupussonic View Post
    Jim, I have zero casting experience, but couldn't you heat the sand / mould In an oven to help keep the metal fluid? Might help with the thin sections...?
    The molding sand needs to be cool to achieve the firmness needed to hold the impression of the pattern. You need to be able to squeeze a handful of sand and have it hold the impression of your hand, while breaking cleanly when snapped in half. As the sand warms this quality degrades.

    The trick is to pour at the correct temperature, which in this case would probably be about 2050 to 2100 degrees, just guessing, but that's the high end of a silicon bronze melt.

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    Default Re: Casting thin shell bronze blocks

    For boat parts I prefer the patina of Manganese bronze, a lovely and stable blue/green. Silicon bronze (Everdur) seems to go to a brown/red color and leaves bits of itself in flakes all over the deck. Unfortunately I was gifted several hundred pounds of silicon bronze, so I will just put up with it...

    Knowing that you are going to be polishing and oiling this stuff daily, it shouldn't be a problem Jim.

    (I have some "White bronze" ingots also (maybe 200 lbs), now what on earth am I to do with that?
    Might make a good anchor...

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