Amazing hollow casting

Hi folks,
Quick question of curiosity here. I bought an old East German scroll saw re
cently and it has a huge cast iron bow. The reach is about 24 inches. The b
ow is a one-piece casting, integral with the base, and it's hollow right th
e way along (there's an air hose inside which blows away the sawdust). Here
's a picture:
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Can someone tell me how it's possible to cast this iron bow in the foundry?
I can't figure how you can reliably and accurately support a sand core ins
ide the mould, and it's making me really curious.
By the way, if anyone recognises the brand of the saw, I'd be interested to
hear. There isn't a maker's name on the machine.
Best wishes,
Chris
Reply to
Christopher Tidy
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The hole on the front of the top bow is the key . I'll bet there's a similar one on the bottom . Between those 2 points and the large hollow base it would be fairly straightforward to support a sand core .
Reply to
Snag
There are holes for the air hose, both at the top and bottom. Not very big, though. The bottom hole is only about 3/8".
Reply to
Christopher Tidy
I guess maybe the core was built on a steel hoop made from bent round bar, or something like that? Not sure how you hold the back end positioned to 1/8" accuracy, though.
Reply to
Christopher Tidy
Just noticed there's a round protrusion on the top surface of the bow, near the back. Perhaps a bar came out of there support the core? Any foundry experts here?
Thanks for the ideas!
Chris
Reply to
Christopher Tidy
I guess maybe the core was built on a steel hoop made from bent round bar, or something like that? Not sure how you hold the back end positioned to 1/8" accuracy, though.
=============================== Chaplets.
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This notorious collapse of a cast iron bridge brought attention to the difficulties of sand casting and prompted improvements.
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Fortunately stronger rolled structural steel was just then becoming available to displace wrought and cast iron.
Many defects were found in the column castings, including shifted cores that left a thin wall on one side. The foundry supervisor couldn't be interrogated as he had emigrated (fled?) to Australia. Railway management had pushed the designer to cheapen, and thus weaken, this bridge compared to his earlier projects.
Two cylinder steam locomotives are inherently unbalanced. The wheel counterweights are sized to divert the vibration from horizontal which the passengers would feel, to vertical which increases the pounding load on the tracks and bridges. The effect increases with speed, and trains reportedly ran over the Tay bridge much faster than they were supposed to.
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Reply to
Jim Wilkins
Then how would you get the rod out of the casting?
Reply to
Bob Engelhardt
This describes the "Burning On" method of repairing a casting:
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Perhaps the preheated base could be placed in a sand mold of a good foot and filled in with cast iron using acetylene?
I've built up eroded starter motor contacts that way, using brass which melts lower than copper. It also wears away faster but was easy enough to redo. When I was practicing TIG puddle control I piled up a column of aluminum about 4~5mm in diameter and 25mm high.
Reply to
Jim Wilkins
Then how would you get the rod out of the casting?
============================
Here's another ancient way to make a hollow casting:
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My mother's antique pewter teapots showed the resulting rough finish inside the S-curved spouts.
Reply to
Jim Wilkins
I guess you'd have to make some kind of rod framework which could be unscrewed or broken up from the ends. Not sure though. The core positioning just looks really hard, and these guys did it perfectly.
Reply to
Christopher Tidy
I guess you'd have to make some kind of rod framework which could be unscrewed or broken up from the ends. Not sure though. The core positioning just looks really hard, and these guys did it perfectly. ======================================
The "chaplets", metal shims that hold the sand core centered, partly melt and disappear into the casting.
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Reply to
Jim Wilkins
Am Samstag, 16. Mai 2020 16:43:31 UTC+2 schrieb Jim Wilkins:
Ah, now I understand. The chaplets only partially melt. Brilliant. Thanks for the answer, Jim!
Reply to
Christopher Tidy
Am Samstag, 16. Mai 2020 16:43:31 UTC+2 schrieb Jim Wilkins:
Ah, now I understand. The chaplets only partially melt. Brilliant. Thanks for the answer, Jim!
===============================
That's what the Wiki said:
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You may have to append a final ")" to visit the page. "Since the chaplets become part of the casting, the chaplets must be of the same or similar material as the metal being cast. Moreover, their design must be optimized because if they are too small they will completely melt and allow the core to move, but if they are too big then their whole surface cannot melt and fuse with the poured metal."
Reply to
Jim Wilkins

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