Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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Default Amazing hollow casting

Hi folks,

Quick question of curiosity here. I bought an old East German scroll saw recently and it has a huge cast iron bow. The reach is about 24 inches. The bow is a one-piece casting, integral with the base, and it's hollow right the way along (there's an air hose inside which blows away the sawdust). Here's a pictu

http://sphinx.mythic-beasts.com/~cdt...ll_Saw_DDR.JPG

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 inside 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
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Default Amazing hollow casting

On 5/14/2020 7:04 PM, Christopher Tidy wrote:
Hi folks,

Quick question of curiosity here. I bought an old East German scroll saw recently and it has a huge cast iron bow. The reach is about 24 inches. The bow is a one-piece casting, integral with the base, and it's hollow right the way along (there's an air hose inside which blows away the sawdust). Here's a pictu

http://sphinx.mythic-beasts.com/~cdt...ll_Saw_DDR.JPG

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 inside 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


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 .
--
Snag
Yes , I'm old
and crotchety - and armed .
Get outta my woods !
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Default Amazing hollow casting

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 .


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".

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Default Amazing hollow casting

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.
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Default Amazing hollow casting

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


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Default Amazing hollow casting

"Christopher Tidy" wrote in message
...

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.

https://en.wikipedia.org/wiki/Core_(manufacturing)

This notorious collapse of a cast iron bridge brought attention to the
difficulties of sand casting and prompted improvements.
https://www.open.edu/openlearn/ocw/m...able=1&id=3524

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.
https://en.wikipedia.org/wiki/Hammer_blow

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Default Amazing hollow casting

On 5/14/2020 9:44 PM, Christopher Tidy wrote:
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.


Then how would you get the rod out of the casting?
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Default Amazing hollow casting

"Jim Wilkins" wrote in message ...

https://www.open.edu/openlearn/ocw/m...able=1&id=3524


This describes the "Burning On" method of repairing a casting:
https://www.scientificamerican.com/a...ast-iron-by-a/

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.


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"Bob Engelhardt" wrote in message ...

On 5/14/2020 9:44 PM, Christopher Tidy wrote:
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.


Then how would you get the rod out of the casting?

============================

Here's another ancient way to make a hollow casting:
https://thelibraryofmanufacturing.co...h_casting.html

My mother's antique pewter teapots showed the resulting rough finish inside
the S-curved spouts.

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Am Freitag, 15. Mai 2020 14:12:09 UTC+2 schrieb Bob Engelhardt:
On 5/14/2020 9:44 PM, Christopher Tidy wrote:
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.


Then how would you get the rod out of the casting?


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.


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Default Amazing hollow casting

"Christopher Tidy" wrote in message
...

Am Freitag, 15. Mai 2020 14:12:09 UTC+2 schrieb Bob Engelhardt:
On 5/14/2020 9:44 PM, Christopher Tidy wrote:
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.


Then how would you get the rod out of the casting?


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.

https://www.giessereilexikon.com/gie...2619cd2ceb9f1c

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Am Samstag, 16. Mai 2020 16:43:31 UTC+2 schrieb Jim Wilkins:

The "chaplets", metal shims that hold the sand core centered, partly melt
and disappear into the casting.

https://www.giessereilexikon.com/gie...2619cd2ceb9f1c


Ah, now I understand. The chaplets only partially melt. Brilliant. Thanks for the answer, Jim!

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"Christopher Tidy" wrote in message
...

Am Samstag, 16. Mai 2020 16:43:31 UTC+2 schrieb Jim Wilkins:

The "chaplets", metal shims that hold the sand core centered, partly melt
and disappear into the casting.

https://www.giessereilexikon.com/gie...2619cd2ceb9f1c


Ah, now I understand. The chaplets only partially melt. Brilliant. Thanks
for the answer, Jim!

===============================

That's what the Wiki said:
https://en.wikipedia.org/wiki/Core_(manufacturing)
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."


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