Subject: Re: repairing pot metal and cast iron

From: [email-masked] (Rich Feldman)
Date: Tue, 27 Jul 1993 02:37:20 GMT

[email-masked] (James W. Swonger) wrote:

>  You might oughtta be sure this piece isn't cast aluminun, not pot metal.
...
> Maybe somebody here has a good simple test for telling one from the other?

Specific gravity is one clue -- Aluminum alloys are typically 2.5 to 3, 
zinc alloys are up with iron and steel at about 7 (with respect to water).
If you don't trust your judgement simply looking at the part and hefting it,
it's easy to measure the sp. gr. (actually the weight of water displaced)
if you can immerse the part in a bucket (or lake!) of water: 
* measure weight or volume of H2O that spills over
* measure weight or volume of H2O to refill the bucket
* suspend part from scale and measure loss of weight when under water
* set bucket on scale, measure increase in weight when part is immersed

You can also test it chemically.
Aluminum is rapidly etched by (fizzes in) a lye solution.
Zinc alloys are rapidly etched by hydrochloric (muriatic) acid solution.
(this can be handy for de-galvanizing steel parts in areas you want to weld).

> Pot metal is cheesy stuff and I don't think it's the material any 
> reputable manufacturer of heavy duty equipment would use. 

IMHO that depends on what parts they use it for.  Virgin zinc alloys 
such as Zamak allow die-casting of parts with thin sections, excellent
surface finish and dimensional accuracy, and well-controlled physical 
strength (for example, laser printer base plates).
All bets are off when a foundry throws a heap of random zinc-based
scrap in the pot.  The same goes for lightweight aluminum-based "pot 
metal"; brasses and bronzes are even more sensitive to contamination
of the batch by a small piece of an incompatible alloy.

-- 

Rich Feldman		Megatest Corp., San Jose, CA 95131
[email-masked]  	{decwrl,sun}!megatest!rich
home (415) 967-6966	work (408) 441-3027

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From: [email-masked] (f.w.kerfoot..iii)
Date: Tue, 27 Jul 1993 17:06:17 GMT

>From article <[email-masked]>, by [email-masked] (Rich Feldman):

While we are on the subject, does anyone have methods, or reference to
methods, of identifying copper alloys, even into general types? Just
being able to determine if the alloy contains significant zinc, or lead,
for instance, would be a big help. A more detailed analysis technique
would be even nicer, if it could be done at home with relatively simple
techniques. I believe that some sorting can be done by color, but that
doesn't work well for me, since I am red/green color blind.
McMaster-Carr has a kit of chemicals for about $500 to identify brass
and bronze, but that is a bit rich for my taste.

Frank Kerfoot
[email-masked]

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From: [email-masked] (Robert Mitchell)
Date: 28 Jul 93 17:37:14 GMT

>> (this can be handy for de-galvanizing steel parts in areas you want to weld).

Along these lines, does anyone have any good methods for selectively	
disolving small broken iron screws from brass, as in clock plates?
Also the reverse; I have some broken brass stuck in a VERY small
high carbon steel screw plate.  Too small for a jeweler saw...
 - Thanks - Robert Mitchell

-- 
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From: [email-masked] (bill nelson)
Date: 28 Jul 93 20:21:46 GMT

[email-masked] (Robert Mitchell) writes:
: 
: Along these lines, does anyone have any good methods for selectively	
: disolving small broken iron screws from brass, as in clock plates?
: Also the reverse; I have some broken brass stuck in a VERY small
: high carbon steel screw plate.  Too small for a jeweler saw...

Hm, I have not checked the way the metals would react to various acids.
However, brass tends to be far more reactive than iron, so it is likely
to be rather difficult to remove iron from brass.

It should be possible to remove brass from an iron plate, by using the
different reactivities of the metals.

Bill

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From: [email-masked] (David Jenkins)
Date: 29 Jul 1993 08:08:21 GMT

In article <[email-masked]>,
[email-masked] (bill nelson) writes:
|> [email-masked] (Robert Mitchell) writes:
|> : 
|> : Along these lines, does anyone have any good methods for
|> selectively	
|> : disolving small broken iron screws from brass, as in clock plates?
|> : Also the reverse; I have some broken brass stuck in a VERY small
|> : high carbon steel screw plate.  Too small for a jeweler saw...
|> 
|> Hm, I have not checked the way the metals would react to various
|> acids.
|> However, brass tends to be far more reactive than iron, so it is
|> likely
|> to be rather difficult to remove iron from brass.
|> 
|> It should be possible to remove brass from an iron plate, by using
|> the
|> different reactivities of the metals.
|> 
|> Bill

The only time I've removed steel screws was by means of a spark
erosion machine - worth finding a workshop locally who owns one.  If
the piece is valuable enough then the few Pounds/Dollars are well
spent.

Chemically, I don't think acids will work - you will have to corrode
the steel with something like salt solution, or something which
induces rapid rusting.  Over a short period this shouldn't affect
brass, apart from some discolouration.  Once rusted to dust, you
should be able to run a tap in to clear out the junk.

Still prefer the spark eroder!

-- 

David Jenkins

* All incorrect spelling and faulty grammar is intentional.   *  
* Corrections are superfluous and will be ignored.  So there. *         
*

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From: [email-masked] (ron ginger)
Date: Fri, 30 Jul 1993 12:21:51 GMT

In article <[email-masked]>, [email-masked] (David Jenkins) writes:

|>The only time I've removed steel screws was by means of a spark
|>erosion machine - worth finding a workshop locally who owns one.  If
|>the piece is valuable enough then the few Pounds/Dollars are well
|>spent.

Not that I've ever broken off a tap and needed one of these devices, but
I have looked at a couple articles on building them. One uses a virbating
engraving tool as the drive mechanism, the other was a very old ME article
which used a step motor driver, but based on very old - mid 70's- parts. The 
next issue of STRICTLY IC magazine promises an article.

Has anyone worked with one of these? Can you tell me about the proper power
supply- voltage and current, AC/DC, etc. 

I watched a Bridgeport add-on at a trade show drill .020 dia holes in a 1"
stainless steel rod. The drive was clearly by stepper motor, but the 
power control dials were simply calibrated on a 0-100 scale, and
the operator couldnt tell me real voltages.

ron ginger

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From: [email-masked] ()
Date: 30 Jul 1993 14:33:32 GMT

I'm posting this for my husband who makes jewelery.  He does lost wax
casting in the usualy way--with plaster, rubber molds, etc.  Out of
curiousity, he wants to try a different technique.  It's a historical
technique used at least by the Celts, and probably other groups.  They
would carve a template out of bone (my husband will use wax or lead). 
Then they would press clay half-way around the template, and pull the
template out.  They would do the other side similarly.  They would let
the clay dry, fire it, and fuse the halves together (or maybe they do
those last two steps in the opposite order).  Then they would use the
clay mold to make a metal copy.  For multiple copies of the same piece,
they just pressed the original template into many pieces of clay.

My husband's wondering if anyone has tried anything similar, and if so,
what kind of clay did you use?  What were your results?  He would like
to find a clay that is soft, holds fine detail, fires into a hard,
strong shell, has minimal shrinkage, and doesn't crack when molten metal
is poured into it.

E-mail or post, and thanks in advance for your help.

Donna
[email-masked]