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Posted by Richard on June 8, 2008, 5:59 am
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I thought this may be of interest to the group as it's been a topic a
few times inthe recent past. I recently discovered a model steam
engine casting I'd buggered as a teenager some 35 odd years ago by
breaking a 7BA tap in a hole. Given the prices Stuarts are charging
these days I thought it might be worth trying to recover the
situation.
I don't know anyone near me who might do spark erosion and briefly
considered a home-brew attempt, but some research indicated it would
need a fair investment in time and basic equipment just to 'have a
go'. However, along the way I also noted some of the details for
electro chemical machining, which seemed to offer an alternative with
potentially less overhead to start with.
So armed with a battery charger and a couple of pints of salt water
(no idea what concentration, it was sea-water) I set to work. The tap
was broken off just 'under flush' as one would expect. Initially I
used a piece of 1.5mm2 core from some lighting wire, cut off flush at
the end ie only the end of the wire exposed. I held it against the end
of the relevant hole. Switched on the charger and there were
immediately some bubbles from the hole and then nothing. I concluded
that the bubbles effectively drove the electrolyte out and then the
current stopped so I shaved the sides of the insulation to allow the
brine to flow. Continued bubbles - promising. I left it for half an
hour and came back 'just to see if anything had happened' and was
staggered to find the hole had been opened up to the diameter of the
insulation an down to the top of the tap. So I swapped the 1.5mm2 for
a piece of single filament from some telephone wire and re-started the
charger.
Having seen the effect I made sure I came back every 15~20 mins to
stir the sludge in the hole and push the tip of the wire further down
and ensure it was in the centre of the hole and not resting against
the side. In this way I disolved the entire tap in under 2 hours.
Given the extremely rudimetary setup I had and the complete lack of
accurate control of either the current or the electrode position I
consider this an amazing success. I may just have been very lucky, and
I would make a somewhat more engineered rig for use if I need it
again. I don't often break taps, but such a simple in-house means of
sorting the problem seems much better than having to hunt round for
tool makers with spark eroders or the restrictions of sulphuric acid
provided it's in brass etc. Just make sure the work-piece is the anode
or you'll build-up the broken end, not disolve it!
Richard
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Posted by steamer on June 8, 2008, 11:48 am
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--Well I hate to tell ya but I've got a little EDM machine, but it
might be a bit of a commute to California! ;-) FWIW it's a fun bit of kit to
add to your shop if you can fine one. Besides tap busting it's possible to
modify it to use it as a sinker and I've made a few injection molding dies
this way. IIRC there were a series of articles in either ME or MEW many
moons ago describing how to build one.
--
"Steamboat Ed" Haas : "Hold on! we're passing
Hacking the Trailing Edge! : through the moronosphere!"
www.nmpproducts.com
---Decks a-wash in a sea of words---
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Posted by Christopher Tidy on June 8, 2008, 5:35 pm
Please log in for more thread options Richard wrote:
> I thought this may be of interest to the group as it's been a topic a
> few times inthe recent past. I recently discovered a model steam
> engine casting I'd buggered as a teenager some 35 odd years ago by
> breaking a 7BA tap in a hole. Given the prices Stuarts are charging
> these days I thought it might be worth trying to recover the
> situation.
>
> I don't know anyone near me who might do spark erosion and briefly
> considered a home-brew attempt, but some research indicated it would
> need a fair investment in time and basic equipment just to 'have a
> go'. However, along the way I also noted some of the details for
> electro chemical machining, which seemed to offer an alternative with
> potentially less overhead to start with.
>
> So armed with a battery charger and a couple of pints of salt water
> (no idea what concentration, it was sea-water) I set to work. The tap
> was broken off just 'under flush' as one would expect. Initially I
> used a piece of 1.5mm2 core from some lighting wire, cut off flush at
> the end ie only the end of the wire exposed. I held it against the end
> of the relevant hole. Switched on the charger and there were
> immediately some bubbles from the hole and then nothing. I concluded
> that the bubbles effectively drove the electrolyte out and then the
> current stopped so I shaved the sides of the insulation to allow the
> brine to flow. Continued bubbles - promising. I left it for half an
> hour and came back 'just to see if anything had happened' and was
> staggered to find the hole had been opened up to the diameter of the
> insulation an down to the top of the tap. So I swapped the 1.5mm2 for
> a piece of single filament from some telephone wire and re-started the
> charger.
>
> Having seen the effect I made sure I came back every 15~20 mins to
> stir the sludge in the hole and push the tip of the wire further down
> and ensure it was in the centre of the hole and not resting against
> the side. In this way I disolved the entire tap in under 2 hours.
>
> Given the extremely rudimetary setup I had and the complete lack of
> accurate control of either the current or the electrode position I
> consider this an amazing success. I may just have been very lucky, and
> I would make a somewhat more engineered rig for use if I need it
> again. I don't often break taps, but such a simple in-house means of
> sorting the problem seems much better than having to hunt round for
> tool makers with spark eroders or the restrictions of sulphuric acid
> provided it's in brass etc. Just make sure the work-piece is the anode
> or you'll build-up the broken end, not disolve it!
That sounds very promising. I have an aluminium air-raid siren housing
with broken off screws in it, waiting for an idea like this. What metal
is your Stuart casting made from?
Best wishes,
Chris
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Posted by Richard on June 10, 2008, 5:41 am
Please log in for more thread options Sorry, should have said, the Stuart casting is iron. As I frankly
didn't have a great deal of hope in the first instance I made very
little attempt to control anything - the wire was lodged in place with
a dob of plasticine and a rubber band and tended to move about a bit.
I would put in a bit more effort next time. After the first go it was
immediately obvious that the finer the electrode, the finer 'hole'
you'd machine. Blindingly obvious really.......
I was careful to cut the 'electrode' off flush with the insulation so
that current could flow only out of the end and not radially. Given
your situation of something you don't want to make worse, I'd try with
a steel screw in any old bit of ally first.
If you want to read a bit of technical stuff on the subject look at
this:
http://www.osti.gov/energycitations/servlets/purl/15015141-slv9mp/native/15015141.PDF
It _is_ quite theoretical, but skipping through it picks up a fair bit
of useful pointers.
Richard
>
>That sounds very promising. I have an aluminium air-raid siren housing
>with broken off screws in it, waiting for an idea like this. What metal
>is your Stuart casting made from?
>
>Best wishes,
>
>Chris
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Posted by Tim Nash (aka TMN) on June 11, 2008, 3:06 am
Please log in for more thread options > Sorry, should have said, the Stuart casting is iron. As I frankly
> didn't have a great deal of hope in the first instance I made very
> little attempt to control anything - the wire was lodged in place with
> a dob of plasticine and a rubber band and tended to move about a bit.
> I would put in a bit more effort next time. After the first go it was
> immediately obvious that the finer the electrode, the finer 'hole'
> you'd machine. Blindingly obvious really.......
>
> I was careful to cut the 'electrode' off flush with the insulation so
> that current could flow only out of the end and not radially. Given
> your situation of something you don't want to make worse, I'd try with
> a steel screw in any old bit of ally first.
>
> If you want to read a bit of technical stuff on the subject look at
> this:http://www.osti.gov/energycitations/servlets/purl/15015141-slv9mp/nat...
>
> It _is_ quite theoretical, but skipping through it picks up a fair bit
> of useful pointers.
>
> Richard
>
>
>
> >That sounds very promising. I have an aluminium air-raid siren housing
> >with broken off screws in it, waiting for an idea like this. What metal
> >is your Stuart casting made from?
>
> >Best wishes,
>
> >Chris
Hi Richard
What made you go for sea water - I thought the fluid should be non
conductive?
Tim
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> few times inthe recent past. I recently discovered a model steam
> engine casting I'd buggered as a teenager some 35 odd years ago by
> breaking a 7BA tap in a hole. Given the prices Stuarts are charging
> these days I thought it might be worth trying to recover the
> situation.
>
> I don't know anyone near me who might do spark erosion and briefly
> considered a home-brew attempt, but some research indicated it would
> need a fair investment in time and basic equipment just to 'have a
> go'. However, along the way I also noted some of the details for
> electro chemical machining, which seemed to offer an alternative with
> potentially less overhead to start with.
>
> So armed with a battery charger and a couple of pints of salt water
> (no idea what concentration, it was sea-water) I set to work. The tap
> was broken off just 'under flush' as one would expect. Initially I
> used a piece of 1.5mm2 core from some lighting wire, cut off flush at
> the end ie only the end of the wire exposed. I held it against the end
> of the relevant hole. Switched on the charger and there were
> immediately some bubbles from the hole and then nothing. I concluded
> that the bubbles effectively drove the electrolyte out and then the
> current stopped so I shaved the sides of the insulation to allow the
> brine to flow. Continued bubbles - promising. I left it for half an
> hour and came back 'just to see if anything had happened' and was
> staggered to find the hole had been opened up to the diameter of the
> insulation an down to the top of the tap. So I swapped the 1.5mm2 for
> a piece of single filament from some telephone wire and re-started the
> charger.
>
> Having seen the effect I made sure I came back every 15~20 mins to
> stir the sludge in the hole and push the tip of the wire further down
> and ensure it was in the centre of the hole and not resting against
> the side. In this way I disolved the entire tap in under 2 hours.
>
> Given the extremely rudimetary setup I had and the complete lack of
> accurate control of either the current or the electrode position I
> consider this an amazing success. I may just have been very lucky, and
> I would make a somewhat more engineered rig for use if I need it
> again. I don't often break taps, but such a simple in-house means of
> sorting the problem seems much better than having to hunt round for
> tool makers with spark eroders or the restrictions of sulphuric acid
> provided it's in brass etc. Just make sure the work-piece is the anode
> or you'll build-up the broken end, not disolve it!