MadModder
Gallery, Projects and General => How do I?? => Topic started by: AdeV on July 27, 2010, 04:55:40 PM
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Help!
I managed to break 2 (yes, 2) taps tonight, both M6x1.0, in the ali sump :(
I know there's some fancy chemical thingybob you can use to dissolve the tap out without damaging the aluminium (it will be interesting to see what it does to Loctite!), but my search karma is failing me & I can't find any details.
Can someone give me the idiot's guide - in particular, what chemicals do I need to buy - and can I just nip to Tescos to buy them (& if so, what are likely trade names)? Unfortunately, I don't think American product names will help, as most stuff is either unobtanium or a different name over here...
Any assistance gratefully received :thumbup: :nrocks:
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I've read on this and other forums that Alum dissolves taps without harming aluminum and brass. Can't use it on steel though, it will dissolve it. Supposedly you can get it a the grocery store. I've not used it but have heard good reports about it.
Maybe some one that has used it can shed more light on the subject such as how long it takes and what strength to mix it up as.
Hope you get it sorted out. Be a shame to lose all that work.
Ron
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Tough, Ade!
Spark erosion is probably the quickest route, by people like this: http://www.sparkeroding.co.uk/ but I bet they know how to charge, because they will mainly work on jobs where a lot is at stake, either in money or time (now, whose job does that remind me of?)
You can get tap extractors with long fingers to go down the flutes, but they don't get a good press.
Alum (from swimming pool suppliers) will dissolve the tap without touching the aluminium, but it can take a looooong time - weeks rather than hours. I don't know whether the "Vissin" discussed here http://www.cnczone.com/forums/archive/index.php/t-23044.html is the same stuff, and if it isn't it might hurt aluminium.
I suppose that with careful setting up and a 4mm carbide slot drill (that should be wide enough to break into the flutes) you might be able to mill it out.
Try Googling "extracting broken taps" to read up on the subject.
Fingers crossed for you!
Andy
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I bet if you got a high conc. of alum and warmed it steadily in a pan it`ll work fast but I`m guessing it depends on what your tap is made from..
Let us know how it turns out if you do use it.
Chris
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This looks a decent price and the right stuff... I think :ddb:
http://cgi.ebay.co.uk/ALUM-Aluminium-Potassium-Sulphate-ALUM-250g-Tub-/290456957643?cmd=ViewItem&pt=UK_BOI_Medical_Lab_Equipment_Lab_Supplies_ET&hash=item43a09572cb
I`d pour it all out and dissolve it in pretty much as little water as you can and put your piece to soak in that. Ideally somewhere hot or even warm it on the pan. I`m just guessing...I`m sure others have better advice who have tried it!
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Hmm, taxidermists use it apparently (according to that ebay item, thanks Chris), I may contact our local one - there must be one... - & see if they've any spare.
I've not tried it, but I think I can get the end of the sump into the toaster oven, so I can heat it to "hot, but not too hot" to accelerate the reaction. I don't have any carbide end mills/drills :( but that would be an alternative.
I just can't believe I broke 2 taps. That's just careless... As far as I know, both are regular carbon steel (one, that broke almost by being looked at) claims to be "fine carbon steel"...
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Hi AdeV,
Do you have any gun shops near you? If so pop along and ask them for some "cartridge case cleaner". The main ingredient is Nitric acid, which will dissolve steel but leave Aluminium alone. It works a treat on broken taps, I completely dissolved a 2.5mm tap in a couple of hours, all that was left was a black sludge. A 6mm tap will take longer but you don't have to dissolve it completely, just enough so that the core can be pulled out.
My stock, bought years ago at an ME exhibition, came from Delway Technical Services. I don't know if they are still going, recessions being what they are, but their phone number was 051-639 3201 although the prefix needs updating.
Ned
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Ade,
I have dissolved a few taps with Alum. Got mine in a grocery store here in the states. Your piece is to big to do it the way I did, but I am sure you can figure out how to do something similar. I had water and alum in an aluminum pan and brought up to heat. Just below boiling... threw my part in and waited. Took most of the day (better of 12 hours) to dissolve the tap. I didn't go all the way, just enough to remove it.
HERE (http://www.homemodelenginemachinist.com/index.php?topic=270.0) is a thread to read. Helped me.
Eric
**fingers crossed. Don't want to see it become a paper weight!
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Found a neat trick for removing tapsbroken off in steel. Heat the work piece, with the heat concentrated on the tap. When the tap gets red hot, let it air cool. The heating takes the temper out of the tap and a drill bit can be used to drill it out with no problems. The heating doesn't effect the work piece since it was never tempered to begin with. Sometimes the last resort is the solution to the problem after all. Note.... this trick does not work with non-ferrous metals
Steve
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Ade, if you get some alum, (which should be called aluminum potassium sulfate just about anywhere, as that's the chemical name),
start with a pint of hot water from the tap. Start dissolving the alum in the water, and keep adding it until no more will dissolve.
Now get some of that modeling clay that folks use for hobby projects. Using the clay, make a well around each hole with the broken taps.
Make them large enough to hold a couple of ounces.
Set your sump on something warm. It doesn't have to be very hot. Just good and warm. Maybe use a hot plate set on low.
Pour the solution into the wells, and tap the side of the sump with a plastic hammer to dislodge the bubbles in the tap holes. You
will probably have to leave it a few days, and change the solution in the wells a couple of times. It will turn the taps into crumbs.
Won't hurt copper alloys or aluminum.
Dean
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Hi AdeV,
Do you have any gun shops near you? If so pop along and ask them for some "cartridge case cleaner". The main ingredient is Nitric acid, which will dissolve steel but leave Aluminium alone. It works a treat on broken taps, I completely dissolved a 2.5mm tap in a couple of hours, all that was left was a black sludge. A 6mm tap will take longer but you don't have to dissolve it completely, just enough so that the core can be pulled out.
My stock, bought years ago at an ME exhibition, came from Delway Technical Services. I don't know if they are still going, recessions being what they are, but their phone number was 051-639 3201 although the prefix needs updating.
Ned
Perfect that Ned, thanks! That's only a couple of miles away from me, all I need to do now is get someone to answer the phone...
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I've obtained some concentrated nitric acid, but due to the late hour it arrived, I didn't have time to test it out. All will be revealed tomorrow (I'll be trying out a test piece first...)
Edit to add: I will be taking all proper precautions, I am suitably drilled in the nastiness of concentrated acids (and alkalies, for that matter). :zap:
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If you don`t mind me asking, where did you get it from as Conc. Nitric Acid - I thought best you could get without some registration would be about 40%. Only place I could think of was artist supplies as they use it for etching copper but then again, this wasn`t conc.? No reason...just curious to know!
Nitric acid will be great for this, really quick and won`t touch the ally, or at least shouldn`t do! Let us know.
Chris
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Wikipedia sez (http://en.wikipedia.org/wiki/Nitric_acid):
Nitric acid is miscible with water and distillation gives a maximum-boiling azeotrope with a concentration of 68% HNO3 and a boiling temperature of 120.5 °C at 1 atm, which is the ordinary concentrated nitric acid of commerce.
I think 40% HNO3 is plenty strong. I certainly wouldn't want to mess about with anything stronger without a complete lab setup (fume hood, granite bench, 3mil nitrile gloves, et cetera).
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(I'll be trying out a test piece first...)
Edit to add: I will be taking all proper precautions, I am suitably drilled in the nastiness of concentrated acids (and alkalies, for that matter). :zap:
Good luck Ade! :thumbup:
Be careful....... (http://serve.mysmiley.net/confused/confused0092.gif) (http://www.mysmiley.net/free-sick-smileys.php)
David D
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If you don`t mind me asking, where did you get it from as Conc. Nitric Acid - I thought best you could get without some registration would be about 40%. Only place I could think of was artist supplies as they use it for etching copper but then again, this wasn`t conc.? No reason...just curious to know!
Nitric acid will be great for this, really quick and won`t touch the ally, or at least shouldn`t do! Let us know.
Well, you _can_ buy it straight off eBay, 1ltr @ 70% for less than £25 delivered... but mine came from a friendly chemist at Dad's works. The strength of the acid hasn't been written on the bottle, unfortunately, but I'm told it's "probably" 70%.
I plan to test on a sample, I'll drill a hole, jam the broken off end of the tap in there with a smidgeon of WD40 (to match the real deal), then I'll add acid & see what happens. Getting the acid back out of the hole will be the interesting part... I will have to try not to overdo it, so that it all gets converted to whatever it turns into when it's eaten steel...
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Wikipedia sez (http://en.wikipedia.org/wiki/Nitric_acid):
Nitric acid is miscible with water and distillation gives a maximum-boiling azeotrope with a concentration of 68% HNO3 and a boiling temperature of 120.5 °C at 1 atm, which is the ordinary concentrated nitric acid of commerce.
I think 40% HNO3 is plenty strong. I certainly wouldn't want to mess about with anything stronger without a complete lab setup (fume hood, granite bench, 3mil nitrile gloves, et cetera).
Other than the gloves (which aren't 3mm), I don't have any of that stuff... what I do have is a very big building, so if anything does start fuming wildly, I have plenty of "get the hell out of there" space. Although I could do with it -not- raining nitric acid inside the building....
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AdeV, ahh good job. To be honest, concentration won`t matter too much...just the stronger the quicker it`ll work. I`d watch it doesn`t get too hot. I`ve stripped the copper off an old 2p coin a few times with Nitric Acid and it does get very hot. You might want to do it in a container and try and put the container in a cold water bath to cool the acid down.
I have an idea that you may want to keep prodding the tap with something. Sounds silly but I think it`ll probably crumble in a shorter time rather than completely dissolve away.
Regarding cleaning up. I think the HNO3 will be reacting with the iron and so it`ll form iron(III) nitrate, Fe(NO3)2 and H2 gas but you`ll still have HNO3 there because it`ll have been in excess. I`d give it a good clean in water to dilute it and then put it in a good bath of Bicarbonate of Soda to neutralise any acid left on there.
Good luck...take a video!! :worthless:
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Wikipedia sez (http://en.wikipedia.org/wiki/Nitric_acid):
Nitric acid is miscible with water and distillation gives a maximum-boiling azeotrope with a concentration of 68% HNO3 and a boiling temperature of 120.5 °C at 1 atm, which is the ordinary concentrated nitric acid of commerce.
I think 40% HNO3 is plenty strong. I certainly wouldn't want to mess about with anything stronger without a complete lab setup (fume hood, granite bench, 3mil nitrile gloves, et cetera).
Other than the gloves (which aren't 3mm), I don't have any of that stuff... what I do have is a very big building, so if anything does start fuming wildly, I have plenty of "get the hell out of there" space. Although I could do with it -not- raining nitric acid inside the building....
Guys, do not attempt this inside a building without a fume hood! Set it up outside (with a fan set to blow away the fumes)! Wear a face mask and feed a "light trickle" of air into the mask. You do not want to get a lung full of any concentrated acid fumes!
I work with concentrated acids fairly regularly. I have a pressure regulator that I set to 3 psi that I feed air into my face mask with (using a piece of 1/4 inch ID tubing with small holes punched in it to distribute the air). I have a board (1 X 4) screwed to the side of my house to which I can attach a piece of polyethylene sheeting (camping tarp) to give me a dry area to work (I live in western Washington State where drizzle is to be expected). I have cinder blocks set into the ground on which I can stand a fan to assure that the fumes are blown away from my house. I have several polyethylene storage containers to hold whatever I am working on and the acid. A brick on the "lid" keeps animals and kids away from the dangerous stuff (and all the kids in the neighborhood know not to mess with things in this area). I place a piece of polyethylene sheeting on the ground under the container with raised edges to make a "catch basin", cover that with newspapers covered in baking soda to catch spills -- and have an empty plastic bucket at hand to dump excess (or used) acid and a hose "at the ready." I have another plastic bucket "at hand" with a baking soda/water mixture ready for the pieces I pull out of the acid processing. This all adds about an hour (set-up and clean up) to any "acid job," but it is cheap at twice the price!
About half of what I do with acids is desmutting hot rolled steel (using hydrochloric acid). About one-third of what I do is passivating stainless steel (using sulfuric acid). The balance are processes dealing with other acids. Getting a lung full of acid fumes is not a "good thing."
Desmutting hot rolled steel: If time is not an issue, salt and vinegar will do this job for you -- just be prepared to wait a couple of days for good results. If time is an issue, muriatic (moderately weak hydrochloric) acid speeds things up. Use care as the fumes are nasty! After you have removed the material from either bath, neutralize the acid (baking soda works quite well) and rinse thoroughly. I find that a coating of phosphoric acid (Naval Jelly), rinsed well, and dried -- followed by a light waxing with a carnauba-based wax (Treewax is the most common brand in the U.S.) lets such material sit on my "stock shelf" for years without rusting.
As with anything, RTFI -- Read The Fine Instructions!
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Thanks for the details, Lew. I could see feeding the air into the mask from the top to blow any fumes downward away from your nose -- and cool your forehead while it's at it.. :thumbup:
What sort of mask are you using?
:offtopic: Speaking of masks, I saw this (http://www.harborfreight.com/grey-and-red-skull-welding-helmet-66177.html) in a recent flyer and burst out laughing so hard I startled my SBH -- and the cat. :lol:
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Lew - chill. To say you need that sort of setup all this time is crazy. Yes, you don`t want to be breathing in acid fumes but a well ventilated room (i.e a door and window) and you will be OK there. You use a fume hood when the products are toxic...like my example of copper with nitric acid, you produce nitrogen dioxide which isn`t good stuff. But to say any use of conc acid needs the following is nuts...
"Wear a face mask and feed a "light trickle" of air into the mask. You do not want to get a lung full of any concentrated acid fumes!
I work with concentrated acids fairly regularly. I have a pressure regulator that I set to 3 psi that I feed air into my face mask with (using a piece of 1/4 inch ID tubing with small holes punched in it to distribute the air). I have a board (1 X 4) screwed to the side of my house to which I can attach a piece of polyethylene sheeting (camping tarp) to give me a dry area to work (I live in western Washington State where drizzle is to be expected). I have cinder blocks set into the ground on which I can stand a fan to assure that the fumes are blown away from my house. I have several polyethylene storage containers to hold whatever I am working on and the acid
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We poor from conc acids, hydrochloric, phosphoric and sulphuric almost everyday in an open lab and only use a fume hood when the reaction is likely to be unpredictable or products toxic. We never wear a face mask.
I guess however, you can never be too careful.
Chris
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Hi Lew,
As already said CHILL, we are talking about, maybe, half a teaspoons worth of not too concentrated Nitric acid, not buckets full. You will have to find your own equivalent to a teaspoon on the other side of the pond, but you should get the idea of the quantities involved.
In reply to another earlier suggestion about spraying with WD40 before acid treatment, I would suggest you degrease and dry instead. You want the acid to dissolve the steel and not get diluted or used up by dealing with WD40 first.
I await the results with baited breath, I know it works and I hope others find out too.
Ned
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OK.... the results (so far)...
At about 3.30pm I started the acid working on the back end of the tap in a test piece. Mainly, I wanted to make sure the acid didn't attack the aluminium (or, rather, one of the alloying metals). I definitely have "fuming Nitric acid" - you could see the haze as soon as the bottle was opened. Lew, I don't have face masks or anything fancy like that. I used gloves, eye protection, and did everything at arms length. I reckon I put about 1.5ml (maybe a smidge less) in the hole. I caught the occasional whiff of the acid smell, but certainly nothing to worry about.
Left it until about 9pm, then drained out the acid & had a look in the hole.... OK, it's definitely been doing some kind of dissolving in there, but if I'm honest, it hadn't completely eaten it as I'd hoped. Ah well.
I've now set up the sump with its two holes soaking away. Unfortunately, one of the holes is leaking, so the acid simply drains away. I forsee a long boring day ahead as I drip feed fresh acid down the hole.... I know where it's going, I just hope it's not sneaking across the top of the plug & eating into the hole I successfully tapped, which has a steel bolt in it...
Scores on the board (so far):
Taps dissolved: 0.1 (estimated)
Acid used: 5ml (estimated)
Fatalities: None (yet)
I may write facetiously, but I treat dangerous stuff like concentrated acid with the respect it deserves.
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AdeV - I haven`t a clue what the part is your tap is stuck in but the problem is that as your tap reacts your basically using up your HNO3 so if your only using a small volume it`ll be diluting. The HNO3 really shouldn`t reactively touch the aluminium and so you need to put the part in more of the solution if you can?? You may also want to warm it a little, maybe put the container in a warm water bath! ... . but don`t shoot me if :zap: :lol:
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Thanks for the details, Lew. I could see feeding the air into the mask from the top to blow any fumes downward away from your nose -- and cool your forehead while it's at it..
What sort of mask are you using?
I am using a standard "flip-down" splash & dust shield mask. I loop the flexible tubing to the bottom of my face (similar to a surgical nostril feed -- just not in my nostrils) and let the rising air keep everything clear.
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As already said CHILL, we are talking about, maybe, half a teaspoons worth of not too concentrated Nitric acid, not buckets full. You will have to find your own equivalent to a teaspoon on the other side of the pond, but you should get the idea of the quantities involved.
Ned, A suggestion of "70%" concentration was made. That's a lot more than "not too concentrated." I regularly work with hypergolic fuels, so my "measure" is fuming red nitric acid -- and I think accordingly. Garden variety muriatic acid (15% concentration of HCl) sends several dozen people to the hospital every year just in western Washington because of inhaled fumes. I was not hearing enough think safety in this discussion (at least as I see things). Concentrated acids have dangers. It is not "dangerous materials" that frighten me, it's people handling "dangerous materials" without having though things through that scares me.
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I was not hearing enough think safety in this discussion (at least as I see things). Concentrated acids have dangers. It is not "dangerous materials" that frighten me, it's people handling "dangerous materials" without having though things through that scares me.
Lew,
It's true that I've not handled concentrated acids since my school days, which is more years ago than I care to remember. It's also true that I'm not sure what acid dissolves what - I had absolutely no idea that Nitric acid would leave aluminium alone whilst dissolving steel, for example. Google helped there.
What I DO have left over from my school days is a very healthy respect for the dangers of concentrated acid, and the need to handle them with extreme caution. Not quite as much caution as you, perhaps, but I'm expecting that if you're de-scaling steel, you're using quite a lot more acid than I am...
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I regularly work with hypergolic fuels, so my "measure" is fuming red nitric acid
I can`t imagine the OP is working with the red fuming rocket propellant type! - I`m pretty convinced it`ll be good old white HNO3.
Garden variety muriatic acid (15% concentration of HCl) sends several dozen people to the hospital every year just in western Washington because of inhaled fumes.
It wouldn`t supprise me that the vast majority of irresponsible people were sniffing the stuff from the bottle! ... but I did hear that 87% of statistics are made up.
Anyway...AdeV - any more progress today????
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... but I did hear that 87% of statistics are made up.
I heard 88.67 but don't know how true it is ?
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Hi Lew,
If you look back to when I mentioned using Nitric acid, in the form of "cartridge case cleaner", I mentioned no specific concentration. The Delway product is already diluted and the label says it can be diluted further by up to 90% with water. This I do not consider to be 'concentrated' or particularly dangerous. I remember one teacher at school using Nitric acid to clean a Brass fitting on a book and deliberately putting some on his finger to show how safe it was! Although I personally would not do this, as the thought of it getting in a cut makes me wince. You might only think in terms of exotic compounds used upper echelon technologies but WE are talking of commercial over the counter products. I doubt in this day and age of those-who-should-know-better's paranoia, that concentrated Nitric acid is easily available to the general public because of its use in things that go "whoosh-bang-ooh-nasty". :(
So, I repeat "Chill", and stop looking for the worst and start looking for the best. Credit some people with a little common sense, you might be surprised that the majority of the population can survive even without your education or experience, both of which I respect :bow:, but please keep it "real".
Ned
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Anyway...AdeV - any more progress today????
Not much to report at the moment, I'm afraid.... last time I prodded at it, there's still a chunk of tap in the hole.
I may have to move on to plan C: Buy a small carbide drill, drill into the tap, then use an easy-out to try to extricate it. Or try to tap the tap with a left-handed tap, if one can buy such things... [google] I see you can, why did I ever doubt t'internet... hopefully as the inner tap starts to bite, it'll simply rotate the original tap (what's left of it) out of the hole. Then I can finish the job with the proper HSS taps I've bought...
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Well…I did it a lot of times but not on a tap, you weld on the broken stud/screw using stick welder very carefully till you make a welding buildup out of the hole. After you can weld there a nut and take everything out. Usually we use E312 16 electrodes. I hope this can help!!!!
Andreas
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Guys,
AdeV wrote, "Well, you _can_ buy it straight off eBay, 1ltr @ 70% for less than £25 delivered." That is what kicked my reactions into gear.
One of my longtime friend's wife is an ER nurse. I had gone over to their house with a bottle of muriatic acid to help with some regrouting. She just about freaked (not seeing my air supply system) as she sees people brought in suffering from acid damage to their lungs from using muriatic acid with fair regularity. I agree that getting into that position qualifies as "dumb" if you have any idea what you are doing, but too many people do not know the dangers of things they can buy off the shelf.
I was visiting Guy Lautard (I assume most of you are aware of him and his The Machinist's Bedside Reader series) a few years back. He had a small (plastic) bottle of concentrated nitric acid that he had wrapped in cotton balls and placed inside a larger glass jar on his shelf. The cotton balls had turned into gun cotton and all the steel within a couple of feet of his "storage location" had been etched to a nice medium brown by the fumes that escaped the jar. (A) Gun cotton (nitrocellulose) can be "highly entertaining" in the most unexpected ways; and (B) etching by acid fumes can reach into all sorts of areas where you do not want it to reach (hence my warnings against using it indoors). The storage container was located about four feet from the headstock of his lathe.
The fact is that many of the things we do can be quite dangerous. So long as we are aware of the dangers and take appropriate precautions, there is nothing wrong with this. However, when I see "advice" accumulating calling for something like "70% nitric acid" coupled with "using it indoors," I think that a bit of, "Guys, think about this..." is not out of order.
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Very nicely said Lew.
Ok now guy's let's keep under control here.
Bernd
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:ddb:
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Well, there was a blob of rustyness in one of the holes today... I guess it's working, then, just slower than I'd hoped... :coffee:
I've refreshed the acid, will leave it another day, repeat until done...
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A quick update: After leaving it a couple more days, there's more rusty snot appearing - this time out of both 'oles. Conclusion: The taps are dying! Slowly.... it has to be said, but shurely. Hopefully there'll be enough aluminium left at the end of the process to take a thread!
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AdeV - chemically it shouldn`t, but is the ally being affected?
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AdeV - chemically it shouldn`t, but is the ally being affected?
It's taken on a bleached appearance where the acid's been around... apparently concentrated nitric creates a protective oxide layer on ali (wheras dilute nitric will, in fact, dissolve it! Thanks to Ralph for that heads up...!). I don't think it's going to cause a problem for my purposes, but I'm definitely glad I've got some excess material to mill off the top...
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Nice one, didn`t know that and think maybe I should have :whip:
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Ade, are the broken taps out yet? It's been a fortnight since you were last cursing them.
Andy
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Ade, are the broken taps out yet? It's been a fortnight since you were last cursing them.
Not yet... I think it may be time for Plan B. I just need to figure out what Plan B is....
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Are you sure you are actually using the right acid? You would have thought that by now your problem would be over long ago. As I said I dissolved a 2 or was it 2.5mm tap in a couple of hours.
You haven't found a source of stainless steel taps, have you?
Ned
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Ade,
Not sure of what the status is currently but since you didn't have much luck with the acid, this might work for you: http://pico-systems.com/edm.html
bob
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Has anyone heard of using bleach to dessolve broken taps??
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alum you should also be able to get from a chemist . i used to ( many years ago ) work in the factory that made the stuff . we made it mainly for the city council and it was used by them in water treatment . i used to eat the stuff every day and go home covered in it .
it is made from mixing bauxite powder , water and sulphuric acid in the correct ratio and order ,then it generates a hell of a lot of heat and foams up to at least 10 times in volume ( we would sometimes throw liquid soap on it to knock it back down a bit if it looked like over flowing the huge mixing pot ) then when the reaction is finished it was poured out at a very high temerature onto into a bay ( looked like wet cement at this stage ) then left to cool and set , then smashed up with boncats and fed through the crusher and milld to the required size granule .oh yeah it used to eat the titanium mixing paddle in the mixing pot over a few months of production and stainless steel disapeared in a matter of seconds .
johno
oh and dad used to put it in a hollow tooth to kill the nerve when he had tooth ache , this i can not recommend though cos i tried it and it aint nice , he told me it was a common fix for a tooth ache while at sea in the merchant navy .
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The reason alum is available in grocery stores here is that it is used in making pickles crisp. I think it's found in the spices section of the store.
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I got some alum from flebay, I had it a couple of weeks so it was time to try it. I made a strong solution in a cup then dug out the electric slow cooker then stood the cup containing the solution of alum with the offending alloy manifold with broken tap, I then surounded the cup with boiling water and turned on the slow cooker. I took about 3 hours or more to desolve the tap but it did, a bit to my suprise & joy. :thumbup:
I have a lot of alum left, lets hope I dont need it!
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I've found alum in halal food stores. They're pretty good for finding some of the harder to get stuff, like citric acid and monosodium glutamate (the secret behind making pizza dough as good as the takeaway's).
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Gosh, this topic brings back memories...
Ned, if you're reading this - yeah, pretty sure it's the right acid (fumic nitric acid - it certainly fumed alright). It was having a go at the taps, I think the problem was, none of the acid was getting down past the shank, so the total surface area the acid was working on was only the top of the broken off piece. Hence the slowness to dissolve.
As for EDM/spark erosion machines, oh yes, I want one of those. Having seen awemawson's machine in another post elsewhere... Definitely want one of those...
Alum - interesting, I wish I'd known that, I'd have tried it as a solution.
As I never really finished this thread off, here's what eventually happened: I got fed up with the acid, and ended up milling a rectangle around the offending taps. Once I'd got low enough past the broken off shanks, I was able to extract them with pliers. I then cleaned up the holes, used the broken-off ends of a couple of old HSS cutters as pins (to hold the circular plugs in place), milled out a rectangular bung to go in the hole I'd made (I made it about 0.002" bigger than the hole), stuck it in the freezer for a while, then whipped it out & banged it in with a big birmingham screwdriver (er, hammer). Then, when it was properly gripped, milled it flat. You have to go looking for the join these days, which is nice :)