Broken drill bit soaked in Alum : it is not working

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Any chance of a link, please, Julian ?

I deduced that it might be found here ... But no such luck.

MichaelG.

Was the tap you dissolved Carbon Steel? Is the drill Carbon Steel?

Regards Ian.

I had a poke around and I think it is here...

**LINK**

Jon

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Thanks, Jon

MichaelG.

Chris : this is the second base plate I have made and I am NOT making a third one ! The first one had two holes drilled in slightly the wrong places...I have no idea how that happened.

The tap and the drill bit were both high speed steel. The difference may lie in the fact that the tap has 4 flutes running down the sides so enabling the alum to reach all the broken tap at once. The drill bit is only experiencing the reaction at the broken end ; the soft aluminium is plugging the spiral flutes on the drill bit. That is my theory.

I followed the advice given here : I turned the pimple into a dimple using a small punch. Once I had that marked then I sacrificed a 1.4mm drill bit to knock it out. It all worked perfectly. The base plate has now been drilled and tapped for M2 using plenty of tap magic. I just need to give it a final polish.

Many thanks for the suggestions. It is nice to have a win sometimes !

Brian, the process is electrolytic, and it doesn't seem to work quite so well on aluminium. I would suggest that you get a little piece of copper wire (a strand out of an electrical flex),poke that down one of the drills flutes if you can and leave it there. You have to get the mixture down in the hole. It will make a white powdery mess of the aluminium pot, I put any bits in a glass jar, then put that in a pot of water, just got to keep an eye on it or it will boil dry. Ian S C

Brian, glad to hear that you have resolved the problem. I to would have been reluctant to make a third item.

Ian, so you agree that it does not work quite so well on aluminium ? That is good to know for next time.

I'm no chemist, but I think it's a reaction between the steel, and copper, some aluminium alloys have a percentage of copper, perhaps that would do it. Ian S C

Geo Thomas advised the method many years ago & in his inimitable way, explained the "best" way to do it. (it's on page 291 of the collection of his words of wisdom in "The Model Engineers Workshop Manual" Vol 1 (it'll also be in ME magazine of course). Well worth reading if you're having problems

There's nothing magic about using alum. All it is doing is accelerating corrosion (rusting) of the steel or rather the Fe to be precise. Dilute mineral acids work just as well.

As an expert, with years of experience in breaking drills and taps in bronzes, there are two major reasons why alum or acid corrosion doesn't appear to be working. The first is because a thread lubricant, other than saliva, was used before breakage. Not surprisingly, this prevents surface action on the broken drill or tap. I've seldom had any success getting rid of such lubricant with broken small drills & taps in situ as the flutes are choked with it. If the part is small enough and you have access to an ultrasonic cleaning bath, this will remove most of the trapped oil & swarf but by Sodde's law, the part with the snapped tap or drill is invariably too big to fit the bath.

The other problem is trapped gas .......bubbles preventing access of alum soln or acid so these need to regularly dislodged.

If you persist with using a copper wire to agitate the alum solution on the rear (broken) end of the drill you will succeed, I had to do this with a drill jammed in copper. Took time but worked. Also Note I was forced to make a bund of blutac covered with clingfilm to provide extra solution and to stop the water evaporating over the weeks.

It's amazing, there doesn't seem to be any explanation of the phenomenon on the web. Even wikipedia references this website! On a chemistry forum the question is asked but goes unanswered.

Potassium Aluminium Sulphate is clear not going to react directly with iron in a simple displacement reaction as iron is the least reactive of the three metals.

My guess is that using an alum salt ensures the aluminium isn't corroded but creates an environment where local electrolysis can take place and it somehow catalyses oxidation of the iron. Interestingly, corrosion resistant nickel irons eroded (at 0.5 mm per year rates) in alum and twice as fast in pure aluminium sulphate. Fastest in aluminium chloride. Again this seems to be by facilitating oxidation of the iron.

If only we had a tame inorganic chemist to hand...

Neil

I'm no inorganic chemist but..

Wikepedia tells us that Alum soln is acid to litmus so it has to be causing some dissociation of hydrogen ions in the water. It'd be a fair guess that we also get loose sulphate radicals..effectively sulphuric acid with the aluminium and potassium soaking up spare OH groups as the iron turns to soluble ferrous sulphate????

(it's sounds a good theory but may just be awesome bull-cr@p)

Aluminium naturally forms an oxide film on exposure to air and/or water If you put aluminium into sulphuric acid it will react and dissolve. This is because the acid dissolves the protective surface oxide layer and then the acid can attack the underlying metal. Eventually the acid become to weak to dissolve the oxide film and the reaction stops. The solution then contains aluminium sulphate. It is still quite acidic.

The oxide that forms on iron is not protective at all and iron will rapidly react in acidic solutions to give iron ions (Fe2+) and liberate hydrogen. This is what happens when iron reacts with aluminium sulphate. The iron dissolves to form iron sulphate and the aluminium comes out of solution as aluminium hydroxide.

The potassium content of alum (potassium aluminium sulphate) plays no part in these reactions.

Some of the posters on this topic have suggested poking copper wire down the flutes of the drill. "to ensure that the solution goes down the flute". Copper wire may also be effecting the reaction in another way. The copper in contact with the iron creates a galvanic couple and this can greatly speed up the rate of corrosion of the iron. This is why you should not use brass fittings on steel pipes because they will cause rapid corrosion of the steel adjacent to the fitting.

Mike

The copper wire was solely to tease out the black ferric oxide, once cleared it continued to work until the next slowing down. It is a simple corrosion cell with the oxygen coming off at the iron "electrode" causing "rust" in this case, black ferric oxide as in the central heating example above.

"It's amazing, there doesn't seem to be any explanation of the phenomenon on the web. Even wikipedia references this website! On a chemistry forum the question is asked but goes unanswered"

Neil,

Musn't teach granny to suck eggs, but if you put ["aluminium sulphate"+"iron corrosion"] into Google you find several references. Herewith one of the most informative:-

**LINK**

That great source of info on engineering "The Engineering Toolbox" lists a table of metals vs. various agents.

**LINK**

Although it doesn't show HSS, carbon steel is listed and given a "3" for contact with aluminum sulfate (sic), that is, 3 = "not usable". However my earlier point about alum not having magical qualities is born out if you look down for hydrochloric & sulphuric acids which also have "3".

Michael Cox 1 above has explained - the potassium radical is irrelevant for the enforced corrosion of iron . I've used dilute hydrochloric acid in a covered jam jar for a 10BA broken tap in gunmetal. It works well and just by warming on top of a radiator overnight. Do this in the kitchen though as the HCl fumes will rust any nearby iron/steel in the workshop so it's best to get downstairs before SWMBO does next morning !

I did find one reference and it showed Aluminium sulf(ph)ate caused corrosion about twice as fast as alum (potassium aluminium sulphate) Aluminium chloride was three times quicker again, so it may be worth trying and less hazardous than HCL. Obviously HCl may not be the best idea for aluminium.

It's interesting isn't it, the length you have to go to sort out a broken drill or tap, it's not a subject often touched upon but the general advice i garnered from books is to avoid the instance in the first place, as even they at least acknowledge what a pain it is to sort it once it has gone wrong.

Michael W

Well yes. My own metal mangling efforts are mainly guided by info from books and t'interweb. The videos from MIT are particuarly good - my possibly faulty recollection is that the advice given there was that you can drill twice the diameter of the bit on the first plunge, then half the diameter subsequently before withdrawing the bit to clear the flutes. For a small bit that really means pecking away at it. The OP said that the flutes were pretty much clogged so maybe that's why the bit broke.

Rob.

The flutes were clogged and I was forcing it...pretty dumb thing to do upon reflection. I now make small passes of the drill bit on a pre-punched dimple which has had a drop of tap magic. You can feel the difference the cutting fluid makes.

Edited By Brian John on 10/06/2016 02:05:51