I've managed, through nothing but carelessness, to break a 1.5mm drill bit in the cylinder head of the little glow motor I'm (still) making. Only one hole left to complete and I broke the soddin' bit into the head. I've been dripping sulphuric acid into the hole for the past couple of days, without seemingly much progress.
Any better ideas for getting the broken bit out? Is there anything better than H2SO4 to do this job?
It *will* dissolve, but takes time. Warming the metal helps, as does prodding it with a stick as often as you can be bothered. Also, you need to change the acid fairly frequently as the volume of acid down the hole is completely inadequate to dissolve that much steel. Don't use concentrated acid, it will actually be very slow to react; something like 20-30% concentrated acid to water (add acid to water, slowly and with stirring) will be best.
If the block is aluminium I do not recommend complete immersion, as sulphuric acid may cause pitting - I know it did on mine when I had to do this a few weeks ago
If it were carbon steel in an aluminium head then dilute nitric acid would get the steel out without harming the aluminium. HSS is, I think, more work. Can check tomorrow.
Is the hole for a head mounting screw? If so, how about:-
Is the drill broken off flush? if not then drill 1.5mm into a bit of steel bolted to the drill press table to act as a locator, then put a pin in the hole to locate the buggered hole with. Failing that, make a template with the hole locations out of a bit of scrap and screw that to the underside of the head with some of the existing holes.
Once you've done that, carefully drill from the other side until you meet the broken screw. Then punch the broken screw out with a 1mm, or less, punch. Finally, clean up the hole.
What've got is an aluminium head machined from bar stock, and the hole is one of four for holding down the exhaust manifold flange. The drill is a
1.5mm HSS hertel and its broken off below the top of the hole (only just).
I can't get at the hole from the other side.
I'm using conc sulphuric, hadn't thought that diluting it would actually work faster! The anodising trick sounds interesting.
Not sure how to acquire smal quantities of Nitric acid locally either. Our local chemist is nervous about selling me 500ml bottles of IsoPropyl Alcohol, so asking for Nitric will probably give him a heart attack.
Thanks for the accumulated wisdom - back into the shed to have another go.
Concentrated acid tends to "passivate" the iron unless it is heated, which can be dangerous.
If your local pharmacist* is reluctant to sell acid (many are these days, used to be so much easier in my youth) then I suggest you try jewellery trade suppliers. I bought 5 litres (1 litre packages are available but not much cheaper) of concentrated H2SO4 from H S Walsh a couple of months ago for exactly this purpose - over the counter, as carriage is a problem. They will take your details and expect you to say what it's for - "pickling and etching metal" is fine, and quite truthful in our hobby. Just treat it with reasonable reverence - it's nasty stuff, but I prefer it to nitric or hydrochloric acids, both of which give off highly corrosive vapours which will cause mayhem with your tools. Wear eye protection, *always* add acid slowly to water, stirring continuously. The reaction of H2SO4 with water is very exothermic (gets hot) and if you add water to the top of the very dense acid a very sharp boundary layer will heat up alarmingly and may well crack the vessel, and much spitting is almost certain. If you get a drop on your fingers, you probably have 10 seconds or so to wash it off before it causes damage; if on your clothes, wash immediately at least twice in copious amounts of water, or holes will appear fairly quickly.
*As one who studied and practised chemistry in my early career, I dislike the way pill peddlers call themselves "chemists". Just like the way medical practitioners have quite shamefully stolen the term "doctor".
Nitric Acid can be purchased in small quantities as "Cartridge Case Cleaner"- Try a Gunsmiths or shooting accessories shop, even in these paranoid days there are still some about, just. I bought mine many years ago at an ME show from a company called "DELWAY TECHNICAL SERVICES" very old phone number 051 639 3201 they market Metal Blacking kits. or at least they used to. It does work, my "apprentice" broke a M2 tap in a microscope I was making, took a couple of hours to dissolve away, leaving a black sludge in an untouched hole. T.W.
Could you use differential expansion to your profit? Aluminium has a coeff. of expansion about double that of steel. Perhaps drop the whole bollixed lot into boiling water for a while, lift out and give it a sharp rap. Let us know. Mike in BC
By far an away the most satisfactory method would be to use EDM. As it's a small part I would offer that you post it to me, but my machine is still in storage awaiting completion of the new workshop. There is bound to be someone local to you who can pop it out in a matter of minutes on a sinker machine. Look for local diemakers.
I diluted the acid as recommended and kept the head warm in the oven most of the the day, dripping in the acid and prodding at it with a sharp stick every hour or so. It's still solid though. I don't want to heat the head too much though.