Broken 6BA Tap in steam cylinder

[John Ross 4Participant @johnross4]

Hi

I have a broken 6BA tap that has broken off when taping the cylinder cover, all was going great and then all of a sudden snap. Have been trying for the whole day to drill, mill and even tried stud extractor but to no avail. There are 8 planned bolts to hold on each cover, I have them spaced out evenly leaving a larger gap for the steam coming in. I have put so much time into the cylinders that I am loathe to staring again. What are peoples feelings should I just put a dummy bolt head in or is the steam pressure just going to leak, maybe drilling just one or both sides of the broken tap and putting in another 2 bolts, but this would not look to good. This is my first train a 3 1/2″ Juliet, and am very disappointed with myself for such a stupid mistake. I look forward to your replies.

Thank you.

[Peter Cook 6Participant @petercook6]

If the cylinders are gunmetal, you could try the old clockmakers trick of soaking the part in a warm solution of Alum. It will gradually dissolve the steel of the tap without (I don’t think) damaging the gunmetal cylinder.

[John Ross 4Participant @johnross4]

Hi Peter

Thanks for your thoughts how long do you think it might take. The cylinders are gunmetal.

Best wishes John

[noel shelleyParticipant @noelshelley55608]

One seldom hears of any one with a simple spark eroder but they are just the tool for the job. Not having needed mine for a while I have not got round to repairing it yet – even though I’ve got the parts. If your a member of a club ask around, the job will take under1/2 an hour and does not touch the thread. Good luck.  Noel.

[Dave HalfordParticipant @davehalford22513]

I believe there are two different ‘Alums’. Only one works.

[Peter Cook 6Participant @petercook6]

John, it may take a while – depending on how hot you make things, hours to days seems to be the  consensus. Fortunately I have never had the need to try it (touch wood!). There is a thread here which discusses the process and identifies the correct  form of Alum.

[Andrew TinsleyParticipant @andrewtinsley63637]

I am with Noel on this one. I don’t often need a spark eroder, but am I glad I have one just in case.

Andrew.

[Nigel Graham 2Participant @nigelgraham2]

How long alum takes obviously depends on the volume of steel it has to dissolve, and it’s worth starting with a hot, strong solution of it.

Time’s not that important. If it takes a few days rather than overnight, it takes a few days. How long would it take to make a complete new cylinder, including obtaining the material if you don’t already have it?

I had to use the method once with a tap broken in stainless-steel, and I think that was over about three days, with occasional refreshing of the solution. I would also agitate the thing each time I passed it, to remove the debris (iron sulphate?) from around the bit of tap.

 

[DC31kParticipant @dc31k]

On the ‘agitate and keep warm’ theme, would an ultrasonic cleaner assist?

Some have an inbuilt heater and periodic operation would circulate and flush the fluid in the hole.

You can put part and chemical in a strong ziplock bag and then plain water as a surround.

[Stuart Smith 5Participant @stuartsmith5]

John

If you are near Lancashire, one of the members of my club found a company in Blackburn with a spark eroder. He had a similar problem and they sorted it out for him at a reasonable price.

Let me know if this is any use to you and I will get the details.

Stuart

[bernard towersParticipant @bernardtowers37738]

The ones I have done in the past I have used a hypo with a fine needle to go down the tap flutes to clear the debris an flush with new fluid

[JAParticipant @ja]

Have a chat with your local car or motorcycle engineers. They will know of a local firm with a spark eroder. However such firms, and engineers, are now few and far between. It is also a “distress” job so it could be a bit expensive.

Where are you based?

JA

[Robert Atkinson 2Participant @robertatkinson2]

I’ve used Alum on a m3 tap in aluminium. You want Potassium alum.  Using warm solution speeds things up. I used a cheap peristaltic pump (liquid only touches the inside of of the plastic tube) from ebay to keep the fluid flowing. I like bernards idea of using a small tube down one of the flutes.

Robert.

[MacolmParticipant @macolm]

A long time ago, the machine shop of the firm I worked for broke 6BA and similar size taps in expensive aluminium parts occasionally. They had a home made spark eroder which consisted of a 12V or perhaps 24V solenoid (presumed ex vehicle) mounted on a retort stand, with  a short length of single strand copper wire as the eroder. It was self acting, with the spark in series with the solenoid. No idea what the radio interference was like however.

The spark area was under paraffin. It took a bit of tweaking to get it going, but then ran with little attention. It was slow, taking a couple of days or more to get through a tap, with the paraffin needing to be swilled occasionally to flush out debris.

It would be pretty easy to try.

[Howard LewisParticipant @howardlewis46836]

+1 for a spark eroder!

Will remove the tap without damaging the parent metal.

Which is cheaper, a new cylinder and all the time and energy to machine, or paying someone to spark erode it?

6 BA is small, so shouldn’t take long

[duncan webster 1Participant @duncanwebster1]

Thread on simple EDM here https://www.model-engineer.co.uk/forums/topic/a-simple-self-acting-spark-erosion-machine/

But if you can find someone local if saves making a tool for a one off

[Dave HalfordParticipant @davehalford22513]

if it’s not a blind hole (though I believe this one is) you can also blow out a tap with a plasma cutter, obviously the placement accuracy is the same as an EDM. On an 6ba that I broke it took about a second to blow it out.

[John Ross 4Participant @johnross4]

Hi Stuart

Thanks for your comments, unfortunately don’t live near, i am in Fleet Hampshire.

Will look out for a local company.

Best wishes John

[BazParticipant @baz89810]

John, you should have a PM.

[Nigel Graham 2Participant @nigelgraham2]

Well, John, you and me both now.

And unlike everyone else here I have no spark-eroders and plasma-cutters to hand.

 

This afternoon I was milling phosphor-bronze, awful stuff to drill and tap. The 4mm diameter drill jammed and snapped without warning, and I was unable to extract the fragment even after carefully milling the surface around it down to near finished level.

I completed the rest of that phase of the machining, and degreased the part with meths prior to treating with alum.

It is now in a glass jar of new Potassium Aluminium Sulphate.

The old solution I started it with seemed contaminated, but it had started corroding the drill noticeably after only an hour and a half. The bronze is untouched.

I’ll leave it overnight, view progress in the morning; strengthen the solution if necessary; then leave it for the rest of the day.

I do not know if the drill had snatched on breaking through into another hole it is supposed to meet, or the broze had clamped it before break-through. If the former the residue has somewhere to go.

Previously, drilling that first hole on the lathe, the drill stuck enough to loosen the chuck in the tailstock taper. I should have used brass- a perfectly good alternative material for a 3-axis connector for a vacuum-brake system so will only be carrying nowtness!

 

So I will be back with a Progress Report….

 

Incidentally I knew of this, and have the Alum, thanks to my having sought the same advice on this Forum a few years ago. In that case, I had snapped a tap in stainless-steel.

[John Ross 4Participant @johnross4]

Hi Nigel

Sorry to hear of your dillema, hopefully you will sort it out. I look forward to hearing how you get on.

Best wishes John

[duncan webster 1]

On duncan webster 1 Said:

Thread on simple EDM here https://www.model-engineer.co.uk/forums/topic/a-simple-self-acting-spark-erosion-machine/

But if you can find someone local if saves making a tool for a one off

i built this when i broke a 6BA tap. did not take long to build, cheap to build and burnt through the tap quickly.

[Nigel Graham 2Participant @nigelgraham2]

All these people with access to / able to build plasma-cutters (sounds like a recipe for disaster) and spark eroders…

I persevered with the potassium aluminium sulphate, successfully.

It went something like this:

Friday eveing: put the part in the solution overnight.

Saturday morning: rinse it to remove residue (the drill had jammed in a blind hole), add a bit more sulphate to the solution. Leave it all day.

Saturday evening: make a bain mairie by standing the glass jar of solution with the bronze part, in a medium-size saucepan of water. Raise the temperature very gradually from cold so as not to crack the jar. Set the water simmering on the gas ring’s mimumum, at slightly below boiling.

A fine plume of what I took to be microscopic bubbles steadily rose from the hole, but not being a chemist I don’t know what gas they would be. At a guess, possibly hydrogen.

I turned the gas off after about three hours and left it all as-was overnight.

Sunday (this) morning: The broken steel was still in residence though diminished in length. I re-lit the gas, set the water just simmering again. When I next looked, about three hours later (apart from occasionally verifying there was still water in the saucepan), the steel had all gone but the bronze was untouched, not even discoloured.

 

So yes, the method does work. It doesn’t need specialist equipment or trying to find companies willing to carry out the work for you – at a fee. Just patience.

I accelerated it by keeping the solution hot: once it was all cool I put the lid on the labelled jar of K-Al-SO and put it away for future reference.

 

Carried on machining the part, only to have one problem after another with trying to hold the wretched thing, a three-way pipe connector, securely, square and centred on the holes so I could machine the exterior around them. It is very small, too small for my Myford’s hefty lathe chucks, and an unusual shape. It ended in failure and I have spent this evening starting another to the same form but using a different machining approach. And brass not phosphor-bronze.

 

 

[Alan CharlestonParticipant @alancharleston78882]

Hi John,

Would it be possible to drill it out using a carbide slotting mill? The diameter would need to be the diameter of the tap at the bottom of the flutes so you would have a good clearance from the cylinder.

Regards,

Alan C.

[Nigel Graham 2]

On Nigel Graham 2 Said:

I persevered with the potassium aluminium sulphate, successfully.

It went something like this:

Friday eveing: put the part in the solution overnight.

Saturday morning: rinse it to remove residue (the drill had jammed in a blind hole), add a bit more sulphate to the solution. Leave it all day.

Saturday evening: make a bain mairie by standing the glass jar of solution with the bronze part, in a medium-size saucepan of water. Raise the temperature very gradually from cold so as not to crack the jar. Set the water simmering on the gas ring’s mimumum, at slightly below boiling.

A fine plume of what I took to be microscopic bubbles steadily rose from the hole, but not being a chemist I don’t know what gas they would be. At a guess, possibly hydrogen.

I turned the gas off after about three hours and left it all as-was overnight.

Sunday (this) morning: The broken steel was still in residence though diminished in length. I re-lit the gas, set the water just simmering again. When I next looked, about three hours later (apart from occasionally verifying there was still water in the saucepan), the steel had all gone but the bronze was untouched, not even discoloured.

 

So yes, the method does work. It doesn’t need specialist equipment or trying to find companies willing to carry out the work for you – at a fee. Just patience.

I accelerated it by keeping the solution hot: once it was all cool I put the lid on the labelled jar of K-Al-SO and put it away for future reference.

 

 

 

That’s a good description of how to do the job Nigel, perhaps that should go in the magazine under “readers tips”

The K.Al.So is readily available and a lot cheaper than a spark eroder.

Andy

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