On small threads like that although you were backing off to break the chips they will build up around the tap so worth withdrawing and cleaning the tap maybe half way through. Alternative is a spiral flute tap that will push the swarf up out of the hole.

I would use a larger tapping drill than 2.05mm; you could probably get away with 2.2mm, which considerably reduces the load on the tap and provided you’ve got more than 3 or 4 threads, won’t significantly reduce the strength of the assembly.

Try soaking the existing ball in a saturated solution of alum. Watchmakers use it to get broken steel screws out of brass. May discolour the ball but should turn the broken tap to mush.

Hi Steve,

Sulfuric acid should eat the tap out leaving the bronze ball relatively untouched.

Hope this helps.

Peter.

+1 for Alum (Potassium Aluminium Sulphate, AKA Fatakdi, Fitkari).

I use a peristaltic pump to direct a jet of solution (under the surface of the liquid). This keeps fresh solution on the hole and flushes the debris out. I happen to have a lab pump, but you can get small ones on ebay and amazon. Sometimes called dosing pumps. They have the advantage that the liquid only touches the inside of the plastic pipe so no corrosion.

Robert.

On 20 May 2024 at 19:22 Dalboy Said:

Something worth looking at when tapping any material with a T handle is the way the tap is held.

This video explains it better than I could.

https://youtu.be/C0PBPGjd0Pc?si=Lg1M7CO4v21t_ikD

 

Thanks for the link I have seen the video before but re  watched anyway

H

Spark erosion is MUCH quicker if you know someone with such a machine. As has been said a slightly bigger tapping drill. Brass would be cheaper and do the same job. Noel

Hi Steve – as I said help is at hand but to add to the other good advice

Phosphor Bronze – as opposed to gunmetal – is probably one of the worst materials outside of some stainless that the average ME comes up against. It is very prone to work hardening and will seize and break a drill/tap quicker than you can see or feel it happening. Reaming is also fraught with difficulties rapid heat build up and seizure quite common to the uninitiated. Best advice is to avoid it when ever possible.

I have never tried it but old school advice is to use alum – whether it works efficiently I have no idea.

On something like this – or a casting for instance it is better to drill the hole over size then Loctite in a pre-tapped insert. If you are unfortunate to break a tap in the insert then at least the part is not affected.

I can’t help with a spare ball that size. If you really want non ferrous then I’m not sure where to get them but you can purchase steel ‘soft balls’ from Simply Bearings which is a more realistic alternative.

Be in touch later – Tug

I bet the full size had cast iron balls. Not worth messing, just buy some soft steel or brass.

This reminds me of the rugby song version of ‘do ya Ken John Peel’ where the named gentleman had genitalia made from steel and brass, and a ‘cast iron **se’

It looks like ARC’s is from the picture, can Ketan confirm? If so I’ll buy a set, looks like a useful bit of kit

On 21 May 2024 at 10:07 SillyOldDuffer Said:

On 20 May 2024 at 22:38 MikeK Said:

Having recently tried alum to dissolve a tap broken in aluminum, I wish anyone good luck.  Heat helps, and may even be necessary.  Take care to not use a steel pot.  I used a Pyrex glass dish sitting on a hotplate.  Lots of bubbles, but didn’t see any progress after two [8-hour] days.

(I got my alum in the grocery store, it being a common item used for pickling food items.)

Chances are it’s the wrong Alum!   Our forefathers in trade called a number of different chemicals ‘Alum’, and we are still confused by their imprecise nomenclature today.

There are 3 common Alums on sale: Potassium Alum, Sodium Alum, and Ammonium Alum.   Of these, only Potassium Alum (E522), dissolves steel.  (Robert’s post identifies the correct Alum.)

What may have happened is that Ammonium Alum is a cheaper substitute for Potassium Alum, satisfactory for some purposes, but not dissolving taps.  Or Sodium Alum is also food-grade product (E521), opening the door to buying another Alum that won’t work.

What the tap is made of may be a problem too.   Carbon-steel taps are easy meat for Alum, but High-Speed Steels contain Chromium and other elements with distinct anti-corrosion properties.  Their similarity to Stainless Steel suggests an HSS tap may resist Alum for  longer than expected.   Ditto coated taps, because the likes of Titanium Nitride will make it hard for the Alum to get at the underlying steel.   It will work eventually, but may take an age…

Dave

 

Mine does say “potassium aluminum sulfate” on the jar.  Tap might be HSS, but knowing how cheap I am I would have thought carbon-steel.