Tapping threads in bronze

[preserved_killick]

This issue is sort of a pre-issue to recoring my cockpit floor. I had a heck of a time getting my tiller head off from the rudder shaft.



YYesterday, after days of heating, spraying, tapping, heating and waiting..I put the wrench on the one bolt bolt that holds my tiller
head to the shaft, and of course the head came off. Luckily, there was just a little stump of metal sticking out past the shaft that
prevented me from removing the tiller head which was easy to grind out allowing me to remove the tiller head.



Today, with the tiller head out of the picture, I got myself a couple of screw extraction kits, which of course failed. The bolt isn't that
corroded, it just seems to be part of the shaft now. When I drilled the hole for the extraction bit, I drilled all the way through the
bolt since the hole in the shaft also goes clean through.

I could see with a mirror the back side and clear through so with my hole evenly centered I, drilled out the hole in the bolt larger and
larger, moving up in drill bit sizes to where there's not much bolt left. I used a chain saw file to further file out the bolt. I got the
bolt to where just the edges are left. Thinking I could remove this like a cutless bearing I put a modified hacksaw blade in the hole to
create a split, thinking I'd dork up the threads in the bronze but cleanly in just the one line. I got the cut finished and still cannot
remove the steel edges of the bolt.



What I'm thinking now is that I should give up on saving the threads. Just drill it all out and tap threads to the next larger size.
Currently I believe the bolt was 3/8. The hole in front of my tiller head measures 7/16. Perhaps I could drill this all out to 7/16 and cut
some threads for the new 7/16 size. How difficult to tap bronze? I'm not expecting much trouble, but one lever knows.

Once (if) successful, should I use a steel or bronze bolt for this?? It would seem that 7/16 bronze is strong enough, but then again the
bolt is pretty important. Stainless and some anti corrosion substance like tef-gel??
PK

[Quetzalsailor]

Oh, what undeserved fun! Can you file just a smidge further and expose the first few thread tips? Then pick the remains of the bolt's threads out with a dentist's pick? Then you could probably run a tap through and clean out the rest.

Bronze should tap pretty sweetly.

[Zach]

Find a chart that has the tap size for the size thread you are working with. Normally they are numbered, or lettered. If the hole is centered, drill it out again to the tap size.

Then take a pair of tiny needle nose pliers and pull the threads out like a coil spring. If the hole is off centered, drill it out to the next size and tap it out.

Silicon Bronze can be ridiculously hard. It does not like normal metal cutting twist drills. Trying to describe this... You need a bench grinder or dremel type tool to modify the relief side of the drill, the metal behind the cutting edge of the drill is to flat of an angle. If you narrow the flat to an 1/8th or so, and taper the "thickness" of each spiral so that the bevel is not quite twice that of what you started with.

Zach

[Tallystick]

These drill bits for copper and bronze from McMaster work well for bronze. Direct link isn't working for some reason. Go to this page and scroll down to High-Speed Steel Jobbers' Drill Bits for Brass and Bronze http://www.mcmaster.com/#standard-drill-bits/=h1wowq

I'd try drilling it out with with a 5/16" bit (if the fastener is 3/8"). Then try chasing the threads with a tapered 3/8" tap.

[preserved_killick]

Finally got back to this. Using a flashlight and a mirror to see the back I was able to (painstakingly) file the steel out and just barely touch the tip of each thread in the bronze. Then was able to clean out the first few threads using a sharpened punch which let me start the tap to chase the steel out of the rest of the threads! It worked. The threads look pretty good too. What a little project this has been! A test of patience!

Thanks for all the advice!

-Jeff