tig brazing cast-iron with Sif Al Bronze no32

[carl0s]

I have a small hole to fill in a cast iron piece. It's where a grub screw used to intercept the toolpost stud of my lathe's compound slide. The whole thing is rounded out (toolpost mounting) so I want to fill the grub screw hole, then mill/drill/tap the stud hole itself out to m18 or m20 (can't remember actual sizes and numbers off hand. existing rounded out threads are m16 or thereabouts)

I'm going to try some Sif Aluminium Bronze No32. I picked up some short lengths from an eBay seller.

I'm a bit confused though. eBay seller shows a Sif data sheet which seems to say 'DC Positive'. Sif brochure seems to show DC Negative. Other people on the internet are using it in AC. I suspect the latter (AC) are actually welding the base metal with it, while the DC use is actually brazing. Does that sound correct? I think for what I am doing, brazing will be perfectly fine and presumably much easier ?

 

[carl0s]

This is the bit I'm talking about - the grub screw bit. I'm intending to fill with Sif AlBronze no32.

I could even try to braze around the entire edge and re-cut at original size. maybe.. not sure though. I seem to remember it's an odd M16. like m16x1.75, which probably isn't actually a metric thread at all. The lathe is imperial, but this is a non-original stud for a multifix QCTP.

 

[Pete.]

Definitely I would not put brazing heat anywhere near that compound lest you deform it permanently. Turn a slug of steel or iron to fit in the hole. Bore it to accept your new stud then glue it in so that it's just below the surface, Then simply bolt your tool post to it and call it done. If you don't like the idea of gluing it in you can make it a slip fit and scotch key it in but any half-tidy glue or bearing fit will fix it in there so that it's never coming out.

 

[Brad93]

You don't want Aluminium Bronze either really. You want silicon bronze. Easier to machine or grind, and more ductile.

 

[Brad93]

AC or DCEN is the only way it will run.

DCEP on a tig will just ball a massive tungsten blob

 

[Brad93]

Direct you to @doubleboost videos on youtube

 

[carl0s]

Thanks very much. I'm sure I'll do something with the AlBronze another day. Glad I didn't buy much of it - quite spendy.

In terms of heat and warp-factor, wouldn't TIG brazing such a small bit be OK ?

However, what you have suggested Pete (a slug insert) is actually exactly what my father in law suggested, and was going to be the plan. with a large long grub screw through the side.

 

[Pete.]

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Turn it up to a loose sliding fit, degrease with acetone and glue it in with that stuff. It ain't ever coming out.

 

[Martin Hocking]

Definitely I would not put brazing heat anywhere near that compound lest you deform it permanently. Turn a slug of steel or iron to fit in the hole. Bore it to accept your new stud then glue it in so that it's just below the surface, Then simply bolt your tool post to it and call it done. If you don't like the idea of gluing it in you can make it a slip fit and scotch key it in but any half-tidy glue or bearing fit will fix it in there so that it's never coming out.

I tig brazed a cracked cast iron milling vice sliding jaw once and it pulled in so much it didnt fit on the lower afterwards. I probably did it badly mind you but it would seem to concur with warping being likely…