Cast iron repair part two

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I need to repair the following cast iron motor cover that is hanging on the right side of the tablesaw.  It is cast iron around 1/4" thick.  What appears to have happened is someone dropped it and now there is a super tight crack from the lip going 3" in.  The only reason I want to attempt a repair is I know if I repaint it without stabilizing it, the crack will show through the paint.  The part is 70 years old so I only have one chance to do this right.  My initial though, and what was recommended here, was to oxy/fuel braze it, but I am worried about all the heat that is going to be put into the part.  At my disposal is a TIG welder, MiG, and oxy/fuel.  I am a beginner with almost all technologies.  Now what I am thinking about doing is carving a small spot weld size crater with a carbide bur near the edge and either filling it with a 99 nickel rod using DC stick or trying to tig braze with some silicon bronze rod.  Luckily I do have some scrap cast iron to try on.  The piece is not structural, I just don't want the paint to crack once I refinish it.I think that a quick hit with the 99 electrode would be the least invasive and would not require a preheat.Thoughts/comments?Chris
Reply:I would drill a stop hole at the end of the crack, and if it were a long crack, maybe stitch in a few spots with nickel, but smooth it all with bondo.  You don't want to take the chance and have it split in half.
Reply:I would drill and radius a hole at the end of the crack, bevel it, and TIG it with silicon bronze, especially if you're concerned with overall heat input.  You're not going to get by with no/minimal preheat and low amperage with a nickel rod.  The silicon bronze is also going to be more ductile and easier to dress when finished.
Reply:Just prepare as described above, the fill with epoxy, sand and paint.
Reply:Whatever method you decide...think about doing the repair to the inside of that piece,since it's obvious you are doing a super job restoring that Delta saw. At least that way the repair won't be obvious or difficult to finish that way. FWIW
Reply:Thanks guys, I will try some runs tonight.  I only have 1/16" electrodes currently, what current should I run?  The material is 1/4" thick.Chris
Reply:I'd say O/A braze it.  I think the heat from the TIG would be a bit on the concentrated side, while the O/A flame could be danced around a bit more to have the heat spread out a bit.  Pre-heat with a rosebud or weed-burner and slow-cool it (wrap with a welding blanket, bury it in vermiculite or DRY sand/etc) would also minimize the delta-T and chances for cracking after the repair.The Ni99 stick should also work, but unless there is a lot more stress/load going on than just a cover, it is probably 'overkill'.Check to see how much the crack opens/closes as you tighten/loosen the mounting bolts/screws.  That will give you more of an idea as to how strong you need to make your repair.Pretty much anything other than just grinding a small groove and filling with Bondo will mess up the paint job to varying degrees.  And the Bondo would probably crack when the cover flexes slightly when you tighten the bolts/screws anyway.  But since you said you are going to refinish it anyway, not really a problem.  The best laid schemes ... Gang oft agley ...
Reply:Originally Posted by MoonRiseAnd the Bondo would probably crack when the cover flexes slightly when you tighten the bolts/screws anyway.  But since you said you are going to refinish it anyway, not really a problem.
Reply:Alright, I made some test welds in a cast iron sander table that I had lying around.  I cut slots into it with a portaband and sanded the surface fairly clean.  First the NI99 rod:These came out pretty well, for some reason 2 of them had porosity, but I didn't do a perfect cleaning job.  They almost run like 6013 rods where you drag them, I maintained a very small arc.  These are 1/8" rods, and was running them 100 amps DCEN.  I think this could work for me.  I tried whipping the rod a bit, but that didn't seem to work well, it seems like slowing moving it along the joint is the best way to use it.  TIG brazing was a lot harder, I was using 100amps DCEN with a 1/16" electrode and 3/32" silicon bronze rod.  I am really not sure what the technique is with the torch and filler, but I kept balling up the filler.  I tried to maintain an arc length around 1/8" waited until the CI puddled and dipped the rod without letting up on the pedal.  Am I supposed to heat the CI with the TIG until it will melt the rod without any arc?  Can anyone give me a play by play?  As you can see I ran a bunch of beads in a short time frame and the piece "pinged" twice.  Couldn't find the cracks though.  ALthough they look like bird crap, hitting them with a slag hammer did not cause them to come off, so the bond is pretty good.  Obviously I need a lot more practice before continuing.  I only got a TIG welder last week Is a 1/16" electrode not the right size for 3/32" filler?  What size bead should I be shooting for?
Reply:If you are trying to TIG-braze (or anything-braze, O/A-braze, MAPP-braze, whatever), you are trying to heat the base metal but NOT melt it.  Heat the base metal until it is hot enough to melt the brazing filler.  Let the hot metal melt the filler, not the torch flame or the arc.If you melt the base metal, you are welding.And you really can't expect molten cast iron and silicon-bronze brazing filler to mix and form a good puddle.That's pretty much why I suggested using the O-A to braze the cast iron instead of the TIG.  You can use a more spread out flame or pull back slightly with the O-A torch to distribute the heat through the cast iron a little better than the focused arc heat from the TIG.  And pre-heat.A "PING" usually means somthing is cracking somewhere in the cast iron.Also, use flux.  Either separate flux, or flux-coated brazing rods.  Maybe both.  The flux is pretty important to get the metal clean and not oxidized and let the filler flow.  The best laid schemes ... Gang oft agley ...