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Brazing Help!


11echo

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...I'm a hobbyist machinist, and I have just picked up a little knee mill that's probably about 70 years old. I'm in the process now of cleaning it up. In this process I noticed that the cast iron pedestal is cracked in two places. Not real long cracks, one is about a 1 1/4" and the other is about 2 1/2". This is at the base that has cast in webs, so varying widths of cast iron. Which is kind of the problem, I'm finding it hard to get the area to braze hot enough to do the job! One problem with my brazing technique that I

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I did stop the one small crack; the other goes clean thru to the edge, so nothing to stop. I have a #2 tip, but that's too small to put out the heat I need. Don't think the #3 would do much better!?? ...I do have a rosebud tip which I planned to slow the cooling process with. I guess I could try to pre-heat with it!!?

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May I suggest getting a #4 tip. A good preheat will help, as John B said. 1/8" rod should be plenty. The slow cool should be accomplished by wrapping in kaolwool or completely covering with DRY sand. Good cast, as this most definitly will be, is not that difficult to braze, IF you can get it hot enough. I have brazed broken valve yokes on 24" valves with no problem, ya jest gotta getter HOT! :) Good luck, and be patient

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What is the likely cause of the crack?

Was it dropped or hit?

It may be a crack caused by poor casting design.

If I can see no cause for the crack I might be tempted to drill a hole at the end of the cracks to keep them from spreading and call it good.

I have done a fair amount of cast iron welding and brazing and I am always concerned that I make the situation worse by thermal sresses

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If you have a TIG welder, you can braze weld it with 1/8 bare silicon bronze rod. This would help with not being a able to get it hot enough and it will look ALOT better too. In the event that you don't have a TIG machine, there is nothing wrong with brazing with a cutting torch. It is a little cumbersome, but works better, IMHO, than a welding/brazing tip. I have a W-6 brazing tip that would certainly do the job that you have and it's a bit lighter than the full cutting head. You CAN use the rosebud to get a good preheat, but I wouldn't advise doing so because you want to be able to control the heat input. IDK what your brazing background/skills are, but with some practice, your cutting head should be completely suitable. I would also try and make a groove into the crack with a grinder (if accessible) and build it up from there to the full cross-sectional area with some re-enforcement. Another tip, you don't want to make the braze too fluid because it won't build up like it should. I do 99.9% of all brazing jobs that come into our shop at school. I also had to braze a tab back onto the firepot of my forge cause I broke it off trying to get a bolt out and I just used bare electrical copper wire with no flux. It works like a charm! :) Hope I helped. Good luck and fill us in as to how it turned out.

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Unfortunately I don't have a TIG welder! :( ...So I'm stuck with the gas welder. Someone has suggested to use a carbonizing flame when I brazing! ...Any thoughts on this? ...And to what degree ...how much of a "feather" off the tip of the blue flame?
"Cause of the crack"!?? ...Not a 100% sure, but 2 things stick out for me! Poor casting is a good part of it, but design of the casting is also a part. There is a rectangular hole near the corner of this pedestal (coolant or lubrication return I’m guessing), and the cracks have come off the apposing corners of this hole (would have been nice if they used a round hole!*G*), reaching to the edge of the outside web! I don't see any damage as if dropped.
Another question I have, is it better to try to braze this thing from the inside to the outside edge, OR the other way around??

...I would also like to THX everyone that has taken the time to respond to my question here! I find it VERY helpful!!! ...Mark

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books all call for an oxidizing flame, but I learned from old masters and a neutral flame is best, everybody is saying a certain size tip but not the brand name, there is a world of difference in tip size by brand. I use Smith torches and the biggest I have for brazing is a #12 which has about a 1/8 hole in it.

The trick to brazing is the tinning of the base metal, the best flux for this is the Oxweld flux for brazing cast iron, it comes in a can with a green label.

I have done several brazing jobs where I used 1/4 brazing rod and put upwards of 20#'s of filler in.

The biggest job I have ever done was on a rubber machine for Chicago Rawhide seal plant. Used one of the large WS Acetylene tanks and 5 large K oxygen tanks to get it done. not to mention about 40 lbs of charcoal briquettes and about a 5 hour preheat with them, then another 30#'s of charcoal briquettes for post heat.

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Irnsrgn is right about the flame. To save on heating he has a good Idea with charcoal or wood. Get a good hot clean fire and let it soak up the heat and it shouldn't take so much from the torch. then stick it back in the fire to cool slowly. It would be the same as putting it in the oven.
Travis

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I understand about pre-heating, but there are two issues with that! One this thing is cast iron about 4'-6" tall with a 18"x 24" base ...i.e. weights a ton! So I'm limited to "freely" moving it around (especially hot!) And Two where I live building a fire outside is fronded upon by the "Air Care Board" ...HOWEVER I have come up with an idea, I have a propane shop heater, I could fire-up and park next to it, that should get it pretty hot!!?

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I don't want to be to repetative (sp) but I would again ask the question why do want to repair the crack? Are they in a critcal location?

If they are in critcal location stiching and pinning could be the preferred repair. (there was a post and a link on stiching and pinning on IFI a month or so ago)

How long have the cracks exisited? Where they loaded with grease and years of dirt?

Are you concerned about structural strength? Cracked castings in non crtical locations have existed in in some machines for years with no adverse affects

If your concern is asthetic Bondo will do wonders

Just some encourage to reconsider and not make some of the mistakes I made.

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reminds me of a local farmer, one of them know it all types, if you don't believe him just ask him. He brought half of the variable speed drive pulley in that had a crack and insisted I gob it up with some nickel rod. I did what was the critical part to him, and left some of the crack cause I knew what would happen. He was very frustrated with me, so he purchased some of the few nickel rods I had and went next door to the impl dealer and borrowed their welder, and proceeded to daub it up with nickel. He never came back, but one of the mechanics told me what happened, just as I had forseen, Bang and he had two pieces. It broke right thru the middle and he had to wait 2 weeks for a new $300+ replacement part. To this day he still won't speak to me when we happen to meet.

LOL

There is a long Crack in the Anvil of my Depew Helve hammer caused from shrinkage when the casting cooled. No one has ever attempted to fix it over its life span and I have no intention of every trying either, It doesn't hurt anything at all. Why put the same stress back into it that caused it to crack in the first place. I learned a lot from the old timers I was around as a kid and learned a lot more from my own experiences over the time I was a shopman.

The important thing is, are the cracks getting worse with use or just lounging around.

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