Brazing Help!

[11echo]

...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’m aware of is that I'm using my cutting torch to braze with. I find this is the only thing I have that can put enough heat into the cast iron to do the job! This pedestal is pretty thick in places! I have ground out the cracks, and I've sand blasted the edges (seems to help with the braze adhering to the area to be brazed), I use flux and a large diameter brazing rod, and I try to puddle the braze to get a solid area. BUT trying to do this with a cutting torch is abit trick, especially in tight areas. SO my questions ....what size tip should I be using? …Mark

[jimmy seale]

maybe # 2 or 3?

[John B]

It may be advisable to drill a small hole at each end of the crack to try and stop it splitting further even when brazed, preheating by use of a large burner head would also help, and a slow cooling rate after the job is done.

[11echo]

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!!?

[Thomas Dean]

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

[R Funk]

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

[Hillbillysmith]

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.

[Covforge]

If you have tig get a nickle ac rod and nock all the flux off and sand it bare. V it out heat to just smokeing and weld away. The more nickle the better it will weld. I've tryed Brazing, Ac welding, Mig , but Tig is the best for me.
Just my Idea
Travis

[11echo]

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

[irnsrgn]

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.