Cast iron brazeing repair, joint prep

I'm repairing the cross-slide dovetail on my lathe, the last couple inches on the back side were snapped off some time ago.



Back when the saddle got busted it was repaired by 4 long capscrews pulling the rear wing onto the bridge. Once I get the new dovetail pieces fit & screwed down along with a couple other keys elsewhere, then I'd like to braze the entire repair- including the old joint.

So my question is since the repair is stable and the two halves seem to fit very well as is, should I take it apart to prep the surfaces or will it be enough to vee out the joints and rely on flux to get the filler down into the crack?

I think I'll have plenty of heat for the job, I'm borrowing the burner from my brother's turkey cooker to help with the preheat and I'll have an oxy/acetylene rig.

Regards,

Gregm

I don't think bronze will penetrate a close fitting crack very well. Its just not flowable enough, IMO.

I don't really like brazing because of how much heat it takes, and this heat can cause permanent distortion. I would count on remachining the dovetail afterwards, to make it clean up and bear some load, since its purpose is more than just to be a decoration

I might try to jig it up so that the broken pieces were purposely offset a wee bit into the dovetail guideway. This would permit a light cleanup cut to be taken on the welded in pieces, without cutting the rest of the good dovetail.

I think I would use a ferro-nickel rod and arc weld it: better strength and better color match.

I have made some successful cast iron repairs and some that were not. I found that your have to remove all the oil that has permeated the cast iron. Preheating the whole part(s)in a BBQ works good. I let it bake for a while way after it stops smoking from burning off the oil. I use the same for post heat treatment. Grind to clean metal. I like to dowel with threaded set screws or unplated all thread where you can prior to welding.

As mentioned expect warpage.
Sloooow cooling will avoid creating white (hard) cast iron.
Good luck.

You will not get any penetration to speak of into the crack. My best results have been with grinding or milling the braze preparation, draw filing the surface to get rid of graphite smearing. Washdown with acetone and then tin the surfaces with braze material. Then I jig the parts in place and flow braze into the tinned surfaces to fill the joint.

My best results have been with Harris Welco 17FC which is a nickel copper alloy rod that is flux coated nd then I use an additional liberal amount of borax on the joint.

Yup, JPfalt is on it. I also like the heat method of getting the oil out of the parts. Heat it to just barely red hot and let cool. You can then brush it with a wire brush to get the carbon scale off and then wash with hot soapy water. V the joint and then tin it if you can. The more tinning you do, the better the flow will be.

The main thing to remember here is that the brazing is not the only thing holding the parts together in this joint. The primary strength is going to be the screws, which will be serving just like rebar in concrete. The braze is just there to keep things from squirming around, wallowing, loosening, or otherwise working the joint. I always try to make the screw repair heavy duty enough so that it is capable of holding the load itself (which it obviously is if the carriage has help up this long). In this case, super deep brazing penetration is not going to be critical.

I can't imagine a preheated and carefully brazed repair of this type would create any more or less warpage than an arc weld, and it will certainly be less likely to break again. A part that large will be VERY hard to properly cool without a post-heat furnace.

What about siver solder? I understand it can be as strong as brazing and you can tin both sides first.

Take a look at this: http://www.locknstitch.com/CastIronWelding.htm

Cool- thanks guys. I've not worked out the post-heat yet, gotta give that some thought. Thanks for the locknstitch site- very helpful.

Regards,

Gregm

You will definitely have less warpage and lower residual stresses with the braze using an OA torch. The reason is that you are heating a larger area more slowly, which lets the heat move through the part. One issue with arc welding is that the heat is intense in a localized area which leads to higher residual tensile stresses when the weld cools. The solution for arc welding is to preheat the workpiece above 500 deg F before starting welding. It doesn't hurt to do the same for the OA braze.

One reaason I like the 17FC is that it has a tensile up to 100,000 psi, wets clean cast iron well and actually ends up stronger than the surrounding cast iron. Usually on repairs I chamfer back from the original break to increase the surface area of the braze and to put more of the 17FC in the higher stressed areas as it is much stronger than the cast iron. By using preheat and jigging the parts you can minimize the warpage and machining required to get back in shape after the repair.

Someone else had asked a question in the forum back in 2003 and I passed on my method for brasing Cast iron.

[http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=002555]

As to the idea that you will permanently distort the piece I say this: It will warp only if you dont heat the piece evenly.
Awesome metal Iron It conducts energy very well.
And
As for cleanup if you follow my method you will have a seem of bronse and some bronse tint bonded into the surface around the repair. A thorough scuffing with 000 steel wool will clean up most of that. If you stick weld it you are most likly going to get micro fractures in the surrounding metal due to the fact that the rate of cooling unless performed in an oven will be uneven. Bronse like brass is mostly copper which will only cool as fast as the iron surrounding it. This is why you need to heat the repair areato just the right temp to achieve the sparkle effect I discribed in the post in 2003. Its the point where the bronse flow reches an idealfusion with the cast iron. I learned this method from reading several years of blacksmithing text as well as trial and error while working at repairing antique iron ornamental pieces. The true test for a welder of cast iron is to repair a thin plate which has to fit into a frame as in a victorian (about 1840-1900) era Fireplace shield. These were locked into an iron frame set into the face of a fireplace when not in use. A stick weld job even if done cold destroys the details in the face of the piece as well as shows the thermal fracturing. Most modern books only describe one or two types of cast iron but there are actualy 3: Black Grey or White. black is durable but not very workable for machining, White like black is also not a desirable metal for machining owing to its ceramic-glass like britleness, and grey which ias the most commonly used because it has the ability to be worked hot or cold. all can be repaired but since most all modern materials are some grade of steel alloy and less a base iron product real iron repair isn't taught much.
Another upside of brazing is that since it is more like soldering you need not gring the Holy BaJee-bus out of the thing since the key is to draw the bronze into the crack. You can also re heat and free the pice if you find that it has shifted and bcome mis aligned that you can't do with the stick weld

Hope this will help
John

Its the even heating and cooldown thats somewhat problematic. The piece is a lathe saddle weighing about 100 lbs and is pretty bulky. The existing repair is robust, intact and stable so an argument could be made that I might just replace the busted portions of the dovetails and use silver bearing solder just in those joints. Thanks for the pointer though, I'm giving it some thought.

Gregm