# Difficulty with silver soldering broken tooth on back gear



## ericc (Feb 20, 2018)

Hi.  I read in the Internet that a broken back gear tooth can be fixed by making a new one out of cast iron, slotting the broken spot in the gear, and silver soldering the replacement tooth in.  I tried this and had problems.  It was difficult to get the assembly hot enough without overheating the tooth.  The problem is that the tooth does not have good thermal contact with the rest of the gear, and the flame is just too hot.  A National propane-oxygen torch was used.

Since there are suggestions all over the Internet about silver soldering gear teeth, I decided to look at Youtube.  Interestingly enough, there are no clear examples.  There is a video by Halligan showing him building up silver solder with an air acetylene torch, which has a much softer flame.  The problem about this is it does not involve that pesky small piece of cast iron.  The most intriguing example is a video by Keith Rucker which clearly states that he is silver soldering in a tooth machined out of a cast iron bar.  There is a beautiful illustration of him machining the tooth.  But the part where the tooth is silver soldered in is missing!!!!!!!!!!!!  He states that the camera ran out of memory, so all the footage was lost.  Way down in the comments, there is someone who voices his suspicion that the silver soldering didn't go so well.  The commenter points to a visible gap on the side of the tooth after silver soldering.  It is pretty well known to those well practiced in the art that there should be a fillet in the gap.  Anyway the commenter makes some pointed statements about the missing segment of video.  Normally, I do not take most comments to Youtube videos seriously, since there is a lot of noise.  But there is one noise that is unmistakable. That is the ping sound of an incompletely silver soldered cast iron replacement tooth breaking free during a test and followed a fraction of a second later by the sickening sound of that tooth hitting the floor.  This noise made me really sensitive to that negative comment to Keith Rucker's video, which was otherwise excellent.  Are there any videos showing the job done successfully?


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## DAT510 (Feb 20, 2018)

Many years ago I watched/helped my boss braze a cast iron part back together.  We had similar issue with even heat as you describe.  He ended up borrowing an electric furnace where we heated both parts up together,  very close to the brazing temp.  My job was to pull the parts out of the furnace, while he was at the table torch ready to braze, before everything cooled off. 

Not exactly silver soldering, but a furnace might help.  I've also seen furnace's used to do the complete solder process.  The parts were fluxed and placed in the furnace with strips of ribbon solder along the joints.  Once parts reached temp, the solder would melt, fusing the parts together. 

Hope this helps.


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## coherent (Feb 20, 2018)

If you have or if you know anyone with a tig, I think tig with nickel rod would be a ok option. Aluminum bronze rod will work and is lots cheaper than the 90% or higher nickel rod. I read somewhere about a mig wire for cast iron which is a iron-nickel composite.  I think a number of different maufacturers make it. Don't remember what gas is recommended but think it was just argon. The wire was pretty pricey if I remember correctly.


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## benmychree (Feb 20, 2018)

The trick to doing a job such as this is first have the parts absolutely clean; bead blast that portion of the gear where the tooth fits and apply flux liberally, and I agree that silver solder ribbon is the way to go.  Then do the heating INDIRECTLY,  on both sides UNDER the area where the tooth is, and do not directly heat the tooth, if you do it will overheat and likely the silver solder may not adhere due to overheating/ oxidation.
As a sidebar, I have brazed a fair amount of cast iron; my secret (not carefully guarded) is to do the veeing out and also grind at a shallow angle adjacent to the vee, and LIBERALLY apply silver solder flux to the whole prepared area and preheat the assembly, and when near brazing temp (still black heat) play the torch on a small area until very low red and apply cast iron brazing flux by sprinkling it on to that spot and then apply a fluxes brazing rod and heat until it flows and wets out on the prepared area' I make several of these 'tacks" to keep things in position, then start brazing along the unbrazed area, not worrying about filling things in, but just wetting the brass out, then going back and filling in the prep area to finish height.  My best kept secret is that most all brazing fluxes, even "cast iron brazing fluxes do a poor job; I use Anti Borax EZ #3, it makes a world of difference.
If I were repairing a lathe back gear due to broken teeth, likely I would make a new gear from scratch.  That would be more difficult on the bull gear, but the cone gear and the backshaft gears it is a fairly easy matter; a short time ago, I repaired the cone pulley gear on a Monarch 9" lathe from the 'teens or 20s, just machined off the old one, undersize enough to have about 3/16" wall thickness at the bottom of the teeth on the new gear, turned a new blank to press over where the old gear was, cut the teeth, pressed it on with a light press fit and drilled and tapped for 3 setscrews at the interface of the press fit to "key" the two parts together.  
Doing the repair as has bee suggested above may not be the best avenue of attack, but I think if I did it in that manner, I would make the tooth of steel and after soldering it in (made oversize) I would then machine to  finish size and form.  Another thought would be to dovetail the slot in the gear and make the tooth a tight fit and perhaps just swage it in place, the cut the form.


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## benmychree (Feb 20, 2018)

coherent said:


> If you have or if you know anyone with a tig, I think tig with nickel rod would be a ok option. Aluminum bronze rod will work and is lots cheaper than the 90% or higher nickel rod. I read somewhere about a mig wire for cast iron which is a iron-nickel composite.  I think a number of different maufacturers make it. Don't remember what gas is recommended but think it was just argon. The wire was pretty pricey if I remember correctly.


That would likely result in a hard weld that could not be machined.


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## markba633csi (Feb 20, 2018)

Not an easy job no matter how you do it, but if you could do it well and the word got out, you'd have all the business in town and the next two towns over.
Mark


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## cathead (Feb 20, 2018)

I would follow John York's plan with the dovetail approach.


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## Uglydog (Feb 20, 2018)

Couple summers ago I built up the broken teeth on a spur gear for using nickel bronze braze then turned her to OD.  
Followed up with cutting the profile with an invloute cutter.
The biggest challenge of the entire project was getting the cutter to line up with the good teeth.

Daryl
MN


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## Briney Eye (Feb 20, 2018)

I've had some success brazing cast iron this past year with Harris Safety-Silv 45 alloy.  On a small job I used Harris Stay-Silv paste flux, and on the bigger jobs Hot Max 24000 powder flux.  I used two MAPP gas torches, fire brick, and patience to get things hot enough.  The small job, the brake ring for my Clausing 8520 mill, was a clean break with a surface around 1/2" square.  The whole piece probably weighs less than a pound, and the joint heated up quickly enough that the paste flux was sufficient.  I made a light spring clamp from mild steel to hold it together, fluxed liberally, and heated it to glowing.  If the surface isn't wetting, it's not fluxed or hot enough.  The bigger job, a broken milling vise handle, was more difficult.  The paste flux was burning off before the workpiece was hot enough.  The powder flux worked beautifully, though.  I just kept sprinkling it on the lightly clamped joint and touching the rod to it.  It took a few minutes, but things finally got happy, the surface wetted, and I got a beautiful joint.  The medium job was a broken miter gear carrier from a lathe apron, and the powder flux is what worked for that as well.  Same thing, once the piece was hot enough the surface wetted and the braze was sucked right into the joint.

I didn't grind on things beforehand, just clamped the clean joints together lightly.

The lesson that I took was that fairly diffuse heat will get the job done.  The key is patience and the right flux.  For a broken tooth I would get my firebrick perfectly flat, lay the gear on it, apply paste flux and insert the replacement tooth with a light clamp from mild steel flat bar, and start putting the heat to it.  Then I would just sprinkle a little powder flux over the joint every so often while waiting for things to glow.  Once you've got a good red-orange going start alternately dabbing the rod on the surface and sprinkling on a pinch of flux.  Try to direct the heat so that it's the surface of the work melting the rod, not the flame.  The rod should instantly wet the surface if it's properly fluxed and hot enough, and be pulled right into the joint.  Then wick just enough rod into the joint to fill it, and stop.  It's probably a good idea to cool things down gradually from there.  I just started backing off the torches until the glow faded, turned off one torch, backed off further, turned off the other torch, and let it cool off the rest of the way on its own.


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## pmat (Feb 21, 2018)

I recently repaired a broken backgear tooth using techniques gleaned from watching a Keith Rucker video and some advice on the practical machinest forum. First I machined out a decent channel at the base of the broken off tooth, then filled in the area between the existing teeth with silicone bronze brazing. My first attempt at brazing, and I was surprised that it was fairly easy. In the Rucker video, he uses a dividing head and special gear cutter to cut out the gear tooth. Since I don’t have a dividing head or special gear cutters, I used the technique from the practical machinest forum which uses a jig to index the cutter placement using existing good teeth. The cutter is ground to fit the void between the teeth. It worked pretty good but did require some hand filing. 
	

		
			
		

		
	







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## cjtoombs (Feb 21, 2018)

I did a repair on a bevel gear with all but one of it's teeth missing by building it up with brazing material and then filing it to shape.  It was a bit dedious and looked like crap when it was done, but it worked.  It was on the table lift of a shaper, so as long as you loosen the bolts holding the table firm before you try to crank it, it doesn't see much stress.  Here's the tread:

https://www.hobby-machinist.com/threads/bevel-gear-repair.50352/#post-433389


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## NortonDommi (Feb 21, 2018)

I've repaired quite a few damaged/ chipped/ missing teeth on cast iron gears and if it is a broken gear that you are putting back together or for odd teeth Bronze welding is the way to go. Build up the missing area and cut a new tooth/teeth. If filing use new files. Rough shape with a thin cutting disk on a small grinder. Die grinders are good.
  Trying to Silver solder cast iron parts is not a good idea. Brazing and Bronze welding are the recommended methods for cast iron repair and it is easier to make new teeth from Bronze than try and reattach a broken tooth. Oxy-Act is the way to go as the heat helps alleviate stress, a pre-heat helps with the weld as well. Several of the Bronzes are suitable.
  An old Audels book on welding usually has a section on this repair. Have a look at The Internet Archive.


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## ericc (Feb 21, 2018)

Hi Fellas.  Thanks for the tips.  I should have fired up the forge, but my forges are all hot enough to melt cast iron.  They could be dialed down, but I just am leery of putting something that delicate into a forge.  I have plenty of fire bricks, but no mounted burners.  Seems like there is some more prep work to do.  That suggestion about tig welding with nickel rod sounds interesting.  I realize that it is easy to make a nickel arc weld non-machinable, and the more nickel you lay down, the riskier it is.  But I suspect that the hybrid method with nickel tack welds and aluminum or silicon bronze tig welding might be possible.  I really prefer tig welding.  I have arc welded cast iron before, and this appears to be less challenging due to the geometry.  Two little tacks on each side, then go to work with the bronze.


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## ericc (Feb 23, 2018)

Would this wire work for tack welding the ends before silicon bronze brazing the replacement tooth? It is Techalloy KZ 9525-529-0443 wire nickel, chrome, iron alloy.  It probably has a high melting point, but looks like a high nickel stainless.  It is non-magnetic.  Hmmm.  I cannot seem to attach a picture.  I'll try later.


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## Silverbullet (Feb 23, 2018)

Stick with the old way , brazing and flux then a slow Kool down wrapped in a blanket set in an oven overnight . Some use an old charcoal fire braze it after heating then place in the old coals to die and keep warm for several hours. Seems like the old way still works .


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## ericc (Feb 25, 2018)

Hi silverbullet. You are right. I sort of wanted to come up with a new clever way making use of something I already had, but I failed. The inconel wire melts at too high of a temperature. I thought I could make a quick tack and let it cool, but by the time the filler melted, the tooth was ruined. I could lay down good bead on the flat, but small fillet welds were impossible. Also, the welds ended up being non-machinable.

I ended up putting the gear in a can of perlite with only a bit poking out and used a big propane hose torch to heat it. It worked, of course.


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