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PostPosted: August 19, 2019, 1:16 am 
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For my BEC I want to mount a disc brake carrier hub (aluminum) to my Eaton (I think) Chevy 7.5 diff housing. I tried TIG welding threaded steel lugs to it (just tacked them) and they almost immediately cracked. I think I'm using ER70S2 filler.

The diff housing appears to be pretty hard, I can barely file it.

I'm tacking them with the carrier hub bolted to them for location, I suppose that is resisting shrinkage forces.

If the tacks didn't crack right after welding, they did as I was tapping on the lugs to get them true.

I'm tacking at around 140 amps. Is there a technique to prevent cracking and or a different filler? I'm sure it would help to know what the diff is made out of, but...


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PostPosted: August 19, 2019, 7:47 am 
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Are you sure it isn't cast iron?

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PostPosted: August 19, 2019, 8:19 am 
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It may be cast iron. If so, slow preheat and slow cooling are recommended as is a high nickel content filler. Nickel-bronze if you can find it.

I read this on the internet so YMMV.

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PostPosted: August 19, 2019, 9:48 am 
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Hit it with your grinder.
Yellow sparks = steel
Red sparks = cast iron

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PostPosted: August 19, 2019, 5:32 pm 
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It's iron. You can weld to iron with low carbon steel but it's not easy and would not be a good idea for structural parts. I cut and shut iron exhaust manifolds successfully with a mig to make a series of tacks and a lot of peening as it cooled to prevent cracking from weld shrinkage. You need a rod designed for iron with a lot of nickel so expensive but you don't need a lot. You need a rod that produces a weld that shrinks less as it cools.

Brazing is another option but needs a lapped area to place the brazed area in shear. No butt welds for structure.

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PostPosted: August 19, 2019, 9:56 pm 
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So this is what I'm attempting to mount to the diff.

I suppose I could make a 2 piece mount that clamps around the diff. You can see there are "keying" areas that would prevent it from spinning. The brake adapter plate I made fits pretty nice to that geometry. Then I would think I would want a stop on the outside to prevent it working it's way off the diff.

You can see I welded on it before, just 1/16" thick steel to cover some openings. That seemed to go ok.


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PostPosted: August 20, 2019, 5:55 pm 
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The key to welding iron is technique. If you bridge two immovable objects (iron and thick plate), the weld will shrink and crack. The 1/16 steel allowed the weld to shrink and the iron didn't move. You can build up one side to make a steel base to bridge but even if you can get a puddle not to crack on the surface, this is a highly stressed part.Use the right rod. The worst my iron manifolds can do is crack.


Have a ring flange made thicker than you need for machining true, turn the shoulder of the diff in a lathe to make a flat surface for the flange thickness, press it on, check with a dial indicator for true, braze or weld the ring on, then put it back in the lathe to skim to ensure the rotor will spin true and not slide back and forth on the pins as it rotates with the brakes applied.

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PostPosted: August 21, 2019, 3:17 am 
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I was able to get to Madison Badger Welding before is closed and they gave me some high nickle filler rod.

Attachment:
corect_filler_rod_Iron_Nickle).jpg
corect_filler_rod_Iron_Nickle).jpg [ 554.56 KiB | Viewed 1087 times ]


Well this seems promising. I cleaned the diff and mount and filler rod with acetone.

Attachment:
Iron_Steel_TIG_correct_filler1.jpg
Iron_Steel_TIG_correct_filler1.jpg [ 197.54 KiB | Viewed 1087 times ]


I knocked off my first attempt at this, one of them was actually stuck on there pretty good. So I ground clean all that, mounted it up again, and gave er with my propane torch for a bit. And then TIGed them on with the new filler rod.

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PostPosted: August 24, 2019, 3:46 pm 
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Put the Chevy diff in the mini lathe, they said. It will be fun, they said.


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PostPosted: August 27, 2019, 11:35 am 
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OMG - maybe at 1 RPM? Did it work?

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PostPosted: August 28, 2019, 6:18 pm 
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It "worked", but I wouldn't recommend it. Had to take about .002" cuts and resharpen the carbide tool several times. Once done, I mounted my disc adapter plate and put it in my diff frame on the bearings and it seems to about .002" out in the "wobble" direction and within .001" concentric. Looking at it spin by eye it looks great. I'm not sure how much run-out is acceptable.

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PostPosted: August 29, 2019, 5:32 am 
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If the runout was measured near the disc od, then <.005" is ok. Tolerance measured at the disc mount flange will be much tighter.

Did you use a "live center" to support the diff on the lathe? Bolts to the end of the bed and has a bearing housing with a cone you make to fit into the opening in the diff. If you make the live center, provide a fine screw height adjustment.

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PostPosted: August 29, 2019, 1:26 pm 
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I took the chuck off, mounted an alum plate and turned a flat and a post that fit inside a feature of the large diff half. The concentric runout was about .001" measured on the journal before truing the flange. I suppose it really needs to have both journals on, chucked in a 4 jaw with a live center to do it right.

I didn't use a live center as the lathe is so small. It has one, but I assumed it wouldn't fit.

My runouts were measured near the outside of my aluminum brake adapter plate (which is also probably not perfect).

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PostPosted: August 31, 2019, 4:01 am 
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Just following up on this. I highly don't recommend welding on a diff like this.

I was having a hard time getting it back together, it would get stuck about half way, and these things are already like some kind of a Chinese torture puzzle.

The manufacturer must have a jig to hold all the gears together like a cartridge and then slide the whole thing in. When I took it apart, things just kind of fell everywhere. It's quite an amazing mechanism.

So I started trying to fit the smaller long gears into the case by themselves and they wouldn't go in all the way. There are cylindrical cut-outs that they fit into and they must have gotten distorted during welding.

Now that I look closely at it the thickness of the casting is quite thin in some spots and I could see a colored effect near the welding area.

So now I'm trying to sand those out and get this back together. Ah well...


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