Yesterday I was mowing my yard when I discovered that my mower could suddenly only turn two directions: left, and more left. Being that my yard is too big to mow in one circle, I drove the mower to the shop and went to figure out what broke. (Imagine, for a moment, yours truly, steering a riding mower by frantically kicking the front tires while heading full tilt towards the treeline.)

One thing I like about riding mowers is that the technology they're made with is practically stone-age. Well, it's the technology a stone-age manager would use to cut corners, anyway.

Here's the steering, tie rod already removed (it goes in the hole on the right):

This guy is called a sector gear and it looks like it hasn't been to the dentist in a while:

Those missing teeth make for a nice dead spot in the steering. Some googling later revealed that the design for this part changed in 1997 and it would cost me $50 or so to replace the whole steering assembly. No thanks--I'll modify the mower to take a steering box with better parts availability at some point, but for now, I just need to cut the grass. So, the plan is to weld up the missing teeth, then use a file or something to re-shape the weld into teeth. I figured I'd also try to fix some of the worn teeth while I'm at it.

I used a centerpunch to mark the root of each tooth so I'd know sort of where to cut:

Steel welding won't stick to copper; any sizable piece of copper dissapates heat too quickly to melt when welding. Thus, it makes a nice backing plate when you're trying to weld something up.

Welding attempt #1:

This didn't turn out so well. Color me surprised, but the steel this thing is made from is not exactly high quality, and 20 years of soaking in gasoline, grease, and lawn dirt has made the metal impossible to clean. I got a bunch of porosity in the weld. Take a look at these upside-down steel snowmen:

Those were one round bubble of steel when I welded them; there was enough gas expanding from the crap in the steel to make the second bubble. No good.

Also, the welds weren't wide enough to cut into solid teeth. So, I ground all those down and tried again. I figure that if I weld a whole bunch in one go, I'll get the metal hotter and maybe burn off most of the crap before I weld over it.

Attempt #2 (apologies for the bad photo):

This turned out better. Some porosity, but much less. It took me a few tries to get the welder dialed in; I had to feed the wire a little faster than I usually do so it'd build up. I also ground the top of the weld flat after each pass.

I tried to cut a few teeth freehand, but that didn't go so well:

Fortunately there were a few good teeth on the ends of the gear, so I made a tooth gauge:

I then ran welds all the way across the bad teeth.

Re-cut the teeth:

I cut these with an air cut-off tool that is a bit easier to control than an angle grinder.

Unfortunately, I did not quite follow the arc of the original teeth, so I accidentally included one more tooth than the original. I spent a decent amount of time test fitting the gear and re-cutting the parts that bound up, but eventually I got it to work pretty well.

Installed:

(The tie rod is attached via that red nut.)

I hit it with some white lithium grease to hopefully help lubricate it some. I think that the next thing down here to fail is going to be the gear on the end of the steering wheel, but who knows. If this lasts another few years, I'll be plenty happy.