1986 John Deere 650 Tractor 4WD Splined Shaft Repair

Yesterday I decided to try and fix the broken front drive splined shaft on the JD 650. These first two photos show the before and after condition of the shaft.

This next photo is the broken shaft chucked up in my 1915 Seneca Falls lathe, ready to drill a hole into the shaft to accept a new piece of steel.

I drilled a 3/8" hole about an inch deep into the shaft, then turned down a chunk of 3/4" rod for a press fit into the hole.

Test fitting the two pieces.

This next photo shows the two parts welded together.

I turned down the outer diameter of the new shaft to .660" diameter, then cut off the excess.

It's time for some splines. I've never cut splines before so it was a learning experience. I used a single point cutter holder to hold a tool I'd ground to what looked right for a spline groove shape. I used the yoke splines as a sort-of guide to get the cutter profile close. I mounted the shaft in a dividing head and dead center. The mill I'm using is a Smithy Granite 1340 lathe/mill combo. It's not as rigid as a dedicated mill but I'm cutting fairly fine spline grooves, and it's the only mill I have so it will work beautifully.

When I first started cutting grooves, I didn't think much about the speed of the cutter, so I just cranked it up to maybe 5- to 700 RPM. That didn't go well. After sharpening the cutter for the seventh or eighth time, I thought maybe the cutter tool was too soft, so I ground down another cutter. The second cutter didn't help. It was still getting worn down with hardly any groove being cut. After several more cutter grindings and by trial and error, I finally realized that 90 RPM was about right for this job. I set the cross slide feed rate to .0033 inches per revolution, and each groove was 1.74 inches long, so all those numbers equate to 6 minutes per groove, plus time between each groove to set up the next pass. Probably three hours had passed by the time the grooves were finished (for the first round.... round two coming up in a bit).

This next photo shows the fifth (of 15) spline groove being cut after I'd gotten the process dialed in with the second cutter tool and correct speed. Time to settle in for some quality mill time, listening to the 1-1/2-times-per-second thunk - thunk - thunk of the cutter doing its job.

The yoke also needed serious help, as it had been bashed around a lot. I don't know that I'd want to know the history of this poor tractor. I'm sure it's cringe-worthy.

I turned down a piece of rod such that the splines of the yoke were a very slight press fit onto the shaft, so that I could press the yoke onto the shaft, then turn away the mangled up end of the yoke. I used the Seneca Falls lathe again. I took off about a quarter inch of length.

I also turned down the outer diameter just to make it look nicer.

The spline hole of the yoke is still a little egg-shaped at this end, so I took a small jeweler's file to the grooves.

Once all the shaft grooves were cut and the yoke machining was done, I did a test fit. It was tight but the two parts went partially together with light taps of a hammer. I then had to re-setup the shaft in the mill to cut the grooves .010" deeper. It was a fairly simple task, although the whole process took another hour or so. Since the cut was .010" (and not .030"), I was able to increase the cross slide feed rate from .0033" to .006" per revolution.

The finished products are shown below. I'm hoping that with the amount of use this tractor will get, the new splines will hold up fine. Only time will tell.