OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Reel-Type Mowers Diplomat 590 Repair

Forgot to say that part of the appeal of this project is that it is different being a diplomat and I like the challenge of developing a solution. I do find that I am torn between practical versus production solutions though (ie I can fix the ring gear to work or make a replacemnt that could be produced again).

It looks to me as if you would have needed to make an external gear anyway, to attach to the nylon internal gear and thus produce a complete moulding pattern you can use for making moulded plastic gears.

Of course if it will only be used as a pattern you will be hand-filling it with automotive bog, so it does not have to be visually perfect or structurally sound prior to adding the bog, but it does have to run perfectly true, which can be a challenge with hand-filed parts.

I think for 60 teeth I'd have taken the trouble to put it on the rotary table and drill a hole where the gap between each pair of teeth had to be, then only file the teeth to final shape. I'd use a simple triangular file on the filing machine, plus a depth-stop, rather than hand-file so many teeth. The reason to drill the holes with a rotary table initially, then machine-file each tooth to a depth stop, is to give it the best chance of running perfectly true.

The more I thought about it this morning the more I realised it was worth doing it again and figured to drill it in the manner you suggest. Getting it to run true will be a challenge but with the remaining ring gear as the starting point the crucial aspect will be finding the true centre.

Finding the true center of a round plate should be easy. First, how did you make a circular plate without knowing the center? If you somehow managed that, just grip it around the outside in a self-centering chuck, spin it up, and use a center drill in the tailstock to drill the central hole. Further on in the process, after mounting the disk on a closely-fitted central axle, you turn the outside of the disk precisely true.

If no lathe is available, you can do it by taking multiple points around the perimeter of the circle and scribing arcs with dividers, all of the same radius. Keep adjusting the radius until they are all passing through a common point, which will be the precise center of the circle. Anyway, I'm sure you know all that, so at this point I haven't understood what the problem is.

FWIW, my usual method is to take a roughly-round oversize piece of plate, drill a hole roughly in the center, and bolt a closely-fitted piece of bar through the hole. Then I can mount the bar in a lathe chuck and turn the outside of the plate to the exact diameter required, true to the central bar. If you prefer you can mount the bar in a chuck mounted on the rotary table, and end-mill around the perimeter of the disk to bring it to the right diameter. Ideally you would then mill the gear teeth using a gear cutter of the same diametral pitch as the gear that has to mesh with it. Since I don't keep gear cutters, let alone a whole selection of them, I'd have to drill a series of equally-separated holes around the outside, just by indexing the rotary table. Then I'd use those holes as index marks to file equally-spaced triangular teeth around the outside. None of it is difficult but you end up with triangular teeth instead of involute ones, after doing even more work than cutting involute teeth with the correct cutter.

As I indicated I have redone the metal plate by marking and drilling each tooth. The finished piece is a lot better than the first attempt. But, it is still a bit rough. Therefore, I have purchased the silicone molding rubber and polyurethane resin to make a mold and cast a feplacement part instead.

You might consider thickening the web for your plastic ones, which only requires adding a glued-on filler disk of plastic or metal to the pattern you have just made. (Preferably make it very thick at the center near the hub, and quite thin at the outside so you still have almost the whole face width of the internal gear available.) However I think a plastic hub most likely won't stand up: I think the key will mash the plastic, under load. The simplest solution I've thought of so far, is to machine or grind four large flats on the outside of the original steel hub, so it is just about square. That should reduce the stress on the plastic to around a fifth of what it would be with a plastic hub. If you eventually take up making these gears for other people whose plastic gears have broken, you could require them to supply the old hubs, with their orders. All you'd need then is a simple fixture to hold the hub while you create the flats.

I don't know about making others but I am certainly interested in the challenge of redesigning the setup so that it will be more durable than the original. Of course this also means maintaining the original metal hub because I do not have the machinery to manufacture an alternative. That is where the challenge comes into it.
The approach you outlined is something I have been pondering as I think there needs to be a greater key between the hub and the plastic ring gear. I was thinking a basic spline setup (grooves cut with an angle grinder is the simple option but lacks the finesse I am looking for).

I think you can get more-or-less parallel angle grinder grooves fairly easily by clamping the hub to a piece of angle-iron spaced up from the bench, and sliding the angle grinder along the benchtop, guided by another piece of angle iron. I do not recommend sliding the hub, because I think that would be dangerous. The most difficult bit is indexing the hub by an even amount after each groove is cut, then re-clamping it. All you are doing, is making a crude milling machine.

That sounds like a great idea and the finish should be ok. I agree about the spacing but I have a technique that can get them pretty even. I measure the circumference and then lay out a piece of masking tape flat onto something it will not stick to so well. I then mark the required spacings on the flat tape, lift it up and wrap it around the circumference and you should end up with evenly spaced marks on a circle.

I agree that indexing system system should work if you handle the item gently enough throughout the process. I'm not notably gentle, so I'd have to scribe lines onto the hub, through the masking tape.