Back Cutting Gear Teeth

[Zaemo]

Hello Gents,
Brief Intro:
I was encouraged to participate so I will try. I hope this is the correct sub-forum. I bought a Smithy 3 in 1 last year and have no experience in a machine shop. I have been building old Chevy's and older Fords for some time now. I am self taught and have enjoyed learning everything from MIG/TIG Welding to Painting. I have relied on others for the very accurate and tight tolerance stuff. I have been modifying and refurbishing and making some parts mostly by hand up to this point. One attraction to the small machine is creating more professional looking parts.

As I moved into older Harley's a couple years ago, I have been "forced" into another level of DIY. The fit and finish levels on well done bikes is a bit of a different animal. I have also discovered that most bike enthusiasts prefer that no one works on their bikes but them. I have discovered for myself why that is. There seems to be a lot of sub par wannabe motorcycle mechanics out there and a lot of ill fitting aftermarket parts for restoration.

So... this brings me to needing to expand my skills and self reliance.

One project that has me in fits is the kick start ratchet gears. These are notorious for slipping on the old Sportsters for various reasons. One sure fire solution is to back cut or undercut the teeth to remove the 90 degree tooth to tooth contact in favor of an angle cut that causes the teeth to grab each other.

How would you get the exact same cut on each tooth so that every tooth makes contact. Is this what an index is for? Are they accurate enough? The gear and plate are hardened of course to make things more difficult. Does this mean they should only be ground and not cut with a bit? I had a special dovetail bit made but it would jump around the edges and ultimately break a flute.
Is this a job for a CNC machine? Maybe the answer is to just have new gears manufactured to my spec but that's got to be a major investment.
I appreciate any input or opinion.
Thanks,
Chad



Stock 90 degree

 

[rgray]

You might be better to make a jig to hold those to be ground.
Here's an example: http://www.ebay.com/itm/CINCINNATI-...AR-CUTTER-SHARPENING-ATTACHMENT-/121312471437

That's a way over priced example but just to get the idea. On the gear you could possibly put the stop on a gear tooth and be accurate for each tooth. On the opposing part you may have to get creative to find an accurate location for the stop. Typically sharpening a gear cutter (what that jig is made for) the gear cutter is set up and the backside of the cutter is ground first, because the front side is accurate. This only needs done one time on a gear cutter, as once the backside of the cutter is ground accurate the cutter can be resharpened many times as the stop is used on the ground back of the tooth.
A diamond cup wheel might be nice to grind it with as very little stock will be removed.

 

[Tony Wells]

On the ratchet clutch teeth, yes they do need to be accurately spaced to spread the load out. That's exactly what dividing heads are made for. Since it is hardened, it's going to be tough going with almost any cutting tool. Even a carbide cutter will wear prematurely if it is case hardened, as I suspect it is. I'm a little surprised no one has made an aftermarket version to address this slippage. But then again, you don't see that many Sporties that are kickers on the showroom floor, I expect. I haven't set foot in the last 3 dealers here, so I'm very out of the loop on what H-D offers not, but last I was involved, most of them were electric start only. If that's still the case, the market for an improved part would be very small.

One thing to consider is that whatever material you are able to remove, regardless of the method, will make the case hardened layer thinner, and ever so slightly softer. Are you thinking of altering both halves? Making them complete?

 

[Zaemo]

Thanks Russ and Louis. I'll have to search for some videos on how that gear cut sharpener works as I've never seen that in action. I understand what you're saying about a stop. That may be more accurate than a dividing head.
Louis, the kicker left the sporty for good around '69. 52-69 used this ratchet setup. Other H-D models had more saw tooth type gears. !!! One company claims their gears are back cut but they are not. If you've ever thrown your knee out on a kicker slip, you'd happily pony up the cash for a set of gears that don't slip. It would be more cost effective to modify the existing aftermarket gears I suspect.

 

[British Steel]

As the ratchet is hardened it would be less fraught with tool / cutter breakage to grind the engagement faces, could you brew something like a tool/cutter grinder, index plate (another ratchet gear?) to position for the cut, slide to move ratchet or grinding spindle? A Borazon cup wheel (the conical type) could cut a few degrees of undercut, I guess you wouldn't need much of an angle as the force from the kickstart would lock the ratchet together?

Dave H. (the other one)

 

[Tony Wells]

Zaemo, around here we do not talk down other people's machinery and tools at all, so please don't take this the wrong way. When you are machining, rigidity is everything. And most 3 n 1 machines are not known for that. Smithy is supposedly one of the best, but I have never actually run one myself. So take this as an observation. Even hardened material can often be cut with the correct tooling if it's on the right machine. I have been thinking about what I said about the case hardening possibility. It's highly likely that those part are going to be either carburized or nitrided to give them wear resistance. BUT, since obviously there is a wear problem, they are not all that hard. So it is possible they are not case hardened but through hardened. That would be to your advantage. Can you file either half of the pair? If so, then you can machine them. If the file simply skates off, then it's going to be tough to get a cutter to last even if you get one designed for hard-milling. I say that because of your machine. Hard-milling takes a very rigid, tough machine. It might be that you could cut them with a heavier machine and the correct coating on the cutter. This is going to get pretty costly fast though unless you find a standard cutter with a profile you can live with. You can get a custom cutter coated with just about anything, but it isn't cheap. And you still have a fairly limber machine working against you.


Oh, and the name's Tony.....The name Louis is just part of my sig line. One of my favorite quotes. And yes, I have kicked a worn out Sportster a time or two, and yes, I would pay to not have it happen too many times, especially these days. It's been a while. It might not be as bad as you think to actually make that pair from scratch. I'll think about it some more.

 

[bosephus]

i can see a concern with just simply back cutting the gear , my biggest concern would be with after a few overzealous kicks instead of the gear slipping its going to break the teeth off .
or the first time you kick it over without being up at top dead center and it kicks back its going to give your foot/ knee/ankle a bad day or again strip the teeth off .

just something to think about ,.... being stranded and looking for a hill or someone to give you a push to get your scooter going is never a good time

 

[Tony Wells]

Yeah a kickback is no fun....big twin can send you over the bars if you're a lightweight and aren't ready for it. Otherwise yep.....tough on the joints.

I think the amount of negative angle needed is minimal, on the order of a couple of degrees maybe, so not too much weakening.

 

[bosephus]

Something else that comes to mind .
Is this really a gear problem , or a problem caused by the case flexing .

The more I think on it I tend to think this is most likely caused by the case and assembly deflection .
If so will there be much gained by going threw the effort of back cutting the gear ?

 

[Tony Wells]

Probably, because you are changing the direction of any forces that would be flexing the case. In fact, you could be virtually eliminating them by preventing any tendency for the mating pair of dogs to push apart.