[Newbie] Trying to turn down a motorcycle sprocket

[DaveBarbier]

Hi everyone, I’m trying to thin a motorcycle sprocket from size 530 to 520. I’m doing a test with another sprocket first and I’m getting sparks and the tool is stopping the lathe sometimes. Maybe I’m going too deep but I don’t think so. Honestly maybe 5 thou at a time. I’m using carbide and the sprocket seems like it’s case hardened. Should this go smoothly or is it normal to seem like the tool is skating on the sprocket? I wasn’t expecting much of an issue but here we are.

I’m on a Southbend 9” if that matters. And I’m using carbide that I got from China. Hope that’s not the issue, haha because I have a lot of it.

 

[markba633csi]

The sprocket probably is case-hardened and the tool is probably not a good fit for the machine and the material. I would try with high speed steel cobalt tool bits ground with a fairly aggressive rake and smallish tip radius. Remember also that the surface feet per minute will be very high at the outer edge of the sprocket compared with the center, so you may have to use backgears. Use some type of cutting fluid- even nasty old motor oil is better than nothing
-Mark

 

[DaveBarbier]

Hmm ok, I can try that. I have high speed steel but I am not where near proficient at grinding tool bits yet. There might be something I can use in the assortment of has tool bits that the old timer gave me when I bought his lathe. And yeah, I was thinking of going into back gears for slower. All I really need to machine is the outside edge where the chain rides anyway. Thanks for the tips.

 

[NortonDommi]

The sprocket is most likely induction hardened in the tooth area,(most likely),or hard chromed. Less hassle to get the size sprocket you want or better up the chain size.

 

[markba633csi]

Yes that's true you only need to do the outside edge, easier but not a piece of cake. Grinding would be the best approach
By the way, grinding tool bits isn't all that difficult. A nice sharp edge is much more important than having the exact profile. My tool bits look like dog poop but they cut well.

 

[Ed ke6bnl]

we had to do it years ago for my boys Motorcycle ended up grinding with a right angle. today I would use my surface grinder. You may need to put a tool grinder on the lathe.

 

[pontiac428]

Yes indeed, it takes more depth of cut to punch through hardening like that. Grinding works, lots of ways to rig up a grinder to your toolpost that will do the job. I'd try a cut with a sharp tool, but I'd stop if I noticed any flexing under cutting pressure. If it's too hard to single point and you don't have means to grind on the lathe, then I would consider a correct replacement cog (because sprockets are on the driven side of a geartrain). If you meant sprocket, then hoo-wee would that be hard steel. (For some reason I visualized this on the wheel side and not on the gearbox output side).

 

[markba633csi]

Good point, I thought this was the wheel sprocket too, but the engine side gear would be even harder

 

[DaveBarbier]

Well, both sprockets would have to be turned down the same. Front and rear. So the one attached to the output shaft on the motor and the one attached to the rear wheel. The larger one that attached to the rear wheel is what I’m trying now and I quickly grabbed a high quality new carbide tool and it performed better but still not at all nice. Finish is terrible and it’s loud and going super slow.

I have a tool post for my dremel but I’ve tried using that before and it doesn’t work well for me. Can only take off a tiny tiny bit at a time and I have I remove about 100 thou. I would only be able to use small dremel bits and I assume they’re way too small and the shaft is too thin to avoid deflecting.

The only reason I’m not sticking with 530 is because I have a new $90 x-ring 520 chain that I bought a few years ago by accident and never returned and I can get both 530 sprockets for $40. They don’t make sprockets for my bike in 520 with the right tooth count so they would have to be custom made (something I had done for my cafe racer but not worth the $$ for this one).

Thanks for all the help, I’m actually glad this is a more difficult task to complete. Means I’m not as crazy as I thought I was. I was thinking, eh, big deal, get past the case hardening and you’re good to go. Easy peasy. Looks like I’ll get the cheaper standard 530 chain and sprockets because they’re a dime a dozen.

Thanks again!

 

[cathead]

Your post caught my eye as I was replacing a 530 chain and sprocket on my 1975 Yamaha XS 650 yesterday.
After replacing the rear sprocket and using a new 530 O-ring chain, it still didn't seem just right. After removing
the side cover, I found that the teeth were essentially gone on the front sprocket, just a bunch of knuckles,
looking like the shut off valve on a propane tank! The bike has 21,000 miles on it and I seem to remember
changing the sprocket once long ago but not sure on that detail. On your sprockets, only the outside perimeter
would have to be machined I would think so some sort of grinding operation might work OK. Good luck on
your sprocket project Dave. Somewhere around here I have a book called "Zen and the art of motorcycle maintenance".
Maybe I need to read it again........