Chain sprockets are a key drivetrain component in a lot of builds. Unfortunately they can be difficult to source, particularly for those outside the reach of retailers like McMaster-Carr. In such situations, you might consider making your own.
The toothed profile on a chain sprocket can be produced in a simple manner by drawing a base circle, along with a series of circles spaced appropriately for the chain in question. This involves measuring the pitch and roller diameter of the chain. With these measurements in hand, a template can be created to produce the sprocket.
From there a series of holes are drilled to rough out the basic shape of the teeth, before the sprocket is then cut down to its appropriate outer diameter. The finishing work consists of chamfering the sprocket’s thickness, as well as the filing the sharp edges of the teeth for smooth engagement.
It’s a quick and easy method for producing sprockets with well-defined, accurate profiles. We’ve featured other rough and ready methods before, too. Video after the break.
You don’t have to restrict yourself to round shapes when following this technique.
Sprockets from mild steel – might as well make them from used toilet paper.
Same here.
Even though I have a decent mill with a dividing head I would not be easily tempted to make such sprockets out of toilet paper.
I once looked up the prices of industrial chains, and they are considerably more expensive than bicycle or moped chains, which are easily available and relatively cheap.
Bicicle repair chops also sell the sprockets as spare parts, and those are made from much better steel.
You bring up a good point about using bicycle parts. I suspect that there are few settled places in the world that lack at least partial access to bicycles and the parts to keep them going.
Bicycle sprockets are usually made thin and light, not from solid steel. They’re pretty flimsy overall. You’re better off trying motorcycle and moped parts.
Besides, if you buy a bicycle sprocket, you’d then have to modify it to fit your application anyways, which easily ruins any heat treatments already made to it especially if you have to weld stuff on. Alternatively, you need to make adapter plates.
It’s much more elegant to just make the whole sprocket out of a piece of steel you have, rather than spend time shoehorning in parts made for something else.
Then you wrap them in toilet paper, some clay, and toss them in a fire. Burn for a few hours and you have carburized the steel – also known as case hardening.
https://en.wikipedia.org/wiki/Carburizing
There’s certain advantages to using mild steel and then case hardening it. Obviously it’s easy to work with, it welds easily, it’s cheap, etc. but mild steel also has good toughness and impact resistance, so it won’t crack and shatter on you so easily. By using mild steel, you can use pretty basic shop equipment, hand tools, and then use some basic blacksmith techniques to harden the surface to make it last.
https://bortec.de/en/blog/carburizing-mild-steel-how-it-works/
“Mild steels that have been carburized have a hard surface and a soft core. This means that case hardened low carbon steels are harder but not brittle. The core retains its ductility and toughness to a large degree while being protected by the hard surface.”
Agreed entirely – Tricky part would be not going too far and making it brittle however. It is possible to case harden all the way through and a thin part I would think would make this rather easy.
I’d suggest for most uses just mild steel will be fine. Chain systems wear out in use anyway, just wearing faster isn’t the end of the world. You can always send to a proper machine shop for a copy after it finally does wear out and have one made in the right materials -assuming your prototype actually works well enough to be worth it!
One method to control the amount of case hardening is to control the amount of carbon. Use an acetylene torch to cover the teeth only in a layer of soot, then paint the sprocket in clay or plaster, and then bake it at 700-800 degrees C. Time depends on how deep you want the carbon to diffuse.
The trick being that you only have so much carbon to diffuse into the steel, and it’s only on the teeth – so if you overdo the process with too high heat or too much time, the result will simply be softer than you wanted – you can’t make the steel turn brittle.
The main difficulty is knowing what you have actually made. You’d have to do surface indentation tests etc. to discover the real hardness of the material.
But, as long as it’s significantly harder than the material of the chain, it doesn’t really matter.
hours?? 20-30min under the right temperature should be enough carbon penetration to do usable case hardening, you don’t need (nor really want) to make the sprocket completely out of hard steel…
p.s. once you’re done with carburizing, you also need to quench and subsequently temper the part ;-)
Yes. The caveat is “the right temperature” and how to maintain it for the required amount of time. You’ll probably end up lower than you intended, barely able to get the part red hot, so you need more time.
If you did get carbon in there, then flame hardening the teeth only works as well. You take an oxy-acetylene torch and touch the teeth individually, so they cool down rapidly, and that’s enough to harden the surface. You don’t need to do the whole thing.
Depending on your load I’ve had really good luck making my sprockets out of Delrin – it’s way tougher than I ever thought it would be, and it’s oil infused so it doesn’t implode when you forget to lube the chain (like my metal ones did). There are a bunch of calculators on the web so you just dial in your chain size and the number of teeth, then you can CNC it on just about any mill. Clean it up a little bit on the lathe and you’re good to go.
Guess theres no free CAD programs in Aussie land? I’ve used pedal bicycle chain sprockets with little difficulty for many private projects. They are literally laying around especially on garbage day or local recyclers. Some with useable chain. As many have indicated welding them is not suggested. Distemper easily. Multi speed bikes tend to have over hardened brittle sprockets and so single speed kids bikes work better for me.
Given the wear issues with a homemade sprocket if teeth are not perfectly spaced, shaped, hardened, etc, I don’t see why one would do this unless it was the end of the world. You can source great chains and sprockets at any farm supply store. Cheap enough that I would argue that it’s a waste of time to make one on a mill unless you need something that is not sold. I suspect that china sourced sprockets are even cheaper…
I hear you. Princess Auto in Ontario here has a wide selection for around $5 a pop. Even if I couldn’t get the perfect ratio there, I’d think about buying 4 of them and having an intermediate shaft.