Bicycle Mechanics - What material for a DIY chainring?
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07-19-11, 09:54 PM
I'm not really sure where to post this, but I figured mechanics might be some folks that could help me out.
I happen to be getting a small CNC machine soon, and also have a need for several chain rings. (just started riding track) I am exploring the cost effectiveness of making my own on the CNC machine as both a school project (Industrial Design) and a way to save a few bucks. (nice chain rings can be upwards of $80 a piece)
I can extrapolate the functional aspects of the design from chain rings I have access to, but I'm not sure what material is appropriate. I assume it's some aluminum alloy, but don't know which one, and don't know what heat treating or other post process treating methods should be used. I'm guessing something like 7075/6?
Can anyone point me in the right direction?
07-19-11, 11:31 PM
The chainrings that I have are marked 6061. I'm not sure they're treated in any fashion after machining.
FWIW: This old IHPVA newsletter has a paragraph or two about homemade chainrings, made long before CNC came into existence (page 2): http://www.ihpva.org/HParchive/PDF/05-spring-1980.pdf
07-20-11, 10:19 AM
6061 T-6 or 304 Stainless or 6A14V Titanium.
07-20-11, 10:38 AM
<C>, TA and the other top line companies use 7075-T6.
07-20-11, 10:54 AM
Oops,I guess the better aluminum chainrings are 70 series these days.....guess I'm just cheap....
07-20-11, 02:45 PM
Cheap chainrings are stamped, most other chainrings are CNC'd and often heat treated, and a few high-end rings are forged. You should be able to get away with just CNCing some quality metal.
07-20-11, 02:55 PM
Surly[QBP] has a company make some in Stainless steel ,
for single speed drivetrains and IGH.
T6 is the aluminum heat treatment 7075 the alloy..
if the CNC machine bathes the work in cutting oil,
It may retain the treatment properties.
07-20-11, 04:12 PM
If you're CNCing yourself, and using it for track purposes, steel might be a good option. It's usually easier to work with, and has a longer life than aluminium.
07-21-11, 06:12 AM
I have hacked apart Stronglight chainrings. They come in 2 different grades, soft aluminium and a very tough hard Zircal aluminium alloy. The hard stuff is only used on high-end rings.
07-21-11, 11:57 AM
My TA replacement rings (campy set) use 7075 aluminium. Not sure what the original campy is, but judging wear etc. it's also hard stuff. I have also some old chainrings of a flip bike, which are duralumin.
math is fun
07-21-11, 09:24 PM
If it were me I would use 7 series aluminum until you find a size you like then cut one or two out of stainless. If you are going to be making a bunch you should use aluminum until you find the right size due to the faster feed rate and lower wear and tear on the tooling. Un-heat treated aluminum will wear out quickly but since you can make more and are trying out sizes it shouldn't matter
Of the readily available alloys and tempers I recommend 7075 in the T6 or T7 temper. It’s 84 Hardness Rockwell B (HRB) in the T6 temper and 78 to 82 in the T7 tempers. Alloy 2024 is a good second choice as it runs from 63 HRB in the T3 temper to 83 HRB in the T86 temper while 6061 is 47 HRB in the T6 temper. Generally speaking higher hardness equals higher strength and longer wear. The downside to the 7075 and 2024 alloys is that they are much less corrosion resistant than 6061, but unless you live on the beach and leave you bike outside you’re not likely to notice the difference. If wear is a factor the softest steel or heat treated titanium is well over 100 HRB. The hardnesses are minimum values from Aerospace Materials Specification 2658C. If the material you find has additional digits to the temper designation, such as T6510, the additional digits indicate some tweaks to the basic heat treatment that are not going to affect the hardness or strength much.
Heck, if you have a CNC mill just use what ever is acceptable and you can get a deal on and make several of them in case they wear out.
07-24-11, 09:52 AM
Thanks for the info guys! It's looking like I need to find a source for 7075 stock as well as a place to do the heat treating.
07-24-11, 10:01 AM
You should be able to buy the required material in the T6 condition and not need to heat treat further.
07-24-11, 01:14 PM
Like 30 years ago, we did not have access to some track chainrings and my friends dad borrowed one from me to make a mold and then he just melt the material and did a couple that way. Old fashion way, casting them. Then he just sand them and polish them to give them the desired finishing. No idea if he put it in a late for rectification purposes but I Imagine he did. Another guy did like 3 pairs of old parini (or whatever the name was) cleats in aluminum too casting them in a mold around the same time 30 years ago.
It’s way easier to buy the aluminum stock in the already heat treated condition. OK, some sources:
Aircraft Spruce and Small Parts Inc. are geared to and used to crazy people building stuff in their garage / basement / secret underground lair-bat cave. I should know. McMaster-Carr less so.
Regarding the Aircraft Spruce offerings:
Alclad means the outside 5% of the thickness is a more corrosion resistant alloy than the core. For 0.125 thick stock the outside .00625 is an aluminum alloy with better corrosion resistance than the base alloy. Shouldn’t make much difference for a chainring.
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