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Old 01-12-08, 05:11 PM   #1
rednaxela
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Grinding excess aluminum chainring - right tools?

I'm replacing the chainrings on my Sugino Mighty 500x crank. Unfortunately, the large chainring doesn't quite fit onto the spider - the flanges in which the bolt holes are drilled have about 1mm worth of "extra" material at the bottom which prevents them from seating properly.

I had the same problem on the first set of chainrings I purchased, and was advised by some BF folks, as well as the shop from which I purchased them, that a little grinding would take care of the problem. Wish I'd listened then - I'd have saved quite a bit in shipping parts back and forth.

Anyway, it's too dark out in my shed to start grinding tonight, so I figured I'd ask - is the regular grinding wheel I that came with my Dremel going to do the trick here, or do I need to go purchase a file of some particular type from the hardware store?

Your thoughts appreciated.
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Old 01-12-08, 05:24 PM   #2
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There are aluminum specific abrasives out there but it doesn't sound like you'll be removing alot of material so you should be ok. Of course, aluminum will clog the tool really quick so you may need to make a trip to lowes.
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Old 01-12-08, 05:26 PM   #3
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By the way, it will be difficult to keep a straight line with a dremmel so you might want to look into a decent file. It will be more labor intensive but your not removing much material.
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Old 01-12-08, 07:43 PM   #4
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By the way, it will be difficult to keep a straight line with a dremmel so you might want to look into a decent file. It will be more labor intensive but your not removing much material.
Thanks for the confirmation. The flanges are actually curved at the bottom, so I think I'll scry a line accross the bottom and grind away. We'll see how it works.
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Old 01-12-08, 09:13 PM   #5
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Ware the power tool in this job. Too easy to "whoops". Stick with the elbow grease and file.
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Old 01-12-08, 09:20 PM   #6
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i agree with using a file. slow but sure.
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Old 01-12-08, 10:44 PM   #7
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A file will only take you a few minutes, so I think it would be foolish to go power. Plus, unless you have special aluminum grinding wheel, it will make for a pretty choppy job.

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Old 01-13-08, 02:06 AM   #8
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Also Alum fouls regular grinding wheels, rendering them useless. On a benchtop model they can explode if they get too loaded with aluminum and can't expand properly.
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Old 01-13-08, 02:37 AM   #9
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Actually, I think you are going about this the wrong way around. Why grind the chainring when in all likelihood at some time in the future you are going to have to repeat the process when you put another chainring on.

(I had this "misfit" problem too not so long ago when putting a steel middle chainring on).

I'd suggest being patient, getting a flat file and removing a tiny amount of metal off the spider to get the chainring to fit. This is what I did, and I am sure that the next chainring to go on will fit.

In addition, take your time. The temptation is to go right into it, but you could still end up with a misfit still so that the chainring bolts don't line up properly and thread, or you have a chainring that might be sightly off centre.
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Old 01-13-08, 09:03 AM   #10
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Actually, I think you are going about this the wrong way around.
Quite right - my family motto is "If there's a hard way to do it, we'll find it."

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Why grind the chainring when in all likelihood at some time in the future you are going to have to repeat the process when you put another chainring on.
I don't have a good answer for this, but I have two not so good answers:

1. Taking material off the chainring seems less likely to cause structural problems than taking material off the spider. You're going shorter, rather than effectively extending the tabs that the bolt holes are drilled in. Is that a real concern? Who knows?

2. I guess I'm more comfortable taking the risk of destroying a $30 chainring than I am taking the risk of needing to purchase an entirely new cc/crankset combo. Although, given the number of false starts and paid shipping returns I've engaged in on this little exercise, I probably would have been better off just buying a new mid-range integrated bb crankset to begin with.

But hey, at least I get to buy another tool now!

Good input, all. Thanks very much.
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Old 01-14-08, 09:22 AM   #11
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Conclusion and lessons learned

The chainrings are on the bike. Some lessons learned:

1. Aluminum does, indeed, clog dremel grinding wheels. My four year old daughter reports, however, that they look quite pretty with a thin coat of aluminum on the grinding surface.

2. A half-round double ******* metal file does a pretty good job on aircraft grade aluminum. Takes off a decent amount of material, but leaves a relatively smooth surface behind. Don't forget to acquire a file card (a wire brush, basically) before you start.

3. If you're working with a crank spider that's known to be a problem, use your head and check to see that all three chainrings will fit (with a little elbow grease) before grinding one of them up. I discovered after fitting the large and middle chainrings that the BCD on the inner is not the same as that on the crank. Thankfully, my old inner is still in very good shape and you don't need ramps and pins and whatnot on the inner.

Thanks to all for your input and advice. I was able to bike to work today for the first time in a month - lovely!

N
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Old 01-15-08, 10:56 AM   #12
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Grinding or cutting aluminum with a stone or file is MUCH easier with WD-40.It will almost eliminate any loading of the stone or file(whether by hand or power).
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Old 01-16-08, 01:06 AM   #13
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And I think kerosene is highly advisable when drilling and tapping aluminium.
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