For those who propose drilling halfway through to reduce the liklihood of cracking, I wouldn't assume that without some analysis. It wouldn't surprise me at all if drilling a hole partway through created a higher stress concentration in the arm than if the hole passed completely through. This is especially true if you just use a drill bit or an end mill to make the hole. The sharp point or edge at the bottom of the hole would concern me. A ball end mill would be better. But again, if the diameter is too small, you can still create a stress concentration at the bottom.

If it were me, I'd model the thing in CAD and do some FEA analysis before I started making chips. Hmm....that might be fun (yeah, I know, I need to get out more).

Don't do it......your going to kill yourself........it will break..........you'll do a left turn into a car........which will swerve into a truck..........which will go head-on into oncoming traffic and kill countless people.

To do this correctly,you must first buy a mill,then a rotary table,a clamp set,some center drills,drills......Then after your done,send it out for some stress testing,remember,suicide is against the law,since it will probable fail to any good lawyers standards,your going to have to redesign one to except drilled holes in order to pass the lawyer test.......then your going to have to bone up on your enginneering skills some,unless you want to keep making them and running them through the stress tests until one passes.......then and ONLY then is it safe enough to ride around with.....

So I wish you good luck my friend with your drilling adventures....be save and contact a good lawyer......sounds like your going to need it!

And people wonder why there is no work in the U.S........

Just a slight correction, the lightweight/drillium fad was at it's highest during the second half of the 70's not the middle 80's Actually, the mid 80's saw a movement away from drillium and ultra lightweight designs in general specially when Aero designs took over and smooth profiles on components was more important than ultra light weight. Drillium never really came back since then except for the present interest in it from reminiscing older and new "retro aesthetic conscious" riders.

Chombi

I'm only 23 and have a decent head on my shoulders. No plans to die tomorrow and pass off my drilled cranks to some unsuspecting new owner >.>
I'll make sure I leave a note in my will that the crank arms could be unsafe. That being said I don't think a crank arm snapping while riding is the end of the world, in fact I doubt you'd even fall off unless you were climbing some pretty knarly hill and really laying into it, at which point you probably aren't going very fast anyways and a fall wouldn't be too hard.

Edit: They just arrived in the mail today. Going out in the garage to take care of them now. Wish me luck (except for all the negative Nancies. I don't want their negative luck)

I'm a structural engineer, I can tell you that in general you are correct. A crankarm is stressed in flexure and the drilled holes along the center of the crank arm have very little affect on the flexural stresses in the crankarm. Those spiders with the centers completely milled out are affected a lot more than just have some holes drilled in them. Also the crankarm is stessed in torsion since the pedals hang out some distance from the centerline of the crank arm. Torsion causes tensile stresses around the perimeter of the crankarm and the holes drilled in it will have an affect on these tension stresses as well as causing stress concentrations (layman's term stress risers). Personally I don't believe I would be overly concerned about drilled holes as in the first post. The stresses at the ends of the crankarm, especially the non-drive side, are much more complex and more likely to cause a failure than the stresses along the shaft of the crankarm. BUT we know that the holes do have an affect on some of the stresses, and we know that aluminum is susceptible to fatigue and fracture, so anything can happen.

Thanks for all the input. For now I have just drilled the holes in my chainring and the milled out portion of the spider and will leave it at that. I'm not feeling overly compelled to go crazy at this point and drill down the arm fluting since this is my primary bike and I don't have money to blow snapping crank arms while experimenting.
Part of the reason I decided against the holes in the arms was the surprising weight of the cranks. No bolts or anything, just the two arms came to only 435g. My 2009 Miche Primato crank arms are 500g. The IRD remake of these cranks are 600g. Really shocked me that something made in 1972 was so much lighter. I was expecting something more around 700g where there would be a lot more material.

Anyways I'll post some picks when I get everything polished. Thanks again

Old-school cranks tended to use a lot less metal.

I guess they must have been flexy though.

They did? If so, why are modern aluminum (not even carbon) cranks so much lighter than their predecessors?

Are they, though?

https://weightweenies.starbike.com/li...type=cranksets

For a show bike i would drill even a shape to the crank but for riding?? no way, you can make hollow cranks and they will last long time but drilled??? I would not do it if I had to ride it often. For a short TT in a track I would because races are really short but in the road no way...

Uh, they're like, totally not. Older aluminium cranks are barely larger than steel ones.

And IIRC ally cranks got no lighter than Topline ones in the 90s, at under 400g a pair.

Here we have '74 DA and 6400. I don't have scales, but I'd say there's at least 25% more metal on the 600s.



Move along another few years to the Campy Daytona cranks currently on my bike (prolly the last of their square-taper ones), and they're about 20-25% beefier than the 600s.

I don't know why. The DA cranks can't have been too bad, and I can't imagine why you'd want more metal than the 6400s use. The Daytonas are only on my bike because they're 175s.

So, weight weenies - old square-taper cranks may well be lighter than a you-beaut new set, particularly if paired with a Ti BB. If you don't like the idea of em flexing you'll want to be pretty light yourself though, I'm tipping.

As promised here are two pics of the finished product. Drilled 3 holes in each web of the spider, counter sunk them on the back side, smoothed the transitions a little with a rotary tool, and polished using steel wool followed by a buffing wheel and compounds

While aluminium is soft, it's not that soft. You'd need to be really energetic to remove anything like a burr with a pipe cleaner. Threading a tight fitting piece of string through, smearing rubbing compound on the string, and pulling a good foot or two back & forth might work. You could even attach it instead of a blade in a hacksaw frame for a chance of a fairly decent result.

Nice job.