We’re looking at buying about 60 feet of 1/2" precision ground 7075. Mcmaster’s prices are killer though, and I know they’re buying full lengths from someone. I’ve been calling suppliers all day with no luck.
If you know anyone who sells it, please let me know.
McMaster has it listed as $50 for 72inches with a tolerance of +0, -.0005. I could find 60inches for less than $15 with a tolerance of ±0.0015. What do you need and do you have the ability to turn it down yourself if it will save you $35?
I think it is a bit more considering he is talking about 60 feet
I hope that’s a typo…
Or else he needs a LOT of axles (or whatever it’ll be used for) in the next few years…
Good catch. I saw 60 and assumed inches. If he is talking about feet thats quite a few rollers (or something) but I could understand why cost is more of an issue then.
Thanks all, it’s a lot of axles, and a lot of mating parts. So, for overall ease of assembly and good running, we’d need some tight tolerance precision ground.
I know Cal Poly has some centerless grinders, we may just buy some 12’ lengths of 7075 rod and grind it ourselves (assuming the school lets us).
We have the ability to turn it ourselves or even take rod and “grind” it by polishing with emory paper, but the sheer amount of it would tie up our lathes and machinists for too long.
Mcmaster is the last options, as we’d need 10 72" lengths. I’m assuming a supplier would cost half that.
Eric, it’s all for this year 
a total tolerance of 0.003" is junk for the application Adam is looking for.
To reasonably expect a shaft to fit into a bearing you need +0 -0.001ish or so. So basically precision ground. Anything else is going to be a PITA. You can’t reliably (and concentrically) turn 0.0015" off a 20"+ long shaft, so you basically need to use emory cloth and sand it.
you can find a center less grind shop they will have that on the shelf you will need to but them at 12 ft and should be around $25 to $30 each. not sure y you need 7075. but if 6061 will work it is cheeper alum to buy.
There’s gotta be another way to build it?
PG shaft isnt what you really want or need. It would be easier than sanding down a shaft with a tolerence of ±.003, but PG shaft with a tolerence of +0 -.0005 still has a good chance of not fitting, due to the simple fact that you can’t put two things together with a near line-to-line tolerence. If the shaft is even close to .5000 (which .4995 is), it’s likely not going to fit.
In the past we have purchased PG 1/2" shaft and stuck it on a drill with some sandpaper/emery cloth and sanded it down the thou or two required to make it slide in the bearings.
What you really need is undersized 1/2" shaft at .498 -.499 to get the nice slip fit I imagine you are looking for. If you can find it undersized, it doesn’t even need to be PG. What we did in the past was excessive and wasteful. If you can do without it being PG, you’ll save quite a bit of money.
Not knowing your design I cannot say for sure, but some things to think of…
#1 really think about your design, can you use normal 1/2" round bar and just turn the ends down where the bearings are to size? Further do you really need 7075? 6061 does pretty good for most uses.
#2 that much AL weighs about 50LBS:ahh: Could you turn tube down to the proper size?
You may try igus, they make AL shafting meant to fit their plastic bearings (they work really well) website here http://www.igus.com/drylins.asp They list their shafting as .5 however I couldn’t find true dimensions. They have a really nice program where you can get their stuff for free for school projects, they just ask you send them a photo and a writeup of what you did with it.
Good luck
If it were me, I’d try some of the stuff that is said to be +/- .0015" and is $20 for 8 feet. Now if it’s a bit oversize, I might try using one bearing inner race as die, lube the axles and press them through, thus precision sizing them. (Or pull a long length through.) That or do a quick spin in a lathe with some abrasive paper or drawfile.
http://www.onlinemetals.com/merchant.cfm?pid=10440&step=4&showunits=inches&id=192&top_cat=60 (Beware of specs - these seem pretty close for drawn rod.)
If the shaft is undersized, I might try some Loctite 620 Bearing Mount or 641 Bearing Fit.
Just a backyard mechanic’s opinion, and not necessarily suitable for high speed, critical service.
There is some reasoning behind our material choice (not the most sound as Travis pointed out though).
The biggest part is that we don’t want any issues with fits. period. at all. Even if the shafts are the perfect size we want with the perfect fits and slide in nicely, assembly will be a pain. Having to adjust or custom fit 60’ of shaft is not going to be fun.
As for why we went with 7075, yes it is stronger than we need, but we found 6061 to not be strong enough. 2024 was another option, but the price difference is marginal between the two. Even when buying 60’, the price different between 7075 and 6061 isn’t much. We’re looking at about $100 for all the 7075 we need, and we’d pay around $50 for the same amount of 6061; Twice the price, yes, but well worth the extra strength.
Turns out that since PG is still larger than what we really want, and I’m not finding undersize 7075, we’re planning on buying 5 12’ lengths of 7075 rod (about < $20 each) and running them through a centerless grinder at either Cal Poly or a local shop. If that is unavailable, lots of kids, and lots of sand paper.
Once these puppies are to size (ideally before we even buy them 
) I don’t want to any modifications required, I want them all to be interchangeable and with no additions or compounds. It would be well worth our team’s money if we could pay $500 for undersize shafts, and save ourselves the hassle of all that work.
You’ll have to post a photo sometime Adam; what you’re doing sounds like an intricate work of art.