Machining a hub axle and how Titanium strength works

Education
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TLDR: I’m interested in building my manual lathe skills by making a titanium axle for a rim brake rear hub, like a Shimano 105 FH-7000. I’m asking how strong Grade 5 Titanium would be vs the standard steel axle, because I’m unsure of the engineering behind how Young’s modulus, tensile strength, and yield strength play into this.

The way this idea came about is I’ve been planning a wheel build for awhile for my rim brake bike with the brief being: cheapish, quiet freehub, lightish. Cheap and quiet free hub in 2024 means some sort of Shimano hub, but they’re not that light, even if you spend dura ace money. At the same time, I’ve started learning the manual lathe.

The steel axle has significant mass, and it’s little more than a hollow threaded rod. Much of the weight savings from Ultegra 6800 hubs and DA 9000 hubs is probably in their aluminum axle design. They use a larger diameter axle, which like the tubes in a bike frame makes sense for increasing strength with a weaker material like 7075 aluminum. In a normal cup and cone rear hub, there’s normally a few mm of clearance between the axle and cups to play with, but then you have to make new cones, locknuts, etc.

Googling the mechanical properties of Grade 5 titanium vs 4130 steel, it looks like the yield strength and tensile strength of titanium are much greater. Of course, the Young’s modulus is less than steel, but does that matter for a hub axle as much as it does for a frame? If the yield strength is better than steel, then it should be safe to make a titanium axle to the same dimensions as the original steel one and expect it to be lighter and strong enough to not bend, no? I don’t have an engineering background, so I’m very much in the dark about this.

As a lathe project, drilling such a tiny, deep through hole seems like a challenge, but otherwise it’s basically a 150mm threaded rod. The first place I googled sold Grade 5 3/8" round bar for $40/ft (sorry for burger units), so not prohibitively expensive to give it a shot. Winter is closing in, and I’m feeling restless for a project, so I understand if this doesn’t sound like the most pragmatic project ever thought of, but at worst a useful thought exercise for understanding material properties a little better.

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Would be a LOT easier to simply find a Dura Ace 7700 rear hub. Not only has a Ti axle, but also has a Ti freehub body.

I doubt you would be able to drill the hole in a 5.5 inch long 10mm rod without the drill “drifting” off-center. Additionally, Ti loves to work harden and how are you going to get coolant inside that deep of a hole ?

10mm steel axles were very prone to bending on old freewheel hubs. The freehub design was a big improvement, but the basic 10mm hollow axle is still a marginal design due to the built-in stress risers of the threads combined with the thin-walled tube.

Don’t get me wrong - I applaud your learning manual Lathe skills !
To hell with Autocad and CNC - LOL !

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Drilling such a long tiny hole with a regular spiral drill bit into titanium is a bit suicidal I guess… Anything longer than 10x diameter I would classify as risky even in steel, let alone titanium.
You’ll have to factor in the extra cost of all the wrecked drill special deep bore bits as well:
3mm, 30xOD, 364€
3mm. 30xOD, internal cooling, 280€

Or maybe you can find titanium 5x1mm tubing to begin with? Might be worth a shot…

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I’ve never done this type of operation, but it might be a good situation for a gun drill. Carbide and through coolant probably are a must for Ti.

-Jim G

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Two minor points:
(1) You mentioned using 3/8" bar as your raw material; that’s not big enough.

(2) A 5 mm hole is also not big enough for a skewer to pass through. Skewers do vary in diameter a bit, so maybe if you shop around for a smaller one, it could pass through a 5 mm hole, but most axles have a hole a bit larger than that.

After-market M10 Ti axles used to be available from a few vendors, sorry I don’t remember who. Haven’t looked for one in about 30 years, so I have no idea how common or rare they will be today.

You might ask Jim Merz how he made his. His lathe is CNC, but he may have used a normal drill, with an extreme amount of pecking and withdrawing the drill for coolant and chip-clearance. That would be easy with CNC, but quite a chore to do manually.

I drilled a steel axle once, to use a 1950s Sturmey-Archer DynoHub with a QR. This was before Schmidt and others made modern dynamo hubs. With the headlamp attached to the axle (lamp bracket replacing one of the cone locknut washers), I had a wheel I could throw into any bike that needed a headlight. Got the idea from my boss at a shop where I worked, who did the same. Anyway, done on an old manual lathe with a normal twist drill, just an insane amount of pecking and withdrawing, more and more often as you go deeper.

Drill halfway and flip to drill the other end, more chance of coming out centered. And spend extra attention to aligning your tailstock with the headstock. Spot the hole with a spotting drill, and consider beginning the hole with a “stubby” drill before switching to one that’s long enough to reach halfway. Shorter drills flex less, more likely to start on-center.

Solid-carbide drills are lots more rigid, but you’ll cry when you shatter one ($$). Also, unless you are set up for sharpening carbide, you’d better not dull the cutting edge before you’re through. Even if you have a diamond wheel, hand-sharpening such a small drill will likely result in a slightly off-center grind that cuts more with one edge than the other (or one edge does all the cutting), a sure-fire recipe for wandering off-center.

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Thanks for the responses. Work hardening is one of those things I didn’t think of. There are layers upon layers of things to learn with this stuff. I skimmed some Youtube videos of guys drilling deep, accurate holes like this. One guy used about 3 types of drills plus a reamer, had a whole coolant routine, and did half at a time then flipped the part like you’re suggesting. If I did it I’d still start by trying to track down a ready made tube first.

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The aftermarket ti axles mentioned were probably from SRP (Speciality Racing Products).
Rolled thread and a 5.3mm bore.

th-807980126
th-807980126474×632 45.3 KB

SRP folded maybe 20 years ago, but worth a search, they do turn up occasionally on eBay and the RetroBike forum etc.

All the best,
Dan Chambers

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A hefty project that’s worth applauding for! A couple of things to start with.

  1. Titanium is a notorious material to work on unless you have specialized tooling for that. Work-hardening is real. I had my butt handed over to me when trying to increase the ID of a titanium shock bushing from 6 to 8mm. The length of the hardware was only 20mm so I thought it would be a walk in the park to do on a lathe. After ruining the hardware as well as the carbide drill, I was a bit wiser. The feed and speed were probably way off, but I was still stunned at how difficult the task was.

If you’re hellbent on making the axle out of titanium, prepare well.

  1. If you’re looking to hone your manual machining skills, I would make a version (or two) out of aluminium just to see how the task goes and see the potential hiccups. If you’re successful, you will be better prepared for the real deal with titanium.

  2. Should you want to have the titanium axle made, but it’s out of your pay grade at this point in time, have it made. I had a small batch of TI UDH axles made at Meti and can only say good things about the service as well as the products.

Related to that, he knows how to make small and deep holes.

https://www.instagram.com/p/DAc_QCyObNW/

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