Rohloff oil port: cut or formed?

[crg]

Are the threads in Rohloff's oil port cut or formed?

[WorldTourer]

Asking that question with no context will make any reader wonder what on earth has gone wrong with your hub. Come on, don’t leave us hanging.

[crg]

Thinking of drilling and threading an oil port in an alfine 8 hub.
Wondering what Rohloff did.

[martinf]

Thinking of drilling and threading an oil port in an alfine 8 hub.

Done that, but on Nexus 8 Premium hubs rather than Alfine.

At first I used Rohloff drain plugs, but I have since moved on to using 6 mm nylon Allen-head screws cut down to length, with a stainless steel washer compressing one or two Viton O-rings to get a good seal without faffing around with threadlock.

The main thing is to put the hole in the right place, in Shimano 8 speed hubs the alloy shell has some steel inserts which must be avoided.

[PH]

Rohloff shells are machined from a solid billet, so everything on them is cut.
Even if they were forged, drilling and tapping a hole is such a minor task, I doubt they'd consider doing it any other way. 

[crg]

Not sure what a solid billet has to do with threads.

A tap is used whether the threads are cut or formed. Different taps, though.

[PH]

I'm already regretting answering, use whatever you've got, it a plug to stop the oil dripping out, does it matter?

[crg]

I'm already regretting answering, use whatever you've got, it a plug to stop the oil dripping out, does it matter?

So you don't know why.
Why not say so?

[mickeg]

I'm already regretting answering, use whatever you've got, it a plug to stop the oil dripping out, does it matter?

So you don't know why.
Why not say so?

If you are trying to drill and tap a Shimano hub, are you saying that your methods and tools would be different if Rohloff used a different method to make their hub shells?

[crg]

I'm already regretting answering, use whatever you've got, it a plug to stop the oil dripping out, does it matter?

So you don't know why.
Why not say so?

If you are trying to drill and tap a Shimano hub, are you saying that your methods and tools would be different if Rohloff used a different method to make their hub shells?

Yes

[Danneaux]

As a former car tuner-repair shop owner/operator, hobbyist framebuilder and home machinist, I have used both cutting taps of various sorts and forming taps on a variety of materials so can speak to techniques in general, though I cannot say for a certainty what Rohloff used, though I have a good idea from looking at my own hubs.

Briefly --
Forming taps are really only suitable for soft metals like aluminum because they form threads through pressure. There are no flutes on the taps, so no chips are formed to collect or be cleaned out by reliefs on the tap. To draw a broad analogy, they are akin to using a sheetmetal screw to form threads, then backing it out and replacing the screw with a machine screw or bolt.

Cutting taps cut threads in a much wider range of materials.

When you use a forming tap in thin stock (as I would imagine an aluminum hub shell to be depending on location where drilled), it will tend to distort the surface a bit, leaving a formed bulge or collar at the entry point and, sometimes in my own experience, also at the exit -- even if you use the larger pilot drill forming taps require. This bulged collar can be difficult to seal on a flat surface and much harder to seal on a rounded surface.

Because of this formed bulge -- and lack of it on Rohloff hubs -- I assume they use a cutting tap. The hole surround on my hubs is at the same level as the surface of the hub's machined shell, so it sure doesn't appear the threads were formed -- unless the hub was finish-turned after tapping formed threads.

In the end, I'm not sure any of us can say definitively which method is used by Rohloff or its authorized suppliers as that is likely to be a trade secret. If I were to wager, it would be that Rohloff uses cut threads. If I were to cut threads in a Shimano hub, I'd use Martin's proven methods and materials. They're sound and so is the reasoning behind them.

Best, Dan.

[martinf]

Forming taps are really only suitable for soft metals like aluminum because they form threads through pressure. There are no flutes on the taps, so no chips are formed to collect or be cleaned out by reliefs on the tap.

- I now know that I used cutting taps on the eight Shimano 8-speed hubs to which I have fitted drain plugs!

When you use a forming tap in thin stock (as I would imagine an aluminum hub shell to be depending on location where drilled), it will tend to distort the surface a bit, leaving a formed bulge or collar at the entry point and, sometimes in my own experience, also at the exit -- even if you use the larger pilot drill forming taps require. This bulged collar can be difficult to seal on a flat surface and much harder to seal on a rounded surface.

And that it was probably best not to use forming taps for this application.

The threads I cut aren't as good as the ones in my Rohloff hubs, but they are "good enough". The combination of nylon Allen-head screws and O-rings seems to work well as a seal, I suppose that the nylon adapts better to the less than perfect thread. Viton O-rings will probably last longer than the more common neoprene variety, they are more expensive but for the small 6 mm diameter the difference is trivial.

Spoke shortening is one application where I think it would be better not to use cutting. IMO rolled threads should give a stronger spoke than cut threads.

[Andre Jute]

Spoke shortening is one application where I think it would be better not to use cutting. IMO rolled threads should give a stronger spoke than cut threads.

A spoke is a pretty solid thickness of metal, though, and the thread-former supports it on all sides, so that force applied to it is self-stabilising.

My experience with thread-forming in thin metal for tube hifi cases does not inspire confidence, either in my own hands, or the hands of the craftsman in my sometime proto shop (he told me in advance that it was a waste of time), nor in pre-production protos in China. There was never the sort of flat fit that the aesthetics of high-priced gear for an elite demands. In the end I told the Japanese for whom I was designing the amplifiers, including the casework, that we'd wasted enough time and that I was making socket heads a feature of the casework, with locknuts on the inside. And that's for a case that stands on a table forever, because a tube amplifier is HEAVY. In a rotating hub, the least imperfection will spray oil everywhere.

Maybe on a relatively thick aluminium hubshell absolutely firmly and squarely clamped in some kind of fixture under a solidly mounted pillar drill an amateur could cut a near-perfect thread but I wouldn't put my money on a formed thread not giving trouble. But if an amateur has the required machinery and fixture already, he likely also knows which jobs he can tackle and which should go to the local machine shop.
***
Martin, your tips'n'tricks never cease to amaze. Nylon screws with O-rings, of course! I have the perfect application. The port for grease insertion on Shimano roller brakes -- the -75 series anyway, are closed with nothing but minuscule rubber plugs that from the beginning seemed destined to be lost, and were lost. Thanks for the solution.

[mickeg]

Dan, thanks for your description, that added clarity to the situation.

The Shimano hub shell thickness is unknown, I would think that is the more important question.

[martinf]

The Shimano hub shell thickness is unknown, I would think that is the more important question.

Nexus 8 Premium are thick enough to get a thread that holds. I suspect that Alfine 8 are the same thickness, just prettier.

I haven't measured, but they seem to be about the same thickness as a Rohloff hub shell, hence my initial use of Rohloff drain plugs as I had some of these in stock.

The nylon screw variant seems better in practice. Sealing is not depend on thread lock, the nylon will deform to fit the "amateur" tapping, whereas steel into alloy might end up damaging the thread, and with the larger head and bigger Allen key, unscrewing to inject oil is less fiddly.

Finally, as the screw is white and larger than the tiny Rohloff drain plug, it is easier to find if I drop it on the garage floor or forget where I put it down.