Machining a Square-Taper Crank

Maybe a dremal if they make a bit that will work without taking
forever... You could cut it down at an angle then with a grinding bit
to even it out nicely. That's not really efficient and I'm sure some
people here will be able to help you more than this, but I try.

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'Add wickedbob to your ignore list' (http://tinyurl.com/3dfrko).

My posts don't count they came from MR...would they matter anyways?
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That's strange - what frame are you using? I use prowheels on my 26"
muni (UDC CrMo hub in Nimbus II frame, so standard pressed bearing
holders) and there's quite a big gap between the crank and the bearing
holder. I suppose something like a KH frame with machined bearing
holders would probably have a bit more thickness of metal outside the
bearing, but I'd be surprised if it was enough to touch the crank - but
perhaps it is. Or perhaps you've got a hub where the taper is a bit
skinnier and the cranks go on a bit further.

That doesn't really help with your problem though :o... I'd agree with
the poster above and say if it's only just touching just grind some
away with a grinding wheel of some kind.

Rob

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rob.northcott
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rob.northcott said:

That's strange - what frame are you using?

It's a Telford. The space for the crank is only slightly bigger than
the ring in my original crank. I love that curvy frame, and don't want
to replace it--but with 170 cranks, my kid complains about how slow it
is to ride.

Keep the ideas coming! Thanks!

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pkittle

*==========================
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"The Revolution is just a t-shirt away."
--Billy Bragg
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I'm sure gerblefranklin will chime in here as soon as he gets back from
class, but I'll throw in my two cents in the meantime.

It's sad that UDC calls most every crank an "alloy" crank as that's
about as useful as calling it a "metal" crank. You have to guess what
the composition of the crank is before you can do any work on it. I'm
going to guess that it's an aluminum crank.

Unfortunately aluminum doesn't lend itself to grinding very well.
Being a soft metal, it will quickly gum up your grinding wheel...
shortly thereafter your wheel will be an aluminum wheel and it won't do
its job anymore. Bad news. If you choose to use a grinding wheel
expect to throw out both your crank and your grinding wheel shortly
after you begin your job.

On the flip side, since aluminum is soft, it's easy to cut. If you
have a drill press and a hole saw of the right diameter, you may be
able to do this fix yourself... just make sure to cut very slowly (low
RPM and low feed rate). I wouldn't advise doing it if you aren't
confident with your shop skills, though.

That leaves you with a trip to your local machine shop. Yeah, it'll
cost you a bit to get these cranks reworked, but it should be a
straightforward task for an experienced machinist. This way you can
rest assured the task will be done correctly.

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maestro8

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Sorry, I didn't address these points...

pkittle said:

The old crank had a ring around the square hole that was, I guess, just
machined into it.

Most cranks are cast... machining is quite expensive. You'll know the
difference between a cast and a machined part when you see the price
tag 😄

Quoted message said:

Or, if I have to take it to a metalworker, about how much should I
expect to pay to have this done?

Just off hand, most shop rates are about $100 an hour. It shouldn't
take an hour to get this done (I'm thinking 1/3 hour at most), but your
local shop may have a minimum charge.

If I were in your shoes I'd show up at the local shop around closing
time on Friday with a sixer of good beer, your cranks and a handful of
cash. Maybe you can sweet-talk a machinist into doing your job "on the
side" for a cut rate.

Edit: perhaps bring your old cranks too, so the machinist knows how
much material to remove.

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pkittle said:

The old crank had a ring around the square hole that was, I guess, just
machined into it. This ring allows the crank to get pressed fully onto
the taper. The new crank, obviously, doesn't have this feature, and so
won't press all the way onto the taper (the frame and bearing holders
get in the way).

I'm sorry, I don't quite understand this. I don't see how the frame and
bearing holders get in the way. Are you saying that the crank is too
thick at the square taper and the taper is too deep? Then the crank
touches the frame before the taper starts to get tight? If that's
right, you want someone to mill off the appropriate amount of material
perpendicular to the taper axis. This will certainly cost more than the
$22 per pair for the cranks unless you do it yourself or have a friend
that can do it for you. From the standpoint of cost you may want to
consider a different kind of crank.

--
harper

-Greg Harper

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Yes, pkittle, please clarify your issue. Because not all cranks fit on
square tapers the same. Without describing the confounded technical
design issues, some cranks may only bolt onto the outer half of the
tapered surface. Other cranks may slide on darn near all the way. This
variation is not an issue unless, as Greg suggests, the cranks slide on
so far that they contact the bearing holders.

And if your cranks only slide onto the outer half of the taper, an
appropriately-sized spacer between the cranks and the bearings helps to
prevent any "bearing slide" due to lateral loads.

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Or then there's the cheap-o method, vice and file. I've had to do this
with some old (worn) cranks and narrow unicycles. It might be a big
pain with steel cranks, but the file cuts pretty fast into the
aluminum/alloy.

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John Foss
Email: "jfoss" at "unicycling.com" -- www.unicycling.com
-----------------------------------------------

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4" angle grinder a few steel cutting discs a pair of goggles and a
vice,prepare cold beer in readiness of completion.done.

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unisk8r said:

Yes, pkittle, please clarify your issue. Because not all cranks fit on
square tapers the same. Without describing the confounded technical
design issues, some cranks may only bolt onto the outer half of the
tapered surface. Other cranks may slide on darn near all the way. This
variation is not an issue unless, as Greg suggests, the cranks slide on
so far that they contact the bearing holders.

Sorry for not being clear enough. The problem can't be solved with a
spacer, because the crank hits the bearing holders before getting even
close to being tight on the taper; when they hit the holders, they can
still wiggle back and forth.

Finding the right cranks to fit may be tough. The frame requires cranks
with significant offset (to increase Q) or they hit the frame's rear
seat stay. So I suspect that Torker LX cranks, which I'm pretty sure
are without offset, won't work (at least, when I tried putting a DX
wheel on that frame, the cranks hit the frame).

This is what the uni looks like:
[image:
http://www.unicyclist.com/forums/attachment.php?attachmentid=12756&stc=1&d=1143870259]

--
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*==========================
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"The Revolution is just a t-shirt away."
--Billy Bragg
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