Frame making thread

Follow-on to this
[uml=http://www.unicyclist.com/forums/showthread.php?s=&threadid=33294&perpage=15&pagenu mber=3]thread[/uml]

Gerblefranklin, you're on the right track. The key parameters for
frame-making a

DENSITY
The mass per unit volume.

TENSILE STRENGTH, ULTIMATE

This is the force required to pull apart a bar of metal divided by the
cross sectional area of the metal. The link for titanium says that it's
ultimates strength is 49,900 psi (pounds per square inch), or about 50
ksi (thousand pounds per square inch). That means that it would take a
pull of 25 tons to rupture a 1" x 1" bar of titanium. Similar numbers
for ordinary structural steel (bridge I-beams for example) are from
30,000 - 50,000; and for exotic bike-tubing steel around 160,000 to
200,000.
*
Ultimate strength isn't that important on a unicycle because the frame
is usually bent and ruined before it breaks.

TENSILE STRENGTH, YIELD

This is the force required to permanently deform a bar by pulling on it.
For brittle materials it's the same as the ultimate strength, since
brittle materials crack before they deform.
*
ELONGATION AT BREAK[/b]

THIS IS HOW FAR THE BAR STRETCHES BEFORE IT BREAKS. AGAIN, NOT SO
IMPORTANT SINCE MOST OF THIS ELOGATION IS FROM DUCTILE YEILDING. IT'S
USEFUL AS AN INDICATOR OF HOW THE METAL WILL FAIL, THOUGH, SO IT HAS
SOME USE IN JUDGING HOW SAFE THE FRAME IS. FRAMES THAT SIMPLY SNAP ARE
DANGEROUS.

[b]MODULUS OF ELASTICITY

The modulus of a material is important for determining how stiff the
frame will be. Think of it as if it were the stiffness of a spring -
high modulus materials, like stiff springs, require a lot of pull to
stretch even a little bit, whereas low modulus materials stretch very
easily. Carbon fiber is a very high modulus material; rubber is a very
low modulus material.


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Here is a comparison of various metals used in frame-making.

More metals can be found 'here' (http://tinyurl.com/2c6g3)

Note that all of these frame-making metals have about the same specific
elasticity. That means that if you just vary the wall thicknesses of
the tubing all of them will make frames that ride about the same. Since
aluminum and titanium are lighter than steel they can be formed into
tubes with larger diameters to make much stiffer frames.

Also note that aluminum and steel have about the same specific
strengths, but titanium is twice as high. That means that the titanium
frame is more likely to hold up under abuse.


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Sweet! I get it!

Thanks for all this info. Did you derive this information from the links
I gave you?

I plan on making a few steel hunters exactly like the original, and then
trying one out of titanuim. One hitch I'm runnign into is that the
person who runs the shop I may be using says her shop isn't set up to do
titanuim. She says I need a vacuum chamber for some forms of titanium.
But on the other hand, I talked to Steve Howard, and he says that I can
TIG titanium just as I would steel.

Now, assuming I'm still going to make a titanium hunter-like frame, can
you tell me how much larger diameter the tubes need to be? I originally
said 50", but would 25% work? I'd still like to keep the tubing small.

Also, do you know of anywhere I can get the materials for a titanium
frame? Steve Howard said I would need brand new High-speed steel end
mills for the bearing holders, and I assume this applies to the brake
bosses. He said that they'd be worn out pretty quickly. Also, where do I
get grade 5 titanium? I doubt I can just drop by the scrap yard and pick
some up.

Thanks for all this advice, and I apologise for jacking hte other
thread, although by now it seems dead.


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gerblefranklin - Trials Unicyclist

Don't you think it's a cruel irony that acting like a G.I. Joe in the
army can get you a Medal, while playing with one can get you thrown
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"cyberbellum" . com writes:

The key parameters for frame-making a

DENSITY
TENSILE STRENGTH, ULTIMATE
TENSILE STRENGTH, YIELD
ELONGATION AT BREAK
MODULUS OF ELASTICITY

If you are considering aluminum, it is also necessary to include some
sort of fatigue analysis. Stresses on aluminum must be kept
significantly lower than yield strength to prevent cracking from
fatigue.

Ken

gerblefranklin wrote:
*Sweet! I get it!

Thanks for all this info. Did you derive this information from the
links I gave you?
*


Yup. First year engineering schools stuff. It's called Mechanics of
Materials. You're math is probably good enough already for you to read
a textbook on this topic. If you run into anything you don't understand
one of your math or physics teachers can help.

*
I plan on making a few steel hunters exactly like the original, and
then trying one out of titanuim. One hitch I'm runnign into is that
the person who runs the shop I may be using says her shop isn't set up
to do titanuim. She says I need a vacuum chamber for some forms of
titanium. But on the other hand, I talked to Steve Howard, and he says
that I can TIG titanium just as I would steel.
*


Steve is going to be a MUCH better source of information on metalworking
than I am. Aerospace engineers just come up with the design
specifications. We don't actually know how those specifications are
turned into parts.

Titanium LOVES oxygen, so your shop person wasn't far off. But Steve is
right - the inert gas displaces all the air and allows the molten
titanium to stay a metal instead of burning into an oxide.

One of my engineering 101 professors explained the reactivity of
different metals in this way. He said, "A freshly cut surface of steel
will oxidize in a few days or weeks, and will keep oxidizing until the
steel is gone. Freshly cut aluminum oxidizes in a seconds or minutes
but the oxide coating is so tight that it stops the reaction. Titanium
is the same way, but it forms the oxide coating in a few milliseconds."
It's safe to say that no one ever really sees freshly cut titanium, and
that molten titanium will react furiously with air if you ever give it
the chance.

The Russians built a couple of subs out of titanium during the cold war.
No one was sure exactly how they did it. It turns out they built the
subs in a giant airtight building filled with argon. All the workers
wore space suits.

*
Now, assuming I'm still going to make a titanium hunter-like frame,
can you tell me how much larger diameter the tubes need to be? I
originally said 50", but would 25% work? I'd still like to keep the
tubing small.

Also, do you know of anywhere I can get the materials for a titanium
frame?
*


I'll have to get back to you on that. My sense is that 25% would be
fine, but you are going to be limited by whatever tubing size you can
find.

You might call Lightspeed or some other titanium bicycle frame
manufacturer and ask them where you can get materials. They might find
your project amusing and offer some help.

* Steve Howard said I would need brand new High-speed steel end mills
for the bearing holders, and I assume this applies to the brake
bosses. He said that they'd be worn out pretty quickly. Also, where do
I get grade 5 titanium? I doubt I can just drop by the scrap yard and
pick some up.
*


Steve is telling you something quite valuable. Titanium is reactive,
right? Very reactive? Those top-end cutting tools are going to get
dull very, very fast because the titanium will weld itself to the
cutting edges as it is being cut. Those welds will round the edges of
the tools in no time, either because tiny titanium blobs will coat the
edge, or more likely the titanium will pull chips of steel off the
edges.

Steve is also cluing you in on the possibility that you may need to
spend a lot more money on tools than you will on materials. What Steve
is saying is don't waste your time with cheap tools because they will
become useless almost instantly. The really good stuff will cut
titanium for a minute or so before it goes dull.
The machinists who worked on the SR-71 (a mostly titanium aircraft) used
to go through a truckload of tools just to get one assembly built.

There are a couple of processes that have been invented for working
titanium without welding or cutting. It's so reactive that if you just
get two parts hot and press them together they will diffuse into each
other and be virtually welded without ever melting. Titanium also has
this neat property that it will slowly flow or into a new shape if you
get it hot and apply enough pressure. I think the process is called
superplastic forming. If it's not too hard to do perhaps you could make
the bearing holders this way by "mushing" them on to a 40 mm steel form.
It might be cheaper than buying a few sets of cutting bits.

So to recap, I know just enough about making things to be dangerous.
Listen to Steve. He's a pro.


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I was standing in the park wondering why frisbees get bigger as they get
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Oops! Hit quote instead of edit.


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How hot do I need to get it to do superplastic forming? Would 500
degrees farenheight do? Then, I could just stick it on an open flame and
press until it was formed. How much pressing are we talking about.
200p.s.i? 1,000p.s.i? Also, how much are those kinds of cutting tools?
Would a slow enough speed running the bits enlongate their lifespan?
Also, how does one make a 40mm cut on a non-CNC milling machine? Do I
need a 40mm end-mill?


--
gerblefranklin - Trials Unicyclist

Don't you think it's a cruel irony that acting like a G.I. Joe in the
army can get you a Medal, while playing with one can get you thrown
out?

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On Sun, 20 Jun 2004 00:03:43 -0500, "cyberbellum" wrote:

Oops! Hit quote instead of edit.

In such a case, hit your "Back" button.

Klaas Bil - Newsgroup Addict
--
I was right when I made the mistake. - doofe

This is a fascinating discussion about framebuilding. But I want to go
back to the original question of building a Hunter frame out of
aluminum. The engineering aspect of doing this is only one part of the
equation. You have left out "why a Hunter frame?"

Purpose of the Hunter design:
- Fatter tires require wider frames. Wider frames bang into your legs.
Remember, this design was developed before and during the time people
started using 3" tires. Making a wide frame (for 3" tire) out of fat
tubing would make for a very wide outside width; more chance to
interfere with your knees.
- Good quality bicycle tubing comes in wide sizes, a little too wide for
a good unicycle frame, and narrow sizes, for the rear triangle on the
bike. The Hunter (and Telford) design is based on doubling up on this
skinny tubing to maintain strength, while keeping the overall width
down.
- Conclusion: Making a Hunter-style frame out of wide tubing doesn't
make a lot of sense. If you're going to use wider tube, a different
frame design should be considered.

I think this is part of why the Wilder frames are shaped the way they
are. They essentially have rectangular tubing on the fork legs. Wide
front-to-back for strength, but narrow side-to-side to keep the overall
width down.

For titanium, you can probably do a more standard fork design. It might
not look as cool, but it should be easier (and cheaper) to build.

For aluminum, it would be nice if you could fine some ovalized tubing.
Otherwise, you're just going to have to live with a wider frame. This
may not be a problem, depending on rider height. I think the shorter you
are the more problems you're likely to have with a wide frame.


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"jfoss" at "unicycling.com"
www.unicycling.com

"That sucks." -- Ryan Atkins, at the bottom of a 10' cliff he just
jumped off at the 2003 California MUni Weekend, looking at his bent
seat.
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I got to see Kris Holms latest prototype that he was riding on while he
was in Telluride for the Mountain film fest. It's an alluminum frame
made with oval tubing that has a flat crown like his others but it
tapers in at the top of the vertical legs so it's not as wide of a flat
top to reduce leg scrapes. It seemed very stiff and also light, I hope
it makes it to production it was a sweet frame. It would be great to see
a low cost production alluminum frame muni available. Kris Holm has done
great things making high quality munis more affordable, I hope he keeps
it up.


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