bicycle_disciple said:
What manufacturing methods are being used in high end components these
days and how do each one affect strength, failure due to fatigue,
stiffness etc?
There are more factors than manufacturing method that dictate the
properties of a part.
Know first of all that stiffness is a product of basic material type
and part dimensions, not manufacturing technique. A cast stem made
from weak, soft aluminum would be as stiff as a cold-forged and
machined stem made from 7075-T6 alloy, if they both had the same
shape, size, and weight. So for stiffness, you are concerned about
gross categories of material (e.g. steel vs. titanium vs. aluminum vs.
magnesium), part weight, and part form. It makes no difference how
the part is made.
For the purposes of the discussion, I am going to exclude frames,
forks, rims, etc-- things that can be considered structures unto
themselves. Such parts have more in common with each other than they
do with components that function as mechanisms.
The highest quality components these days are either cold forged from
metal or laid up and cured from carbon/epoxy. (I'm not going to talk
about carbon parts because I think they're goofy.) Forging is
basically smashing material into the desired shape between tools or
forms made of harder stuff. It makes the metal stronger and reduces
the size and effect of internal flaws, as well as being a material-
efficient way of making parts. The tradeoff is that the dies (shaped
tools) for forging are very expensive, and the finish quality of parts
in as-forged condition can be pretty crude.
Forging can be done on "cold" metal (in the same microcrystalline
state as at room temperature), which causes work hardening--
strengthening-- of the metal itself. It can also be done hot, with
metal that is softened and easier to smash into shape. Hot forged
parts have the structural advantages of "grain" that follows the shape
of the part and diminished internal flaws, but the metal will be in a
relatively soft state after the part cools. Thus it will be both
weaker and more ductile (bendable; the opposite of brittle) in
comparison to a cold forged part made from the same alloy. You can't
usually tell just by looking whether a forged part was cold- or hot-
forged.
The most common sort of stem I see on new bikes is forged from
aluminum around a mandrel (making the finished product hollow):
http://www.sheldonbrown.com/harris/images/stem-dimension-thless-sm4523.jpg
Most aluminum brake arms, seatpost heads, crank arms, pedal bodies,
and hub shells on decent quality bikes are also forged.
Casting allows the the same sorts of complex shapes as forging, with
even more detail and finer surface finish. (Castings can also be
incredibly crude and poorly finished.) Cast metals tend to be softer
and weaker than cold-forged metals, but much more brittle than hot-
forged metals. They also contain the largest flaws from which cracks
can propagate. Casting dies are very expensive, but the incremental
unit cost of cast parts is tiny compared to most other processes. As
a result, cast parts are common on cheap pedestrian bikes which are to
be sold in great numbers-- think Huffies and other bicycle-shaped
objects.
Machining is the term for cutting material away from a casting,
forging, or blank of raw metal. Any bike part with threads for screws
in it has been machined to some degree. Machining parts from plain
bar stock is one of the most economical ways to make small numbers of
a part, but one of the most expensive ways to mass-produce parts. It
produces a huge amount of swarf (chips) which must be recycled or
discarded.
Machining includes milling (which would be used to make a crank or
fancy chainring) and turning (which would be used to make a hub shell
or pedal spindle), as well as other processes like drilling, tapping
(making threads), broaching (making splines and keyways), flycutting,
and surface grinding. CNC stands for "computer numeric control",
meaning the machine runs under the command of a computer which has
been programmed with toolpaths by its operator. CNC machining allows
shapes and finish quality which were not feasible when the machines
were controlled exclusively by hand cranks and levers.
It's common these days for forgings to be machined on all exposed
surfaces. This accomplishes two primary things: It brings the part
to a very uniform shape and finish, and it makes the part shiny and
attractive to the consumer without the need for much more surface
treatment.
Stamping is another process that can be seen in some metal bike
parts. Stamping uses punches and dies to strike flat forms from metal
sheet or plate, sometimes bashing curved surfaces into the parts in
the same process or a subsequent step. Cheap steel single-pivot
calipers are probably the best-known stampings used in bikes, though
there are plenty of other examples. Seat guts are stamped, as are
some brake levers. Steel chainrings, sprockets, derailleurs, and hubs
are almost always stamped. Lots of steel stems are made by stamping
or a combination of stamping and welding. Pedal cages are stamped.
The benefit of stamping is entirely in its low manufacturing cost.
Parts made this way usually lack rigidity for their weight, owing to
their thin, flattened and mostly open sections.
Welding is used for some parts, like stems and a few cranks and
seatposts. Welding from sections of tube results in a naturally stiff
and efficient structure with all its mass close to the part surface,
where the material can best resist stresses. The drawbacks are
relatively high labor cost and significant variations in uniformity
and quality control. Almost all welded parts will need some amount of
machining before and after welding.
I may have left out some noteworthy processes, but at the moment I
think that's about all the common metalworking techniques used in the
manufacturing of bike parts.
Chalo
