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Forging, Casting and CNC Machining
by Jeff del Papa
with Sheldon "Swarf" Brown
Spoke Divider

Forging

To forge something you make a mold out of very good steel. You put a plate of the metal you want to forge between the two halves of a mold, and squeeze the mold. HARD! As in many tens (and for large things, hundreds) of tons, from a press that may outweigh your house. (The moveable half of the mold is called a "tool"; the stationary half is called a "die.")

The metal flows like toothpaste, and when you open the mold, you have your part. A little finishing, and its ready to use. Its an expensive process to set up (the molds are very expensive to make, and the press, while quite simple, is huge, and costs accordingly) but if you make a lot of something, it can be quite cheap. The lump of metal can be cold (by metal standards, 1/3 of its melting point) or hot (near the melting point, about the only option if you want to forge iron).

Other common processes are casting (pouring liquid metal into a mold) and CNC (Computer Numeric Control) machining.

The strongest result is usually from forging. As metal cools, it forms "grains". The grains are strong, but they don't "stick" to each other perfectly. So castings may well break, and if you look at the break, it will show a bumpy "matte" surface, as it will have separated between individual grains. In metalurgists terms, it shows poor ductility.

When you forge a piece of metal, the high pressures "collapses" the individual grains. The result is a little denser, and will tend to bend rather than break. It can be a lot stronger than a casting, or the same shape cut out of a flat lump of metal. The surface, since it took the highest loads will often be the strongest, and the less you can disturb this surface, the tougher the part. A break may actualy show a fairly shiny surface, as the space between the "grains" is gone. The direction of flow as the metal is squeezed imparts a grain structure to the metal that is a bit like that of wood, making it substantially stronger in specific directions. Well-designed forging tools and dies control this flow so as to make the part strongest in the directions it is expected to be stressed in.

CNC Machining

CNC starts with a peice of metal, sometimes called a "billet."

(Billet:
   pretentious word for "lump of metal," used by machinests and marketeers to confuse outsiders.)

That peice of metal might have been cast, forged, or rolled (squeezed between rollers, sort of a limited forging, only capable of making flat things with straight grain like a board).

It is put into a fairly standard machine tool, that has had position sensing and motors on the control knobs installed. This is basically just a robot machinist. You use a rotating cutting tool to cut away all the metal that isn't your crank. 3D metal etch-a-sketch, with the computer interpolating so the circles come out looking pretty smooth.

The down-side of CNC machining

There are a couple of issues. First, it wastes a lot of metal. The stuff removed is just metal shavings, and can only be sold for scrap. By comparison, forging uses almost all of the metal, except for a little bit of "flash" that seeps into the crack between the tool and the die. The process can be time consuming -- you can remove a couple of cubic inches of metal per minute. (limited mostly by your ability to keep the friction of cutting from overheating, and possibly melting things. This is especially important for the cutting tool, which may be severely weakened if you get it too hot, never mind near to melting), A part that is "sprawling" like your right crank, can take 10 minutes or more to make, compared to the small number of seconds that it takes a press to cycle. (A large press can make several parts per squish, providing even higher productivity.)

They are complicated machines, full of servomechanisims, and measuring technology that can measure to 0.005 mm (0.0001") while covered in oil. A CNC machine has a minimum of 6 motors (including some to change tools, and one or more to pump oil and coolant various places). This translates to running costs that may be well over $1/minute. (The computer is not a significant part of the cost any more.)

Oh yeah, strength. Well, if you cut away metal, it doesn't have the tightly packed surface finish of a forging. Worse, there may be inside corners that have a sharp junction. These are "stress risers," places that cracks can start (in any metal, but aluminum is particularly sensitive to it. Titanium is even worse.)

Advantages of CNC machining

You can't use an acute inside angle on a forging, you would never be able to get the part out of the mold. So all inside corners must be wider than 90 degrees, and have radiused edges (if you had a die (mold) that tried to form a sharp corner, it would cut rather than push the metal into place.

CNC doesn't impose such restrictions, though to get nicely radiused corners, you might have to change tools, to make the last pass. (you use a flat tool to get rid of the bulk of the metal over the flat areas, and use a round nosed tool to form the inside radius where needed.) So eliminating stress risers means more expensive machining time.

Why CNC?

So why CNC at all? Well its good at making small numbers of compicated shapes. In fact, they are just the thing to make the molds (called tools and dies) to do your forging in. (As a result, CNC technology has in fact lowered the "tooling" costs associated with forging!) It got its biggest boost from the missle folks. If you only plan to build 30 of something, CNC is just the thing for parts with a complicated shape, like that landing gear strut on that fighter.

The peace dividend left a bunch of shops with excess CNC capacity. Since the cost of the machine "just sitting idle" can easily be over half what it costs running full out on a billable job, it was find something for it to do, or the bank may be calling the auctioneer. They cast around for things that would get some money in to make the lease payments. Boutique bike parts and other things, where "rocket science" adds enough marketing appeal to overcome CNC's inefficencies, were something these shops latched onto. (for others, take a look at golf clubs or mototcycle and car hop up parts)

A press, while big and heavy is a very simple "low tech" machine, that has very low maintence requirements. For the most part, they are too stupid to break. The most complicated part of a press are the sensors that make sure that the operator's body is out of the way before it starts moving. It's either a single motor connected to a pump, and a big hydraulic piston, or a "drop forge", a big lump of metal, with a mechanisim that picks it up, and drops it. (CLANG!!!!!). No fancy measuring gear. No computer, unless they have a robot next to it to load and unload the finished parts.

Why Forge?

CNC is a good compliment to forging. There are a lot of shapes that forging can't produce. Internal threads, for instance. So you might take your raw forged crank blank out of the press, then put it in a CNC machine to tap the holes for the pedals, crank puller and chainring bolts. Sure, if you were producing them in the millions, you would just set up a simple machine that could only drill and tap crank holes.

If you know that you will be making 50,000 a year, and its always going to be 5 holes in a 110 mm circle, you program the CNC to make a fixture that holds the 5 drill bits in that circle, and has them turn in unison. Add another bit in the middle geared down to cut the puller threads, and # 7 to thread the pedal hole. Its faster than the computer (which will do one or at most two holes at a time, and will spend time moving the part to the place for the next hole.) and a whole lot cheaper to run. Neither takes any skill on the operators part, and there may indeed be a pick and place machine that specializes in stacking parts. It isn't perfect, it can only do that one job. Want to change bolt circles? You have to make a new machine. It might be worth it to make it changable so it can do 170's and 175's, it might be better to just make two different machines.

[Actually, I think they generally use the same forgings/castings for all crank lengths, just cut them down at different place, except, perhaps for super-top-of-the-line models.]

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