What is CNC Machining?
 

What does it stand for and what does it really mean?

Computer numerical control=computer runs milling machine or whatever device they are using to fabricate a part.These days you can literally design a part via computer,insert a chunk of metal into a milling machine,and sometime later the thing is done....literally carved out of solid metal.

Yes, and CNC gives you the ability to economically make one or two items, where its older counterpart (CASTING) requires the making of a mold and pouring each casting so the cost of making only a few is tremendous.

It also means fast development to market time. An idea can go from a CAD drawing to a motion profile computer driving the machining equipment and a new innovative design can be reality in a short period of time.

It is invaluable for companies that like to be on the forefront of technology.

CNC milling/lathe machines come in a variety of sizes. and can manufacture shafts as small as 1mm to shafts as long as 40ft(well, at least thats the largest i have seen)...

we use CNC to grind rolls(paper industry)... good thing about this is you only have to feed your specs and the machine does the rest in extreme precision....

Thanks for the info. Now I feel like I can understand what they are talking about.

One more thing- CNC means REPEATABILITY. When machines were run manually part to part variability was a function of the operators' skill. With a computer running the show, the machine does EXACTLY the same thing each time and the parts that come out at the end are identical and the cost of quality is low.

And another thing, CNC stuff is not as strong as a forged part. Metals have a grain stucture, kind of like wood. A forged part has had its grain realigned in the forging/shaping process. A CNC part may be cut out with its grain structure in a less than ideal direction, say, going across the width of a crank rather than down its length. Some companies use cold forged blanks and then use CNC to finish the job. This gives you a strong part at a lighter weight. I believe Race Face does this on their turbine cranks. Or if you watch one of Discoverys chopper shows you will see them use a forged wheel blank,for strength, to start the CNC process, while they would CNC a cylinder head out of blank stock because strength is not a critical factor.

We have a CNC machine here at work. Awesome to watch it do it's thing.

CNC vs forging is really a matter of economies of scale. For large scale manufacturers, forging is more economic; Shimano is a cold-forging operation. The components come out with superb grain structure and finish.
Smaller boutique shops can't afford a massive forging press, or the moulds, so CNC is more cost effective. Most CNC time used to be assigned to military/aerospace production, but post Cold-War, there was spare capacity, so people started turning out cool bike parts.

I'm going to take MichaelW's response a step further in an effort to give a more generic answer. You probably know what a machine tool is; lathe, machining center, etc. In ye dayes of olde, a person equipped with micrometers, calipers, etc. would stand at said machine, and working methodically from a blueprint, turn a chunk of metal into something useful. A central powerplant, and later an electrical motor, turned gears which, with the help of the machinist, would interpolate curves, generate ratios, and cut the metal. With the introduction of assembly lines and mass production in the first half of the 1900s somebody figured out that it would be useful to program these machines to perform repeatable functions. Various techniques were used, but as the technology came around, rolls of paper tape with punched holes became the popular programming media, and a language of "G codes" was developed to program motion. Gearing started to be replaced by servo systems, first hydraulic, then electronic. This was called Numerical Control. Jump ahead a few more decades, and add computers to the mix. At first they are huge, filling rooms. Later, they are much smaller. Programs, "profiles," can be generated graphically, by programming the machine to "scan" dimensions from an existing part, by remembering what a machinist did to produce an original part, or other mechanisms. Some are networked to large databases, controlled remotely, and synchronized with other operations along an assembly line. Others stand alone, ready to repeat a programmed operation pretty much at the touch of a button. With the continuing advances in technology, controls become less expensive, and some models are a reasonable investment even to "mom and pop" shops. Not quite a robot, but much more than a machine tool. Voila, CNC... Computer Numerical Control.

Roadbuzz, this also makes old school machinists more valuable, because there is less and less emphasis on hand skills and more on computer skills. Makes it hard to find a guy who can make one replacement part.
Tulio Campagnolo, did his machine work with files.
MichealW, I remember seeing many a Kooka crank, aCNC'd crank, fail right through the K in the logo on the arm. This could partially be attributed to the foolishness of milling a stress riser into the middle of the arm.

a CNC is just a machine, it doesnt really have an effect on the quality of the final product.
quality of final product is a result of design and quality of materials used. CNC's are used on both casted and forged materials.
the benifit of a CNC is the speed with which you can make multiple parts, and speed from design to finished product.
also some items of extremely high tolerances need a computer oprerated machine to be made.
when it comes to bike parts saying something is cnc'd probably doesnt really mean anything. i think its just an advertising gimmick. whats important is the quality of the material, the design and the way in which it was machined.
you could put a piece of balsa wood in a CNC and it would cut out a crank for you, or you could carve one by hand, it might look good, but you wouldnt want to put it on your bike.