【文章內(nèi)容簡介】 ace, 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. Welldesigned 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 MachiningCNC 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 isn39。t your crank. 3D metal etchasketch, with the puter interpolating so the circles e out looking pretty smooth. The downside of CNC machiningThere 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 parison, 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, pared 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 plicated machines, full of servomechanisims, and measuring technology that can measure to () 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 puter is not a significant part of the cost any more.) Oh yeah, strength. Well, if you cut away metal, it doesn39。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 machiningYou can39。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 doesn39。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 picated 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 plicated 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 overe CNC39。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 plicated part of a press are the sensors that make sure that the operator39。s body is out of the way before it starts moving. It39。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 puter, unless they have a robot next to it to load and unload the finished parts. Why Forge?CNC is a good pliment to forging. There are a lot of shapes that forging can’t produce .internal thread ,for instance .If you know that you will be making 50,000 a year, and its always going to be 5 holes in a 110mm 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