MAIN FUNCTIONAL REQUIREMENT: Provide a low-force means of accurately machining a wide variety of materials in two dimensions.
DESIGN PARAMETER: An Abrasive Waterjet Machine
As a means of replacing the processes of drilling, sawing, broaching, gear cutting, profile milling, plmching, slitting, spline cutting and hlanking, abrasive jets are used to machine materials with remarkable accuracy and quality of finish; abrasive jets are typically advertised to be accurate within +/- 0.005in.
There are many advantages to using an abrasive waterjet to rnachine a piece. Among them are the following:
· Abrasive jets can cut through any material softer than garnet (material it uses to do the cutting, whose Moh hardness is at least 6.5).
· The finish of a piece cut with an abrasive jet is of
high quality and the piece looks as though it had been sandblasted. Often, deburring is
not necessary after a piece has been machined by an abrasive jet.
EXPLANATION OF HOW IT WORKS/ IS USED:
The big picture is that a motor is connected to a piston pump, which pumps fluid through a tube. For a nifty description of piston pumps and a pretty cool picture of one, see reference 2. The fluid is mixed with an abrasive and the mixture comes out of a nozzle aimed at the sheet of material to be machined. Figure la, is a schematic of the basic parts of an abrasive jet, and figure 1b is a photograph of the abrasive jet in building 35 at MIT.
These variables are used to write down expressions for the various forms of power, itemized below.
Flow out, of the pump is turbulent (high Reynold's number) at a pressure on the order of 40,000psi. The fluid enters a ceramic tube, a section of which contains a sapphire orifice. The orifice is made of sapphire because the design specifications require that the orifice remain at a constant, size and shape in spite of the high pressure forces and wear and tear. Sapphire and diamond can both maintain these dimensions for a long period of time, but sapphire is relatively cheap (the assembly containing the saphire orifice can be purchased from OMAX separately for just $39 [see ref 1]). The fluid is forced through the sapphire orifice at a speed on the order of 2500 ft/sec, or about Mach 3. The pressure of the flow at the location of the orifice is lower than atomospheric (increase of fluid speed across the orifice and their is an accompanying pressure drop with the increased fluid velocity). At this point, the abrasive is sucked into the stream of fluid, in what is called a "mixing chamber." See Figure 3 for a cross section of the insides of the mixing chamber and the tubing surrounding the sapphire orifice. Beyond the sapphire orifice, the speed of the fluid-garnet mixture is comparable to the speed of sound (V ~ 740 ft/s). The mixture exits the tube through a nozzle 0.030 inches in diameter at approximately 0.06 inches above the material to be cut.
The cutting is achieved through a grinding process where a fluid-garnet mixture is used instead of a grinding wheel. Figure 4 is a sketch of the cross-section of a piece of material being machined. The fluid jet drags the abrasive through the material in a curved path as the fluid lags the nozzle. This phenomenon is analogous to what happens when you whirl a rope around in the air; the drag force on the rope causes the rope to lag your hand. Similarly, the fluid lags the moving nozzle, and the mixture can not cut straight through the piece. Because of this fluid lag, it is especially important that the nozzle slow down when turning sharp corners. Machining can go much faster when the edges of pieces are rounded.
WHERE TO FIND ABRASIVE WATEJETS:
References Most of my research was dome on the web at the following sites, listed in order of relevance
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