Friday, December 12, 2008

factors to deciding cutting speed

Material being cut

Hardness: Generally speaking, harder materials cut slower than soft materials. However, there are a lot of exceptions to this. For example, granite, which is quite hard, cuts significantly faster than Copper, which is quite soft. This is because the granite easily breaks up because it is brittle. It is also interesting to note that hardened tool steel cuts almost as quickly as mild steel. (Though "absolute black" granite, which is tough as nails, actually cuts a bit slower than copper.)

Thickness: The thicker the material, the slower the cut. For example, a part that might take 1 minute in 1/8" (3mm) steel, might take a half hour in 2" (50mm) thick steel, and maybe 20 hours in 10 inch (250mm) thick steel.

Geometry of the part

It is necessary to slow the cutting down in order to navigate sharp corners and curves. It also takes additional time to pierce the material. Therefore, parts with lots of holes and sharp corners will cut much slower than simpler shapes.
Desired Result

If you want a high tolerance part and / or a smooth surface finish, then the part will take longer to make. Note that you can make some areas of a part high tolerance and other areas fast, so you can mix and match to get the optimal balance between cutting speed and final part quality.
Software controlling the motion

This is probably one of the most overlooked aspects of abrasivejet machining by novice users. You would not think that software would have much to do with the speed of cutting. In fact, this is (mostly) true if all you are doing is cutting in a straight line. However, as soon as you introduce any complexity to the part, such as a corner, there is great opportunity for software to optimize the cutting speed.

Power at the nozzle (pressure and water flow rate)

The more horsepower at the nozzle, the faster it can cut. How much horsepower makes it to the nozzle is a function of the pressure and the orifice that it is being squeezed through. (Note: do not confuse "horsepower of the motor" with "horsepower at the nozzle". It is the power that actually makes it to the nozzle that is most important. Having a big motor makes no difference, if the power all goes into wasted heat!)

Simply put, the higher the pressure, the faster the cut. The more water you flow, the faster the cut. Unfortunately, as the pressure increases, so does the cost and maintenance, so this is not as simple as it seems.

A good way to learn more about how pressure and jewel size effect cutting rates, and to calculate "nozzle horsepower" is to run the Abrasivejet Feed Rate Calculator, which you can download from this web site by clicking here.

Quantity and Quality of abrasive used

Type of abrasive: In the industry, most machines run 80 mesh garnet for abrasive. However, it is possible to cut slightly faster with harder abrasives, but the harder abrasives also cause the mixing tube on the nozzle to wear rapidly. So, pretty much everyone uses garnet. It is worth mentioning that not all garnet is the same. There are big variations between purity, hardness, sharpness, etc, that can also effect the cutting speed, accuracy, reliability, and operating cost.

Quality of abrasive: Typically, abrasivejets consume between 0.5 and 1 Lb (0.25 and 0.5Kg) of abrasive per minute. There is a sweet spot for every nozzle size and pressure as to what amount of abrasive flow rate will cut the fastest, and what amount will cut the cheapest.

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