Cutting speeds

This page contains information from before 2010. It is left here for archival reasons only.  Although in most cases, the information here should still be relevant and useful, please be aware that the information contained on this page may be out of date.  For the most up to date information please navigate back to the home page.

A typical part in ½” (1.3 cm) aluminum might take 2 to 10 minutes to make. In ½” (1.3 cm) steel, it takes twice that time. A 2″ (5 cm) thick part might take hours, while a 1/16″ (1.5 mm) part will cut in seconds.

Time to make parts will also vary based on the equipment used in making the waterjet machine. If you need a specific time, you should contact a waterjet jobshop or a waterjet manufacturer, but be sure to have a drawing of your part ready. As you will see in the discussion below, the shape of the part affects how long it takes to make.

The chart below shows how long a few typical waterjet parts take to machine.

Picture of part Description Approximate Cutting time
 2.5″ x 2.5″ Box cut from 0.5″ thick mild steel
(6 x 6 cm from 1.2 cm steel)
5 minutes
box cut from 3" mild steel Same part as above, only in 3″ (7.6 cm) mild steel 2.25 hours
8" wide Electrical Panel cut from 0.06" mild steel 8″ wide Electrical Panel cut from 0.06″ mild steel
(20 cm from 1.5 mm steel)
1 to 3 minutes
3" wide gear cut from 0.25" thick nylon 3″ wide gear cut from 0.25″ thick nylon
(7.5 cm from 6 mm nylon)
1.25 minutes
10" wide part cut from 1" thick titanium 10″ wide part cut from 1″ thick titanium
(25 cm wide from 2.5 cm thick titanium)
22 minutes
7" tall horse cut from 0.25" thick aluminum 7″ tall horse cut from 0.25″ thick aluminum
(18 cm tall cut from 6 mm thick aluminum)
4.8 minutes

Below are the primary factors that determine cutting speed.

Material being cut and thickness

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 brittle granite easily breaks up under the waterjet stream. It is also interesting to note that hardened tool steel cuts almost as quickly as mild steel.

Even with granite, there is some variation in cutting speeds. “Absolute black” granite, which is tough as nails, cuts a little slower than copper.

Thickness

The thicker the material, the slower the cut, with no exceptions For example, a part that might take one minute in 1/8″ (3 mm) steel, might take half an hour in 2″ (5 cm) thick steel, and as much as 20 hours in 10 inch (25 cm) thick steel.

Geometry of the part

The waterjet stream cuts most efficiently when it is moving in a straight line. The cutting head must slow down to navigate sharp corners and curves. A part with a lot of curves and angles will take longer to make than a part with long straight lines.

It also takes additional time for a waterjet to pierce the material. Therefore, parts with lots of holes requiring pierces will cut much slower than simpler shapes.

Desired Result

If you want a high tolerance part or one with a smooth surface finish, then the part will take longer to make. By changing the machining speed, 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

The software controlling the waterjet head is probably one of the most overlooked aspects of abrasivejet machining. 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.

Software makes a difference

A part to be machined from ½” (1.2 cm) mild steel

Notice the subtle difference between the two pictures show above. (The colors represent cutting speeds, with yellow being the fastest areas, and blue being the slowest.)

The part on the left took 3.3 minutes to machine, while the part on the right took 4.4 minutes to machine. That’s a one minute difference, or about 25%. The difference, as it turns out, is the software that automatically optimizes the tool path to provide the desired precision in the least amount of time.

The software looks at the geometry of the part, and then modifies the feed rates and adjusts the cutting to get the maximum speed. The optimizations include finding the best speeds and accelerations for all curves and corners, setting the best length and feed rate for all pierce points, adding special “corner pass” elements at corners to allow the cutting to go right past the corners where it can, and so forth.

In the last 15 years, software has gone from simply optimizing corners to adjusting more and more factors as processing speed improves and cutting models improve. Predicting the behavior of a high-pressure stream of water mixed with abrasive as it strikes different materials is becoming more refined, which lets software better predict the ideal speed and acceleration for a part.

Power at the nozzle

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 the water passes through. Note that there is a difference between “horsepower of the motor” and “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. This is also why you rarely see production pumps that run more than 60,000 PSI (4,100 bar).

A good way to learn more about how pressure and jewel size affect cutting rates, and to calculate “nozzle horsepower” is to download and run Waterjet Web Reference Calculator .

Quality and amount of abrasive

Quality 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. Garnet turns out to be a good compromise.

Not all garnet is the same, however, and the differences start with where the garnet was obtained (alluvial garnet is rounder and smoother than crushed garnet from a mine). There are also big variations in purity and uniformity between brands of garnet that can affect the cutting speed and accuracy of waterjet machining.

Amount of abrasive

Typically, waterjets consume between 0.5 and 1 lb (0.25 and 0.5 kg) of abrasive per minute. More abrasive will, up to a point, improve cutting efficiency, although at a higher cost. There is a sweet spot for every nozzle size and pressure as to what abrasive flow rate will cut the fastest, and what flow rate will cut the cheapest.