A waterjet is a beam type cutter, somewhat like a plasma, oxy acetylene, CO2 laser, or fiber laser. Programming a waterjet, and operating one for that matter, is actually simple. In fact, a waterjet is usually considered to be easier to program and operate than most other machine tools. I say this because a waterjet typically does not require a change in parameters when going from one material to another. The tool is non-contact, pierces material easily to start a cut, and cuts without heat so materials don’t change or warp from the cutting process. If you are newer to this blog, a previous article on what makes up a waterjet might be a good precursor to reading this post.
Everyone who cuts parts out of raw stock or a work-piece knows you can’t cut a good part if it isn’t sufficiently held in place. So, what do we have to consider when we’re talking about waterjet cutting? The good news is a waterjet cuts with low force. Where a milling machine might force a rigid cutting tool into a material at 10, 100, 300 pounds of force (4.5, 45, 136 kg), the waterjet head doesn’t touch the part — just the supersonic stream that exits the head touches the part.The machine can’t tell if the jet is cutting material or just shooting into nothingness. The part, however, does feel low forces during cutting.
Although the picture is of pure waterjet cutting pizza, I’m going to focus on abrasive waterjet cutting applications in this post. Fixturing requirements are different in pure waterjet cutting, partially because the material is often very light and the jet forces are an order of magnitude (10x) lower compared to abrasive waterjet.
A waterjet stream acts like a beam when cutting, much like plasma cutting and laser cutting. These types of non-rigid cutting tools have to address the beam flexing and changing within the target material to minimize part cutting errors.
What is taper?
Taper in waterjet cutting is when the entrance width of cut is different than the exit width of cut.
What is stream lag?
Stream lag causes corner damage when the exit point lags behind the entrance point, shown in the bottom of the part below.
Plasma cut parts often exhibit an upside-down V-shaped taper where the width of cut is wider at the bottom. Laser and waterjet exhibit a normal V-shaped taper (more narrow width of cut at the bottom). Plasma, laser and waterjet can all yield stream lag errors when cutting a part.
There are two types of pumps used today in waterjet cutting: the linear intensifier pump and the rotary direct drive pump.
Today, both intensifier and direct drive pumps are capable of reliably delivering ultrahigh-pressure water, and both are successfully used in industry. The two pumps have certain components in common. They both have a motor, water filters, control system, and sensors, among other similarities.
Before we start looking at these two pumps separately, let’s take a look at how the industry defines differences in pressure levels. Please note that pressure ranges follow typical high pressure plumbing runs (water delivery lines, T’s, elbows, etc.).
What did you say?
I can’t say I’ve heard them all, but I’ve heard a bunch of them: strange misconceptions about waterjets.
It’s not surprising.
After all, we are cutting with a supersonic waterjet stream (often with a garnet sand added to it) and yet it can cut through a foot thick (300 mm) of metal. People say, “No it can’t!” Actually, yes it can.
As we know, waterjets cut via a supersonic stream. Although there are hand held waterjets used to remove paint, most waterjets are typically moved around with some type of motion equipment.
Waterjets are rather easy to plumb to a machine using elbows, T’s, and other fittings common, in concept, to plumbing a house. Rather than using PVC pipe, we use stainless steel lines of 1/4″ to 9/16″ outside diameter and stainless steel fittings. And motion can be obtained by swivel joints or simply by making the stainless steel line long enough to allow for flexing within the elastic deformation range. Backthrust on the machine is rather small (under 20 pounds [9 kg] for most waterjet applications) even though we are pressurizing the water to very high levels (up to 94,000 psi, or 6480 bar) because not very much water is used (approximately 1 gallon per minute).
Here is a high level overview of the motion equipment that moves around the waterjets.