Some recent emails from readers have come in asking whether or not they should leave their pump running in a ‘dead head’ state. Or put another way: what is a reasonable amount of time that a pump should be left under pressure when it is not being used to process material?
Garnet abrasive is used on 95% of all waterjet machines. The size of the garnet abrasive typically used today for waterjet cutting ranges from 50 mesh to 220 mesh, with the most common being 80 mesh. Mesh values do not represent particles of an exact dimension, but represent a distribution of particle sizes. An 80 mesh abrasive will have some particles larger and smaller than exactly 80 mesh. Mesh sizes are usually determined by allowing abrasive to fall through a series of screens – each screen smaller in mesh size from top to bottom. A known quantity of abrasive is placed on the top and vibrated for a fixed period of time, and then the amount of abrasive on each screen is weighed to obtain the distribution.
Taper left on a part was by far the biggest complaint from our customers across the world. As we have learned in prior posts, the faster you cut through a material with an abrasive waterjet, the greater the v-shaped taper. At the time, the only solution was to slow down your cut speed. The problem was, slowing down only minimized the taper – but rarely get rid of it; and slowing down costs shops a lot of money per part.
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.
It’s true. When our customers quote waterjet work, there are different types of quotes that work better for different jobs. You might think that there is a significant difference in quoting needs between those of our customers who are job shops versus those who conduct in-house cutting, but that is not the case; both have similar job quoting needs for either internal or external customers.
In recent focus groups, Brian Kent (Global Shapecutting Product Manager) and I have had the privilege of talking frankly and in detail with our customers. One of the many things we learned was that our shop owners and programmers have two types of quotes they create each week: the quick quote and the detailed quote. The quick quote is, well, quick – where they hope to be accurate to about 10 percent. With the detailed quote, the stakes are higher. These quotes are done for large dollar projects, recurring projects, or a first-time project for a new, important customer.
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.).
In part two of our common misconceptions series we get into some machine design topics and specific cutting applications. If you haven’t read it, be sure not to miss what I discussed in part one.
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.
FlowBlog 
