I’m excited to bring you our first guest post by Tim Fabian. Tim is a renowned waterjet expert, well versed in everything from machine design to daily operation. It is my pleasure to have him as our guest blogger for this week. Without further ado…. Tim Fabian.
A reader asked about Slurryjet, and why there are no ultrahigh-pressure units out there in the world today cutting in production.
First of all, let’s make sure everyone who has not studied the subject understands what we’re talking about here. Abrasive waterjets today are created by pressurizing water, forcing it through a small jewel orifice where the pressure is converted to velocity, and then the abrasive particles are metered into a mixing chamber and accelerated like a bullet out of a rifle down the mixing tube. Abrasive slurry jet is where a water/abrasive slurry is pressurized and pushed through an orifice. It is inherently more efficient because the water and abrasive are going the same speed, and no momentum transfer from the water to the abrasive is taking place.
Having more than one cutting head on an abrasive waterjet should be much more productive than running one head, right?
By understanding waterjet efficiency, and the relationship between pressure and power, you can equip your shop with the most productive system possible. In short, raising the pressure and putting the power through one head is more efficient than running two heads with normal pressure.
In 1984 the abrasive waterjet was released as a commercially available product. It was powerful, it was incredibly versatile; but it took a special operator to run it. Someone who was into it – who had caught the waterjet bug. Why? Because at the time, waterjet wasn’t all that easy to run.
I had the unique opportunity to be such a waterjet operator by getting into abrasive waterjet just one year after it was released. While in undergrad and grad school running the waterjet laboratory and demo center I could reference basic cut speed tables for what was believed (at that time) to be the best operating parameters. The table included water pressure, orifice size, mixing tube size, abrasive size, abrasive flow rate and the maximum cut speed for a dozen common materials at one or two thickness levels.
The standoff height is the distance between the tip of the mixing tube and the material. A standoff too high causes dimensional problems and alters the top edge created by the jet as it gradually widens in open air. Standoff too low risks scratching material on rapid traverse between cut paths, or snapping the tube causing downtime, cost, and angry operators. Standoff height should be approximately the thickness of a coin (varies depending on your nationality, obviously) – about 0.100” (2.54 mm).
I guess I could end the blog right here, but I think there are a few more things you might want to know about standoff height.
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.