By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology
KMT Waterjet Systems Weekly Waterjet Blog
In the last Waterjet Weekly Blog, I wrote about the uses of waterjet technology. It is important to note this week that the savings in time that waterjet cutting brings to an operation underlines the old adage about operational costs – “Time and Materials.” The time-savings become particularly true where the high-pressure cutting system is integrated into the modern cutting tables and both cutting and milling operations can be integrated under precise computer control. It is that control (with the fine adjustment of cutting angles) that allows cuts with an edge alignment of one-thousandth of an inch through half-inch titanium at commercially viable rates – since achieving with speed control alone is often too expensive in time.
Figure 1. A test block cut at Missouri University Science & Technology showing ribs of less than 3 mm thickness between adjacent milled pockets
This is where the use of the better computer nesting programs becomes cost effective, where in the cutting of many parts from a single sheet, the move traverse time between cuts and the travel distances are minimized to the overall benefit of reduced cutting time. It is, in this regard, also worth a comment over the waterjet nozzle choice and wear.
University Research of Waterjet Cutting Nozzles
Although it might appear that the cutting table in a University Research Center might not get much use, in fact, many years ago, the student design teams that build components for National and International competitions (the solar car, the solar house, the heavy lift vehicle, the concrete canoe, the strongest model bridge etc) discovered that we were willing to let them use it, in the evening hours. They could then design and make parts to the most efficient size to carry the loads needed, rather than having to go and buy the next largest commercially available piece, and be forced to design to that larger, heavier size. As a result the vehicles they build are generally smaller and lighter – which means that they usually win or place. (This has not been lost on the competition and places such as MIT now have several waterjet cutting tables in their shops too). The result is that the water jet table runs much longer in the evening than during the day, but it also means that nozzle life and cutting efficiency over that life became important factors for us to assess.
Figure 2. Missouri University Science & Technology solar car
We therefore set out to compare nozzles, and to look at how they cut. Our way of doing it isn’t likely to be the way that any other shop would do it, but in our case we prepared triangles of a mild steel, and cut them in the middle of the plane of the sheet. I.e. we set the ¼-inch thick sheet on its edge, and cut down through the middle, starting at the sharp end of the triangle and cutting to the thick end, so that we could see how far along the triangle the jet would cut before it stopped cutting all the way through. Then we cut off the metal on one side of the cut so that we could see what the quality of the cut was, and the average depth of the cut. In our case we made the cuts at 40,000 psi at a cutting speed of 1.25 inches a minute, and, when we were doing the time effects we would run a “triangle” test after every hour of cutting other things. We set a performance requirement for the cut so that when the cuts fell below a certain depth we would consider the nozzle to have worn out. Our results varied, between nozzles, from 10 to 40-hours of operational life, since we had a fairly high standard of cut that was needed.
Figure 3. Two steel triangles made from ¼-inch ASTM-108 steel, cut along the middle of the plane and one side removed by milling along the edge of the cut.
Now this is not to say that you should follow the same path, but it is useful to know how well different nozzle designs cut your particular materials, and how long they last. We were (despite being in “the business” for decades, quite surprised at the range of results we found.
One disadvantage in our case in working with so many work teams is that we rarely get to be there when they cut the parts out of stock, and teaching them to conserve materials by proper placing of the parts on the sheet is not always successful. Yet the “Materials” part of the “Time and Materials” cost has, if anything become an even more controlling part of any operation that it has in the past. Where once we could afford the small amounts of material each student would use, we now have to charge as prices of raw stock keep rising. And this is true not only for us but for all shops. So what can be done.
Obviously the best nesting of parts on a sheet is one way of achieving this, but when there are large bits being removed from sheets of material, we can make some savings by cutting small parts out of the pieces of material that would otherwise be left as scrap from the internal cuts. This is one advantage of waterjet cutting over conventional milling since, as an extreme example, we made a circular cut down through a 2.5-inch thick block of Hastalloy, removing that core for re-use, whereas with conventional cutting it would have come out as chips and have to be sold as just scrap.
Now these are all fairly mundane considerations, but I would like to close with a different opportunity, that is only just becoming more evident. This is in the integration of this new tool in creating forms of art. Vanessa Cutler has just come out with a new book “New Technologies in Glass” (http://www.amazon.co.uk/New-Technologies-Glass-Vanessa-Cutler/dp/1408139545) in which she shows some of the fascinating designs that can be achieved in using, among other tools, abrasive waterjet systems to cut glass.