Waterjet Technology- Conventional Machining compared to Waterjet Tables cutting metals

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

The first two posts in this section described how, in cutting through rock, the tool and the rock would be compressed together so that temperatures could be created in and around the tool that would exceed 2,000 deg C. That temperature is sufficient to melt the cutting tool, and in other situations is hot enough that it can ignite pockets of gas in underground operations that can have fatal results. However, by adding a small flow (less than 1 gpm) of water to the cutting pick not only is this risk of gas ignition or pick melting significantly diminished, but the water acts to remove the fragments of the rock as they are broken under the bit. This has two beneficial effects, first it removes the small rock that would otherwise be re-crushed and rub against the bit, causing the temperature rise due to friction. The second is that by also keeping the tool cool and sharp it can penetrate much deeper into the rock under the same forces, improving the efficiency of the cutting. When a cutting tool is used to cut metal instead, the processes are somewhat different. However, because the tool rubs against the metal and cuts and deforms the metal that will be removed as a chip heat will still build up around the cutting zone.

Figure 1. Temperatures around a cutting tool in metal (Gear Solutions Magazine )

Figure 1. Temperatures around a cutting tool in metal (Gear Solutions Magazine )

If you look closely at the temperature contours you will see that the lines stretch beyond the point where the cut is being made, and both the chip and the machined surface of the metal heat up to 500 degC. This narrow strip of metal on the surface of the piece is referred to as the Heat Affected Zone or HAZ, since the metal in this region has had its properties changed by the heat and deformation. And while the impact is more severe with a thermal method of cutting (such as plasma) there is some affect with mechanical cutting. This can be seen, for example, if a metal piece is machined without cooling of the interface between the bit and the chip. Depending on the material being cut, this can lead to chips that are thermally damaged, are long and can be dangerously hot.

Figure 2. Strips of metal milled without cooling (Dr. Galecki)

Figure 2. Strips of metal milled without cooling (Dr. Galecki)

If the surface of the chips are examined then the amount of heat damage is evident.

 Figure 3. Surface of the chip showing the damage from the heat during cutting. (Dr. Galecki)


Figure 3. Surface of the chip showing the damage from the heat during cutting. (Dr. Galecki)

However this problem with the heat generated during cutting has been widely recognized, and so it has become standard practice to play a cooling fluid over the cutting zone during machining. To be effective the water must pass into the passage along the tool face and down into the cutting zone. It thus acts both to lubricate the passage of the chip up the blade, and separating it from the cutting tool, while cooling the bit and keeping it sharp.

Figure 4. Insertion of the jet into the cutting zone. (Dr. Mazurkiewicz)

Figure 4. Insertion of the jet into the cutting zone. (Dr. Mazurkiewicz)

When this is properly placed, and as with the jet assisted cutting of rock the precision required in placing the jet is around 1.10th of an inch, then the chip and metal surface are cooled and the tool remains sharp. However, with conventional, lower pressure cooling, while the chip length is reduced and the surface is somewhat improved, overall cutting forces do not change.

Figure 5. Chips formed with conventional cooling (note the poor edge quality). (Dr. Galecki)

Figure 5. Chips formed with conventional cooling (note the poor edge quality). (Dr. Galecki)

When the waterjet pressure is increased to the ultra-high pressure range, so as to ensure that adequate water reaches the tool, then the cutting forces are reduced and the amount of damage to the metal is further reduced The result can be seen in the form of the chips that are removed, which are now much shinier in appearance:

Figure 6. Chips from high-pressure jet assisted cutting (Dr. Galecki)

Figure 6. Chips from high-pressure jet assisted cutting (Dr. Galecki)

 

Note that the surface of the chips are shiny, and that they are relatively small in size. The shiny surface is similarly reflected in that left on the machined part.

Figure 7. Cut surface left after high-pressure jet assistance to the cutting tool.

Figure 7. Cut surface left after high-pressure jet assistance to the cutting tool.

The resulting reduction in damage to the machined surface, as well as the lower machine forces, and the consequent lowering of the potential for “chatter” during cutting gives a higher cut surface quality which, because of the reduced damage to the surface has a higher fatigue resistance. The amount of modification required to the equipment is not necessarily large, since the high pressure water can be carried to the tool through relative small tubing that has a small footprint. The pump can be located elsewhere. Further, while conventional cooling requires additives to the water (which make it more costly to treat the scrap) the clean water used in the jet makes this less of a concern.

Figure 8. Arrangement with a jet added to the cutting tool on a lathe. There are also instruments on the platform. (Dr. Galecki)

Figure 8. Arrangement with a jet added to the cutting tool on a lathe. There are also instruments on the platform. (Dr. Galecki)

These results show that the heat damage that can be anticipated with conventional machining of metal can be significantly reduced with the addition of high-pressure water. This becomes even more clear where abrasive is added to the jet stream, and fortunately, thanks to colleagues in Germany, we have thermal images of this, which I will share, next time. (For further reading see Mazurkiewicz, M., Kabala, Z., And Chow, J., “Metal Machining With High Pressure Water Cooling Assistance – A New Possibility,” ASME Journal of Engineering for Industry, Vol. 111, February, 1989.)

Waterjet Cutting-Time and Materials

David Summers WW

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.

Waterjet cutting, Test Cut waterjet

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.

University of Missouri Science and Technology Solar Car

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.

Two steel triangles waterjet cut

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.

Next Week Blog: Waterjet Technology-Creative Folding Artwork with STEEL!

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