Waterjet Technology – Milling without a Mask

Dr. Summers Waterjet Blog

KMT Waterjet Systems Waterjet Series

As abrasive waterjets have developed they have been used to both cut through materials, and, in more recent work, have been used to mill pockets within the internal part of the piece.

Waterjet milled pocket in glass

Waterjet milled pocket in glass

In the early parts of pocket milling simple linear cuts were made adjacent to one another across the space where the pocket needed to be created. However, with the need to slow the head down and reverse direction, the edges of the pocket were being cut deeper than the inside floor, and this could cause some problems with part life and utility.

The first step to overcome this problem was to provide a mask, cut to the size of the pocket to be cut, but made out of a harder material, such as steel. By placing the mask over the piece, and setting the machine so that the cuts were made at constant speed over the pocket, a flat floor could be cut. All the slowing and reversing of the head takes place over the mask, so that it is destroyed fairly quickly. But if it survives one milling, then for some parts this provides a process that cannot be achieved in other ways.

Consider, for example, the sheet of glass cut in figure 1. The corners of the pockets are relatively sharp and of consistent radius all the way down the wall, which is relatively straight. A conventional mechanical milling tool transmits high levels of force between the part being milled and tool holder. Therefore, to prevent vibrations, the tool diameter must be no less than a quarter of the tool length. This means that the radius of the pocket wall cannot be less than one-eighth of the pocket depth. That restriction does not exist with an abrasive waterjet milled pocket, where the radius can be much tighter.

This is a critical issue in the milling of parts, where the milling is to get weight out of the component. In many parts that are made for the aircraft industry the part can be designed so that much of the internal volume is not needed for strength, and can be removed to lower the weight. But with conventional tools there are limits to how much can come from a single pocket, not with the AWJ system.

As the above figure shows, and masking and other techniques allow, the radius of the corner can fall below a tenth of an inch even when milling pockets more than eight-inches deep.

There remain, however, a number of problems with the use of the masking technique. It takes time to make and install the mask, and it costs an additional expense that makes the process less competitive. One of the problems that arise with the use of masking comes with rebound of the abrasive from the mask. Dr. Hashish has illustrated this problem with a diagram.

Abrasive rebound from a worn mask (Dr. Hashish)

Abrasive rebound from a worn mask (Dr. Hashish)

If the mask is not shaped properly, or if it has been used before and is worn, then it may have a chamfered edge. When the abrasive waterjet stream strikes the curved surface it can be reflected back onto the work piece, giving an unwanted erosion shadow along the edge of the pocket.

Another problem can arise if the speed of the nozzle, and the distance that the nozzle moves between passes is not controlled to ensure a smooth and even cut over the pocket surface.

As mentioned in an earlier post, (http://bittooth.blogspot.com/2013/06/waterjetting-10c-abrasive-waterjet.html)

the roughness of the cut increases if the abrasive particles are allowed to bounce and make a second cut within the piece. To ensure quality, as a result, the nozzle should be moved, relatively quickly, over the workpiece. Yet the inertia of the cutting head, and the drive assembly in the table motion controller make this difficult to do at relatively high speed. John Shepherd at PIW Corp came up with an answer to this problem, that coincidentally did away with masking.

The Wobbler showing the nozzle motion.

The Wobbler showing the nozzle motion.

The concept behind the device is that, by slightly oribiting the motion of the focusing tube around an axis, the jet will sweep out a circular path on the workpiece. Because it is only the end of the focusing tube that is moving the forces involved are small, and easily provided through a small motor on the device. The relative speed with which the nozzle moves over the surface is now much higher, while the speed of the main arm remains relatively low. The device was studied at MS& T:

(http://books.google.com/books/about/Three_Dimensional_Milling_Using_an_Abras.html?id=-WjHuH7wQaAC)

and the parameters that controlled the depth and quality of cut were found by Dr. Shijin Zhang as part of his doctoral research.

As with the control of single passes of a non-oscillating nozzle, the distance between adjacent passes is critical to the satisfactory performance. If the distance is too great then ridges will be generated in the floor that are almost impossibly to remove using abrasive waterjets alone. Dr. Hashish, in an early paper on milling, for example, showed that if the upper layers of a pocket are aggressively milled with higher pressures and larger grit sizes, that this floor roughness cannot be later removed by using finer grit sizes. This is because the finer grit, while removing some surface asperities will still erode the surface relatively evenly, so that the roughness pattern shown in figure 4, cannot be later removed entirely.

Rough floor to the pocket where the distance between adjacent passes is too great. (Dr. Zhang)

Rough floor to the pocket where the distance between adjacent passes is too great. (Dr. Zhang)

On the other hand it is not always necessary to have a high quality surface for the pocket. For example MS&T have made a number of plaques where metal plates, cut and lettered with the AWJ are then inset into pockets in polished samples of marble or granite. Since these are not strength-bearing, and the plates are glued in place, the pocket floor does not have to be of that high a quality.

Milled pocket in the shape of the United States, Note the edge sharpness and the narrow cutting radii.

Milled pocket in the shape of the United States, Note the edge sharpness and the narrow cutting radii.

On the other hand, where a smooth surface is required then this can be equally well achieved through programming the path of the overall head movement, so that the nozzle sweeps the floor evenly. The glass plate in Figure 1 was also milled with the Wobbler.

Pocket cut into metal without a mask, using the Wobbler. Note the smooth floor.

Pocket cut into metal without a mask, using the Wobbler. Note the smooth floor.

Note the smooth floor. I will come back to this topic next time.

 

Waterjet Technology-An Introduction to Waterjet Milling

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.

In contrast with the earlier use of high-pressure waterjets in material removal in civil engineering and mining, when industrial waterjet cutting first began it was used to make thin cuts through different materials (in the early days often paper and wood products). Through cutting, particularly in relatively thin stock, has a wide range of industrial uses, particularly when the pieces are cut “cold” and with edge qualities that are, even with the first cut acceptable as the final surface cut needed for the part.

Over time the advantages of this new cutting tool became more apparent, and the range of materials that the AWJ jet could viably cut was extended into metals and ceramics. Yet conventional machine tools do more than just cut the edges of parts, and so questions arose as to the best way to achieve the milling of internal pockets within different materials. Within relatively soft rock, and with pressurized water alone, it is possible to generate interesting shapes.

When we first started experimenting with cutting rock at Missouri University of Science and Technology (MO S&T) the support equipment that we had was very basic, and the budget similarly restricted. In order to achieve precise positioning and control of the speeds during the cutting process, we therefore mounted the nozzle and support lance on the traverse of a conventional lathe. The samples were mounted into the chuck, so that we could achieve controlled cutting speeds. To get a number of sample cuts in a single test we placed a sheet of metal, with slots cut into it, between the nozzle and the rock.

Figure 1. Rock rotates in a lathe while the nozzle traverses across the face.

Figure 1. Rock rotates in a lathe while the nozzle traverses across the face.

The notches cut into the metal plate were cut wide enough to allow the jet to make a single pass over the rock surface as the rock rotated and the nozzle swept past the slot, and they were widely enough spaced that the cut made through one slot did not interfere with the adjacent cut made through another.

Figure-2.-Slots-cut-through-the-mask-into-the-rock-target

Figure-2.-Slots-cut-through-the-mask-into-the-rock-target

After a while we became a little more adventurous and realized that, by making the mask an interesting shape that we could leave part of the rock uncut, but mill out all the rest of the material exposed to the jet, by adjusting the feed rate of the nozzle relative to the rotational speed of the rock.

We thought at first that the feed of the nozzle (easy to set with the lathe) should be one jet diameter for each rotation of the rock, but the jet spreads as it moves away from the nozzle and this turned out to be a little too small a distance, and we ended up setting the feed at about 1.5 times the jet width. This “incremental distance” is going to vary between systems, as a function of nozzle design and size, jet pressure and the distance between the nozzle and the target. In this early work in the technology (this was back around 1972 IIRC) the nozzle stood back from the rock at about one inch standoff. In more modern applications that distance can be quite a bit less, and this changes the incremental distance. Also bear in mind that the speeds at which plain high-pressure waterjet cuts are most efficient are much higher, across the target surface, than the optimal speeds for AWJ work.

So, since there was a need to remind folk that waterjetting could be dangerous if proper care was not taken during its use, we used this idea and made a sculpture.

Figure-3.-Skull-figure-carved-out-of-sandstone

Figure-3.-Skull-figure-carved-out-of-sandstone

For simple lettering and shapes such as that shown above, the practice was to cut the desired shape into a metal plate, using perhaps a cutting torch, and then attach this over the rock. The two locations for the retaining wire can be seen on the sides of the piece. This allowed the plate to rotate with the rock piece as the lathe turned, and did away with the stationary plate between the nozzle and the sample.

By adjusting the feed speed and the rotation speed of the piece a relatively smooth surface could be left in the excavated pocket. (See the depths of the eye sockets). The process is known as “Masked” milling, since the plate masks the sections of the rock that the jet should not be allowed to mill into.

This works well when the work piece allows the use of plain high-pressure water, since it is relatively simple to make the mask out of a material (in this case steel) that the jet would not erode significantly. Thus the same mask could be used repeatedly to make copies of the original (though I think, in this case we only made around three or four).

But what happens when the jet is an abrasive waterjet, and we want to make pockets in the same way as I have just described. Because the AWJ will cut through a thin mask it was not an optimal choice for the process.

One can, with precise control of the nozzle position, have the jet move back and forwards over the desired pocket geometry. With the more accurate controls available today it is possible to slow the nozzle as it reaches the end of the pocket, increment it over the desired distance, and then have it cut an adjacent path back along the material to the start side of the pocket. Here the process would be repeated, moving backwards and forwards until the desired pocket geometry had been covered.

The problem with this approach is that the depth of cut into the target is controlled, in part, by the length of time that the jet plays on any one point, or inversely as the speed with which the nozzle is moving over the surface. So moving the nozzle more slowly as it approached the edge of the pocket (which you have to do because the robotic arm driving the move can’t instantaneously stop, increment over, and reverse direction because of the inertia in the system) is problematic. This is true only however if the pocket has to have a smooth regular floor of a fixed depth but most, unfortunately, do. And slowing the nozzle at the end of the cuts means that the depth of the pocket would be deeper along the pocket profile, relative to the body of the cut.

And so, for lack initially of an alternative approach, for some time the industry used masks that would protect the sides of the pocket, and provide a space over which the nozzle could decelerate, increment over, and turn back. The mask would be eroded away, but in desirable parts (often expensive to make in the desired material) the ability of the abrasive waterjet to make the pocket in the first place allowed the expense of the mask to be written into the cost of making each part.

There is, however, at least one other way of doing this, and I will discuss that, next time.