Waterjetting 34c - Holes, pressure and delamination

If you ever go to an Old-Time Miners celebration, you may watch a group of competitors drilling holes through rock by hand with a cold chisel and a hammer. (You can see an example here). In the competition the contestant has 5 minutes to drill either a ¾” or 1-inch diameter hole as deep as possible typically using a 4-lb hammer. The best results will reach around 8-inches deep in that time.

It was the way that miners, and others, have driven holes into rock for millennia, but the skill that gives the highest penetration rate isn’t based on the person with the largest strength and fastest striker arm. No, rather it is the driller that controls the twist of the chisel correctly between successive blows, turning it just enough that the rock between the new strike and the old is chipped off by the impact.

By indexing the drill around the hole (the distance varies a little with rock type) the volume of rock removed by crushing under the chisel impact is magnified several-fold by the chip that is broken off to the side.

Figure 1. Relative volume of rock crushed, and chipped by lateral wedging to the next cut over.

Obviously the chipping makes much better use of the energy than would be the case if the driller just tried to completely crush all the rock using the chisel. However the chisel has to crush some of the rock in order to penetrate below the surface and get a better purchase for the chipping to be effective.

This makes sense in many other cases as well. And in order to make the best use of a cutting or drilling tool you need to understand how it works, how the target material responds – and how these two factors can be combined to give the best performance.

However, the use of a waterjet cutting tool brings a little extra to the table, since as the jet cuts down into the material, it will not, in the first few milliseconds of penetration, put any great lateral pressure on the sides of the hole, but will only focus on removing material in front and to an extent to the immediate side of the jet path.

The change and growth of the lateral pressure in the walls around the hole, and the widening of the bottom of the cut, occurs as it becomes more difficult for the spent water to escape from the cutting region, and the increasing turbulence of the water at the bottom of the cut starts to eat into the walls of the slot.

Figure 2. Widening of a slot at the bottom as the pressure distribution at the bottom of the cut changes. (Cuts were made at different pressures and AFR into granite, at a constant traverse speed) The view is of the end of the block showing the lengths of the cuts made down into the black as the nozzle traversed on the top of the block and towards the camera.

This build up of pressure at the bottom of the cut can become a problem. As the resistance to the water flowing away increases, so the water can penetrate into any larger cracks, or layers in the material, and apply that higher pressure to the plane of weakness. This can, in turn, lead to delamination of the part, or in some rock types it can cause some severe spalling around the impact hole, which may not be the intended result. (Or the sample may split.)

Figure 3. Spalling around an impact point as a jet penetrated into a block of rock.

The way to minimize this build-up is to make sure that the parameters of cutting (the traverse speed and pressure particularly) are chosen so that this does not occur (lower pressure, faster speed). Where this choice of parameters means that the jet won’t cut all the way through the part on a single pass, then it is usually better to plan on making a series of passes along the cutting path, keeping a relatively smooth wall to the cut, and reducing the chances of getting delamination.

This also holds true when cutting glass, although one has also to consider the size of the abrasive in this case since that will control the size of the cracks that are made in the sides of the cut, and the smaller these are, then the higher the pressure required before they will grow.

Figure 4. The effect of particle size on the crack lengths generated on the sides of a cut into glass. (The cuts were made from left to right with particles of SS-70 (0.0117 in diameter); SS-230 (0.0278 in diameter); SS-110 (0.0139 in diameter). (Shotpeener gives size ranges)

As a result in borderline cases it may be helpful to use a finer mesh abrasive to reduce crack size on the interface, where there is a chance of pressure buildup in the bottom of the cut.

Incidentally modern machines allow considerable precision in making multiple cuts – so that repeated passes can be achieved with relatively consistent precision. Perhaps I can illustrate this with a slightly out-of-focus picture of the insert cut from a counter-sunk hole using two passes of a jet in comparison with the pile of chips that resulted from the conventional removal.

Figure 5. Single piece insert removed from a counter-sunk hole cut with a chamfered edge, and removed as a single piece, in contrast with conventional chips.

However there are occasions where the ability to use the down-hole pressure to penetrate and break out the central core of material can be an advantage. One such occurs in mining applications where the rock is held under confinement. Where the jet first cuts a slot around the outside perimeter of the hole, this relieves the ground stress on the material in the core of the hole. That expands a little, opening the cracks in its structure. (In some cases, where the ground stress is high, this stress relief alone is sufficient to either break the core material into disks or to pulverize it into small pieces, but in these cases the ground is often sufficiently close to breaking already that most sensible folk would not be there).

I will return to talk about the break-up of such cores next time.