ABSTRACT
One would not normally think of using water when a metal cutting application arises. Our nature would be to think that we need something harder and sharper than what we are cutting.

Mother Nature however, might disagree with you. After all, nature doesn't have fancy equipment availŽable when it needs to shape someŽthing, such as the Himalayas, so it uses what it has plenty of, water.

However, nature's process is very inefficient, taking millions of years. In today's bottom line business world, we just don't have that that kind of time on our hands.

In the last 30 years, mankind has figured out how to speed up the process and apply it to a manufacŽturing environment.
 
HISTORY
Abrasive waterjet cutting came about in the mid to late 70's when quarry companies were looking for a more efficient way to cut stone over the traditional diamond coated wet saws. It was soon discovered that directing a very high velocity narrow stream of water at an object could cut it. No doubt this idea came from watching rivers cut channels.

The cutting process can be made more effective by introducing an abrasive such as garnet into the water stream. Today, cutting of stone and tile is mostly done by abrasive waterjet because it is much more effiŽcient and less costly. It also allows the flexibility of cutting intricate shapes not possible with a wet saw. However, waterjet cutting has greatly expanded its presence into almost every industry imaginable. In fact the waterjet machine tool market has emerged as the fastest growing marŽket segment.

Water cutting is actually divided into two categories. Waterjet cutting, which is cutting with plain water, and abrasive waterjet cutting (AWJ), which uses water imbedded with a fine abrasive. Plain waterjet cutting is useful on "soft materials" such as plastic, wood, paper, textiles and food. It is widely used in those indusŽtries to cut everything from frozen chickens to disposable diapers. When harder materials need to be cut, then the addition of a fine abraŽsive such as garnet allows one to cut any material whether it be marble or tool steel, and in thickness' up to 8 inches in some cases. 

THEORY OF OPERATION
Waterjets work by taking water and compressing it to very high presssures, by means of an intensifier pump.

Since water is in fact non-comŽpressible, it will be looking to escape somewhere. The highly pressurized water is then directed along high pressure piping through a tiny oriŽfice or jewel. The water is forced through an opening by the pressure and will exit at very high velocities, up to twice the speed of sound. When the application is abraŽsive waterjet, there is a mixing tube employed after the jewel. The jet then exits the mixing tube loaded with abrasive, ready to cut anything in its way.

The requirements for a waterjet are simple but important, very clean water and even flow and pressure to the pump. Hard water or water with sediment will quickly wear out the check valves on the pump, causing leaking and eventual failure. Starving the pump of water due to poor flow or pressure can damage the pump. 

ADVANTAGES
The advantages of using an abraŽsive waterjet over other methods such as EDM or lasers are many and some of them are significant. For example, AWJ produces no heat-effected zone, as lasers and to some degree EDM does. AWJ can cut any type of material, whether they are reflective, something lasers can't handle well or non-metallic, and something EDM can't touch.

AWJ is much faster than EDM, and can cut much thicker materials than a laser. AWJ produces so little side force compared with convenŽtional cutting tools that you can cut a slice of tubing .0025" wide without any warpage of deflection.

Unlike Wire EDM, no start hole is required with AWJ.

We have only been comparing AWJ to EDM and lasers but in fact, this technology can in some cases replace stamping or blanking operaŽtions. Its advantage in that field is that hard tooling for presses takes a long time to develop and can be expensive. This makes stamping a poor choice for low production runs. However, AWJ, although slower than a press in cutting out a blank, let's say for a coining or forming operaŽtion, requires no tooling. Also, there are no dies or punches to break or wear out. Through special programŽming, parts can be separated by as little as a .050" wall, greatly redŽucing scrap generation over conventional blanking operations. Like an EDM or laser it is program driven and can accept .dxf files from AutoCad' programs.
 
CUTTING PLATINUM
As all of us who have worked with Platinum alloys know, it is perhaps one of the most difficult metals to machine. Its very high density and gummy characteristics plays havoc with tooling.

Typically the only way to get superior surface finishes is with PCD tooling, which is quite expensive. However, for any type of cutoff work, such as cutting ring blanks from tubing, PCD cut off tools will not survive. Even typical carbide or high-speed steel tools produce wide kerfs, creating plenty of scrap turnŽings and taking a fair amount of time. Obviously, the weakest link in cutting platinum is the tooling. What if we could get rid of the tooling? This is where AWJ produces a defiŽnite advantage. There is no tooling to wear out and re-sharpen, and the waterjet doesn't care about Platinum's density or gummy tendencies. It doesn't care about its temper.

Cutting platinum sheet stock is fairly easy and straightforward. It treats it no differently that any other material and in fact will cut it fairly easily and quickly. Cutting tubing is much trickier.  

APPLICATION
How do you cut a tube? Running the jet straight across the tube will only cut the top 180 degrees. What is realŽly needed is to rotate the tube into the abrasive stream. This would cut the tube in an inward spiral from OD to ID. Working with the manuŽfacturer of the equipment, a lathe was mounted to the inside bed of the machine.

This lathe was driven by an elecŽtric motor (waterproofed of course).

In order to support the tube to keep it from wobbling and to hold the pieces so they wouldn't fall into the tank when cut, a mandrel would have to be employed. You don't want to spend any more time than necessary to cut the tube before pulling out otherwise you are cutting the manŽdrel and further eroding the tube, making you kerf wider and putting a taper in the cut, which is undesirŽable.

The program would be designed to cut into the tube, then pull out and advance a distance equal to the width of blank you are looking for plus the kerf of the jet. With this method we were able to cut a platinum blank with a wall thickness of .090" in about 30 seconds. We were easily able to cut 600-700 blanks in an 8-hour shift.

The program never brings the jet over the top of the tube because to do so would risk piercing the tube and the mandrel before the jet pulls out. One wants to advance the jet only slightly past the inside wall of the tube. This will reduce wear on the mandrel. Then it is just a matter of picking the right speed for the advancing jet to insure it cuts through the tube. 

DISADVANTAGES OF AWJ
There aren't many disadvantages to a waterjet. It is a very loud machine pushing 100dBs in sound level so hearing protection is a must as well as its own room.

It produces a matte finish on the cut surface. If you want a bright cut, you're not going to get it. Its water requirements are strict. The water must be very clean, free of any sediŽment and minerals. If not you will need to filter it and soften it. The machine also needs the feed pressure and flow to remain constant. This is an aftermarket system that is a closed loop system the takes the overflow water, filters it, treats it, refrigerates it and returns it back to the machine. The treated water is purer than the original feed water. This saves water and results in less down time from pump maintenance.

Perhaps the biggest disadvantage of the waterjet, especially for our industry, has to be dealing with the spent abrasive and metal. The tank will eventually fill up with abrasive and need to be cleaned out. This involves shoveling and vacuuming out the wet abrasive into drums. After cutŽting platinum out, abrasive is laden with metal that we want to recover.

There are companies out there that make sediment removal and drying / separation systems. These systems automatically siphon the abrasive out of the tank on a continual basis, then dry, screen and separate clean dry abrasive which can be reused, from the metal and abrasive too finely ground to be of use. Most industries are more interŽested in recovering abrasive to lower costs.

FUTURE
With the introduction of tube cutŽting, AWJ is moving beyond cutting flat stock. 3D systems and waterjet milling machines are already on the market. Some highly specialized applications use liquid nitrogen instead of water as a medium, which evaporates leaving just dry abrasive and metal. Indexing chucks instead of rotating chucks would allow cutŽting fancy shapes into tubing, instead of just slices. The future is indeed looking bright for this technology.

CONCLUSION
So, is cutting platinum tubing with a waterjet a better mousetrap. Certainly one will garner plenty of opinion from industry people over which method is preferred, whether it be casting blanks, stamping blanks, machining them or even making them from powder. Our experience from the last two years lead us to feel that in today's market, where product has to be delivered yesterday and labor costs are conŽstantly increasing, we were able to produce 800 rings per day with two people and three pieces of equipment.

Having an AWJ system was inteŽgral in making this manufacturing cell work. The efficiency of the process can not to be overstated. In today's market, efficiency and proŽductivity separate the winners from the losers.

V10N3

Cutting Precious Metals with Abrasive Waterjet
Tino Volpe
TIFFANY & CO

This is an abbreviated version of the original work. For full technical details, please consult the original paper.