Many people do not understand how piercing with waterjets work. Because waterjet is actually a “controlled erosion process” and you are dealing with a fluid tool, as opposed to a rigid metal cutting tool, things work completely different.
Drilling Holes by Conventional Means
Most mechanical cutters will go through materials with a cutting action and a spiral cutter that actually lifts the chips, grit, debris, or whatever you want to call it, from the hole as you go through the material. With EDM the electrode will burn away the material by arcing and the debris is washed out by flooding with a liquid. In drilling glass you may use a diamond bit and the glass will be pulverized or ground away and you use a low volume water or other liquid to carry the grit away. This is a pretty straight forward process in these instances.
Piercing with Waterjets
However, in piercing with waterjets, or waterjet drilling it is a quite different process. There is no spinning spiral cutter to remove the debris, and as you apply very high pressure water with abrasive to the surface of a piece of material it will start to erode and burrow down into the material. Since there is no means of removing the debris as you go through the material, it has to come back out of the same hole that the jet goes into. Because the jet is moving at a very high speed and under extreme pressure naturally it will come back out at a proportionally high speed and pressure.
Piercing with waterjets is a very messy process and is why any time you go into a waterjet shop, if they have been in business very long, you will probably see dried mud up the walls and very likely on the ceiling from the dirty water spraying back from the material during piercing. It is almost impossible to get away from this process, so no they are not just bad housekeepers.
The stream that the waterjet uses will normally be around .030″ Ø and as it goes into the material it theoretically will make a hole the same diameter. However, once it starts going through the material, if it doesn’t pop through the material almost instantly then the water and abrasive that is coming out of the cutting nozzle have to exit the hole somehow. It has to come back out of the same hole it is going into. If you think about it, that is impossible without making the hole bigger so that the stream which is drilling the hole can go down the center of the hole and the debris that is being removed has to come back up the outside of the hole in order to escape. So the hole that is .030 Ø if you can pop through immediately will quickly become a .040″ Ø hole as the water escapes eroding the hole more.
The deeper you go, the longer it takes to go through the material. As the debris comes back out around the stream it is eroding the hole more.
So, you can drill holes down to approximately .020″ Ø (with a .018-.020″ Ø nozzle) through a .125″ thick piece of material in a couple of seconds under the best conditions. But, when you try drilling a hole through 6″ thick material or more, it will end up being .030″ at the bottom, or exit side of the part, but it will be in the range of .125″ Ø at the top where you start out the pierce, and sometimes may take 5 minutes to get through.
When you are drilling deep holes like that it is not easy with any process and waterjet is no different. You don’t have much control over the process down inside of a hole so it will rarely comes out truly round, for a number of reasons. At $2 a minute or more, that makes it a potentially $10-12 hole, that is not a consistent size through the part nor really round, which may not be a satisfactory solution.
If you are cutting parts with large internal features you can have lead-ins that are long enough so that the waterjet will pop through to make a hole for the path to start very quickly, unless you are working with very thick materials. On the lead-in pierce you use what is actually a “dynamic pierce”, or the jet is eroding the material as it is moving down the lead-in to start cutting the feature. As the jet is moving along the lead-in path the debris is escaping back out of the cut you are leaving behind which will allow the kerf that you are cutting to actually be maintained at the same width as the cutting stream and the jet will get through the material much quicker.
If you are cutting 1/4″ aluminum you can use a .100″ lead-in and as the head moves the jet will have gotten through the material by the time that it moves that distance and will take a second or less to do so. If you are working with 6″ Stainless Steel or some other harder material however you will find that it could take 30 seconds or more and you may need to have a lead-in of 1″ or more in order to give the jet time to get through the material. All the while you will have a “rooster tail” of muddy water shooting across the room. That is why you never want to stand next to a waterjet machine while it is piercing, while wearing a clean white shirt because it may never be white again.
As a side note here, one of the distinct advantages of using the Ultra High Pressure (UHP) Waterjet systems are that the higher the pressure you have, the quicker the jet will make it through the material and start its cut.
So, hopefully now you understand a little more about how the whole process of piercing with waterjets works, you will be able to make a more informed decision as to whether you want or need to drill holes with the waterjet or if possibly another process would serve the purpose better.
To watch the full video of cutting runners for the USA Luge team, that the above clip was extracted from, go here.