Many processes use heat or produce heat when cutting material.

The problem with heat
Lasers and plasmas both use heat to melt the target material, so obviously the material endures a great deal of heat. Material is melted by the beam and blown away by an assist gas. Wire EDM (electro discharge machining) removes material by tiny lightning bolts (which is fascinating, but again, is heat based). The lightning jumps from the wire to the target material to melt material and then is washed away with the flushing fluid. Even saws and machining (mills/lathes) produce heat and work hardening at the tool/chip interface due to the high shearing forces at play.

Laser_heat

Material that has been damaged by cutting with heat is said to have a Heat Affected Zone (HAZ). HAZ surfaces have been hardened and possibly made more brittle, making it more difficult to complete subsequent operations, such as tapping or beveling. Engineers don’t like it when material properties are changed during processing because it means the material they have specified might not act as it should in the final application. For example, critical aerospace components are often specified to be cut without heat, or if cut with heat the edge must be “finished” where a thin layer of heat affected material is removed in a gentle way.

How waterjet is different
Abrasive waterjet removes material in a different way than other processes. A very small amount of material is removed with the very tip of a grain of sand at very high speed. It is a supersonic erosion process. There is so little heat generated that even when cutting thermocouple heat detectors in half, not enough heat was measured to change a material.

Waterjet_erosion

Dr. Hashish says, “In a PhD study conducted at Flow in conjunction with Michigan Tech, AWJ cutting through thermocouples imbedded in Aluminum samples was performed. Data showed that the local temperature rise is in the order of a few tens of degrees, not enough to cause any mechanical, metallurgical, or thermal effects.”

One way to understand why waterjet is a cold cutting process is to understand that the material removed is so small and the grain of sand is moving so fast (supersonic) that there is not TIME for the heat to get into the material left behind. The heat leaves with the tiny chip of material scooped away and with the tiny grain of abrasive. Heat needs time to transfer (move your finger through a candle flame fast and slow and you’ll see what I mean).

What’s the benefit of cold cutting?
So, waterjet cuts without heat. That means the cut edge left behind is virgin material. Engineers love virgin material; such material behaves as predicted and intended. The cut part will retain the original material properties. Secondary processes like tapping and bevel cutting can be performed without concern for HAZ.

plasma_vs_waterjet_v2

Another big benefit of cutting with a waterjet is that it does not induce any warp on the target material. If your material is stress free when you start, it will be stress free after you’ve finished cutting with a waterjet.

However, there are some materials you might work with that already have stresses in them when you lay them on your waterjet worktable. For example, tempered glass has, by design, high stress in it so that if it breaks it will shatter to millions of small pieces. Don’t cut tempered glass, not even with a waterjet. Another example is if you make a cut from outside the edge of a piece of cold formed metal you can induce stress relief. Stainless steel sheet metal is famous for containing stresses caused during cold roll sequences. In such cases the material might be already full of stress and when the waterjet cuts through this outer edge the material can stress relieve, which causes material to warp. The abrasive waterjet didn’t induce the stress, it relieved it. For this reason many users simply start the cut just inside the edge of the stressed material, never cutting the picture frame edge and minimizing the warping caused by stress relief.

In addition, today’s waterjet machines are often equipped with contour following devices that will ride up and down a warped piece of plate material. This can help, but keep in mind that cutting a precise part from a warped plate is not possible without flattening the plate during cutting and ensure it is also flat in its’ final application. I have had many discussions with customers over the years explaining that if your material is really warped, then your part can’t be really accurate. Hopefully that makes sense – send me a note if it doesn’t and I’ll try again.

waterjets_cutting

Summary
The waterjet and abrasive waterjet processes remove material by supersonic erosion that does not produce heat affected zones or induce stress. When cutting virtually any material with an abrasive waterjet – glass, stone, metal, wood, ceramic, composite, etc. – a satin smooth surface is left behind that appears sand blasted. Original material properties are retained after cutting with a waterjet. Waterjet is a cold cutting process.

As always, please let me know what you think in the comments below or give me suggestions for future topics.
chip

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