## Saturday, December 5, 2015

### Amazing Water Jet Cutter

In recent classes we learned about motion of fluid in the tube, and we know that mass flow rate is related to cross section area of the tube, velocity of fluid, time, etc according to the equation

Δm/Δt=ρAv

Since fluid has both mass and velocity in the tube, it is easy to relate it to the idea of momentum. So in real life, scientists use the motion of water to make a very useful technique---water jet cutter. It is an industrial tool capable of cutting aside variety of materials using a very high pressure jet of water.

Check the video below:

It is so amazing that water, which is considered as soft and fluent, is able to cut some hard metals. So I am wondering what kind of pressure the water jet cutter is able to generate, and what types of material it is able to cut. I searched the parameters of a specific type of water jet cut online, and want to calculate the water pressure.

Here are the parameters I found:

Diameter of the cross section area: 0.2 mm
Velocity of water: 800 m/s

And I want to calculate the pressure of water as it leaves the tube by using the equations we learned in class
Δm/Δt=ρAv
Δm=ρAvΔt
F=mΔv/Δt=ρA(v^2)Δt/Δt=ρA(v^2)
P=F/A=ρA(v^2)/A=ρ(v^2)=1*10^3*800^2=(6.4*10^8)Pa

Because of the high velocity of water, the pressure is very big. And I realised that the cross section area does not affect the pressure, but smaller cross section area is just easier to control, and more suitable to cut metal.

I also searched the maximum pressure that some materials can bear:

rubber: 5*10^7 Pa
granite: 2*10^8 Pa
cast iron: 8.8*10^8 Pa

Compared with the pressure generated by water jet cutter, I can conclude that for this type of water jet cutter, rubber and granite can be cut. However, in order to cut harder material like cast iron, higher velocity is required.

1. Great post! Thank u for giving the best information....
Water jet Coimbatore