Friday, October 9, 2009

Orifice / Nozzle / Pressure / Abrasive Question

Occasionally questions are raised regarding different aspects of waterjet cutting.   If you have any questions please feel free to post them on the blog and I will answer them.  The following is my take on these questions.

The question is stated below.

 

“I am trying to find out a bit more information on nozzles and rubies.  We currently use 0.8mm nozzle diameters matched to rubies of the correct diameter- we run with the 120 mesh garnet exclusively. This setup is perfect for our main work but I'm wondering if we are missing a trick or two by sticking to this setup.

What are the advantages of running bigger nozzle and ruby diameters? If we need to cut thick steel, say 15mm, would a larger nozzle diameter be useful?

What we are finding is the 0.8mm nozzles stall on heavy garnet loads, for instance, if we set the garnet flow above 3 on the secondary hopper the mixing chamber backs up because the 0.8mm nozzle can't drag the garnet through the small internal diameter fast enough.

I am also curious about stone cutting and the advantages (or not) of running with 80 mesh garnet?

So far we've stuck with a setup which gives good results on the thin gauge steels we work with but is there more we could be doing? Are there advantages we're missing out on?

Thanks for any input.”

 

Answers to the questions

Most answers to these questions are at www.waterjetuniversity.com or at http://www.wardjet.com/Waterjet-University.asp

Orifice / Nozzle ratio

The orifice / nozzle ratio is critical to your cutting power generated by the cutting head.  Generally a ratio of 3 : 1 is held, so the orifice should be about 1/3rd inside diameter of the nozzle ID.  There are some companies that push this to as low as 2.3 : 1  The effect of this is a more powerful stream but you run the risk of accelerated wear on the nozzle ID and wearing it out-of-round more rapidly.  Companies that push a lower ratio will typically suggest you rotate the nozzle every day so any out-of-round wear is evened out. 

Larger orifice / nozzle combination

Make sure you don’t exceed the limit of the orifice size per the next paragraph.  But as long as you are within the limits of the pump capacity, an increase in orifice/nozzle balanced with an increase in abrasive will result in faster cutting.  See the charts   http://www.wardjet.com/waterjet-university/02-Waterjet-Relationship-Parameters.asp

Size of Orifice

There is an optimum orifice that should be used with your pump.  The charts found at http://www.wardjet.com/waterjet-university/02-Waterjet-Relationship-Parameters.asp  will indicate what these are.  If you use a smaller orifice than the pump is capable of supporting, you will not be working at maximum cutting power.  If you use a larger orifice, your pump will not be able to keep up with the demand for high pressure water and a few things will happen.   1 – Your pump will try to keep up with the increased demand, over stroke and shut down.  Bear in mind not all pumps have sensors that will shut down the pump.  2 – Your pump will stroke harder and the pressure will drop to balance the amount of water going out the orifice.  This is not a good situation as your pump will be slamming and working too hard.  I am presuming you have an intensifier pump here.  Have a look on the chart and make sure you are using the correct orifice.  If you are not sure let me know what horse power you have on your pump.

Abrasive Flow

There is an optimum amount of abrasive that should be used with any orifice / nozzle combination operating at a certain pressure.  The chart here shows this.    You will notice the graphs show that too much abrasive will choke the system and result in slowing down your cut.  So check where you are in your system and adjust the flow accordingly.

Size of Abrasive

The size of abrasive you use will also affect cutting speed, edge finish, taper and burr.  Size of abrasive is also critical when it comes to piercing fragile and brittle materials.  So if you are cutting mainly thin alloys and steel, a 120 mesh will give you less burr than if you were using 80 mesh.  The difference in cutting speed in thin alloys is also not so obvious with different size mesh abrasives.  However if you start to cut thicker materials, you will quickly find mesh size makes a big difference.  A larger mesh is more aggressive and has more momentum to impart in the cutting process as it erodes its way through the material.  Also burr generally is not an issue in thicker materials.  When we were a job shop we ran 50 mesh as much as possible.

Stone Cutting

Generally 80 mesh is fine for cutting stone.  Make sure you perfect low pressure pierce procedures and that you always do a dynamic / moving pierce.  Cutting stone is another whole blog, but certainly waterjet is very successful at cutting glass, stone and many other brittle and fragile materials.

I hope these pointers help.  It looks like you are in the UK?  I will be at an open house we are having in Sheffield at the Boeing / Rolls Royce / University of Sheffield Facility October 21st and 22nd.  You are welcome to come and visit and chat.

Rich

 

 

 

  
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