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Design guide - drawing for waterjet profiling

If you are a designer or involved in the drawing up of components for waterjet profiling, we recommend that you take the following points into account, as some will help you save money and other things are physical limitations of the technology, which may need you to change or modify your original design.

1. Toughened / Tempered glass cutting (impossible)

Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass.

Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards, making cutting it impossible. (The jet would start cutting the toughened glass, then at some point the jet will initiate a crack in the glass and will instantaneously shatter into lots of small chunks).                  

2. Hollow section cutting (impossible / very difficult)

Can you cut tube / box section with a waterjet?

A. Yes….BUT….the waterjet will cut one wall of the hollow section, then as the jet enters the air inside the section it will spread out. When the unfocused jet then hits the other wall of the hollow section it will blow a large hole in it (Area A in diagram 1). There a two ways to deal with this issue:

B. Entirely fill the void with another substantial sacrificial material (steel) - this will probably only have a one use life and therefore will add to the cost of the work.C. Place a substantial plate of sacrificial material over area A (Diagram 2). For example a box section with an internal void 700 mm deep; this will disrupt the jet but can be re-used as it is possible to inspect it after every use. Some internal  areas  of  the  hollow sections  will  get  scoured  by the disrupted jet (similar to sand blasting).

So depending on the material / section size, this may be an option, but is not one to be relied upon without discussing it with us first.                

3. Jet size

Waterjets cutting hard materials (metals/hard plastics etc) use an abrasive jet (a beam of water which is entrained with particles of a cutting medium such as garnet). The garnet is what is doing the cutting; therefore the softer and thinner materials can be cut with water only.

Why is this important? The garnet has to mix with the water and exit from the equipment through a nozzle. In simplistic terms, the bigger the nozzle and garnet particle size – the quicker you can cut. However in reality, most waterjets run with jet sizes between 1.2mm and 0.9mm diameter.

The smaller size jet is commonly used when cutting with two cutting heads at one time. If you only require single parts, rather than multiples of the same part, then it would be normal to cut with the 1.2mm dia jet. You could use the smaller jet but it will just take longer to do the cut and cost more. Possibly the only exception may be where the detail you require is too fine to cut with a 1.2mm dia jet.

Cutting with pure water will allow you to use much smaller jet sizes – but you will be limited on material and thickness.

4. Holes

It follows from the above point that your smallest hole size is also dictated by the jet size. Holes are disproportionately more expensive than a straight line cut (which is the fastest cut speed).

Firstly the jet has to pierce the material (thicker & harder takes longer). The jet then has to cut the hole and it is unlikely to ever reach its fastest cutting speed for that material. Instead it has to run at a reduced speed to make sure the bottom of the jet is in line with the top of the jet.

This 25mm fibreglass is cutting on a straight line cut, cutting from left to right – notice the bottom of jet is considerably lagging the top of jet.

However, it is highly likely that cutting holes as part of the waterjet profiling operation will be much cheaper than drilling them manually after profiling and the hole location will be far more accurate.

Screens / mesh / perforations:

These are profiles where there is a large density of holes / cut outs (see diagram 3 & 4). The cost will be disproportionately high for seemingly small amounts of cutting; therefore to reduce costs, consider having slots /longer slots rather than circular holes or short slots.

5. Part separation / Nesting

Cut profiles only need to be separated by a small amount - we usually leave 3.5mm. We will usually nest your profiles into sheet layouts ourselves, as we have lots of experience and software to help us, and we will always do this to maximise shield yield and maintain an efficient use of material.

Here you can see that nesting has reduced material requirement by nearly 30%.

This is an example of general sheet nesting, with nesting of various parts - sometimes inside other larger parts.

Sheet sizes

Standard sheet sizes for metal are usually as follows:

1 x 2, 1.25 x 2.5, 1.5 x 3, 2 x 4, 3 x 6 (in metres)

However if you want a profile that is fractionally longer than a standard sheet size, it may cost you significantly more, as it will probably require the use of the next largest sheet size, potentially wasting a lot of it.

We also usually allow 20mm all round the edge of the sheet to allow for setting up and is therefore ignored in the sheet yield.

6. Sharp internal corners / Narrow cut outs

The waterjet cuts with a circular jet of water, therefore any internal corner can only be cut with a small radius in the corner, as opposed to a perfectly straight edge corner. A lot of time this will not be noticeable, but once the angle reduces under 90 degrees the problem become exacerbated.

The letter ‘L’ is being cut out (the internal is waste). If un-altered, the jet would have cut the red profile (detail A). To prevent this we would alter the profile so that it stopped cutting at the position of the jet as shown.

However this does affect the cut shape. The width of the jet can also add a further problem (detail B), where the jet would overcut, in this case, the cutting profile would have to be changed to accommodate for this. For an ‘artistic’ profile these issues may not be a problem, but for a engineered profile with tight tolerances, this would have to be discussed and resolved and a revised profile agreed upon


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