Parts, materials and finishes
Why waterjet cutting does not save you the machining
A waterjet quotation can look like a giveaway compared with a machining quotation. And it is, because you are not comparing the same thing.
You request a quotation for an aluminium part. A machine shop gives you one figure. A waterjet company gives you another that is a fraction of it, sometimes so low it looks as though they are charging only for the material.
In a way, that is exactly what is happening. The problem is the conclusion people draw: “then I will waterjet it and save the machining”.
It does not work like that, and it is worth understanding why before ordering a batch.
What waterjet does very well
It is an excellent tool, and this article is not disputing that. Waterjet cutting:
- Does not put heat into the part. No heat-affected zone, no local annealing, no distortion from expansion. For sensitive materials, that is valuable.
- Cuts thicknesses other processes cannot touch, and cuts practically any material.
- Follows curves. A complex contour is just as inexpensive as a straight one.
- Produces profiles extremely quickly from sheet or thick plate.
If what you need is a contour, it is hard to beat.
What it does not do
The edge does not come out square.
That is the key point and the one that catches almost everyone out. The jet is not a line: it is a cone. It leaves the nozzle concentrated and spreads as it passes through the material. The result is that the entry and exit faces do not have the same dimensions, and the edge has a slight taper.
How much depends on thickness, traverse speed, pressure and abrasive. A fast cut in thick material can leave a very visible taper; a slow, well-adjusted cut reduces it considerably. But reducing it takes machine time, which is precisely what made the cut cheap.
Also:
- The cut face has a texture, with more pronounced striations towards the exit.
- There is no geometrical tolerance. Nobody guarantees perpendicularity, flatness or parallelism.
- There are no accurately sized holes. A waterjet hole has the same taper problem. If the dimension is functional, it has to be reworked.
- There are no threads, depth-controlled pockets, chamfers or recesses. That is milling work.
What that means for your part
If your part is a loose profile that does not fit against anything, waterjet is enough.
If it has to support, fit, guide, hold or align with another part, the tapered edge is a problem. And if it has functional holes, threads or toleranced dimensions, waterjet has left you a shaped blank, not a part.
That shaped blank still has to go through the milling machine.
So is it useful or not?
It is, but not as a finishing process: as material preparation.
And there it creates real savings, because it saves the two most expensive elements of machining:
- Material. You start with an approximate contour rather than a whole plate.
- Spindle hours. You do not have to rough the entire exterior. The mill only finishes what needs to be right.
A waterjet contour followed by finishing the functional surfaces can be clearly cheaper than machining from plate. That calculation works. The calculation that does not work is “waterjet instead of machining”.
The right way to think about it: waterjet is a saw that can follow curves. A very good saw, but a saw.
What to ask before ordering
If you will use it as preparation, ask these questions and put the answers in writing:
- How much allowance is left on the contour. You need stock for the subsequent finishing. If they cut to nominal size, you have no margin left.
- What taper to expect for your thickness and material. Some machines compensate by tilting the head; others do not.
- Which face is used as the reference. Entry and exit faces differ in size, so you need to know which one is correct.
- What the actual plate thickness is. That has its own story, because nominal material is almost never actual material.
What you learn the second time
Here is the part people do not usually mention: the savings evaporate if you do not plan the finishing from the start.
If you order the cut expecting a finished part and later discover it needs milling, you find you left no allowance, have no reliable locating surface for workholding, and the tapered edge complicates setup.
You end up machining more than you would have from plate. The cheap cut has become expensive.
At 7Axis Industrial we manufacture parts, spares and short runs for machines in production. When the cheapest route combines processes, we plan the whole route from the start.