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Laser Engraving (metals)

 

"Gordon" is here and has been installed, the above image is annealing on an Amazon inox business card case.

Before we start, there are three words that we need to define;

  1. Marking
  2. Annealing
  3. Engraving

Might be worth reading this blog post first - link

Marking is making a permanent mark, such as we do to metals on the CO2 plotter laser using an extra compound such as cermark or marksolid, it is a mark, not an annealing process or an engraving process.

The Fibre galvo process can mark all sorts of materials directly without the need to use any additional compounds, and these materials include such things as polycarbonate, ABS, Delrin, leather, stone, rubber, polypropylene, carbon fibre, etc.

Annealing is a process that only applies to metals, no material is removed, but the heat applied changes the colour and appearance of the area that has been annealed, and because no material is removed annealing is an excellent process to use for things like cutlery and surgical instruments, where you want to make sure you are not creating any more places where dirt can accumulate, but you still want a permanent mark.

The only downside of annealing is that an annealed mark can be destroyed by heating up the whole object red hot.. so it's not an ideal process for permanent marking of things that can either get very hot, like brake disks, or things that are supposed to have a mark that is very hard to alter or remove, like firearms.

Engraving (sometimes called ablation too) is a process where material is actually removed from the item being marked, so it is both permanent and secure and the only real way to destroy it is with an angle grinder or other similar abrasive method, which of course makes it very obvious that something has been ground off.

Together these three processes are often lumped under the heading of "fibre engraving" and it is worth mentioning in passing that generally speaking for a given design engraving is slower than annealing, and annealing is slower than marking, so time per job can vary not just because of the design itself or the material the work is made from, but also because of the desired end process and finish used.

Here at EL (from Sept 2017) we will be using machine "Gordon" which is a Fibre galvo machine (more details on CO2 v Fibre etc in the FAQ section of the menu on the left) for all our fibre galvo laser work.

The FAQ > Fibre Galvo page lists the materials compatibility for the Fibre Galvo machine "Gordon"

The FAQ > CO2 Plotter page lists the materials compatibility for the CO2 Plotter machine "George"

The fibre galvo uses a fibre laser source producing energy at 1 micron wavelength, the beam is then steered by XY galvo mirrors (hence the galvo machine type) and finally focused through a compound f-theta lens.

The precise specification of the f-theta lens determines focal length, focal point spot size, and work area in one shot.

The f-theta lens choices are

  • 70 mm x 70 mm work area – ~15 µm spot size
  • 110 mm x 110 mm work area – ~25 µm spot size – about 2.5x less energy density than 70 mm
  • 180 mm x 180 mm work area – ~45 µm spot size - about 7x less energy density than 70 mmthis is the standard lens used in Gordon.
  • 220 mm x 220 mm work area – ~50 µm spot size - about 11x less energy density than 70 mm

From mid / late September 2017 we will be adding a CNC precision motorised X axis to Gordon, this will have a stroke of 800 mm and a repeatable accuracy of 0.030 mm (30µm) so for example with a 180 x 180 mm f-theta lens, maximum work area will increase to 975 x 180 mm, which is ideal for larger control panels, signage, rulers and so on.

It is important to point out two things at this stage.

  1. the comments in green text about energy density are all based on the same beam input power, and that means for any given beam power the standard 180 mm lens will remove material at a rate several time slower than the same beam power in a 70 mm lens, it's about work rate and work area more than anything else, roundabouts and swings, to some extent you can counter the work rate issue by increasing beam power, but to be brutally honest with you, "Gordon" already has first rate components and a maximum scan speed of 3,000 mm/sec so for 99% of jobs it simply isn't relevant, the remaining <1% of jobs may be covered by the requirement for incredibly fine detail and a 15 micron spot size, eg coin mould/stamp engraving.
  2. being brutally and technically honest, when we talk about for example a given lens having a 180 x 180 mm work area, in one sense this is entirely true, and no naked eye or even naked eye + magnifying glass inspecting the finished work will ever know the difference, the microscope might, because the nature of optical systems means that while the XY galvo mirrors and f-theta lens will scan over the full 180 x 180 mm square area of a 180 mm lens, it is only within the 180 mm diameter circle that sits inside that square, that there is no optical distortion, the four corners will have an ever so slightly lower quality beam delivered, but with primo components such as are used in Gordon this effect is minute.

1 µm = 1 micron = 1 thousandth of a millimeter, or 0.001 mm,  since there are 25.4 mm in 1 inch this means 1 thousandth of an inch, or "one thou" is 0.0254 mm, so "one thou" = approx 25 micron. You can see from the spot sizes above for the various f-theta lenses that the spot size for the standard 180 mm lens used in Gordon is 45 micron, or 0.045 mm, or just under 2 thou, so a fibre galvo is capable of incredible levels of detail.

Delivered power to any given point on the work surface in ONE pass of the beam is then determined by four things;

  1. spot size for that particular f-theta lens and focal length set (defocus or not)
  2. beam power specified (usually a % number, eg 22% of the rated fibre source power)
  3. Beam scan speed specified (usually in mm/sec)
  4. Beam PWM frequency

Delivered power required to any point on the work surface is determined by the material in question and the process required for that particular job.

e.g. annealing requires much less delivered power than engraving / ablating, but partly that is done by defocusing, hence earlier comments about the relative speeds of the various processes... but you want to anneal slowly or the effect gets ruined... being technical we could then talk about PWM beam frequency and line widths when scanning, as they are all factors that must be considered and set correctly for each job and material

The 655 nm "light mark" feature of "Gordon" doesn't just give a "bounding box" of the design to be applied, it can also trace the exact outline, which is incredibly useful for placing logo's on mechanical parts and at the same time avoiding features such as a fastener or cut out.

However, for precision placement "by eye and by hand" isn't enough, particularly for production runs on multiples of the same object, for precision placement the only real answer is a jig.
We can either fabricate this ourselves or you can supply your own if you prefer.

Within the given work area of the f-theta lens used, you can mark as many objects as you like if they are arrayed in a jig, this can eliminate significant amounts of production time, you can even mark different objects in one jig, eg various parts that are later assembled into one component.

Other features of "Gordon" include;

  • Automatic Z axis (height) adjustment for each layer of iterative / deep engraving to maintain perfect focus.
  • Automatic Z axis adjustment for different parts of the same job, so that an object with more than one height can still be marked in one shot, but each area will still have a perfectly focused beam.
  • Automagic import of data from CSV or spreadsheet for batch runs, eg serial numbers or such like.
  • Full rotary capability for parts up to 200 mm long, 100 mm in diameter and up to 2 Kg in weight, plus inside marking for narrow objects such as rings.
  • 175 mm max part height (no minimum) with 180 mm standard lens, 315 mm max part height with 100 mm lens etc
  • 400 x 260 mm part size with door closed in class 1 mode, much much much larger with door open in class 4 mode, 30 Kg max part weight (see photo on right)
  • Dot size and max work area are dependent upon f-theta lens, 180 mm is the standard/default f-theta lens used in Gordon.
  • Scan speed infinitely variable between 1 - 3,000 mm/sec
  • Suitable for anything from one off custom jobs to very large production runs.
  • Extremely tight tolerances for all aspects of the machine including beam quality and power and so on, ensuring perfect repeatability and uniform work results even across large production runs.
  • It's *fast*, you'll have to wait for the videos on the Exeter Laser Vimeo channel to see how fast, but it is fast enough to render every other method of marking / annealing / engraving metals obsolete both from a time per item perspective, and a cost per item perspective.
  • please note that all of the above numbers etc are "standard", if your requirements are outside of them it does not necessarily mean we CANNOT handle them, it just means we can't do it with bog standard plug and play settings and tools...

Please see this blog post - http://exeter-laser.co.uk/blog/2017/09/22/inox-business-cards/

Video