Arthur Turner, specialist in micromachining
Some of the challenges of microengineering include pushing the boundaries as to how small we can make a component, the surface finish and how we actually make the part. A list of conventional technologies such as mechanical micro-drilling and -turning can only get you so far in machining small parts and then consideration must be given to the influence of the cutting tools and the machine being used.
As an example, when mechanically drilling small holes, a drill of 20 µm diameter can be obtained with either a 60 µm, a 120 µm or a 240 µm flute length. However, to ensure it works correctly without breaking, it is not only the tool holding that needs to be good but also the concentricity of the spindle. A 20 µm diameter drill only needs to be running a couple of microns off-centre and it will break.
While not a conventional technology, laser micromachining can help and should be viewed not as a competitive technology but a complementary technology. That said, there are times when laser micromachining offers significant benefits and other times when it is the wrong solution for the challenge in mind.
Another consideration is the number of holes that need to be produced. If the figure is in single digits, then possibly mechanical drilling is the quickest and cheapest way, but as the volume of holes goes up, then the speed of laser micromachining becomes very economical. There is no drill breakage, so the process does not have to be halted while the drill is changed (and possibly removed from the workpiece).
An example of a drilling challenge is shown in figures 1a and b, which show mechanically drilled holes with a 20 µm diameter in 0.1 mm thick stainless steel. The holes look easy enough to produce if you look at figure 1a, but the scale of the problem can be realised if you look at the closeup image of figure 1b. Setting the tolerance on the holes at +/-1 µm means that straight drilling is not an option, and if reaming, the time to manufacture then becomes a big problem. The ability of a femtosecond laser really helps in this case, not only by keeping the manufacturing time to a minimum but, more importantly, by ensuring the dimensions and tolerances specified are met, thus resulting in a high-quality component.
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Figure 1a
1a and 1b: Mechanically drilled holes in stainless steel.
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Figure 1b
1a and 1b: Mechanically drilled holes in stainless steel.
Also challenging is the drilling of small holes in glass or ceramic, as shown in figure 2. These holes can be produced using other conventional technologies such as microwaterjetting, but the hole size, diameter tolerance and cycle time become problematical, therefore laser micromachining is ideal.
Figure 2
Mechanically drilled holes in ceramic.
It is a comparable situation when turning small cylindrical parts. How many diverse types of lathes are there available to machine the Blue Pivot Steel bobbin in figure 3 and long, tapered pin in figure 4? The capabilities of these lathes are not what you would expect in normal manufacture; the size of the tools, ensuring the tool tip is on centre and having a feed motion that is sensitive enough not to have deviation so that all movements are the same are all critical factors.
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Figure 3
A laser micromachined, Blue Pivot Steel bobbin.
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Figure 4
A laser micromachined, long, tapered, Blue Pivot Steel pin.
What if you can machine exceedingly small cylindrical parts easily? How would you attempt to cut your material? Would you ever consider laser micromachining? These are questions being posed by the clock and watchmaking industries.
The bobbin and pin were laser micromachined since their miniscule features would have created difficulties had a turning tool been used. The magnified view of the top section of the bobbin looks reasonably easy to manufacture, however this point of view changes if you consider that the outside diameter is only 0.7 mm with the grooves 50 µm wide with a depth of only 14 µm.
The pin is a similarly complicated component to manufacture; it measures 3.5 mm long from the shoulder of the groove, starting at a diameter of 0.4 mm and finishing at a point, which would create so many problems on a lathe. In particular, it would be necessary to ensure that the cutting tool tip was exactly on the centreline as the smaller the diameter being machined, the greater the likelihood of a component such as this lifting on top of the turning tool. The risk of this problem occurring is significantly reduced if using laser micromachining.
A key point in laser micromachining is the correct selection of a 4th axis drive. The rotational drive must be in the size magnitude of the laser source and afford a high rotational accuracy.
Laser micromachining, while initially hard to comprehend as being a solution to a microturning problem, can be the ideal solution. However, the decision to invest in a new manufacturing technology depends largely on the volume of parts to be produced. The price of a basic femtosecond laser machine is around US$400,000 plus, but the price to put a machine into a production cell is substantially more, as the specification of the machine in respect of raw material changeover and automation to remove finished parts increases the final cost dramatically, so do not be surprised if figures of US$1,000,000 are quoted.
So, is the laser micromachining process viable when drilling small holes and turning small components? The answer is yes if there is a sufficient volume of parts. Of course, a notable incentive is if or how much machining time can be saved. In the case of drilled parts such as those shown in figures 1a and b, the time that could be saved is significant. However, in the case of turned objects such as those shown in figures 3 and 4, the machining time that could be saved is debateable depending on their features and diameters. Once you have finalised a stable process, the components should be consistent and pass quality control checks.
Finally, it is worth remembering that with laser micromachining, there are no cutting tool costs and therefore there is no machine downtime while the tools are being replaced.
Arthur Turner, specialist in micromachining