The Round Milling Cutter and the Sharp Edge

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When talking about milled parts, the round corners of our CNC produced milled parts are frequently referred to. I would like to make some basic comments on this, which may not be known to many modelers.

Let me start out with the production. We mainly use milling machines with a diameter of 2 mm for a cutting length of approx. 12 mm. These specially produced milling machines are a good compromise between minimum cutter diameter and, at the same time, maximum cutting speed through several layers of wood stacked onto one another. IN the process, we are cutting with a speed of approx. 3,000 mm per minute.

Due to theses requirements, it is not possible to use milling machines with a smaller diameter. They would not stand up to the strain. The cutting speed would have to be drastically reduced and, due to the inferior cutting length, only 1 to 2 layers of wood could be processed simultaneously. This would cause higher production costs, since the production time per model would be considerably increased.

Let us take a closer look at such a cutout:



The radius in the corner of the spar cutout is approx. 1 mm.

The cutout for the wing spar shown above measures 10 x 9 mm. At the first glance, a 1 mm radius in the cutout appears to be very small. If you try to insert this rib onto the spar, this will not be possible. The rib will get stuck approx. 1 mm above the construction board and not assume its constructionally intended position. Of course, this is not as it should be.



You could then proceed to enlarge the cutout by this 1 mm. This would, however, have the consequence that the gap at the top of the rib would remain, not assurting a tight fit against the spar. Thus, this option is to be discarded.

Another possibility, which I have frequently seen, is the so-called “overmilling” of the corners of such a cutout. I consider this a very dubious method. Due to the notch effect, marked predetermined breaking points are created on these corners. Anyway, I would advise against using this method.



Therefore, actually only 2 possibilities remain for resolving this problem.

The first possibility consists in simply reworking all corners with a small file. Most of all modelers are doing so.

The second possibility as an alternative to the first, i.e. rounding off the spar edges, is only very rarely used. I find this quite strange, since filing the cutouts quickly turns into a chore lasting all evening, while rounding off the spars only takes a few minutes.

To this effect, a 80 or 100 grade abrasive paper is placed into one´s hollow hand and the top of the spar pulled through. Few grinding strokes suffice for producing a radius that will provide a “sucking” fit of the spar in the cutout.

Another aspect of rounding off the spar edges is, however, even more important than making the work much easier and is barely known.

If a spar or fuselage stringer is being subjected to loads during the flight, this will not only occur in the spar´s longitudinal direction, but – due to simultaneous torsion – also in the transverse direction. If such a spar or stringer has slightly rounded edges, the loads encountered can be absorbed much more harmoniously by the component part.

This will have as a consequence that such a spar can take greater loads, while being equally dimensioned. Sharp edges favoring the notch effect also make an “untreated” spar break earlier.



Therefore, you should generally “break the edges” on all strips, spars and stringers.


In this way, a simple measure, such as rounding off the edges, not only eliminates the problem related to the “round corners”, but also significantly enhances the strength of the construction.
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