I have little experience with dental drills (although I have indeed just come from the dentist), but I have written a few milling programs on a CNC mill, so that I think: I can answer this one!
It's not entirely clear to me whether milling or grinding is generally used in dental treatment. But the considerations are similar.
Let's start with milling:
A milling cutter cuts chips from the workpiece (in this case, the tooth). At the very least, the cutting edge should be harder than the material being cut, otherwise it will not survive for long.
In milling, there are two main parameters that affect the process: The speed of the cutting edge and the "feed" of the cutter.
- The speed of the cutting edge (the so-called cutting speed) can be
easily calculated if you know the diameter of the cutter and its
v = Pi * D * f. Of course, this only applies to
the outer edge of the cutter. There are milling cutters that have
the cutting edges at the bottom (similar to a metal drill). Then the
cutting speed at the edge is like according to the formula - and it
becomes smaller and smaller towards the inside. In the middle it is
theoretically zero. However, one tries to avoid this situation
if possible. The cutting speed is therefore the speed with which the
cutting edge is moved through the material. It can be varied via
the cutter diameter and the speed of rotation.
- The "feed" is actually about the thickness of the chips: Let's say a cutter with one cutting edge rotates 100 times per second and is moved into the
material at 1 mm per second: Then each chip is 0.01 mm thick. If the
cutter had 2 cutting edges, it would cut chips 0.005mm thick,
because then a cutting edge would be fed through the material twice
as often. The feed rate is always what you have to set for CNC
milling. There it is specified in mm/min. If you want Chips with thickness d, your tool has n cutting edges and is turning with f turns per second, then the feedrate is
d * n * f.
There are a few different angles at which the cutting edge of the milling cutter can be set. Nowadays, you just buy a milling-tool for a certain material, where the manufacturer has it all worked out. That is why I know almost nothing about that.
Now to the question of why dental drills turn so fast:
I am not an expert in the field of dental treatment, so as a physicist I would just like to make an educated guess:
I believe that the high rotational speed is about small forces that are supposed to act on the tool, the handle and, above all, the dentist's hand.
The feed is literally in the dentist's hand. He moves the tool with his hand.
I know from experience that the forces on the milling cutter can be very high if the material removal rate is too high (that is the chips are to thick).
(I still remember how my milling machine rattled when I once accidentally removed too much material at once).
The finer the chips you cut, the lower the forces acting on the cutting edge and therefore the whole tool.
In addition, the high rotation frequency means that vibrations from the milling are also transmitted to the handle at this frequency.
(Is that ultrasound then?)
I assume that the dentist is less disturbed by this when working.
When I think about it, the laws of grinding are very similar. The only difference is that the material is not cut, but rather "scratched out" with small particles.
Optimizing the milling process is always a trial and error process. You try out which cutting speed works with which feed rate for a certain material.
And of one thing I am sure: they do it because it works better!
Hope I could help.
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