Sunday, March 5th, 2017
Pretty much everyone in north America has had, or has played with, an Etch A Sketch.
And everyone who has done so knows, you can draw anything with an Etch a Sketch, so long as it’s a straight line. If you try to do a circle, unless you have practically infinite manual dexterity, you will find that it is fairly difficult.
Not so for machine tools. Programming machine tools is fairly simple, from a programming standpoint, because if you want a circle you use a G code (In most cases G2 or G3) combined with, for instance, a startpoint, an endpoint, and a radius (There are other ways but this is probably the simplest).
This tells the machine to cut a circle. But it only tells it WHERE the circle is to be cut. Another, deeper level of code that nobody sees is what actually tells the machine HOW to cut the circle. To understand that, you have to understand that, for the most part, all cnc machines can really only cut straight lines. So we’re back to the etch-a-sketch, only the machine can cut straight lines of practically infinitely small length. So an algorithm is used to plot a number of points that lie on the surface of the circle, and goes from point to point to point.
Think of it this way: A circle with only four points is, well, s square. A circle with five points is a pentagon. A circle with ten points is a decagon, a circle with twenty points is an icosagon. The more sides the polygon has, the more circular it looks. The Canadian dollar is a hendecagon (11 sides) but for all practical purposes, it is round.
When you get into the 200-300 sided regular polygons, it becomes practically impossible to distinguish them from circles, and that’s what you are actually getting from a machine tool, an etch-a sketch circle of such a great number of sides or “Facets” that it appears round.
This becomes an issue when you’re making a great number of curves, because a curve of any kind or size is simply a part of a circle, and when you want it to be a smooth surface, you want the individual “Facets” to be tiny. If a machine tool is cutting a polygon with 30 sides, its done in short order, but if you want one with 30,000 sides so the surface is ultra smooth, the amount of processing required is sheerly immense, and it actually slows the machine down. So High Nurbs machines were developed; these are machines that have LOTS of processing power to keep cutting speeds up and surfaces smooth. NURBS means “Non Uniform Rational B spline” more on that later.
Bottomline: IN CNC there is no such thing as a curve, just polygons with very tiny sides, for all practical purposes.