# Polygon App - Can someone identify the units in the "radius" dialog?

The default is “100” and the slider will change the value of the radius, obviously making the polygon larger or smaller, but 100 what?

It is clearly not millimeters because a setting of 100 results in a polygon that has a radius of about 22 mm
It is clearly not 100ths of inches, for the same reason.

Am I asking this question in the wrong place?

I am assuming that the radius in the app refers to the radius of the circumcircle, not the incircle.

I threw the question out to some students who developed a procedure to try to “calibrate” the polygon tool. We will run a test carve series in MDF, creating a series of n side polygons where n is divisible by 2, and labeling each with selected values for R. (Not a mathematician, but I believe R refers to the Radius of the circumcircle while r is the radius of the incircle).

This should give the students a table of values from which they can derive formulae to calculate incircle and circumcircle radii for a given input for polygons with even numbers of sides.

With some basic data, they should then be able to derive a formula that can calculate incircle and circumcircle radii for any n sided polygon, and then we can carve a display “tile” that explains the radii of polygons.

And to think this all stated because I was making a dust extractor…

Ain’t technology just the coolest?

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Oh, and I thought it was cool that the students figured out that we needed to start with polygon with an even number of sides because the Diameter of a polygon with an odd number of sides ends at a vertex on one side, but bisects the opposite side to end at a tangent point on the circumcircle.

This is precisely the kind of inquiry I had in mind when I talked the school into buying me an X-Carve.

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Those values are pixels, which are scaled after importing the shape. I need to convert the app to the new app api version we have (`2.0.0`) before that slider can represent real-world values. Currently that value would translate to 96 pixels/inch.

Thanks Eric, 96 pixels per inch will give us the means to produce a conversion formula that I can put on a poster near the machine.

My students came very close to the real measurement, they took the average from a series of measurements and came up with 100 “mystery units” = 26 mm, so they were within half a millimeter. Not bad for 11 year-olds!

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@ChristopherDahle Hey Chris, I updated the polygon generator app. It now uses inches as the radius’ unit!

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Thank’s Eric! We have used the app since you made the change and it works perfectly.