# Geometry I guess...

Discussion in 'Math' started by mbxs3, Jul 12, 2011.

1. ### mbxs3 Thread Starter Active Member

Oct 14, 2009
143
3
Anything math related = a major head ache for me....

Ok, now that I got that equation out of the way, here is my story/question:

I am building a bezel, or a mounting panel, that will cover an old ashtray compartment. Ultimately, I will have a few small electronic displays and connections that will be within the display. However, right now, I am just trying to fabricate the main body of the bezel before I start making slots for the electronic stuff...

So this is where my math question comes into the picture. The area that the bezel will cover is rectangular with rounded corners. So unfortunately, it is not as easy as measuring the length and width and cutting my material to those dimensions.

What I am trying to figure out is if there is a way to measure the rounded corners and put a number to it. I may be way off but I assume the term I am looking for is the "arc" of the corners. Someone please correct me on that if I am mistaken.

If anyone knows what I am talking about and can shed some light on this subject I would greatly appreciate it. Thanks.

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2. ### blah2222 Well-Known Member

May 3, 2010
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Best way I can think of it is get a compass and try to draw a circle that has that arc, then measure the radius.

JP

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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You can select whatever arc radius you want. It would probably relate to the drill size you might use to pre-drill the rounded corners before completing the full cut-out.

To fix the center of the circle you come in a distance equal to the circle radius from both the corner edges.

It would be easy to draw 4 lines parallel to each of the cut-out edges at a distance in from each edge equal to the arc (drill) radius. The 4 lines intersect at the 4 corner radius centers.

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4. ### mbxs3 Thread Starter Active Member

Oct 14, 2009
143
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Nice explanation t_n_k. I appreciate it and will definitely print your diagram out for future reference. The only issue with what you presented and my situation is that I can not select any arc radius I want. Unfortunately, I am trying to build my panel to fit into a rectangle with rounded corners that is already made by the ash tray console manufacturer.

To be honest, after the many hours I have spent trying to make a cut out for this project, I have determined that I was over thinking it. Since I don't have a CNC or some type of machine that is going to make this part for me, determining any dimensions other than basic length and width are kind of a waste of time. I am just going to take it slow and steady and I am sure that I will do a lot of sanding and fine filing.

5. ### Georacer Moderator

Nov 25, 2009
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1,266
If that was a mathematic problem, it would be easy. However, this is real life and things get complicated.

Since you are going to drill and cut the material by hand, why don't you draw the perimeter of the ashtray against a piece of paper and then use it as a blueprint for your design?

Jul 7, 2009
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This is a common task for machinists and they typically use radius gauges to measure the radius of the corner. However, you probably won't want to spend the money on such things for a single use. Another approach is to use a drafting template for drawing circles -- just match the arc up to the closest-fitting circle and read off the diameter (you'll have to allow for the pencil correction).

The poor man's way is to look for found objects that match the curvature, such as coins, slugs, etc. The other approach is to start drawing arcs with a compass or dividers until you match the radius you're trying to measure. For crude work, it's pretty easy to estimate the arc's radius with a rule.

7. ### mbxs3 Thread Starter Active Member

Oct 14, 2009
143
3
Thanks for all of the suggestions. I ended up using some styrene sticks and just filing them to fit the rounded corners to use as a template. I think it will work, I just like to make things extra complicated sometimes.