Determining Radius of Existing Round over


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Geometry was never my strong suit in school, and perhaps if I’d known then that I would pick up woodworking later in life, I might have paid more attention. As I did not, I’m struggling with determining the round-over of my existing header board. I’m extending the header in order to affix a barn door and would like to match the existing header as much as is possible. I have a 1/4” round over bit that I ran a piece of scrap through, and the existing header definitely has a smaller radius than that, but I’d rather figure it out mathematically rather than buying multiple smaller bits and using trial and error. Anyone have any help for me here?

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I was usually looking out the window in my geometry class or at my teachers cleavage so I can’t help you mathematically. Actually, it looks to be greater than 1/4”. 

I’ve used one of these in the past from Office Depot. You can also place a straight edge along the two edges of the board and mark where the curve starts. Then take a small combo square and draw perpendicular lines at these points. The length of these lines at their intersection, should give you the diameter and half of that would be the radius. I really stepped out on thin limb by saying that! :D

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Have a good combination square?  I have palm  routers with 1/16, 1/8, 3/16, and up to 1/2 quarter rounds.  For larger ones, I have to change a bit.  I can't tell by looking at the picture, but with a good scale on such a combination square, you should be able to measure it pretty close.

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I’d guess 1/8” or 3/16”. If you can estimate where the curve begins and draw a line perpendicular to the horizontal (top) flat and then to the vertical (right) flat, the intersection of those two lines will be the center of the circle. Then you just need to measure from the intersection to the edge (or to any point on the roundover) to find the radius.

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