Is there a way to accurately determine this radius?

[woodchucker]

Take 2 combo squares and put them on the case move them to where the flat starts to roll away the same on the other corner. Read the radius from where they meet. They should both read the same

 

[pontiac428]

How accurate? .001. .010, .05?

Put a square up to the corner and measure from corner to tangent point of arc.
Get a set of female radius gauges.

Jeez, @Asm109, you're gonna put me out of a job. Three thoughts came to me while reading post #1, and all three came out in post #2...

The only two easily obtainable tubing that comes to mind for heavy sheet metal projects to section from are exhaust pipe and electrical conduit. Use a saw (hopefully a power tool) to quarter the pipe. Either butt weld and grind everything smooth, or lap weld, rough grind smooth, and body filler the rest.

 

[C-Bag]

I’m surprised nobody has mentioned radius gauges. I’ve use mine a LOT, and so far I’ve not had anything they can’t measure.

 

[OTmachine]

We always used product of the means = product of the extremes using a perpendicular bisector, (diameter of circle). Use a known length straight edge to make an internal chord, then measure the depth at mid-point of said chord.

 

[Skippy]

Keep it simple. Find out what size of drill bit fits well in the corner.

 

[Parlo]

Add the width to the depth of the cabinet and multiply by 2 to get the perimeter if it had square edges. = A
Measure with a flexible tape the actual perimeter. = B
A -B = Circumference of all 4 rads.
Divide by 2pi = average radius of all corners.

 

[RJSakowski]

Add the width to the depth of the cabinet and multiply by 2 to get the perimeter if it had square edges. = A
Measure with a flexible tape the actual perimeter. = B
A -B = Circumference of all 4 rads.
Divide by 2pi = average radius of all corners.

Don't work.

I laid out a 5" x 10" rectangle with 1" fillets The perimeter of the rectangle is 30". The perimeter of the filleted rectangle is 29.2932". The difference is 1.7168" and divided by 2π = .2732".

Edit: The straight sections on the sides A and B are A-2R and B-2R for a total length of 2(A+B)-8R. The four fillets form a compete circle of circumference 2πR. The total length of the perimeter of the filleted rectangle is 2(A+B) -8R + 2πR. The difference between the unfilleted and filleted rectangles is D = 8R - 2πR. Factoring out R and solving for R, R = D/(8 - 2π) = D/1.7168 = .5825D.

 

[Flyinfool]

Unfortunately for @Parlo 's method to work you must already know the R.
If you take the length of the 4 straight sections to enter for "A", then the rest of the formula will work to give you the radii.
But if you know the length of the straight sections then you also know the radius of the corners.

In this case use what was described in post 2.
Place a square against the corner, slide your thinnest feeler gauge (or a piece of paper) in until it stops, read the radius on the ruler. This will get you within your ability to read the ruler on your square. I am betting that you can not form a piece of sheet metal more accurate than what this will give you.

 

[Parlo]

Don't work.

I laid out a 5" x 10" rectangle with 1" fillets The perimeter of the rectangle is 30". The perimeter of the filleted rectangle is 29.2932". The difference is 1.7168" and divided by 2π = .2732".
View attachment 452878
Edit: The straight sections on the sides A and B are A-2R and B-2R for a total length of 2(A+B)-8R. The four fillets form a compete circle of circumference 2πR. The total length of the perimeter of the filleted rectangle is 2(A+B) -8R + 2πR. The difference between the unfilleted and filleted rectangles is D = 8R - 2πR. Factoring out R and solving for R, R = D/(8 - 2π) = D/1.7168 = .5825D.

Sorry to post what at first seemed logical without checking first. Thanks for checking.
Does the figure 0.5825xD apply to all cases? if it does, that is a handy number.

 

[RJSakowski]

The OP asked for a way to accurately measure the radius of a fillet. The method that I proposed in post #9 will determine the radius to within a few thousandths.

There is an instrument used in optics called a spherometer which will measure the radius of a spherical surface. It consists of three points on an equilateral triangle and a fourth adjustable point at the center of the triangle.. The instrument is placed on the surface of the sphere with a ll four points touching. The distance the fourth point is above the plane of the three points determines the radius of the sphere.

I made a 2D version of this which will measure the radius of a cylindrical curve. It is essentially two points a known distance apart with a third adjustable point midway between the first two. Like the spherometer, the third point is adjusted so all three points are touching and the distance of the third point above the line between the first two points determines the radius. This can be used for both positive and negative radii.

It is much like a depth mike except the bar is replaced by the two points.

The radius of a curved surface, r, is given by the distance of the midpoint, h, above the line between the two outpoints, separated by a distance, d as r = (d^2)/8h +h/2. This device is particularly useful for large radii where only a small portion of the curve is available. For more accurate measurements, the screw would be replaced with a micrometer barrel.