Corners on Swaine Adeney Brigg Bag

[esantoro]

I like the shape of the flap on the SAB bag. To me, it looks that the corners are symmetrically rounded, meaning that a round object of the proper size, such as a dinner plate, could be used. Or does it seem that the vertical and horizontal cuts begin at different lengths from the edge of a squared flap?

I put a ruler up to the image on my laptop screen. It appears in the image that the cuts on both the vertical and horizontal axes begin at 12mm from a squared edge. I'll try to open the image up in photoshop and use the ruler grid there to be more scientific.

Ed

Edited December 14, 2008 by esantoro

[Mike Craw]

Hey Ed!

It may just be an optical illusion on my screen, but it appears that there is a changing radius from the center of the arc getting tighter as it reaches the edge. By that I mean, if you started in the center of the arc and went either to the side edge or the bottom edge, it starts out at about a 4" radius and tightens to about a 2" radius. Does that make any sense?

Mike

[esantoro]

Hey Ed!

It may just be an optical illusion on my screen, but it appears that there is a changing radius from the center of the arc getting tighter as it reaches the edge. By that I mean, if you started in the center of the arc and went either to the side edge or the bottom edge, it starts out at about a 4" radius and tightens to about a 2" radius. Does that make any sense?

Mike

It does make sense, but that would mean that the horizontal and vertical points would have to begin at different lengths from a squared edge. Right? Am I correct in thinking that if the arcs begin at the same distance then the arc is symmetrical all the way around?

Ed

[Mike Craw]

No, I just mean that the curve of the arc gets tighter as it approaches the straight side, not that different radii were drawn from the same point. It's like they drew that curve the way Carlos and I would have in the shop, by tracing the bottom of a large can in the corner and then a smaller can near the intersection with the straight edge, and filling in between them with a French Curve template. Does that make any more sense?

Mike

[esantoro]

I tried out two cuts with some vinyl cloth. One with a 27cm diameter, the other with a 19.5cm diameter.

Ed

[Jordan]

Years ago I read an article in a model railroading book about using some formula to calculate the increase and/or decrease in the radius of a curve. That flap does look like the radius changes just before intersecting the straight portions. Your sample in the middle looks to be the closest.

[esantoro]

Years ago I read an article in a model railroading book about using some formula to calculate the increase and/or decrease in the radius of a curve. That flap does look like the radius changes just before intersecting the straight portions. Your sample in the middle looks to be the closest.

I thought there might be some mathematical formula to figure these things out, and a while ago someone did post such a formula, though I think it got lost in the crash.

I used a dinner plate with a 27cm diameter to shape the flap in the middle picture.

Ed

[esantoro]

I used the measuring tool in Adobe Acrobat to get a better idea of where exactly the arcs begin on the X and Y axes.

It appears that the arc on the Y axis begins approximately .93" inches from the bottom edge of the flap and the arc on the X axis begins approximately 1.08" from the right edge of the flap.

I'll now get some poster board and my french curve and see if I can make a pattern.

Ed

sabcrmeasure.pdf

sabcrmeasure.pdf

[Mike Craw]

OK, I'm going to try to explain what I mean one more time. I re-read my last two, and it's no wonder it's not working...I didn't even understand it. I looks to me that the corner was made by drawing the arc with a radius slightly larger than the center point is from the edges. In other words, lets just say they chose a 4" radius, but the center point was an equal 3 1/2" from each edge. The arc would shoot by the straight edges rather than blend in with the straight edges as it would if the center point was 4" from each edge. So far?

Now those two non-blending intersections were blended with an arc from a 2" circle, or something like that.

Well, that's my guess.

Mike

[esantoro]

Mike,

Thank you for taking the time to explain this. I've taken what you've said and tried to simplify it acoording to my understanding.

Here is what I've now done:

Based on the measurements from the picture, I rounded the ratio to 1:.90. The arc on the Y axis begins 90 percent of the distance between where the X axis arc begins and the right edge of the flap.

I marked off these distances for the arc to begin on both the Y and X axes: X axis arc begins at 12.7 cm from right edge of flap; Y axis arc begins at 11.4 cm (roughly 90 percent of 12.7 cm) from bottom edge of flap.

I moved into position the 27cm dia. dinner plate to meet up with the x-axis 12.7cm mark and the y-axis 11.4cm mark, and then drew my curve.

Here is what I got.

Thanks again for taking the time to explain the geometry.

Ed