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shellyrat

how to figure arch heights

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Newbie question. Do you measure the arch height from the table to the very top of the arch outside or from the table to the underside? 

i e if I set a piece of wood on the table and it was 15.5 mm thick would it be the right height or should I have a slab that was 17 mm etc. if you wanted an finished arch height of say 15 .5 mm. 

I know clear as mud. 

kathy c in ky

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It IS sort of unclear.  I think the answer is yes.   :)   The measure you would want is 15.5mm from the gluing surface to the high point.  If the plate is already hollowed, you have to figure a way to measure, but it's not too hard. 

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Arch height normally includes edge thickness, i.e. from the flat gluing surface to the highest point.  So if you want a finished arch height of 15.5mm, you'd need a wedge thicker than that to start with.

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I do not quite understand the question, but it could be a problem of mine....... :rolleyes:

However, at the minute 4:00 of this video you can see how I measure the finished arching :

 

 

The height of the piece before starting the job (center joint already glued) must be at least 1 mm higher than the finished measure, considering the inside  plane already finished and the outside with a flat strip at least 2 cm wide at the max heigth area.

 

Maybe not clear as water...... :unsure:

 

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Here's a great arch calculator:

 

http://www.handymath.com/cgi-bin/rad2.cgi?submit=Entry

 

I just carved my cello plates from the inside. My plate has a height of 26mm. To arrive a at a decent

plate thickness in the center of the longitudinal arch from the inside, I aimed for 20mm. I changed 

the input unit to cm.

 

length of span      height of arch   radius of arch

66cm                    2.0cm               2.732m

 

The cross arching can be determined at key points ie  bridge line, at upper and lower points, fullness of upper and

lower bouts and some points between.

 

 

length of span      height of arch   radius of arch

 

Bridge line

22cm                    2.0cm               2.732m

Upper bout

33.5cm                 1.4cm               1.00m

Lower bout

43cm                    1.3cm               1.78m

 

I included margin for gluing surface to the ribs and blocks.

I draped a fine chain to check the arching as I refined it.

The chain does what this math does.

 

This is a first for me to carve archings from the inside and graduate on the outside.

It's an adventure.

 

https://www.flickr.com/photos/14995534@N05/sets/72157639727623634/ 

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