Top thickness for OM in WRC

Kia ora everyone,
I'm currently building a carved top OM for a friend. He's specified 575mm scale length as he already has an instrument that he's loved to death at that scale. I've settled on a 2-point shape and the body is about 340mm at its widest point. I'm carving the top to a similar arch profile to the one I'm using successfully on mandolins. But I'm looking for opinions (and I know you guys have those! ;)) on the top thickness. I'm using a nice piece of Western Red Cedar but it's quite light and I'll admit I'm scared of going too thin... My arch height is around 19mm (I think I based this on a combination of guesswork and comments I think I found in John Hamlett's fantastic Mandocello build thread from back in 2014.
Any thoughts on where builders might aim for?
Cheers,
Davy

The mandocello is bigger by a fair amount and the top is red spruce; very stiff and hard compared to any WRC that I've encountered.
I did make a WRC topped mandola of a size a little closer to yours some years back. It is still going strong. I'll try to remember to look it up when I get back to the shop and see what the top thickness is and how long ago it was made. It was unfamiliar wood to me at the time so I'm sure I started thick and thinned it down by feel to final thickness.

That'd be brilliant, thanks John. Much appreciated!

Start with 7.75-8mm in the center and go from there. This is based on my redwood instruments of the same size (15" archtop). It might be able to be as thin as 6mm, but you won't know until you have it roughed out. Also, don't let it get too thin in the recurve. I wouldn't go thinner than 4mm anywhere on it. 4.5 is probably safer.
Keep a nice thick area 7+mm along the core centerline that goes from the neck block to the tailblock, what you do on the treble and bass side are less structurally critical than that primary centerline strip.

Thanks Marty, I think I'm currently sitting about 8.5 in the centre, so good to hear I'm not a million miles away, but with room to shave a little more...
Wish I'd thought of asking the 'cafe earlier - I've been scratching my head, guessing and googling all week! :)

Well, the 'dola is smaller, about 280mm wide. It is an oval hole instrument with a WRC top about 3.0mm thick near the edge and 5.5mm in the bridge area.
I built an octave mandolin with f-holes and an Engelmann spruce top that is a little less than 350mm wide. That top is about 3.3mm near the edge and about 6.0mm in the bridge area.

I just have experience with extremely soft Engelmann on mandolin. I left it 3.8 - 4 mm at recurve (which is actually flat inside with just the recurve carved out) and 6.5 mm under bridge and I used x bracing to adjust extremely low crossgrain stiffness. I had no idea back then how much to add so I left it that thick compared to my spruce tops counting that I can remove some wod from outside but it ended up very fine sounding instrument that thick.

8.5mm top thickness is in the range of contemporary double bass building!

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Originally Posted by j. condino

8.5mm top thickness is in the range of contemporary double bass building!

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But I guess they don't use WRC or Redwood for double-basses. :-)
If we compare mandolins and violins the difference is still there with contemporary violins 2.4-2.7 mm or so for tops and mandolins generally in 3.2-4.5 grads...
BTW, what is difference between tension of 4 bass strings and 8 OM or mandocello strings?

The other thing to consider is arching height. 19mm is around where I would carve a spruce bouzouki soundboard, but with a softer wood, I would add 2mm to that. Using King Billy pine, which I think has genetic connection to WRC I will add 2mm or so to the arching height and start out about 20% thicker than spruce. With redwood it was suggested by a very good mandolin and archtop guitar builder that I add just 20% to the thicknesssing. I only ever made one mandolin following those instructions and it worked very well.

Quote:

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Originally Posted by j. condino

8.5mm top thickness is in the range of contemporary double bass building!

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I've shipped mandolins with 7.5mm thick redwood tops. Sounded great.

I just went back and looked through my huge library of blueprints that I have made over the years.

One of the finest octave mandolins I ever played was made by Stephen Owsley Smith. It was teardrop shaped, similar proportions to an old Gibson A mandocello, and had a western red cedar top 7.0mm in the center, tapering to 3.2mm at the recurve.

Having lived in the land where WRC was a common local tree and handled 1000s of board feet & guitar tops, it varies tremendously in stiffness and density from floppy punky slop to beautifully crisp, resonant, and stiff. Just giving a species and random thicknesses doesn't really offer much of anything. My absolute favorite use for western red cedar trees is to leave them standing in the forest and taking a nap underneath on a sunny afternoon.....;)

Thanks everyone, and sorry for the delay in responding - I was hiding from the news by going camping. This is all really good information and hugely appreciated! I've taken the thickness down a little so we're now sitting about 7.5 in the centre and 4.5 around the recurve, with a bit more meat under the tailpiece. I'm already committed on the arching height, but I'll file that info away for next time (or the new top if this one fails...). However, it sounds good to my ear at this point, and an improvement over the thicker dimensions, so I'll push on and see how the rest of the build goes. If you can contain your excitement I'll try to put an audio clip up when I've finished... probably early 2023 at the rate I'm going :popcorn:

Here's a video of the instrument in action with a cover of Dougie Maclean's wonderful Ready for the Storm.
https://youtu.be/MxY9tgt7YmM

Good looking OM with a nice sound. It seems your dimensions worked out.