mandola graduations

[whyner]

I'd like to carve a mandola. I have plans for mandolin and found some Loar Hacklinger maps (on MC) for mandolin.

But, I'm wondering is there much difference in top and back graduations for a mandola vs mandolin? Would mandolin work?

This will be the first carved instrument, so I don't have the experience for 'feel' and 'flex', etc. I'm gonna need to paint by numbers on this.

Thanks, Steve

[Fhornplayert]

Roger Siminoff sells H4 and H5 plans http://parts.siminoff.net/drawings.aspx

I found these to be an excellent starting point for carving mandola plates.

Travis Fernandez

[sunburst]

Probably should go a little thicker than the Loar mandolin grads. A little too thick is much safer than a little to thin, and since you are not experienced, and you can't be expected to judge the top wood for stiffness and strength and graduate accordingly, you're probably better off assuming it is less stiff and strong than a good piece of red spruce, and therefor should be thicker.
I've only built two carved mandolas, but in each case I was surprised how close to my typical mandolin the graduations I ended up carving them, so don't go overboard on thickness, just a little thicker than most published Loar mandolin thicknesses, especially in the string line under the tailpiece.

[James Sanford]

Another vote for Siminoff's plan. I have built 2 H-5 copies using this plan as well as another mandola of my own design but used the H-5 plan as a starting point for graduation on it.

All of my mandolas have used red spruce tops and I have been very well pleased with the results.

Good luck.

[whyner]

I'm going to use Sitka, which from my reading is somewhat less stiff than Red Spruce. So slightly thicker would make sense..?

I've also read that the Siminoff plans ($45 delivered) are from a Loar era instrument, which is interesting.

But I'm a cheap, contrarian knucklehead, with a DYI curse wanting to make the blasphemous strat looking asymmetric two point. In fact I intend to make one with an oval hole and another with f holes. I might brace both of them the same, not sure which to go, X-brace or tone bars. Maybe I'll do this instead (Dr. Cohen's bracing):



Thanks, Steve

[Dave Cohen]

You can use my bracing pattern if you like. It is not patented or protected in any way. Fwiw, there is no advantage to using that pattern. I only use it because I started with it and have learned to work with it. I did the experiments with different bracing patterns in a single mandolin back in 2004. The differences are way more nuanced than anyone here might think. The main thing to be concerned with is the amount of cross-grain stiffness relative to the stiffness parallel to the grain. F'rinstance, tone bars have less cross-grain stiffness compared to an X-brace, which has relatively more cross-grain stiffness. Two different pieces of top plate wood will have different ratios of cross-grain stiffness to stiffness parallel to the grain, so you may want to choose your bracing pattern based on measured properties of your top plate wood.

Speaking of which, how do you know that your Sitka sample has a lower Young's modulus than a given piece of red spruce? After all, there is considerable variation of properties within any given species. You may have a relatively stiff piece of Sitka, and Joe Doakes down the block may have an unusually soft piece of red spruce. Further, which modulus are you talking about? The one parallel to the grain lines, or the one perpendicular to the grain lines?

Back to bracing patterns; you could just decide to do something far out or off-the-wall. There was an article in American Lutherie way back showing an early classical guitar top made by Boaz Elkayam (originally from Israel) braced in a star-of-David pattern. Probably worked fine.

http://www.Cohenmando.com

[whyner]

Dr. Cohen:

Thanks for the response. and permission! It's good to know that bracing differences can be subtle and there's a wide range of potential. I need all the slack I can get! I remember reading about your removable back experimental instrument sometime ago...

Should I measure the physical properties of my wood unfortunately it would provide no useful direction for graduations or bracing as I have zero accumulation of baseline to work from. The only data I have access to is the Loar maps and the Siminoff plans for gradutaions. But as you point out, with no idea of the properties of those specific pieces of wood renders those plans only gross targets anyway.

I have read about folks doing deflection measurements and I may end up setting up a repeatable system for the possible future instruments, while keeping notes from that. Kind of like making wine and recording tasting notes. I like to keep things simple.

Thanks,

Steve

[Bernie Daniel]

I'm not a builder. But you mentioned that an oval hole was in your plans. Just a a comment about all braces being more or less equal. I'll suggest that a exception is the single transverse brace that Gibson put on their oval hole instruments in the first quarter of the 1900's. I posit that this was certainly an inadequate design. I say this based on the hundreds of these instruments out there with top sag under the bridge.

[whyner]

^^^ Thanks for that, will avoid a single transverse brace.

[Marty Jacobson]

... the single transverse brace as used by Gibson in those instruments. Gilchrist, Coombe, and many, many others have successfully used transverse braces. The brace or tone bar itself is only a part of the structural equation.

[whyner]

Thanks for that, I get it. I just strung up a FT OM with an 'H' bracing system like Graham McDonald suggests in his books. Among other things it induces curve into the top. Seems to work fine...

Thanks,
Steve

[Dave Cohen]

The single transverse brace is essentially no bracing at all. The transverse brace serves to complete the rims, and the modal analysis I have done on vintage Gibsons, L&H A models, etc., all showed the main or trampoline mode to be contained within the rims and the transverse brace.

Softwoods used for instrument top plates are said to be anisotropic. That means that the properties are not the same in every direction. Put most simply, woods are a lot stiffer ("higher modulus") parallel to the grain lines than they are perpendicular to the grain lines. How much so? By a factor of anywhere from 7x to almost 20x! So, it is conceivable that you could have one sample of Sitka spruce that is 7x stiffer parallel than perpendicular, and another sample that is almost 20x stiffer parallel than perpendicular. The difference is almost a factor of 3. The second sample would be a very different piece of wood than the first sample. And that is where bracing comes in. If you use a bracing pattern with relatively little stiffness perpendicular to the grain, e.g., "tone bars", you will get one spacing of modal frequencies. Use a different pattern with a great deal of stiffness perpendicular to the grain, such as ladder bracing (lutes, bowlback mandolins), and you get a different spacing of modal frequencies. Pianos are essentially ladder-braced, and the cross-grain stiffness from the bracing just about cancels the overall anisotropy of the soundboard. In most archtop mandolins (both oval hole and ff-holes), there is a lot of anisotropy left in the soundboard, despite differences in bracing patterns. Ladder-braced lutes and bowlback mandolins are another matter - much less overall anisotropy. In those things, the lower modes occur at much higher frequencies, and you can hear the difference in their characteristic sounds. In archtop mandolins the main body mode occurs much closer in frequency to the main air mode, and the sound you expect to hear from an archtop mandolin is more of a product of the main mode/air mode interaction. I wrote more about that in my chapter in the Rossing book, and supplied some of the supporting data therein. Also included some data on a few mandolas.

http://www.Cohenmando.com

[whyner]

ah ha!!

No two of your mandolins are braced precisely alike!

"The differences are way more nuanced than anyone here might think" and you "have learned to work with" your bracing pattern.

You alter your bracing based on the plate characteristics of each instrument. uh, am I right?

Steve

[Bill Halsey]

From a 1923 Gibson spec. book:



Some confusing, probably directly from dictation. It means that the final thickness of the back is to be 5/32" at the center, graduated to 3/32" at the minimum thickness area located 7/8" from the edge around the sides and tail.

Similarly, the final thickness of the top is to be 3/16" at the center, graduated to 1/8" at the minimum thickness area located 7/8" from the edge around the sides and tail.

FWIW, of course.

[whyner]

Bill:

Sweet! Thank you for that!

Anyone working in metric?

7/32 = 5.56mm
5/32 = 3.96mm
3/32 = 2.38mm
3/16 = 4.76mm
1/8 = 3.18mm
7/8 = 22.22mm.

Steve

[Dave Cohen]

"ah ha!!....."

Yes, but not really an "Ah Ha!" moment. I did say those things explicitly in my post. And, I have written about them before, both in this forum and elsewhere.

http://www.Cohenmando.com