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soliver
Jan-12-2021, 8:57am
I remember reading that when you carve redwood or cedar for an arch top, that you typically leave it thicker than spruce. Does the same rule apply to a flattop?

I bought some redwood for the top of my second build which is milled to about 3/16" (0.19" is what was in the description), which I know is too thick, but I'm wondering how thin to go with it, and if it needs to be a little thicker since its redwood.

Thanks in advance!

sunburst
Jan-12-2021, 10:50am
Yes, redwood will usually end up thicker than spruce when used for similar tops. How much thicker? Depends on the stiffness, density, desired sound and other things that experience tends to teach us.
If you have records of your spruce top's weight and stiffness, and you are happy with the sound, you can thin the redwood until you get close to the same weight and stiffness.
No records? That's what I figured (none of us think of that on our first build), so you'll just have to take a good guess.

soliver
Jan-12-2021, 11:58am
No records? That's what I figured (none of us think of that on our first build), so you'll just have to take a good guess.

HA! yea John, you called it!... I have some leftover pieces, so I can probably extrapolate... I do feel like I should have thinned the top on the first one some (its a little quieter than I would like), I assume I should take that into account when figuring out the thickness of the Redwood?

sunburst
Jan-12-2021, 12:01pm
...I assume I should take that into account when figuring out the thickness of the Redwood?

Could be, but the top thickness may not have much to do with the loudness of the instrument. Loudness significantly depends on coupling between the top and back, so thinner is only better if it brings the top and back into better compliance.

Marty Jacobson
Jan-12-2021, 1:33pm
The results from my redwood builds compared to spruce builds generalizes to redwood specs being about 25% thicker, glancing through ~20 instruments that I have the best notes on.

I start out with my Sitka spruce tops roughed out to .120" (recurve) to .200" thick and then adjust from there based on feel and measurements. They end up being between .110" (tone bars) and .130" (oval) in the recurve, and between .165" and .200" in the center.

I start out with my redwood tops roughed out to .170" (recurve) to .300" thick. Which is a little chunky. But I've ruined 4 mandolins by carving redwood and cedar too thin, so I start out with plenty of room to work.
They end up being between .130" (f holes) and .150" (oval) in the recurve, and between .220" and .280" under the bridge.

Weighing your tops is a good habit to be in. If a top is way too heavy, you're probably safe to thin it down. If your top is way too light, you... have to decide if you want to risk it. My tops end up with +/-10g mass regardless of what they're made out of.

If I had to guess, I'd probably start at .165" and then brace it. You can always thin it from the outside if you feel like you need to later, and the good/bad news is, redwood sands FAST. Don't put a random orbit sander any where near it, you'll have a hole in the top.

soliver
Jan-12-2021, 8:39pm
Well here's a kerfuffle....

I found a piece of the top wood for my first build. It is 36 square inches by 0.1325" thick and weighs 36 grams... easy math... that's 1 g per square inch. I did probably sand some of that thickness off in the final sanding, but probably not much.

The redwood I have is 90.7 square inches and weighs 60 grams at 0.19" thick... that translates (if I'm doing the math correctly) to 0.66 g per square inch.

I'm not sure matching weight is going to be a possibility here since I'm already at a deficit. But in hindsight, I should say, I chose redwood because I am after a darker tone than I got with the Torrified Sitka from the last go round.

Love to hear thoughts on that!

Thanks guys.

Marty Jacobson
Jan-13-2021, 8:51am
Ok, what does "darker tone" mean? To me it means, as a gross over-simplification, "Set the EQ to Bass Boost".
So if you want it to really have those properties, consider reducing the size of the sound hole, increasing the body depth, or both.

mclaugh
Jan-13-2021, 9:59am
If you calculate the density for each piece your spruce is 7.55 g/cubic inch = .46 g/cubic cm which is normal according to an online wood density chart. Your redwood is 3.48 g/cubic inch = .215 g/cubic cm. Judging from Marty's ~25% thickness difference, that seems very low. Have you checked your measurements? I'm not a wood expert, just wondering about the numbers.

soliver
Jan-13-2021, 2:17pm
... To me it means, as a gross over-simplification, "Set the EQ to Bass Boost".
So if you want it to really have those properties, consider reducing the size of the sound hole, increasing the body depth, or both.

Yes by darker, I do mean more emphasis on the bass end... So okay, I'll add those implementations to the plan. Sound Hole reduction should not be too hard since I plan to do ff holes this go round, and I'll plan to add some depth to the body. Does this mean that my presupposition that wood species (ie choosing redwood) to emphasize bass is incorrect.

Marty Jacobson
Jan-13-2021, 10:02pm
Does this mean that my presupposition that wood species (ie choosing redwood) to emphasize bass is incorrect.

Not necessarily. But you don't know anything about the wood you've got on hand, other than that it's lighter than what you used before. And the side depth and the soundhole area are easy to measure, so a much more objective thing to modify.

soliver
Jan-13-2021, 10:20pm
Roger that Marty, any recommendations on how much depth to add to make a notable difference?

You still think .165" is the best starting point for thickness?

Richard500
Jan-14-2021, 8:50am
If you calculate the density for each piece your spruce is 7.55 g/cubic inch = .46 g/cubic cm which is normal according to an online wood density chart. Your redwood is 3.48 g/cubic inch = .215 g/cubic cm. Judging from Marty's ~25% thickness difference, that seems very low. Have you checked your measurements? I'm not a wood expert, just wondering about the numbers.

Density is a factor, but stiffness, elastic modulus and damping are also involved. That is, you can have a very light material that’s strong enough, but also damps vibration, etc. The two jobs of structural integrity and usable elasticity require compromise.

mclaugh
Jan-14-2021, 2:15pm
No argument about other factors. Just observing that as the OP suspects it's not possible to match the weight of his spruce top with that redwood, if that's the goal as recommended above. The redwood would need to be more than twice as thick as the spruce to match the weight.

Tom Haywood
Jan-14-2021, 4:32pm
I don't know if this 2nd build is your own design, is being built from a plan, or is modifying a plan. If it is your own design or a modification of someone's plan, then I think you really are on your own. You'll have to build it and let us know what happens, just as Marty has done with his mandolins and his redwood tops. I'll suggest that thicknesses of carved tops are not directly relevant to flat tops. You have more bracing options with the flat tops. That is something you may want to modify if you are using any of the plans I'm familiar with. Understanding the balance between building light weight and building strong support with any particular wood is something that I think you just have to get a feel for. I think the unique structural designs of Marty's mandolins may or may not be directly informative for your design. I enjoy building my own designs and modifying someone else's plans, but I am mindful from the very beginning until the strings have been on for a year that it could crash. That has never happened - so far. I haven't used redwood for a top, although I have had the privilege to study some of Marty's successes with it - including the failed tops.

Marty Jacobson
Jan-14-2021, 9:40pm
Yep Tom, all good points. I was trying to give a sense of the relative difference in thicknesses I've arrived at between Sitka vs. Redwood, the numbers themselves are meaningless for Spencer's application. But at the end of the day it just comes down to the old adage that it's easier to take more wood off than to put more back on.

soliver
Jan-14-2021, 10:11pm
Thanks Tom and Marty, and to be clear, I am modifying someone else's plans.

Just as with my first I am making a pancake mando (Flatiron style) with plans from Terry Majewski of Crystal Forest Mandolins. The first one was directly off the plans and this one will deviate in that I will be doing f-holes instead of an oval and using X-bracing. Marty and I did exchange emails a little while back about this but I did not discuss wood species at that juncture. I am also using Graham McDonald's book as a guide as well, though not as much as the first time around.

I plan to use this redwood for the top, and I have a back glued up and the sides bent all of which are redbud wood from a tree I cut down from my yard (this tree was planted when my house was build in the 60's). The wood has dried for a few years now and this was my plan all along for my second build. Hoping to do this one as a black top so I hunted for the most uninteresting piece of redwood I could find so I wouldn't feel guilty covering it in black :))

Also in the plan is walnut for the neck, ebony fretboard, ebony headstock overlay and white binding.

soliver
Jan-19-2021, 11:13pm
Here's a fun irony... I contacted a friend who runs a furniture shop to ask if I could thin down my redwood with his Supermax but unfortunately it is out of commission. So I decided to use my home made thickness sander and worked to tune it up and get it working well. Then I measured the Redwood's thickness again to be certain of how much I was removing ... turns out it is already 0.15"!... I swear when I bought it, the seller listed it at 0.19" thick!