Thinning a top with different season ring thickness

[Henning]

Hello, how do you handle to thin a top where the season rings thicknesses varies?
Do you make it in even thickness all over, or how?
It might be the winter and summer wood, where occasionally the winter wood suddenly turns out to be rather thick now and then, making the stiffness of the top to vary, please.

[Alan Carruth]

I'm not entirely sure I understand the question, but that never stopped me from replying in the past...

What I think you're asking is whether the top should be made thinner in those areas where the annual ring lines are 'thicker'. No matter how you define things you're likely to get different answers from different makers, but I'm going to start by asking what 'thicker' means here. Is it that the lines themselves are 'thicker', or that they are more 'thickly' spaced? In the first case we could say that 'thick' lines are 2mm apart on center and 1mm thick, as opposed to 'thin' lines spaced out to the same 2mm O.C. but only .3mm thick. So; 'thick' or 'thin' lines. 'Thick' spacing could have .3mm thick lines that are 1mm on center, as opposed to 'thin' spacing, with .3mm lines 3mm apart or more.

The hard wood in latewood lines of softwoods adds a lot of stiffness, but is also adds density. As it turns out, the weight goes up faster than stiffness along the grain as the latewood lines make up more of the area ('thicker' in the first sense). Grain spacing ('thickness' in the second sense) doesn't matter as much as the ratio between early and late wood line width. If the soft 'early' part makes up 90% of the line, and the hard 'late' wood only 10%, then it doesn't matter much how many lines there are to a centimeter.

Since a high proportion of late wood adds a lot of stiffness and mass some folks like to try to even things out by thinning the areas where there is more late wood. If you try to get the stiffness to be uniform by feel from place to place, you end up with uneven mass distribution because of the way stiffness, mass, and thickness relate. By the same token making the mass more uniform, by looking at the way light shines through, you tend to end up with uneven stiffness. There are makers who advocate for either system, and others who don't bother.

Anyway,I hope this helps...

[John Arnold]

I selectively thin heavy latewood, aka hard lines or compression wood, when I am graduating a top. The goal is to give a more even flex across the lower bout. The main tool I use for selective thinning is a cabinet scraper, which is flexed to create a slightly concave surface. My preferred thickness in flexible scrapers is 0.024", or 0.6mm.

[redir]

But Alan I thought from your studies of top thickness 'grain count' doesn't correlate to stiffness?

Or is it all a matter of density and the count of summerwood has on that affect on stiffness.

I like to say this when this topic comes up that one of the very stiffest tops I have is a Carpathian top that has about 8 lines per inch. I have had it for 20 years because I have no idea what to do with it yet. It's insanely stiff.

[John Arnold]

Thick latewood, aka compression wood, increases the density. It is the proportion of latewood that is pertinent, not grain count. Compression wood tends to be wider-grained, but it can occur in tight grain, too.
Compression wood is the one outlier in the general rule where density and stiffness are proportional.
That said, I have built some very good sounding guitars with compression wood.

[redir]

I had no idea this was a building technique! Always so much to learn.

[Alan Carruth]

redir asked:
"But Alan I thought from your studies of top thickness 'grain count' doesn't correlate to stiffness?"

What counts is primarily the proportion of earlywood to latewood, as John points out. If the latewood takes up 10% of the volume of the wood, it doesn't really seem to matter whether it's a lot of very narrow lines close together, or a few wider ones further apart with a lot of earlywood in between.

Tom Thiel noted in cutting red spruce that you will often see heavier latewood lines lower down in the trunk, and they get thinner higher up, even when the grain count stays the same. The density of the wood drops a bit, but it retains most of it's stiffness. Sometimes a tree will have heavier latewood lines on one side, often toward the south or strong sun exposure, or if the tree leans or has more branches on one side. It's the tree's response to stress; hence 'reaction wood'. You see it below knots as well.