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I think I may of made a mistake with the orientation of the boards, they both have the close grain in the middle but on further inspection of dome of the outer grain marks one half maybe the wrong way round.

Will this cause a difference in shading under finish of the 2 half's?

Thanks

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2 options imo,

1 Live with it
Or
2 Cut it apart and rejoin


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the shading difference is from run out and you will have that no matter what you do

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

If you tilt the soundboard away from you does the shading change sides? If so, you’re looking at runout (the tubules in the wood tilt relative to the surface so you’re looking at the sides of them in one orientation and the ends in the other). If that’s what you’ve got, you can’t fix it and keep the bookmatch. One of the best sounding guitars I ever made has a harlequin top like that.

If I'm understanding you right you're saying you need to flip one piece end for end to get a book match? That won't change the shading caused by run out.
I work with a lot of Engelmann, and some of the tops will have no runout when book matched on one edge and significant run out on the other. I join whichever edge has the least runout. Sometimes this puts the wide grain in the center and sometimes it puts it on the edges.
When checking for runout I have found it to be about the same in "AAA" grade sets as it is in "A" grade sets.
You can check the runout on the outer edge of the top and if it has little runout, then it might be worth sawing the joint apart and gluing up the set on the "outside" edges.
Engelmann is known for spiral growth which causes opposite edges of the plank to have different degrees of runout. Sitka spruce can also be this way, but generally the runout is pretty much the same from edge to edge, so what you see is what you get.

Thanks Clay, that is exactly what i meant, I think I need to flip one side.
The board doesn't seem to change alot on reflect from light, I have just always joint the two as a mirror image of any defects on the outer edges, this one didn't have many so I got it wrong way, it is AAA so lovely straight grain un marked in the build area.

I'm going to sound dumb now, been building for 5 years and still don't understand what run out is. I hear it all the time on here, can you explain it to me mate in a way I might understand lol

Thanks

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Runout:

Look at the board from one side edge. The wood fibres don't run exactly along the length of the side (except in very rare cases). So they run at an angle, and run "out" onto the top at various points.

This is what makes the wood happy to be planed in one direction, but not the other - if your plane blade is digging into the ends of the fibres, the blade snags. Going the other way, it snips off the ends no problem.

If you look at the top surface under angled light, along its length, in one direction you are seeing the ends of the fibres which have run out. In the other direction, you don't see the ends. So it looks a little different in each direction.

If you bookmatch a top, each half has the runout in opposite directions. So the two halves look different under angled light, and the greater the runout, the more different they look.

In a thin plate it should be hard to see the runout because the board is so thin. But splitting a fragment, or trying to plane it, tells you which way the grain runs out.

Have you ever spent time around a short-haired dog or cat? Their hair has an orientation; it grows out at an angle. If you stroke the animal head to tail, you smooth the hair down. If you stroke the animal tail to head, you ruffle the hair, because you are digging underneath it, due to the angle at which the hair grows out.

Wood fibers are somewhat similar; not in how they grow (wood is not hair), but in the practical effect of how the wood is cut. We need wood to be flat and straight, and have certain dimensions. The wood fibers most often don’t grow as straight as that. So, the face of the wood will have exposed wood fiber ends. Those fiber ends have an orientation; they “lean” toward one end of the board or the other. Most of the time, when you open a book matched set, you have one side with grain that leans toward the tail, and the other side with grain that leans in the opposite direction.

This makes it harder to plane the wood if you go against the grain (stroking the animal tail to head). And when light hits the wood after finish is applied, the wood looks different, depending on the grain direction.

I hope that helps. This is the sort of thing that is easier to show in person.

To add to what Profchris said-
Run out can not be easily seen or measured by looking at the edge of the board, that is why many of us cut off a small piece and split it to measure the runout.
When I check for runout I will often do a number of sets at a time, so I take them to the table saw which is set to cut off about a 1/16th sliver about 2 to 3 inches long. I flip the set so opposite sides are cut by the blade, so I can measure both edges for run out (and not loose as much width from the set).
The tools to check for runout are few and simple - a knife for splitting the wood, a ruler for measuring the split, and pair of cheap cheaters for us old guys who don't see well. The knife doesn't cut the wood, but rather takes a bite and then "wedges" it apart. I use a metric ruler because base ten is easier to calculate that fractions. For those who want higher precision a digital caliper could be used instead of a ruler. I'm just looking for close approximates. To find the runout you divide the length of the sliver split off by the thickness of the "fat" end. If the sliver doesn't runout to nothing subtract the thin end from the fat end and divide the length by that number.
Everyone sets their own limits as to how much runout is acceptable.

With the Engelmann tops pictured you can see the runout varies from edge to edge. Notice the "toothpicks" laying against the sides of the ruler. The first set I would join on the 1:55 (one in 55) edge. For a 2mm thick top the grain would runout from top to bottom in 110mm or about 4 inches which I would find acceptable and which structurally and cosmetically is also acceptable to me. The second set I would not use on a guitar. The 1:8 runout would mean on a 2mm thick soundboard it would runout, top to bottom, in 16mm (about 5/8ths in.). It would show it in the way light is reflected in the halves of the top and may not be the best structurally. I might use it for a cheap and cheerful pickin' stick or for some non lutherie use instead.

I don['t understand what you mean by, "dome of the outer grain marks one half..."

But if for example you open a book and you have a page on the left and a page on the right and you ripped one page out and flipped it so that you are now on the back side and all the words are upside down then you should cut it and rejoin it imho.

It has nothing to do with the runout shading but under finish you will probably notice that it's not book matched.

Sorry typo and an unhelpful one, supposed to say *some

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So I'm kind of back at square one with it,
Very grateful for replies but some say it will cause a shading problem others don't I guess that's the joys of ask the audience

I don't know what to do

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Did you understand my book page example? If the top has runout the nit doesn't matter how you orient the book matched edges you will always have alternate shading.

The only way to avoid runout would be to not open the two halves like a book but rather join them edge to edge. So again with the book analogy say you were on page 100 and 101. Instead of opening the book so that page 100 is on the left and page 101 is on the right you would rip out pages 100 and 101 and stack them in order. Then you would slide page 100 across to the right and join the right half of page 101 with the left half of page 100.

That will not give you a symmetrical left right half but it would not make the runout shading apparent either. In a well quartered top with even color and garin lines it might be doable.

To further muddy the waters
The fact that half the soundboard is flipped won't - cause - a shading problem (nor will it solve it). Being flipped end for end bothers some people because it makes the top appear to not be book matched. That is why some people would prefer that it be recut and reglued in a more usual orientation. In this happy time when book matched sets can be had for the click of a mouse there are aesthetic expectations that people have that are more than formerly expected. The "flipped" halves probably won't affect the sound, but will hurt the aesthetics.
A separate issue is that with the photo you posted one half of the top appears to be slightly darker than the other. If this has not been caused by light exposure "tanning" one side of the top (which might sand off) and the other side not being exposed to the light, then the assumption is that we are seeing runout in the top. Again, if it is not severe it is mostly an aesthetic consideration and in many cases there are great sounding guitars that show some runout in the top. Placing the two halves of the top in their proper book matched orientation won't change that.
A third issue is that in some top sets (more common with Engelmann) the run out will change from one edge to the other. Joining it one way may make it appear to not have runout, where joining the opposite edges will show obvious runout. Aesthetically (and possibly structurally) joining the edges with less runout is better (assuming other considerations being equal).
Torres built guitars with soundboards that were not book matched, were not joined at the centerline, were sometimes built up from three or more pieces, had both wide and narrow grain spacing on the same top, and by today's standards would not meet any of the criteria expected of a fine instrument's soundboard. But they sounded good.
The short answer is - cutting and regluing the soundboard in a "normal" book matched fashion may help aesthetically in that the top will show a book match, but it won't change the "shading" if that is caused by runout (exception - if what is now the outside edges have measurably less runout and these edges are joined then the top can show less runout).

As has been pointed out, run out in a top can vary from one edge to the other. When the tree starts growing the fibers normally line up with the axis of the trunk, but as it gets bigger they can start to spiral around like the stripes on a barber pole. The run out goes from none, or very little, on the heart of the tree to possibly very high just under the bark. I'll note that th e twist can reverse, which is what gives 'stripe' figure. It's more common in tropical hardwoods, but I have seen it in spruce. But I digress....

Normally we join tops along the bark side, since this usually gives the best quartered wood, and the tightest annual ring lines. However, if the tree grew with increasingly spiral grain the bark match also gives the largest change in run out across the join line, which shows up as the 'harlequin' effect mentioned. Using a 'heart match' on such a piece of wood can minimize, or even eliminate, the appearance of run out across the joint. The run out is still there, of course, it's just all out in the wings of the top and it's not as obvious.

Run out reduces the stiffness of the wood along the grain, so that, in theory, you would need to leave a run out top a little thicker to gain the necessary stiffness. A bigger problem has to do with the glue line between the top and the bridge. The static string load causes the bridge to rotate forward, and this, along with the shear in the glue line, produces a peeling stress along the back edge of the bridge. When that exceeds the strength of the glue line the bridge peels up. The peeling strength is lower when the grain climbs up from the back of the bridge toward the front. When the run out is the other way the bridge is less likely to peel loose, but when it does the split will dive down into the top. Run out across the center joint leaves you with a bridge that is more likely to peel up than it would on a top that had no run out, and it's also very likely to damage the top when it does. Sadly, it's difficult to find top wood that had no run out at all. Luckily the problems become much less problematic when there's less run out.

I'd check the run out on both sides of those top halves, and join them in the way that will minimize it. If the grain spacing down the center line ends up notably wider than at the edges, tell people you're making an homage to the Martin -35 model....

I've read a number of really helpful OLF discussions about run out, including this one. One particularly helpful post that I have archived and referred to again is this one from John Arnold:
viewtopic.php?f=10101&t=40923&p=541392&hilit=top+without+runout+suggestions#p541292

Thanks again, Mr. Arnold!

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Tim Allen
"Never hurry, never rest."

I have seen that stripe figure in spruce too, particularly Carpathian for some reason. Also with a few Carp sets that I built with was a very odd runout chatoyance. It was split into quarters. Picture the BMW car logo. It looked like that and would flip back and forth in the light.

Probably will be you best sounding build yet. I believe the imperfections in the wood shape the tone “vibration” traveling through the guitar. Let us know how she sounds please.


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Here is an image taken from the ANZLF discussion on runout:
http://www.anzlf.com/download/file.php? ... &mode=view
It shows how the "harlequin effect" (good term for it Alan) increases as runout becomes more pronounced. They show it in a unit dependent measure where I find a ratio more useful ( 1/4in. in 20ins = 1:80 , 2in in 20 ins = 1:10)

Put in some time planing wood, and you will grasp the concept of runout. Flipping a board end-for-end does not change run out direction, but flipping it side-to-side does.
The only reason runout is visible on a book matched top is because flipping one half reverses the direction of the runout. If you have a grain slope of 2 degrees, that means the difference in grain direction in two halves of a bookmarked top is double that, or 4 degrees. That is why a very small amount of runout is so visible, particularly when viewed using a distinct light source.

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John