On various build forums I have read a number of different suggestions over the years as to how one should proceed when extending the width of a top or back plate with wings taken from the off cuts of the waist yet avoid the obvious contrast whch results when light is refracted through the finished instrument and the jointed components beneath have opposing directions in grain runout to anything but the most minor degree.
I recall reading one suggestion in particular which claimed you could align the runout within the components 'and' use the longer 'straight' edge of a the waist off cuts as the jointing or glue edge thereby extending the over all width of the plates to their maximum. The advice was to take the waist scrap from the 'opposite' side plate and simply flip it over so any runout in that piece would be reversed to point from one direction to the other and become a perfect match for that within the plate on the opposite side of the bookmatch. Well when I first read it that idea seemed quite a plausible thing, and considering the source was of course well meaning, I had accepted it without any real thought.
Sometime late, someone else had suggested that flipping an entire plate of a bookmatch would adress the problem of contrast from light refraction when a guitar top has moderate runout. The suggestion here was that doing so would result in the runout in both plates now matching right across and would result in the guitar top looking either 'all light' or 'all dark' at one time depending upon which angle light was entering the surface and this would of course result in a much more acceptable look than the usual alternation of one side of the guitar top appearing darker than the other as we normally see in a bookmatch top set with moderate runout.
Well it was that comment which got me thinking that if it was really 'that' simple to improve the look of cheaper tops that had runout, then why don't all the factories do it?? So when I finally sat down to actually think about it a bit more, I soon realised that the direction of runout will never change by simply flipping a piece of wood over. Rather what you would see is the exact same runout going in exactly the same direction with the only difference being that you would now be viewing it from the other surface of the board....nothing changes. So I fooled around a bit more and come to realise that the only way you could possibly match the direction of runout in a plate with an off cut piece taken from the opposite side of bookmatched plates, would be to turn that piece through 180 degrees.
Well that's all fine and dandy, but when you do this, it will present the 'curved cut edge' of that off cut to the jointing edge of the plate your trying to widen, and not the long straight edge of the off cut at all, and 'that' is the only reason one would have to do the swap in the first place. So it became clear to me..at last..that it is a complete waste of time taking the off cut from the opposite side and or flipping it or anything else. Because regardless of which way you turn it, flip it, or look at it, the same if not a 'better' match of runout will always be found in that off cut taken from the plate you will be gluing it too. You simply take that bit and slide it straight back from where it had been cut from and glue it on after you've shot a nice straight gluing edge....and that's it right there.....You need to shoot the 'inside' curved cut edge to form the glue joint. Either off cut will pretty much match runout in the plates on either side, but 'both' MUST use the inner edge of the scrap as the gluing edge or you will mismatch the runout in the plates..
I know this may all sound pretty bloody obvious to many people who will read this, but I am seriously right brained kind of bloke, so I had to work through this stuff with angled lines drawn onto 2 bits of paper so I could 'see' this happening for myself before I could come to terms with what's really going on..yeah I know, not too sharp at times but when I pick up at least it stays there...well most times anyhow
So after coming to grips with this I headed out to the shed to do a visual aid to help others of the world who struggle to turn things upside down and around in their head and still make head or tail of it....read on if you need to, if you don't, then seeya later smart arse. Here we have our mock up billet of spruce all ready for resaw into a bookmatched top set. Those angled lines on the side represent the grain runout present in our super select, triple, quadrupedaled, special reserve, double master grade billet of fruity bear mackerel bridge spiky leopard striped fury goldfish spruce
....(Sorry Marty, could not help meself) .....The mdf scraps have the left overs of a block mount still attached to one side, this will help identify the orientation of the plates in the bookmatched set as they are moved around in the following images.Here we see off cut 1 with plate 1. It has been turned 180o so the long straight edge will present as the glue edge and as suggested, the runout is going the opposite way to that which we see in the plate.
OK, lets get off cut #2 cut from the opposite plate and present the long straight edge of that to the glue edge of plate 1 to see what happens..
And just in case you need a bit more of a visual to help understand how two bit of wood sliced in bookmatch from the same board can end up with completely opposite directions of runout, then look at two edges of our original billet in this image and it should clear things up for you nicely.
Kim
