What's the best way to fix this?

[seangilb]

I had some very bad tear out damage when routing the binding channels for a new body. I have offcut leftover from the top. I could cut a piece to glue into that gap to match the grain lines quite closely. However, I'm not sure that is the best approach and welcome any advice.

If I proceed with gluing in a new piece of soundboard, I have two questions in mind?

1) Should I be concerned about structural integrity of the top since it will mean that there are about 1.5" - 2" on the perimeter where the top is not anchored directly on the kerfing, but rather attached to a piece that is anchored? Should I just give up on this top completely?

2) If the adding a new piece in to replace the damaged area, any advice on the best way to cleanly cut out a piece? I am thinking about either using an exacto saw or a file to get a clean, straight line against which to glue.

Photos of the damage are here?

https://www.flickr.com/photos/199153...posted-public/

https://www.flickr.com/photos/199153...posted-public/

Thanks,

Seangilb

[runamuck]

No matter how close the grain is on the piece you have to replace the torn out spot you cannot conceal the patch because it's broken across the grain. It will always show and will always be an eyesore UNLESS you do an opaque sunburst to cover it.


Unless you want to consider this project a practice piece I'd replace the top and first look into why that happened. If you want to keep what you have and fix it there are good guitar building and repair videos on youtube. To explain all this here would require too many words/

[nikpearson]

At matching wood grain is always challenging and in this case the repair will always be visible. As the previous poster suggested you’ll need to remake the top - a lot of work - or do are sunburst finish to cover the repair.

The tear out you have is as bad as I’ve seen and I’m guessing this was a combination of cutting in the wrong direction, taking too large a cut, or a not very sharp tool.

When cutting binding channels you need to ensure that the cut direction means the router blade is pushing the wood into itself to avoid this. I also find that using a Gamil or similar device to hand cut a clean line prior to routing can help.

[Fathand]

From your 2nd picture, it appears the damage is 23/64" wide? If your binding is 1/16" and you use some combination of purfling 1/8" wide, you can cover 3/8" or 24/64" of damage.

Herringbone purfling is usually 3.5mm .140" so that would be more than enough. You could also combine 2 or even 3 strips of the typical 1.5mm 1/16" purfling.

I always run the router clockwise around the guitar body, a "climbing cut". This causes most of the pressure towards what's already been mostly cut. When done I do a final counter clockwise cut to smooth it.

[seangilb]

Thanks for pointing out the dimensions. I usually don't build with purflings and assumed that it wouldn't cover, but, as you point out the math, it does work.

I'm curious about your point on the climbing cut. I was taught to start in the middle of the bout and go downhill in either direction. In this case, it did feel like the bit pulled the guitar.

Wouldn't going in a full counter-clockwise circle mean that for one side of the guitar you have the router bit essentially "pulling" against the grain?

[Alan Carruth]

The best way I know to make that sort of repair unobtrusive is a 'finger' patch. Ideally you use an offcut of the same wood, but take the patch pieces from places where the grain spacing is wider. Be sure to match up the cut offs to their proper places, too, so that the run out matches.

The idea is to cut in a lot of short pieces that are as wide as the grain lines, so that the glue joints along the grain all fall on the hard, dark latewood lines. The ends of the patch pieces are cut off at an acute angle across the grain, not straight across. You start from one side of the blowout, and work your way across, making sure that the ends of the patches don't all line up in the same place. You have to have a really sharp knife with a fine point to make the cross grain cuts, or the glue lines will show. Use Titebond or HHG, not CA, as the water based glues will shrink and minimize the glue lines.

This sort of blow out is sadly more common with WRC, as it's more brittle than spruce. A sharp, new router bit (preferably steel, and not carbide), run on high speed, with a light cut, and advanced slowly, helps. Wetting the surface with a light spritz of water on the 10 o'clock and 4 o'clock quadrants, where the bit is running into the grain, helps a lot. I have heard of luthiers who buy a new router bit for each guitar to minimize this: it would probably be worth it, given the cost and diffculty of the repair... ;(

[seangilb]

Thanks Alan - that's very helpful! I had been running my router on a lower speed to reduce heat and wear on the blade. It does want to "pull" as I round the corner on the lower bout. Perhaps a higher speed would help a bit on that.

When you say that the ends of the patches need to be cut at an acute angle across the grain, I am imagining that the seam from the deepest point in the blowout to the edge of the guitar (about 1.5") will be a diagonal (roughly). So the first finger patch might be 1/8" long, the second patch will be shorter, etc. Is that correct?

That would likely have benefits in being more likely to blend the patch into the full board to a degree. I imagine that there is some trade-off in the strength at that point since I would have 4 or 5 strips glued together rather than a single piece. How much would you be concerned about that tradeoff?

Another suggestion was to design an inlay over the area.

[Fathand]

Originally Posted by seangilb

I'm curious about your point on the climbing cut. I was taught to start in the middle of the bout and go downhill in either direction. In this case, it did feel like the bit pulled the guitar.

Wouldn't going in a full counter-clockwise circle mean that for one side of the guitar you have the router bit essentially "pulling" against the grain?

The way you were taught can work too. I used to do that. I find it easier and I think less likely to cause damage going CW all the way. Picture your bit cuting towards a piece of end grain a quarter inch wide or cutting towards an area you've already cut.

See diagram of the CW rotating bit and how it cuts in either direction. https://www.woodmagazine.com/tool-re...ting-explained

Also if you can use a smaller diameter bit it will take less bite per revolution.

[phavriluk]

Everyone offering advice speaks to what works for them. They've gotten to where they are through perception and practice.

Now, my nickel's worth of advice: Take very shallow cuts, many of them, each a bit deeper than the one before.

On the new soundboard.

I think there will be less time spent, at the end of the day, and the result will be a flawless soundboard, which will never be the case doing repairs. Salvaging a damaged soundboard makes sense on finished instruments, but this is a construction problem which can be addressed by backing up the project to before the occurrence of the problem.

One thing I think I found was that the slower the tool speed of the router, the likelier chance of tearout. If the feed rate isn't slowed too, the router bit is trying to take a bigger bite for each rotation and there's the mess. There's reasons why routers were made to run zillions of rpm.

And a shiny new router bit is the cheapest way to keep the router behaving nicely.

[Howard Klepper]

Re preventing blowout: Nothing wrong with making downhill cuts from the center of each bout. It's always worked for me (not that this is what makes it okay). For sure get the router speed up if you have been slowing it. Your rabbeting bit should be the best you can find--no economizing here. The better bits have the blade angled a little to take a shearing cut. If you use a bit that has the blade parallel to the shaft, then make a few lighter cuts instead of all at once (as Peter said). A good carbide blade bit should be good for hundreds of guitars, but I touch up the cutting edges with a little EZ-lap diamond hone (extra fine) every 15-20 guitars or so.

What I'm curious about is your router and how it is guided. What are you using? You want no slop or runout or vibration in that system.

Fixing the problem: I would not go to using purfling wide enough to fill the damage, because that will put your purfling inside the liners, which creates a stress riser and weakens the edge of the top too much. I consider 1/8" of liner inside the purfling to be a minimum, and prefer to get closer to .150". And while you can do elaborate angle cutting that if done perfectly will make the joint in the spruce harder to see, it is highly unlikely that you could come close to invisible; very few people could. Better to sunburst it with a near-opaque outer color. If you haven't done a sunburst, this is your opportunity! Practice on scrap--a lot.

You could try to get a little extension joined to the liners where you are joining new top wood, so the joint is supported. Worth the try; if not, put a cleat under the joint.

[redir]

There was a guy recently who posted to one of the lutheir groups on Facebook the same problem. This was his very first guitar and I like most said no way can you fix that and he proved us all wrong. I've never seen anything like it. He virtually made it disappear with cutoffs from the same top matching the grain but it takes super level of wizardry to do it.

It came up recently again and it was in a place where it would make sense to make a unique looking sound port.

[Alan Carruth]

"When you say that the ends of the patches need to be cut at an acute angle across the grain, I am imagining that the seam from the deepest point in the blowout to the edge of the guitar (about 1.5") will be a diagonal (roughly). "

Not that extreme; something like a 45-60 degree included angle on the end of the 'finger'.