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Best Way To Joint Acoustic Wood?


Daniel Sorbera

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joining the top and back sets together. I have heard that a power jointer doesn't make a good enough cut and you have to use a hand plane, something I lack the ability to do. I tried using a hand plane with horriable results. I'm going to call the guy I know at a local cabinet shop tomorrow (as long as no one here tells me diffrent) and see if I can get him to joint it on his power jointer.

Edited by Godin SD
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I just jointed my first top yesterday, for my koa topped semi-hollow body, rather than an acoustic though.

I'm not gonna lie, I bought a beautiful, brand spanking new Veritas #5 jack plane and started flattening out my 1/4" thick figured koa and it cut beautifully right out of the box, but it wasn't like my cuts were near perfect on the first try. I ended up taking off far more wood than I wanted to in order to get a truly flat joint and I may now have an issue with some knots that will threaten to tearout when routing off the template I made.

I did, however, build myself a shooting board to make sure the plane was orthogonal to the board faces and to stabilize the whole thing. Man did that save my butt. The other thing I learned with hand planes is your blade has to be SHARP to do it well. I took off way way way too much material (was not just evening siades, but taking off a ton of wood in teh process to get it where I wanted it, not smart) and teh blade started to dull. Not knowing this, given I have no real experience with hand planes, I just pushed harder and harder until I switched it up and shimmed beneath the wood and it suddenly cut like butter again, dead straight, super flat, perfect joint.

It took me 4 hours to take off my material, even out my mistakes, and generally figure out how jointing with a shooting board works, but my first jointed top is a super clean joint with no light passing through between faces. Best of all, to me, the tool fits right back into its little box and onto the shelf and I can use it forever if I take care of it. It felt real real nice to do some "real" woodworking by hand. I've jointed with power jointers many a time and had excellent results, but this was a much more pleasing and visceral experience.

My $0.02 is to try making a shooting board, sharpen your blade, and try again. I've seen your work and I know mine, so I'm convinced you could pull it off.

-Dave

Edited by davee5
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do you own a table saw? if you do i'll tell you about a simple cutting method that insures an almost invisable joint.

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Tops and backs are not that bad. You can use a jointer for the first ruff pass. I have two jointers my 6" is used for most tasks, and the 4" is used only for jointing tops. I also use regular planes if I don't feel like walking to my garage. I take the sides and place them back to back on a flat board and spring clamp them down. Then I use a long square sanding block w/150 grit to make my final touch up. Be sure to hold the sanding block square against the board, sand only in one direction using even pressure. If your jointer or plane was set well. It should take next to nothing to have it spot on. The most important thing is to keep your tools dead square when cutting or sanding, and only go one direction(helps avoid wobble). One other note; when you clamp the set down be sure to place a thin veneer or other flat shim to keep the set slightly above the flat board(you will sand a ridge into the edge closest the board if it is not raised slightly).

Hope that helps,

Rich

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Alternative methods would be to use a router with a laminate trimming bit over something straight or just a rebate bit following a straightedge. A plane and a shooting board also works well once you’re cool with planes.

Also, I saw someone here post an article where he sticks 4 bits of sandpaper to a flat surface and rubs the edges of the 2 halves held together along it to finally true them up, holding them against a metal fence to keep them at 90 degrees

As for gluing up I’ve seen illustrated, a method where you either knock pins into a flat board or use some straight stock exactly the width of your blank then you glue the edge and close it in from the outer edge like closing cupboard doors if you see what I mean – put grease proof paper below and clamp it flat to avoid spring back though

Deepest apologies for the nursery school-esque illustration below :D

Gluing.jpg

Edited by NJD
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I just did the my first acoustic back a few weeks ago. We jointed the edges with a power jointer (buzzer) and it was perfect. I glued it using a process similar to what NJD illustrated above. If you need more details on that, just let me know.

Here are the results of that join:

th_PA100423.jpgth_PA240474.jpg

keep in mind that the centreline is marked with pencil on the second photo.

Regards,

Brian.

do you own a table saw? if you do i'll tell you about a simple cutting method that insures an almost invisable joint.

Please do tell!

Thanks.

Brian

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this method depends on your being able to move your rip fence to either side of your blade and having enough room on the left side to run one of your pieces through. it also depends on your having a straight edge on the outside of each piece so that it will run true on the fence.

1) tilt your blade about two/three degrees off of straight up so that you're cutting a slight angle.

2) set your fence on the right of the blade.

3) set it so that when you're running the wood through it's just taking a quarter of an inch or less off.

4) run the right hand piece of wood through the saw so that it's oriented with the top surface facing up and the edge that will be at the neck pointing away from you. it's important that the top surface is up so that if there is any slight tear out or burring due to the cut it will be on the inside of the guitar.

5) move your fence to the left side of the blade setting it so that it cuts a small amount off of the right hand side of the left hand piece of wood. run that piece through oriented the same way as the right...top surface facing up and the neck edge pointing away from you.

if your blade is sharp you now have two mating edges that basically overlap each other and the resulting seam is almost invisible. you have to take care when clamping not to put too much side to side pressure on the wood. the right hand cut wants to act as a ramp and the left had piece will want to slide up it.

by cutting this way you've also created a slightly larger glueing surface than with a ninety degree cut which theoretically should make the joint stronger.

like anything else you'll want to try this setup on a couple of small pieces of scrap wood and see if it works for you.

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I use a no7 plane and a shooting board - I can get a board ready to join from a rough edge in about 5 mins.

Remeber the joint is far more important on the top as the wood's lighter and lines are easier to see.

Edited by jammy
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First and formost. Remeber these joints are not like a drop top. Where looks are your main issue. It has to be a great structural joint. This is why it is critical to be sure your surfaces mate up extreamly close. Take care to not be decieved by a joint that looks like it mates correctly when you hold it up to the light, but is not square when the halves are flat. If your edge is a tiny bit off 90 degrees you will only joint a fraction of the wood surface and be left with a glue filled joint that is structurally weak(hold that 90 deg. requirement if using unclej's method.).

Unclej's method of using a slight angle to derive a wider gluing surface(this would be a very slight increase, but potentially an improved method). I would have a couple concernes about attempting this style of joint. First it would make it impossible to tell if you had evenly mated surfaces(unless you are able to hold the sides flat and view them at an angle with light behind them). Second you will have to take great care to keep the two halves flat, and press them together without having the joint slip a bit(one side raise). I would be game to try this type of joint, but it would require great attension to detail to ensure it is exicuted correctly.

From my experience. A router is not going to cut it on these joints(no pun intended). It is fine for drop or carve tops, but not soundboards and backs. Other fellas may have better routers than me, and better control. These joints just don't have room for slight skips, this is why even with my jointer or hand plane(that makes a darn clean edges) I still touch the surface up to make absolutely sure the surfaces mate nicely. Like Jammy mentioned it only takes a couple minutes to do this. So be sure to pay extra attension to inspecting the joint(tiny adjustments are fast).

Peace,Rich

P.S. On the back of the guitar a EDIT: "backing strip or brace strip"(internal bracing component) is put in place as a brace/joint enhancement. Along the same lines some people add small diamond shaped braces(thin spruce) to the soundboard seam. Only a few, but it is a method that seems to work well(an option to consider). Mattia may be able to shed additional light on the use of this technique.

Edited by fryovanni
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An argument against using this strip is that you do have to be careful that it doesn't cause splitting of the back board over time (looks like stretch marks along the side of the centre join) if it expands or contracts in varying humidities/temps. True; glues have advanced, but then if that's the case you shouldn't need this further enhancement (in my VERY humble opinion) and many acoustics have held up, and indeed continue to hold up very well without them.

It’s another of the many 'each to their own' aspects I think.

Edited by NJD
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An argument against using this strip is that you do have to be careful that it doesn't cause splitting of the back board over time (looks like stretch marks along the side of the centre join) if it expands or contracts in varying humidities/temps. True; glues have advanced, but then if that's the case you shouldn't need this further enhancement (in my VERY humble opinion) and many acoustics have held up, and indeed continue to hold up very well without them.

It’s another of the many 'each to their own' aspects I think.

I was talking about backing strip not "backstrip". Sorry about the confusion.

As far as having issues with decorative back strips. I have not had issues with mine. I would imagine you could have an issue due to the radius stressing the area if you did not account for the tolerance. Again, though the backing strip (bracing inside the box) is what I am talking about.

Peace,Rich

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I used a power jointer with no issues; I can't see how you can go wrong if your blade is sharp and the jointer is set up correctly. I did brace it up betweeen a trued 2x4 and the fence to keep everything flat and 90° to the blade.

I also installed a back strip (router) that covers the glue joint, and a spruce backing strip on the inside of the back, so the back's glue joint is covered on both sides.

I don't have little diamonds under the top yet, but I will. The glue line on the top's show face is almost invisible. :D

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I was talking about backing strip not "backstrip". Sorry about the confusion.

As far as having issues with decorative back strips. I have not had issues with mine. I would imagine you could have an issue due to the radius stressing the area if you did not account for the tolerance. Again, though the backing strip (bracing inside the box) is what I am talking about.

Peace,Rich

The backing strip is what i meant - if shrinkage occurs they have been known to create these cracks in the back board. how much of an issue this is in this day and age with our more advanced lacquers and glues, i don't know :D

i could only see the decorative 'backstrip' causing issues in very rare circumstances, as you've pointed out :D

Edited by NJD
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I was talking about backing strip not "backstrip". Sorry about the confusion.

As far as having issues with decorative back strips. I have not had issues with mine. I would imagine you could have an issue due to the radius stressing the area if you did not account for the tolerance. Again, though the backing strip (bracing inside the box) is what I am talking about.

Peace,Rich

The backing strip is what i meant - if shrinkage occurs they have been known to create these cracks in the back board. how much of an issue this is in this day and age with our more advanced lacquers and glues, i don't know :D

i could only see the decorative 'backstrip' causing issues in very rare circumstances, as you've pointed out :D

Wow. Does the other back bracing have the same effect? Also do you know if that would apply to a strip that has been sanded to accomodate the back radius or if it would apply to flat strips that had been glued and forced into place? Out of curiosity is the stretching and cracking in the wood itself or is this like what you see in some of the 60's vintage laquers.

Peace,Rich

Edited by fryovanni
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This can happen in the wood. I couldn’t tell you whether the affected backs had shaped backing strips or no. As far as the perpendicular bracing goes, I would expect higher tolerance due to the crossing grains.

Is this something that has happened to you after you built an acoustic, or on a repair job? Or is this information from a book or webpage? I ask because it is plausable depending on how a guitar was constructed, or what finishing materials were used(possibly even due to type of material). The thing is it is not an uncommon practice, and I suspect there would be an underlying reason for it to occur. If I am going to change my methods or possibly need to be aware of an issue. I would like to know more.

Peace,Rich

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I would expect higher tolerance due to the crossing grains.

A properly cut backing strip also has crossing grain. In other words, the direction of the grain is the same as the cross braces (and perpendicular to the grain of the back).

That may be the down fall of the effected guitars or/and an unfortunate cumulative effect by chance of a number of factors mentioned by Rich

I worked in a music shop some years ago and a guy occupied their upper floors with his own guitar repair business (spent more time there hassling him than on the shop floor). I’d seen it on an old steel string in college (the joys of studying cabinet making in an art college) and asked him about it, he told me that it can happen for the reasons I’ve posted above and asked me to check if it did indeed have a backing strip, and it did. I can’t offer any more depth of description other than what he gave me to understand I’m afraid.

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Fair enough. There are a couple of things that come to mind that could cause a problem such as you describel. If you used a flat wide and or thick strip, and then braced (radiused bracing of course). That could start a little tension. Then, (and I believe this could be a likely issue) you attach the back properly radiused. This would leave you with a slightly flat area down the center of the back. If you finish sanded it to make the flat area smoothly transition to the radius of the rest of the back. You would thin the back wood along the edges of the backing. This could introduce a weakened area. This could create issues with brittle finishes, and possibly cause issue with the wood. I do not believe it would relate to the joining of the wood directly, or the shrinkage and contraction due to seasonal humidity (again directly). That is the senerio that came to mind when you were talking about the issue.

Peace,Rich

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First: learning to use a handplane ain't difficult, and it's a good skill to have. Hardwood backs will probably come out fine straight off the tablesaw or similar, but spruce I wouldn't trust. Learning to tune a #5 (perfect size for jointing tops and backs, IMO. Bigger is fine, but a #5 or a #5 1/2 has good heft) and use it to joint tops and backs ain't that hard.

Also, in terms of the tiny, less than 1/8" thick cross-grain spruce cap causing actual problems in a back due to expansion and contraction, sorry, but there are millions of guitars with cross-grain strips that do fine. Any 'proper' woodworker will tell you cross-grain joints are dangerous, should never be used, but that's a rule we break all the time in acoustic guitars, because the thin plates have enough flex in them to compensate. If anything, the braces would be the ones to cause problems (large, inflexible, strong bits of cross-grain glued wood across the ENTIRE width of a back, not changing in length with humidity, while the back does expand and contract); this can happen if you go through huge humidity shifts, but that's not a design flaw, that's a user error. If you're letting your guitar get that dry, you pay for that with cracks. If the whole thing's flexible, it will 'merely' cause the back or top to curve a little more, assuming it's not an actual flat flat-top guitar (which can crack more easily than a radiussed top or back instrument).

The reason I put backing strips in (on the inside of the back) is because I like the way they look, and the fact I tend to route out most of the joint on the other side when I put in the centrestrip. I like having a little insurance.

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