Ash's acoustic rabbit hole

[ADFinlayson]

On 7/17/2020 at 6:44 PM, Andyjr1515 said:

That looks a pretty good result!

I'm not sure which Dremel you have - mine shows the rpm up to 25000.

Some of my bits have criss-cross cutting edges that therefore cut on both sides of the slot.  I'll check when I get a moment whether the size I used for the rosette was one of those.

Andy

I looked mine up on line and it goes up to 25000 rpm too so I guess 17000 is the next speed down which is where I was. The result came out acceptable to me so I'm not going to complain now, I'll just get better bits than the one that came with the circle cutter. 

So the funniest thing happened, the the back braces were shaped to the 15' radius dish and glued to the back in the dish and the back took on the shape of the radius dish, a week or so later I find that actually the braces have now taken on the shape of the back which has cupped in the opposite direction.

Any suggestions on how best to proceed? As I see it I could:

A). bin the whole thing, get a new back and start again
B). plane off the braces shape new braces and try my luck again
C). just glue it on to the radiused sides anyway and hope the glue holds it all together

None of the above are particularly appealing to me 

[Andyjr1515]

22 minutes ago, ADFinlayson said:

So the funniest thing happened, the the back braces were shaped to the 15' radius dish and glued to the back in the dish and the back took on the shape of the radius dish, a week or so later I find that actually the braces have now taken on the shape of the back which has cupped in the opposite direction.

Oh wow - that's odd!  What timber and in what grain orientation were your braces?

First off, I would spray the back (outer) side with water (which should make it expand on that side to bow back again) and then pop it back in the radius dish with the go-bars on it and leave it a couple of days to fully dry.  

Have ALL the braces bent backwards?  Even two thinner taller ones at the heel end?  If it's just the area where the wider shallower ones are, I would add another couple of thinner taller ones in between the two wider ones to hold it in it's re-formed shape.  I reckon once the back is glued, I can't see it can move anywhere and so that would hopefully do the trick.  They don't weigh a lot and you can't see that far back through the sound hole...and they are not sonically functional at all - their job, ironically, is just to hold the back in shape!!

 

[ADFinlayson]

4 minutes ago, Andyjr1515 said:

their job, ironically, is just to hold the back in shape!!

I'll try giving the back a spray now, the braces are quartersawn and very hard maple, left over from a neck blank. The heel end just seems to look flat and it's the bottom area around the two wide braces that seems to cup the other way. I'm doing very little guitar building at the moment so leaving it in the dish for a week shouldn't be a problem. Thanks

[ADFinlayson]

Finally getting back to the OM build after a 13 month hiatus. The back and braces have thoroughly warped so I need to remake that. I bought a new walnut back a few months ago and it's been sat in the workshop with the dehumidifier running ever since so hopefully this one will go better. The sound board hasn't warped so that's a blessing, I put a lot of effort into the rosette. 

Did a bit of work on the sides this evening. Made an end wedge out of an ebony offcut and got that installed. Chiselling that bit out of the sides was very satisfying. I've been doing some work on the neck too so another update tomorrow.

[Andyjr1515]

Hi @ADFinlayson !

Good to see this is back in progress

Ref the new back, I have a few tips that might help.  I'll post them later this morning when I'm back on the desktop.

[Andyjr1515]

Hi again @ADFinlayson

You may well already have this all covered, but a few tips and tricks I've picked up over a number of builds in terms of getting the back to stay at the 15' radius you started off with!

In terms of the challenge, it is a large piece of bookmatched timber where one side is trying to cup one way and the second side is trying to cup the other way.  Left to its own devices, looking edge on down the grain, it is trying to warp into an 'S' shape.  Worse than that, it's pretty strong along the grain...and you need to overcome that too.  So everything you do has to be to make the bits you add to create the shape stronger than the walnut itself.

You have to, basically, beat the walnut into submission  

Thickness

1.9mm maximum.  I've just done the backs on my weird semi I'm building and that has bookmatched walnut, using a 15' dish.  Because I've also had issues keeping the back radius in the past, I now test the 'spring' of the back wood in the dish as I go along.  At 2.5mm, you can't actually hit bottom pressing into the dish.  At 2.2mm, you can, but it's firm thumb pressure.  But once you are below 2mm, all of a sudden it becomes much, much easier to press it into shape with only moderate pressure.  That's where you need to be.

Getting there I am no help to you...with no access to a sander thicknesser, I use all sorts of methods.  If you are using a sander thicknesser, then aim for around 1.85mm.  Anything down to around 1.7mm is still going to be strong enough. 

If you don't have access to a sander thicknesser then, assuming you don't already have one, get a long reach dial-gauge - it is impossible to accurately measure the thickness any other way and the edges are almost always thinner than the rest of the sheet so just checking with standard callipers isn't going to be accurate enough. 

Braces

I would strongly recommend tall thin cross braces over wide flat ones.  This type of thing:

They can actually be quite a bit thinner than these, or carved progressively thinner, but they need to be at least 15mm tall and ideally made from plain maple.  Their height increases the 'I' value enormously and gives huge strength in terms of resistance to bending.  At 15mm high, however thin, they will be difficult if not impossible, to bend by hand.

Then, as you already do, radius the bottoms of the braces in the dish in the positions they are going to be, thus maximising the glue contact bond right round the radius

Cross-grain Jointing Strip

This is best done after the cross braces have been glued.  And, as the name implies, it has to be cross grain to the back timber.  Cut each piece so that it is a close fit up to each cross brace when pressed into the radius dish (I go for a firm push fit into position at the braces both sides) and glue the bottom and both ends before letting fully dry in the go bar deck.  Because it was cut and glued in the radiused state, any tendency for the back to straighten will put these strips in compression...and decent maple doesn't like being compressed, so it stays curved.  I get mine from David Dyke and they are no more than 1.5mm thick...but there is no way that the walnut is going to overcome these little powerhouses!

Hope this helps!

Andy

 

[ADFinlayson]

@Andyjr1515 looks like you've diagnosed my issue successfully. My back was > 2.5mm and the bottom two braces were wide and short, the other error I made was to have the centre reinforcement strip grain running in the same direction as the back grain. Lastly I used curly maple for the back bracing which may or may not have contributed to warpage. So looking back now, everything I did to that back was working against me.

I've got a drum sander so I can be fairly accurate with thickness, I'll go thinner on the next one and use something else for bracing. I've got a load of African mahogany offcuts from previous necks so I think I'll try that and keep them all 6mm wide x 15mm tall.

Thanks for the advice, really helpful!

[Andyjr1515]

1 hour ago, ADFinlayson said:

@Andyjr1515 looks like you've diagnosed my issue successfully. My back was > 2.5mm and the bottom two braces were wide and short, the other error I made was to have the centre reinforcement strip grain running in the same direction as the back grain. Lastly I used curly maple for the back bracing which may or may not have contributed to warpage. So looking back now, everything I did to that back was working against me.

I've got a drum sander so I can be fairly accurate with thickness, I'll go thinner on the next one and use something else for bracing. I've got a load of African mahogany offcuts from previous necks so I think I'll try that and keep them all 6mm wide x 15mm tall.

Thanks for the advice, really helpful!

Sounds good.  You will be able to carve an ellipse into the braces to save weight - as long as the total height isn't affected, they will stay as rigid as they were as rectangles

[ADFinlayson]

Bit of neck progress. I made a 13º scarf joint first by bandsawing along the line then thicknessed the headstock piece down to 13.5mm, then double sticked the two pieces together and got the joint finalised with my no5 plane. So far so good, but then I used cocktail sticks as locator pins while glueing and clamping, the cocktail sticks snapped and the headstock slid down hill, so I hurriedly tried screws and the headstock part of the joint split and I threw it across the room and did it all again. Second time around I used some 2.5mm aluminium rod that I was using for side dots which was successful.

Sticking with offucts and cheap timbers (the neck is made from offcuts from previous neck builds) I'm using cheap ebony faceplates I got from Maderas Barber a while back, I think these faceplates were £8 for pieces long enough to get two face plates out of. I thinknessed one half to 1.5mm to make the front and thicknessed the other half to .5mm for a veneer to go on to the back, figured it would look cool, mask the ears I glued on and add some strength... Absolutely nothing to do with my scarf joint being a touch lopsided

I decided to have headstock access for the truss rod because it would make life easier not having to drill a perfectly aligned hole in the transverse bar and I was thinking about doing a butt joint and bolt on contruction but the shorted truss rod I could find at a reasonable price was 360mm about 1/2" too long (the line in the centre of the next pic denotes the 14th fret) so I think I'll have to either make a small recess in the neck block to house the end of the truss rod, or make the neck a heel a mortice and tenon joint - Option 1 sounds easier.

I got some threaded steel bushings to get into the neck joint, M8 on the inside so some m8 hex bolts or similar should do to bolt it on. 

 

I've got the headstock rouged out and I just need to glue the heel block on then I think I'll leave the neck until I've got the top braced and glued on so I can determine what angle I need to cut the heel to. It's funny, when building electric guitars I always find it's easier to build the neck in it's entirety and build the body to suit, acoustics seem to be the opposite way round.

[ADFinlayson]

1 hour ago, Andyjr1515 said:

Sounds good.  You will be able to carve an ellipse into the braces to save weight - as long as the total height isn't affected, they will stay as rigid as they were as rectangles

Thanks Andy, do you know of any reason why I can't use mahogany to the brace the top? I don't have any soft wood that would be suitable without buying something. 

[Andyjr1515]

45 minutes ago, ADFinlayson said:

Thanks Andy, do you know of any reason why I can't use mahogany to the brace the top? I don't have any soft wood that would be suitable without buying something.

On the one hand, the characteristics of the two woods will be different.  However, I'm pretty sure the scooping of the braces when you are tap tuning the top changes the flex much more than that anyway and so I can't see an issue.  It will just end up with a little less scooping or a little more - but as there are so many other things that affect the flexibility I'm pretty sure there would be an imperceptible difference. 

[ADFinlayson]

7 hours ago, Andyjr1515 said:

On the one hand, the characteristics of the two woods will be different.  However, I'm pretty sure the scooping of the braces when you are tap tuning the top changes the flex much more than that anyway and so I can't see an issue.  It will just end up with a little less scooping or a little more - but as there are so many other things that affect the flexibility I'm pretty sure there would be an imperceptible difference. 

Thanks, mahogany braces it is. 

You were totally right about back thickness, once it was down to 2mm I could really feel it start to flex and wobble. Glued it up and went down just under 2mm, My caliper was measure between 1.7-1.85mm (I think the drum sander needs some tuning up). Got it glued up over lunch and made some serious progress this evening.

Cut out and shaped my back braces on the dish - 15x6mm. I couldn't find anything suitable with the grain going in the other direction so I just used the offcut from when I cut the back out and it was just long enough with a few mm to spare, actually I think it looks pretty cool!

Dug the gobar deck out of the shed.

Decided to stop before the last brace, it was starting to look like spaghetti and I only had 3 gobars left and I don't want to rush the clamping 

Also finished shaping the headstock, the veneer on the back came out pretty well though there is a bit of a split at the bottom I think I'm going to have to dust and glue if I don't carve through it. 

[ADFinlayson]

Tonights antics was all about top bracing. I cut all my braces out of quartersawn mahogany to 13mm x 6mm. I made them narrower on purpose than the plans, rationale being that it's a harder wood so braces can be thinner - I may or may not regret that later. Then I flipped the dish over to the 28' side and radiused them all apart from the transverse brace (I think that's what it's called). 

I cut the half lap joint with a fret saw and chisel, it was fairly tight but there was still a little bit of play so I clamped them to my mdf template to make sure they stayed at the correct angle.

Then once that had set, I glued a 2mm strip of ebony over the top of the joint for some reinforcement

Then I radiused the x brace on the dish - Next time I do this I think I'll rough in the radius on the braces before I glue them up because this was time consuming 

Next I used a scalpel to cut a load of holes out of another copy of the plan so I can draw the locations of the braces on the underside of the soundboard. You may notice that the plan is slightly forward on the soundboard in the below pic, the reason for that is that I decided to make the guitar scale 25" instead of the traditional 25.4" for no other reason than I have a fret scale ruler for 25" and I don't to go down the road of manually measuring for my fret slots, so I moved the sound hole and the brace patter forward by 10mm. Another thing I may or may not regret later. 

 

I still need to make the bridge reinforcement strip and glue in the transverse brace, I will do that brace seperately off the dish because I want to keep that section flat so the fretboard has a flat service to stick to, I'll need to flatten that area of the sides to match. 

I was using a razor blade to clean up glue squeeze out which worked really well, but once it started getting busy it was impossible to reach all areas so I chiseled a point into a piece of dowel which worked nicely to get to all the hard to reach areas. 

Lastly I wasn't too sure about those little braces around the sound hole, I figured they were just slight reinforcement for that area so I thicknessed them down to 2mm before glueing them down. I still need to do the one just above the x brace but I couldn't reach to mark out the mitres so it'll have to wait until the weekend.

Really enjoying getting back to this build, it's both exciting and calming all at the same time. Remind me I said that when it comes to making and locating the bridge.

[Andyjr1515]

4 hours ago, ADFinlayson said:

Lastly I wasn't too sure about those little braces around the sound hole, I figured they were just slight reinforcement for that area so I thicknessed them down to 2mm before glueing them down.

Exactly right.  Especially if you fit a hidden sound hole volume/eq which tend to sit over these braces with a clearance notch built in.

[ADFinlayson]

2 hours ago, Andyjr1515 said:

Exactly right.  Especially if you fit a hidden sound hole volume/eq which tend to sit over these braces with a clearance notch built in.

Thanks for the tip, I was originally planning to put a pickup in this one but I've since changed my mind. I'm not expecting my first acoustic to be world changing so I'm going to save the fancy electrics for the next one. P.S Andy where do you get your bracing materials? When I search all my usual sources, I can't find anyone that sells any spruce. 

[Andyjr1515]

25 minutes ago, ADFinlayson said:

Thanks for the tip, I was originally planning to put a pickup in this one but I've since changed my mind. I'm not expecting my first acoustic to be world changing so I'm going to save the fancy electrics for the next one. P.S Andy where do you get your bracing materials? When I search all my usual sources, I can't find anyone that sells any spruce. 

I get all of the acoustic internal woods from David Dykes Luthier Supplies.  If the link works, those components are here.  Bear in mind the prices are pre VAT and shipping. 

Soundboards (luthierssupplies.co.uk)

 

 

[ADFinlayson]

1 hour ago, Andyjr1515 said:

I get all of the acoustic internal woods from David Dykes Luthier Supplies.  If the link works, those components are here.  Bear in mind the prices are pre VAT and shipping. 

Soundboards (luthierssupplies.co.uk)

 

 

Thanks Andy! Next question... My bridge reinforcement strip, I was going to make out of 3mm ebony, should I radius that on the dome before glueing it on or keep if flat? I'm assuming that I will need to sand the underside of the bridge to match the radius of the soundboard anyway.

[Andyjr1515]

43 minutes ago, ADFinlayson said:

Thanks Andy! Next question... My bridge reinforcement strip, I was going to make out of 3mm ebony, should I radius that on the dome before glueing it on or keep if flat? I'm assuming that I will need to sand the underside of the bridge to match the radius of the soundboard anyway.

That's a good question because pretty much no plan or guide tells you!

The first two I built, I left them flat, but now I radius them in the dish (it's a relatively small amount of sanding in this position on the dish).  And yes - you will be matching the shape of the top with the shape of the bottom of the bridge.  I use the 'engineer's blue' method, using blackboard chalk instead of blue.

Shout when you come to fixing the bridge - I have a few hints and tips I've learnt along the way that might help.

[ADFinlayson]

12 minutes ago, Andyjr1515 said:

That's a good question because pretty much no plan or guide tells you!

The first two I built, I left them flat, but now I radius them in the dish (it's a relatively small amount of sanding in this position on the dish).  And yes - you will be matching the shape of the top with the shape of the bottom of the bridge.  I use the 'engineer's blue' method, using blackboard chalk instead of blue.

Shout when you come to fixing the bridge - I have a few hints and tips I've learnt along the way that might help.

Thanks! rest assured I will be asking for tips when it comes to bridge!

[ADFinlayson]

made myself a bridge reinforcement out of a bit of ebony, 2.5mm thick then radiused it on the dish, it's slightly bigger than the one on the plan to give me a it of leeway for bridge placement but it's a bit thinner too, figured it doesn't need to be that thick considering the density

Also did a load of brace shaping. I'd like to say I voiced the top but really I just went at them with a chisel until they looked cool. I was testing the sound but resting the top of the sound hold on my thumb, ear up to the front while knocking the bridge plate with my finger. I was definitely noticing changes to the sound after reducing the braces but I have no idea what I'm actually listening for. 

After all that brace shaping, I glued on the transverse brace and the sound hole brace that was missing, out of the gobar this time because I want the transverse bit to remain flat.

I did a bit of research on the google today and found the off forum posts that say mahogany is not a good bracing material due it's weight to stiffness, i.e it's not particularly hard for a hard wood and hard woods are heavy, and that spruce is the best bracing material because although it is a softwood it has the greatest stiffness to weigh ratio. But that being said, mahogany is a still a stiffer wood that spruce, it's just heavier and I think I must have made the right decision making them narrower at 6mm instead of 8mm. I've also taken off quite a lot of material so they aren't that tall anymore yet the spruce is still taking on the shape of the dish. I wonder how much of this is actually relevant in the real world and how much is just purist talk. 

[Alucard0811]

Really like this build.

I have hugh respect for people able to build accoustics. I want to build one someday also but so many jigs and special skills you need

[mistermikev]

nice work... making it look easy is the mark of true skill.

[ADFinlayson]

On 8/29/2021 at 12:04 AM, mistermikev said:

nice work... making it look easy is the mark of true skill.

Thanks Mike, the beauty of still photos is it looks a lot easier than it is, had there have been video, it would be rather sweary. 

On 8/28/2021 at 2:20 PM, Alucard0811 said:

Really like this build.

I have hugh respect for people able to build accoustics. I want to build one someday also but so many jigs and special skills you need

Thank you, I have a few jigs for this - I bought the radius dish and the OM mould, made the gobars and deck, but no special skills. I have been winging it for a few years and after enough builds some of it just becomes muscle memory, This build however is a step or 10 into the unknown and fully back to winging it. 

[mistermikev]

"sweary" - love it.

[ADFinlayson]

Shaped the top and back braces

Then drilled a couple of hols into the heel block 8.5mm to take M8 screws, I figured a little bit of play might not be a bad thing if I have to tweak neck angles, but that is a guess.

Now came the most difficult bit of the build I found so far which was to notch out the sides where the braces go. I tried to put the sides back in the mould but I struggled to get them to go back in nicely, I guess there may have been a bit of movement over the last year, they have possibly got a hair off square and although I could get them in, they seemed under tension and If I notched the brace locations in the mould, that tension would stay in the guitar forever and it seemed better to locate the notches in it's relaxed state.

So I sat the top on top of the sides locating centre (awkward to get it not to move around), marked the notches with pencil, then used my measure gauge to mark the heigh of the braces so I knew how deep to notch, fret saw to cut to depth then small chisel to carefully remove the material from both sides, outside in and finessed with a jewellers file. had a little bit of slop left to right but figured the important part of the joint was probably top and bottom. x brace notches were by far the most awkward. 

I had it sat on the dish when I did this which helped to keep it still.  

Next was the biggy! glued on the top in the gobar deck, I had the radius dish with 28' radius facing up and glued it top side down to help prevent squeeze out making a mess of the sides. Did a dry run first then glued it and left it in the deck overnight. I noted that the back of the bottle of titebond says clamp for 30 mins but leave 24 hours before stressing the joint - I'm bending the spruce over the radius of the sides which I considered to be "Stressing the joint" 

You may notice I only notched for one of the braces on the back. I decided to stop notching the back just in case anything changed dimensionally with the glue up.

 

glue up was a roaring success, band sawed off the excess then used my trim router with a new flush trim bit to flush the spruce with the sides. I did it hand held because I wanted to climb cut to prevent this insanely soft wood tearing out and doing it on the table would no doubt throw it across the room. I want to reserve that privilege for myself if it implodes.

Close up of some notches, not perfect but I reckon acceptable

At this point I'm considering neck break angle, you can see it's bang on 1.5º but the very front is just a hair off square. I figured I would get the back on then true this up with some sanding and take another measurement as it's bound to affect the neck angle slightly.

I also did some more tapping now that the top was attached to the sides. Holding it with the sound hole balance on my thumb, listening to the front around the tone bars and tapping the back at the bridge plate there is a definite bong sound which seems like it might be alright but I noticed there doesn't appear to be a huge amount of flex if I push against the top. I've got a couple of guitars here to compare, a cheap washburn nylon which has no flex whatever, a taylor gs mini which has no flex and a Camps nylon which has a bit of flex. So I'm still none the wiser   so after shaving a bit more off those mini braces on the bass side, I'm going with it.

Notched out the back braces and got her glued up.

Used all 28 gobars as it needed a little more persuasion to get round the tighter dome of the back. Left in the deck overnight and will see how it went in the morning.

Oh 2 things I forgot to mention. 1. I made 2 vertical braces on each side to braces the sides, I don't know if that's enough, they're 1.5mm thick ebony. 2. I made a bit of a chamfer on the top and the bottom of the end block to create a smaller glueing surface to hopefully enable less stiffness in the top and back. I don't know if that will help tonally or not.