Soldato
- Joined
- 21 Jan 2010
- Posts
- 3,892
heh, I'll just buy and fit new ones- they're only a few pounds.
Maybe I'll have to stop playing in weird tunings? Nah, not gonna happen!
***The Official Guitar Thread***
- Thread starter Nitefly
- Start date
heh, I'll just buy and fit new ones- they're only a few pounds.
Maybe I'll have to stop playing in weird tunings? Nah, not gonna happen!
Soldato
- Joined
- 28 Dec 2017
- Posts
- 9,863
- Location
- Beds
Try fitting tuners that cost a bit more
Schaller used to make some nice ornate ones, they're probably still around.
Try fitting tuners that cost a bit more
Schaller used to make some nice ornate ones, they're probably still around.
tasty 1:18 and 1:14 too..
So I'm finally coming towards the end of the researching step. Designing means you need to know the end goal, precisely, and from that work backward.
You need to have almost a frequency spectrum in your head of what the end goal with be. In my case a personal jazz/blues acoustic - sounds simple doesn't it? The reality is fine until you ask yourself to define it further - what is "personal" what form of "jazz/blues" and it's at this point you start to see how deep the rabbit hole is. I'll abbreviate here to respect TL;DR.
Personal = I don't need/want it to be heavily projecting the volume to the back of a hall filled with loud instruments. I just want it 'in the moment' where I can hear it intimately. This has some steer - I don't need to trade off to achieve volume in the structure, or need directive beaming of sound from it. I can concentrate on tonal balance, care a little less about susceptibility of feedback from amplifiers vibrating the top and back. It also means less fatiguing playing so less boom and fizz - this doesn't mean being dull, far from it, but it means setting a "I don't want shrill" (although all acoustics calm slightly as they break in, so some forwardness is preferred initially). I'll also want a more 'personal' sound port on the top bout, kind of like having your own personal monitor speaker.
jazz/blues = this means I can focus tone tone (the response to frequency) and how fast I want the guitar to start vibrating (response) and how fast I want the vibrations to decay (sustain) which forms how it will drives more or less resonance. So do I want percussive jazz, modern jazz, what style of blues do I want to play - which drives the focus of strumming, picked hard and soft and the sound profile (do I want mid focus for example).
All these points start building a target tone I want.
Design isn't precise, it's like pointing a direction when you start out - choosing woods that align based on stiffness, strength, etc, then moving on to the shape/geometry and slowly iterating towards that target tone.
Point 1. So the first thing to throw way at the door of the rabbit hole, is the concept of a "single pitch" or a single frequency. Everything you do occurs across the entire frequency spectrum - so you get the note, harmonics, and with the picked string creating a traveling wave, it then has even more complexity in that spectrum. Think a box low to high each time someone mentions "tone", "pitch" or "target frequency" because they may be adjusting one part of the box but the rest of the box will change too.. There is no spoon.. it is you that bends..
Point 2. A plucked string has a small amount of energy, the guitar needs some size to make that amplified - the larger the guitar the better than can be done and the lower the note (lower notes have less energy than high). So keep this in mind - a thinner wood is more flexible and takes less energy to move, a thicker wood takes more energy. The more sound you make the more energy you lose across that box of frequencies. I fret an A note, it's got energy at 110Hz (the A itself) but then had a myriad of energy across other frequencies in the box, both at harmonics like 220Hz 440Hz etc. This becomes important - if we move too much at one frequency and hit a natural resonance we get a big "wolf note" where that note is louder than others. If we lack support for that frequency compared to average we'll get a "dead note". Across the range if we have more in certain notes we could favour notes close to different scales - for example if it supported notes in commonly associated in the playing of haunting music, it could end up sounding great for Dracula but not for more upbeat keys.
Next up - what I'm erring towards building, and why in simple description.
17" lower bout. This is a compromise balance between sound and comfort/ease of playing. An upper bout of 13" and a waste of about 11". The body length at the moment is around 20" (still look at this). This provides a good basis for the frequency response, it lacks some support for the lows and lacks some ability to create volume.
To cut out or not - that's the upper bout shaping to access higher frets. This would be useful to be honest and I'll probably target 20 frets. The point that the body will join will align with the 14th fret and the fretboard will float above the top.
Arch hight is a trade off - higher arch (20-30mm) means more focused/beamed sound (louder in a direction), more support for percussive/hard strummed playing, more structural stiffness means less support for lower modes so we loose low frequency and out guitar sounds more mid/treble focused. It's not the only mechanism adjusting our box of frequencies but as I'm less strumming in billy bob jazz, I don't need a high arch.
A lower arch - say 10-14mm would then present a more relaxed tone, support deeper notes, create a mild mid hump which can be tuned further. The less structural stiffness will give a more open balanced tone options later but we will need to support the bridge with structs as the arch isn't supporting the downward pressure of the strings. The thinner top will sound less 'smooth' and more 'detailed' but that can be tuned later with bracing unlike the higher arch.
For this reason I'm considering a med-low arch, possibly around 12mm. Higher also makes the neck/floating fretboard more of a challenge.
The cross profile shape of the arch is important too - there's two main camps:
(a) thick centre that thins toward the outer edge - this makes a stiff centre that operates like a speaker cone, the down side is a thicker top leans towards treble and less bass, it also takes more energy to start moving (less responsive) and thus provide more heavy strumming headroom, but once moving takes longer to stop (more sustain). It can sound smoother as a result of this slowed response to start and stop.
(b) thin centre that thickens towards the outer edge - this allows a top that starts with bass support, it will be more responsive (so quiet and loud) but less heavy playing headroom means it can saturate and distort the sound, the top will also transfer energy to the air quickly and so will stop moving quickly (less sustain but also allows the end of the sound on the string end to be clearer). It can sound less smooth in terms of response and can therefore pickup string sounds too. The less mass, the less sustain but ironically the top will move more with overtones.
I've decided that a thin centre is better for the guitar top, but I will need to listen to the wood and find out what comes out of it best. It's important to note that the top is three dimensional, so you can have a thicker and thinner portions of the top but getting to that is iterative. The back I'm still researching but my tendency here with the maple is to use a thicker centre (the back will be thicker than the front anyway for tuning reasons and contact reasons).
Bracing structure as settled on two main camps for wood bars stuck on that support the top from the string bridge force but also provide additional stiffness to change the response of the top and back:
(a) 'parallel bracing' which is any form that has strong parallel bars that are positioned along the line from neck to tail. This is the older form, and the sound effect is to stiffen the top against low notes (lower 'modes' of top vibration) and creates strong mid-upper sound that is 'punchy' (ie it amplifies the effect of higher arch volume and beaming). If you want a hard playing loud guitar that is 'tuned' to sit within an ensemble on the dance hall, going "thunk thunk" this is where parallel shines. The more mass here the more energy and sustain will appear allowing the sound to be supported in a larger environment.
(b) 'X bracking' is the more modern approach, as amplification can cope with the volume, the box off frequencies can be allowed to be more balanced across the range. The bracing style supports more subtlety, faster response. The crossed braces provide a more balanced support across the top and so each 'mode' of the top gets equal support that can then be tuned. The X gap at the bottom then allows the low notes (modal vibrations) the lower bout to shine and you get more bass extension.
I've talked about 'modes' - this is where, like a skipping rope, you get a bow in the vibration (lowest mode) then as you get harmonics more mode patterns appear. This picture of a top's modes gets complex quickly but the mode support then creates peaks in the box of frequencies we call "tone".
Additional bracing is normally added to these to tune the top's response changing the shape of the mode on the top. If you think of a top's modes being the hills and valleys across the top. Tuning helps cleans these ups making them more pronounced (better support for the tone peak caused), and moves the position of these modes across the top.
Even the kerfling/lining that supports the top/side side/back joints play a role in changing the modes.. so you can't tune the top then assemble because everything changes! As a general trend, each assembly stage sees a the tonal box of frequencies shift down in pitch. Freaky but it's akin to playing darts in the dark.
I'll be looking at X bracing as a basis, although I'm not looking for entirely a flat-top acoustic tone it's a good target to work towards.
F-holes = a space for air inside to escape, but their size and position has an effect on our box of tone, but they also exist to reduce the stiffness of the top presented to the bridge (and less mass to move) which affects our tone. However the size has a paradoxical effect:
Small f-holes change the guitar's tone to a lower frequency tone.
Larger f-holes change the guitar's tone to a higher frequency tone.
The position is usually at a point where these modes exist and so the air is moving in and out of the hole due to the compression caused by the mode's peaks and toughs (bonus point is that the you can have a peak and troughs cancel out across the air hole! Let's gloss over that complexity..). However, typically the higher frequencies modes can be harnessed closer to the edge by holes, whereas the modes that support lower, deeper longer vibrations and "Helmholtz' are better serviced by having a central sound hole where you see them in acoustic guitars.
I'm leaning towards having a top hole - providing sound towards the player, with smaller f-holes however I have a thin top, and an X brace supporting the bridge.
So why 'tune' or tap wood?
There's a couple of reason why people 'tune' or listen to the tone of the wood:
1. Critically - there's a key point in the interaction between the top and the back of a guitar. The back is tuned higher than the front by between 3-5 semitones to prevent the front and back from resonating and creating wolf notes. There's also discussions about tuning for fitfths and octaves - this relates to ensuring that modal frequencies (notes) isn't going to interact heavily with fretboard notes where the guitar starts sounding wolf notes.
2. Tuning the top/back to create a ringing sound creates the clearest resulting TONE (not I don't say note or pitch here) from the resulting guitar. The tapping and testing of the modes of the top attempt to make the best sounding piece of wood (both reinforcing good but calming the bad).. then adding braces, again the best sounding side... etc it's iterative loop moving towards a pleasing target sound.
3. Tapping the wood before you even start can quickly pick wood that rings naturally. If you're mass producing, then you can use bracing to get the required tone out of the wood. However by luck finding a stunning ringing log of wood, a quarter-sawn wedge that rings means you're starting with something good and working on that, the best sounding violins have come from wood that rang like a bell when felled. Acoustic guitars are no different in this respect.
The guitar's scale length is longer, so the strings will have more mass and tension. This changes the box of tone applied to the above. I get more articulate bass notes, heavier strings vibrating, but get less harmonics on the upper notes which is why people state baritones have colder/shrill upper notes.. but that really depends on the guitar supporting it (my baritone solid body sound clean and clear upper notes compared to my strat) and the amplification (especially as a large number of pickups are designed to boost highs/upper mids). Tone is important and the strings/scale impart the starting point of that tone. This means whatever I build really needs to iterate from what the strings give. Lastly - ergonomics.. adding cutouts, etc all make the guitar modes behave differently, so these will need careful thought and inclusion during the process.
I found that in my previous guitar, it's easier to follow the wood. Both in grain of carving but the tonal response that comes out, hence I have:
* European spruce arch tops - this rings like a bell in its current wedge, not a dull thud! This will give a clean balanced tone. X braced with spruce should make for a decent clear, balanced tone.
* Maple arched back - this also was selected on its tonal ring vs dull others. This will support mids and upper, with the back tending to be thicker. This naturally loves mids/mid upper.
* Maple sides - these unexpectedly have flame, but they're quite thin hence the need for additional supports. Especially on the bottom and arm over positions. The sides offer some tonal change but they're typically a weak spot in acoustics.
* Willow lining (this is the better sounding variation of the slotted kerfling supports you see if you look into the guitar - down side is it needs time to heat and bend), the sides will have structs too to prevent the sides splitting.
* Spruce bracing - quarter sawn, straight grain etc to give the best rigidity to weight, there's another subject on shaping/scalloping (adjusting the stiffness) but that's really given by following the modes and ring.
* Laminate neck - this rings like a xylophone block, but is extremely heavy - so this is why I will have a 'spar' of the same material providing both string tension support to the tail and to provide weight to prevent neck dives. The neck will have both a truss rod but also carbon fibre rods, the idea is that the design will use the carbon fibre to carry through into the spar along the length of the guitar. The carbon fibre and spar will help stop any humidity movement and as this will have a thinner neck, it will help.
* bubinga fretboard - this worked nicely on the solid body so I will use the remnant of that on this.
* neck block and end blocks - combination of laminate, hard maple.
* the tail 'trapeze' will be wood, but this and the bridge can be modified to adjust the tone being sent to the top.
* bridge - initially hard maple, but I'll adjust over time (the bridge will be floating).
Frets will be smaller and non-jumbo frets. I'm leading towards 2mm nickel frets but the stainless steel works well with the solid body so the decision is still out..
You need to have almost a frequency spectrum in your head of what the end goal with be. In my case a personal jazz/blues acoustic - sounds simple doesn't it? The reality is fine until you ask yourself to define it further - what is "personal" what form of "jazz/blues" and it's at this point you start to see how deep the rabbit hole is. I'll abbreviate here to respect TL;DR.
Personal = I don't need/want it to be heavily projecting the volume to the back of a hall filled with loud instruments. I just want it 'in the moment' where I can hear it intimately. This has some steer - I don't need to trade off to achieve volume in the structure, or need directive beaming of sound from it. I can concentrate on tonal balance, care a little less about susceptibility of feedback from amplifiers vibrating the top and back. It also means less fatiguing playing so less boom and fizz - this doesn't mean being dull, far from it, but it means setting a "I don't want shrill" (although all acoustics calm slightly as they break in, so some forwardness is preferred initially). I'll also want a more 'personal' sound port on the top bout, kind of like having your own personal monitor speaker.
jazz/blues = this means I can focus tone tone (the response to frequency) and how fast I want the guitar to start vibrating (response) and how fast I want the vibrations to decay (sustain) which forms how it will drives more or less resonance. So do I want percussive jazz, modern jazz, what style of blues do I want to play - which drives the focus of strumming, picked hard and soft and the sound profile (do I want mid focus for example).
All these points start building a target tone I want.
Design isn't precise, it's like pointing a direction when you start out - choosing woods that align based on stiffness, strength, etc, then moving on to the shape/geometry and slowly iterating towards that target tone.
Point 1. So the first thing to throw way at the door of the rabbit hole, is the concept of a "single pitch" or a single frequency. Everything you do occurs across the entire frequency spectrum - so you get the note, harmonics, and with the picked string creating a traveling wave, it then has even more complexity in that spectrum. Think a box low to high each time someone mentions "tone", "pitch" or "target frequency" because they may be adjusting one part of the box but the rest of the box will change too.. There is no spoon.. it is you that bends..
Point 2. A plucked string has a small amount of energy, the guitar needs some size to make that amplified - the larger the guitar the better than can be done and the lower the note (lower notes have less energy than high). So keep this in mind - a thinner wood is more flexible and takes less energy to move, a thicker wood takes more energy. The more sound you make the more energy you lose across that box of frequencies. I fret an A note, it's got energy at 110Hz (the A itself) but then had a myriad of energy across other frequencies in the box, both at harmonics like 220Hz 440Hz etc. This becomes important - if we move too much at one frequency and hit a natural resonance we get a big "wolf note" where that note is louder than others. If we lack support for that frequency compared to average we'll get a "dead note". Across the range if we have more in certain notes we could favour notes close to different scales - for example if it supported notes in commonly associated in the playing of haunting music, it could end up sounding great for Dracula but not for more upbeat keys.
Next up - what I'm erring towards building, and why in simple description.
17" lower bout. This is a compromise balance between sound and comfort/ease of playing. An upper bout of 13" and a waste of about 11". The body length at the moment is around 20" (still look at this). This provides a good basis for the frequency response, it lacks some support for the lows and lacks some ability to create volume.
To cut out or not - that's the upper bout shaping to access higher frets. This would be useful to be honest and I'll probably target 20 frets. The point that the body will join will align with the 14th fret and the fretboard will float above the top.
Arch hight is a trade off - higher arch (20-30mm) means more focused/beamed sound (louder in a direction), more support for percussive/hard strummed playing, more structural stiffness means less support for lower modes so we loose low frequency and out guitar sounds more mid/treble focused. It's not the only mechanism adjusting our box of frequencies but as I'm less strumming in billy bob jazz, I don't need a high arch.
A lower arch - say 10-14mm would then present a more relaxed tone, support deeper notes, create a mild mid hump which can be tuned further. The less structural stiffness will give a more open balanced tone options later but we will need to support the bridge with structs as the arch isn't supporting the downward pressure of the strings. The thinner top will sound less 'smooth' and more 'detailed' but that can be tuned later with bracing unlike the higher arch.
For this reason I'm considering a med-low arch, possibly around 12mm. Higher also makes the neck/floating fretboard more of a challenge.
The cross profile shape of the arch is important too - there's two main camps:
(a) thick centre that thins toward the outer edge - this makes a stiff centre that operates like a speaker cone, the down side is a thicker top leans towards treble and less bass, it also takes more energy to start moving (less responsive) and thus provide more heavy strumming headroom, but once moving takes longer to stop (more sustain). It can sound smoother as a result of this slowed response to start and stop.
(b) thin centre that thickens towards the outer edge - this allows a top that starts with bass support, it will be more responsive (so quiet and loud) but less heavy playing headroom means it can saturate and distort the sound, the top will also transfer energy to the air quickly and so will stop moving quickly (less sustain but also allows the end of the sound on the string end to be clearer). It can sound less smooth in terms of response and can therefore pickup string sounds too. The less mass, the less sustain but ironically the top will move more with overtones.
I've decided that a thin centre is better for the guitar top, but I will need to listen to the wood and find out what comes out of it best. It's important to note that the top is three dimensional, so you can have a thicker and thinner portions of the top but getting to that is iterative. The back I'm still researching but my tendency here with the maple is to use a thicker centre (the back will be thicker than the front anyway for tuning reasons and contact reasons).
Bracing structure as settled on two main camps for wood bars stuck on that support the top from the string bridge force but also provide additional stiffness to change the response of the top and back:
(a) 'parallel bracing' which is any form that has strong parallel bars that are positioned along the line from neck to tail. This is the older form, and the sound effect is to stiffen the top against low notes (lower 'modes' of top vibration) and creates strong mid-upper sound that is 'punchy' (ie it amplifies the effect of higher arch volume and beaming). If you want a hard playing loud guitar that is 'tuned' to sit within an ensemble on the dance hall, going "thunk thunk" this is where parallel shines. The more mass here the more energy and sustain will appear allowing the sound to be supported in a larger environment.
(b) 'X bracking' is the more modern approach, as amplification can cope with the volume, the box off frequencies can be allowed to be more balanced across the range. The bracing style supports more subtlety, faster response. The crossed braces provide a more balanced support across the top and so each 'mode' of the top gets equal support that can then be tuned. The X gap at the bottom then allows the low notes (modal vibrations) the lower bout to shine and you get more bass extension.
I've talked about 'modes' - this is where, like a skipping rope, you get a bow in the vibration (lowest mode) then as you get harmonics more mode patterns appear. This picture of a top's modes gets complex quickly but the mode support then creates peaks in the box of frequencies we call "tone".
Additional bracing is normally added to these to tune the top's response changing the shape of the mode on the top. If you think of a top's modes being the hills and valleys across the top. Tuning helps cleans these ups making them more pronounced (better support for the tone peak caused), and moves the position of these modes across the top.
Even the kerfling/lining that supports the top/side side/back joints play a role in changing the modes.. so you can't tune the top then assemble because everything changes! As a general trend, each assembly stage sees a the tonal box of frequencies shift down in pitch. Freaky but it's akin to playing darts in the dark.
I'll be looking at X bracing as a basis, although I'm not looking for entirely a flat-top acoustic tone it's a good target to work towards.
F-holes = a space for air inside to escape, but their size and position has an effect on our box of tone, but they also exist to reduce the stiffness of the top presented to the bridge (and less mass to move) which affects our tone. However the size has a paradoxical effect:
Small f-holes change the guitar's tone to a lower frequency tone.
Larger f-holes change the guitar's tone to a higher frequency tone.
The position is usually at a point where these modes exist and so the air is moving in and out of the hole due to the compression caused by the mode's peaks and toughs (bonus point is that the you can have a peak and troughs cancel out across the air hole! Let's gloss over that complexity..). However, typically the higher frequencies modes can be harnessed closer to the edge by holes, whereas the modes that support lower, deeper longer vibrations and "Helmholtz' are better serviced by having a central sound hole where you see them in acoustic guitars.
I'm leaning towards having a top hole - providing sound towards the player, with smaller f-holes however I have a thin top, and an X brace supporting the bridge.
So why 'tune' or tap wood?
There's a couple of reason why people 'tune' or listen to the tone of the wood:
1. Critically - there's a key point in the interaction between the top and the back of a guitar. The back is tuned higher than the front by between 3-5 semitones to prevent the front and back from resonating and creating wolf notes. There's also discussions about tuning for fitfths and octaves - this relates to ensuring that modal frequencies (notes) isn't going to interact heavily with fretboard notes where the guitar starts sounding wolf notes.
2. Tuning the top/back to create a ringing sound creates the clearest resulting TONE (not I don't say note or pitch here) from the resulting guitar. The tapping and testing of the modes of the top attempt to make the best sounding piece of wood (both reinforcing good but calming the bad).. then adding braces, again the best sounding side... etc it's iterative loop moving towards a pleasing target sound.
3. Tapping the wood before you even start can quickly pick wood that rings naturally. If you're mass producing, then you can use bracing to get the required tone out of the wood. However by luck finding a stunning ringing log of wood, a quarter-sawn wedge that rings means you're starting with something good and working on that, the best sounding violins have come from wood that rang like a bell when felled. Acoustic guitars are no different in this respect.
The guitar's scale length is longer, so the strings will have more mass and tension. This changes the box of tone applied to the above. I get more articulate bass notes, heavier strings vibrating, but get less harmonics on the upper notes which is why people state baritones have colder/shrill upper notes.. but that really depends on the guitar supporting it (my baritone solid body sound clean and clear upper notes compared to my strat) and the amplification (especially as a large number of pickups are designed to boost highs/upper mids). Tone is important and the strings/scale impart the starting point of that tone. This means whatever I build really needs to iterate from what the strings give. Lastly - ergonomics.. adding cutouts, etc all make the guitar modes behave differently, so these will need careful thought and inclusion during the process.
I found that in my previous guitar, it's easier to follow the wood. Both in grain of carving but the tonal response that comes out, hence I have:
* European spruce arch tops - this rings like a bell in its current wedge, not a dull thud! This will give a clean balanced tone. X braced with spruce should make for a decent clear, balanced tone.
* Maple arched back - this also was selected on its tonal ring vs dull others. This will support mids and upper, with the back tending to be thicker. This naturally loves mids/mid upper.
* Maple sides - these unexpectedly have flame, but they're quite thin hence the need for additional supports. Especially on the bottom and arm over positions. The sides offer some tonal change but they're typically a weak spot in acoustics.
* Willow lining (this is the better sounding variation of the slotted kerfling supports you see if you look into the guitar - down side is it needs time to heat and bend), the sides will have structs too to prevent the sides splitting.
* Spruce bracing - quarter sawn, straight grain etc to give the best rigidity to weight, there's another subject on shaping/scalloping (adjusting the stiffness) but that's really given by following the modes and ring.
* Laminate neck - this rings like a xylophone block, but is extremely heavy - so this is why I will have a 'spar' of the same material providing both string tension support to the tail and to provide weight to prevent neck dives. The neck will have both a truss rod but also carbon fibre rods, the idea is that the design will use the carbon fibre to carry through into the spar along the length of the guitar. The carbon fibre and spar will help stop any humidity movement and as this will have a thinner neck, it will help.
* bubinga fretboard - this worked nicely on the solid body so I will use the remnant of that on this.
* neck block and end blocks - combination of laminate, hard maple.
* the tail 'trapeze' will be wood, but this and the bridge can be modified to adjust the tone being sent to the top.
* bridge - initially hard maple, but I'll adjust over time (the bridge will be floating).
Frets will be smaller and non-jumbo frets. I'm leading towards 2mm nickel frets but the stainless steel works well with the solid body so the decision is still out..
Soldato
- Joined
- 28 Dec 2017
- Posts
- 9,863
- Location
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****ing liar
******* liar
*stewy*Ban Him! <points>*stewy*Someone wrote a book on the subject... $200 and in two volumes.! I spent an age going around trying to work out that.. I spent the afternoon sharpening plane blades, chisels and gouges after they'd been a little abused since the last guitar!
Soldato
- Joined
- 28 Dec 2017
- Posts
- 9,863
- Location
- Beds
You're going to want sharp if you're doing relief carving. Have you seen the Bob Benedetto videos on carving an archtop? Brilliant. Mostly small thumb planes with curved bottoms.
Someone wrote a book on the subject... $200 and in two volumes.! I spent an age going around trying to work out that.. I spent the afternoon sharpening plane blades, chisels and gouges after they'd been a little abused since the last guitar!
I'll end up getting a thumb plane.. I had a little experience on carved tops with the solid body - if blunt it's an absolute pain with unstable flamed maple, and purple heart also rips out too. A very well respected luther took a look at what I was considering and gave some pointers, I worked through on my own and then came to the same conclusion about 3mm top in the centre, but it needs to be consistently smooth graduation.
For this reason the top will have a removable 'support chassis' so I can keep it safe without accidentally putting something through it!
One tip for sharpening chisels and planes to prevent edge lines etc is to very slightly contour the blade (were talking just visible and it won't appear on the wood. Instead of simply rebuilding straight, then going up the grit and finishing straight, just as you go up the grits, for one pass you divide the blade into 5, then do 10-5-3-5-10 passes with finger pressure on those 5 points. the blade will have a slight gleaming profile and will cut from the centre out and not leave edge lines (and the blade edges will disappear into the plane before the centre). Then when you re-strop between sharpening, simply do it normally (it wont take the slight curve off).
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