What is the correct relief and curve for a neck?

[redmarshall]

I am new at this but a long time player now looking to invest in some tools and set up shop.

So I see that there are various methods to leveling frets but what I'd like to know is what is the correct curve for a neck?

Some neck guages measure at the 5th fret some around the 7th, while others use a staight edge and feeler gauges but this all leaves me confused.

Should the neck have a perfect arc to it or should it be more parabolic towards the nut?

I see products like the Katana for fret levelling and although info is limited on the device, it looks like it bows evenly when the nob is tightened.

So this should leave a perfect arc on the neck but is this correct?

Seems to me all my guitars seem to have more curve down at the nut than further up the neck so obviously this is not a perfect arc but more of a parabola but which right?

Has anyone ever studied guitar string motion?

[demonx]

The position of the curve is in relation to the truss rod length. The longer the truss rod, the further down the neck towards the nut the curve will appear. You dont want too short of a truss rod, the rod length should be chosen according to the scale length of the guitar.

You want some curve/backbow in the neck, not too much, but some. Enough to off relief so the strings can be fretted without buz, but not too much that the middle area of the neck is not in proportion height wise to the upper and lower areas.

When adjusting the action, I break it down to three areas, upper lower and middle. The bridge and truss rod have the most effect on all these areas, but the nut height should be the very first to be set. As far as setting up, if the nut height is correct from the beginning, then its simply bridge height and truss rod adjustment.

I set string height at the 12th fret by adjusting the bridge, then make the necessary adjustments to the truss rod, then correct the string height again and make any further adjustments until everything fits into my tollerance.

[redmarshall]

Thank you.

I am aware that this is pretty much the standard way of doing this but I am still confused as to whether this leaves the correct curve on the neck?

I have been playing for decades and have a small arsenal of guitars all of which I have set up more or less along the same method as you describe.

Set the truss rod for a level neck and then level the frets with a fret file or radius block.

Sure no problem .

Then restring and set the truss rod for a few thou clearance.

Pretty standard stuff.

But I see this new device, the Katana which level the frets with the strings on.

Now by the looks of it, it will curve in a perfect arc when the knob is tightened, which should leave a perfect arc on the neck.

But do we want a perfect arc.

I am sure that tightening a guitar truss rod will not leave a perfect arc but accentuate the arc towards the nut and hardly at all past the 12th fret.

In other words, a parabola.

But is this what we want or do we want a perfect arc?

Is there a book or webpage with guitar physics thoroughly explained?

Thanks!

[demonx]

What you are asking is for a black and white answer when it's a whole grey area.

The action/relief/backbow/whatever is personal preference. As long as there's no buzz and you feel comfortable fingering it, then it's correct.

Some people like high, some low, some in between. If you look at spec sheets for all the major companies the all have different standards. The backbow, or "arc" as you keep referring to is only one part of the equasion that I spoke of in my first post, again, it's personal preference.

I used to set up all my builds super low, which is how I used to play, yet nearly every time a customer picked up their guitar they asked for the action be be raised, so these days I set a medium action as standard and I've become to prefer it that way myself. Medium action is a bit more versatile genre wise.

As far as you're "throw a file over it" type levelling presumption (thats how I read it anyway) - you need to be super precise when fret levelling. Jigs to support the neck are essentail so there is zero movement. Not just a basic caul. Not just straighted out the truss rod. If the neck is not rigid then you'll file a banana into the frets and you'll never get a good action. Many "pro" luthiers still cannot get that right and most of the tutes on the net are not that great. I used to use home made timber jog and clamps, but these days I use the amuminium stewmac jig. It's great once you set it up right.

[KeithHowell]

A stretched string will, as far as I am aware, actually follow a catenary and also vibrate following a catenary which is close but not a parabola.

The neck under the influence of the string (and the counter influence) of the truss rod will form a far more complex curve as it will be influenced by the density of the material (wood) at each point along the length and the cross sectional area. Wood being what it is can have varying density and a complicated integral will be necessary to calculate it all.

If you are really interested I can get some formulas from a structural engineer of my acquaintance (my brother actually) and post them.

Keith

[redmarshall]

Thanks for the replies!

It is surprising to me that this is such a gray area.

Hit and miss it seems even for long time professional luthiers.

I have seen the StewMac neck jigs and understand the principle at work but I am not sold that this will result in the neck being left with the correct arc once the strings are retentioned because of the variances in the taper and composition of the wood.

Even though the frets will be level, the neck will form whatever compound curves it wants once the strings are back on.

Hit and miss.

I understand that action and relief is a personal preference but the curve of the neck should be a certain determinable curve based in science and math, not guesswork.

Maybe a caternary is the correct solution?

I still have not been able to find a solution even after googling...

I am disappointed so many professional luthiers are willing to accept this type of hit and miss methodology to their work.

Surely there must be a better way....

[demonx]

The stewmac jig I reference to is not anything to do with the relief in this case, it's in order to get the fret level as precise as possible and keep the neck rigid whilst doing so.

It's not "hit and miss" when you've done it hundreds of times. It's the product of repitition and refinement to your technique. The refinement can be hit and miss and I suppose many "pro's" never refine as much as others. I'm always evolving myself. Always searching for a better way to do things and for now, the way I do it IS the best and most consistant way that I'm aware of.

Unless I'm reading you wrong, what you're suggesting is that there should be one and only one way. If that were the case, if you take that mindset further than there should only be one style of guitar, only one build method, only one choice. So my response is, there is ALWAYS more than one way and different methods work for different people.

If you want a "engineering/scientific" approach to fretwork, google Plek machining. However even a hundred thousand dollar Plek relies on the person who programs the settings, which comes back to what I've been saying all along, it's just that its a cnc machine doing the fretwork as opposed to the human touch.