Fixing back bow in necks

PeteC

Hi 
one of my builds has a slight back bow which I’ve found now that it’s strung up and I am doing the final fretwork 

the neck was perfectly straight when I fretted it - so I guess it’s been caused by changing to a different fretwire which must be causing pressure in the slots.  I glued the frets too which might also be a factor
 
Luckily it has a two way truss rod and I can get sufficient relief into the neck. 

But it got me thinking it would be a good learning exercise to find out how to fix a back bow properly
I’ve been reading about various methods including heat lamps , clamping , etc 

what works best for other people ? 

Cheers 

WezV

you tend to find wood does what it wants, heat treatments can help for a bit but are rarely effective in the long term.

My preference is to level out the back bow.   If its minor this can be done on the frets, but more major deformation may require a re-leveling of the board.

even on a straight neck with a single action rod it makes sense to artificially back bow the neck with the rod prior leveling the frets.   this gives you some adjustment both ways, you can still tighten the rod more for back bow, or loosen it a bit for a little relief

PeteC

Thanks @WezV , 

so for a very mild back bow ( like this particular neck ) would a good approach be to : 

* destring 
* slacken the truss rod off to reintroduce the natural back bow
* re-level the frets while the slight back bow is in the neck 
* re- crown 
* restring and check relief and action 

I’m also interested in how to remove the board and introduce a bit of upbow - would be a great exercise if I can do it without spoiling the board.  I’ve just been reading about using heat lamps or heat blankets but can’t find any detailed tutorials on that yet. 

WezV

if its controllable with the 2 way rod I wouldn't do anything at all.   With 2 way rods you want to avoid the rod sitting at its neutral position as this is when it can rattle.

heat lamps and blankets can work well in the short term, but any heat treated wood will want to return to its original shape.   


wood has to get quite warm to reset its shape effectively, so you need to be aware of glue joins and finishes on something as thick as a guitar neck.    Also, what's going to hold the neck straight over the following years and all the seasonal changes... hopefully string tension, but if the internal forces of the wood warped it before, what is to stop them doing it again?  

PeteC

Thanks @WezV.  It seems to me that I'm having to put a lot of pressure on the rod to get sufficient relief to make it nicely playable be honest- so it does feel as though I need to take some of the back bow out if at all possible. 

Maybe I should just try re-levelling first and see how that goes.  With the rod slackened off now the back bow is back and I could try levelling the whole board in this state.  

Worth trying ? 

WezV

it might help then, just make sure you dial it in for a slight back bow, not a significant one that's going to give you much lower frets in the middle of the neck

PeteC

Will do. ..and thanks yet again @WezV ;


Out of interest - if I did want to relevel the whole fretboard as you mentioned earlier,  how would you go about it ? A case of removing the frets and replacing the board,  or removing it and putting upbow into the neck/board glue joint ? 

WezV

you can often level the board once the frets are out.

A severe bow may require the board removing and neck shaft levelling, but that is extreme

John_A

I would leave it, even if the rod is tight, if the relief is how you want it don't mess.

I once strung a LP with a very light back-bow up with 13's tuned up to A and left it for about 2 months, once I restrung and tuned to E it was right back where it started!

Paul_C

From what you said in your first post, if you get back bow after fretting then you need to cut the slots wide enough that you don't have that problem.

If you're gluing anyway then you can allow a little more room, but obviously you don't want the slot too big or the fret might not be in the right place.

PeteC

Couldn't help myself - so have levelled the frets again after introducing a very slight amount of back bow. 
Crowning and polishing later - fingers crossed it works OK. 

It was definitely playable but I'm really fussy about my set up. 

really appreciate all the advice. 

Danielsguitars

Even with a 2 way rod I'll add a bit of backbow then level the fretboard again before fretting, it's good practice imo

As wez said you don't want to be in neutral, I also think tension on the rod is better for tone 

PeteC

All done.    Much better playability now.    Even with a set of 9s there is enough relief in the neck now without too much tweaking of the truss rod. 

...Just need to cure the sticking bone nut I just made.   Still haven’t quite got the knack of a perfect nut yet.  Each one getting better but still keep getting string sticking issues which seem to be to do with the exit section at the rear of the nut. 

Off to do some reading up...

Danielsguitars

Slope the back of the string exit down a bit more and make sure the back is open enough, that might help 

PeteC

thanks @Danielsguitars ;;

yes - done some fettling/angling of the back of the slots and polished them with superfine grits and all is now good. Smooth tuning and no sticking. 
Also found a very good StewMac video on making and adjusting the nut which is a good resource. 

PeteC

Just following up on this in case it’s of any interest or help to other novice builders in future. 

After levelling, crowning and polishing the board for a second time - this time using a tiny bit of backbow in the neck before levelling to compensate for the natural backbow that was in the neck - I now find that all is ok except that my top E and B strings have some rattles and choking off on bends once I get up around and beyond the 12th fret 

without strings the frets seem perfect when tested with a good quality fret rocker, but once the guitar is strung and tuned to pitch I can find very slight high spots at the 13th, 20th and possibly the last fret. 

It’s a flaw in an otherwise really nice build so I am determined to sort it.  The guitar sounds amazing with the old timer PAFs through my jubilee combo. 

I’m guessing that under compression the neck is going into a very slight S curve and causing a microscopically convex hump where the neck joins the body. 

To sort it out ideally I need to be able to level these frets while the neck is under tension and have been looking at Sam Deeks (Reloved guitar) method using a dual action truss rod adjusted for the neck relief curve as a levelling file. https://youtu.be/MDRfd3MCBIo

Has anyone tried this method, or have any other advice on how to cure this problem ? 

Cheers 

Jazzthat

I wonder if making a fall away would sort this out ?

PeteC

Thanks
I did try but not sure if i really did it properly - was winging it a bit tbh 

Hoping that one of the experts will put me straight !! 

WezV

Yeah, sounds like a little more fall away will help


I haven't done levelling under tension, but if you have a way to do it I think it will help

Jazzthat

PeteC said:

Thanks
I did try but not sure if i really did it properly - was winging it a bit tbh 

Hoping that one of the experts will put me straight !! 

I am not an expert for sure , though on my last leveling ( not fall away done at this stage  ) I had very similar problem when  I put strings on . 
If I remember correctly , the problem  was from  14-15 onwards and so I had to raised action well above I usually  like to have . 
In the end I did fall away ( first time as well ).

I read that most people use 3 stripes of masking tape on 12th fret for this . Well , I used 4 of them as 3 didn't seem to be high enough . 
It is nice and good now again . 
Worth a try before you reach for more complicated solutions .

PeteC

Thanks both.   I’ve ordered a spare dual action truss rod and will try Sam Deeks method carefully to see if I can get the action a bit lower. I might try do the fall away if it’s needed after that. 

I set it up to factory LP specs a couple of days ago ( measured nut height, relief and action ) and it seems to have settled down quite a bit.  There’s certainly less ringing on the b string around the active and above now. So perhaps this is just the whole guitar adjusting to string tension.   To be fair I’ve had the neck ready and fretted for about 2 months - with a big gap in getting the guitar finish courtesy of COVID-19!   Maybe it’s just settling in. 

Still,  I’m a fan of low action and clear tone all the way up the board, so I will persevere to see how close to ideal I can get it.   It’s good learning for my next carve top which just needs the body carve doing now - the neck is ready to fit.