Unavoidable fretboard hump while radiusing

[killemall8]

Ive had this problem on 90% of necks ive ever made.
Regardless of the method of radiusing, including pre radiused boards.
 

With fretboard glued to the neck, either radiused or flat. 8" sanding block or 20".

Fretboard dead flat, with 4 different straight edges.
use either long or short sanding block. 5 passes with it, already a hump in the middle.
Blocks are flat and straight measured with precision straight edge.

I dont understand how people can just clamp a neck to a table, radius with a short block and it be fine.
Ive tried so many d ifferent methods to avoid this and none do anything.

I have to go back and manually knock down the middle and it takes forever.
How do you avoid this and do you have any ideas of why this happens?

With a long sanding beam it doenst seem like it should even be possible.

[Bizman62]

Now that really sounds strange! There's been talk about creating a hollow in the center especially with a shorter block as it gets the most sanding but a hump??? Obviously I don't have any straight answer, but here's some thoughts:

First, do you radius the fretboards before or after carving the neck? Although not the first culprit, sanding the fretboard may release some tension.

How tight is your truss rod when radiusing? Guessing your necks aren't thick and fat even a tiny amount of tension to the release direction can cause only the ends being sanded.

How do you hold your sanding block? Pushing at the ends of the block may cause the ends of the fretboard to be sanded more. I've noticed the same although the full length block I've been using is made of steel and hence has to be clamped to the workbench. But if I hold the neck at the ends there'll be a slope at least for an inch or two at both ends of the fretboard. Basically that means there's a hump in the middle! But I've learned a trick or two to address that:

• A light hand is essential in any sanding. A fellow builder was struggling with his fretboard until I told him not to use any more weight than what the neck had.
• Drawing lines on the fretboard tells whether an area is sanded or not. That's useful especially if the corners have been knocked off with a belt sander.

Here's another idea that popped into my mind: Attach handles similar to a plane to the sanding block - or even attach an entire plane on your block with double sided tape or even flatheaded screws at the ends! That would add all the weight ever needed and give you better control, don't you think?

They even make a commercial power tool based on that idea:

https://maximumguitarworks.com/products/power-block-radius-sanding-block-maximum-guitar-works

 

[mistermikev]

On 8/20/2023 at 12:32 PM, killemall8 said:

Ive had this problem on 90% of necks ive ever made.
Regardless of the method of radiusing, including pre radiused boards.
 

With fretboard glued to the neck, either radiused or flat. 8" sanding block or 20".

Fretboard dead flat, with 4 different straight edges.
use either long or short sanding block. 5 passes with it, already a hump in the middle.
Blocks are flat and straight measured with precision straight edge.

I dont understand how people can just clamp a neck to a table, radius with a short block and it be fine.
Ive tried so many d ifferent methods to avoid this and none do anything.

I have to go back and manually knock down the middle and it takes forever.
How do you avoid this and do you have any ideas of why this happens?

With a long sanding beam it doenst seem like it should even be possible.

afraid I can't add much here... but I'll commiserate.  sanding is really hard.  I try to avoid it at all costs.  to sand fast and true... you need to use less pressure.  I know that's the case but I have somehow yet to learn it.  we all put pressure on... and when you do that it's not going to be even.  

I know folks sand fretboards with blocks... and I would assume some can do it well... but for me it is a lost cause.  The only guaranteed solution is to make it mechanized.  one strategy would be to build the common radius router jig. 

Another technique I've seen... warmoth I believe is where I saw it... they have a 2' long flat sander and they have a jig that is mounted to the ceiling on rope... they mount the fretboard and it just makes contact with the sander... move it to and fro and voilla... radius fretboard.  Theirs was actually setup to do compound radius via dif lengths at either end of the fretboard. probably hard to dial a system like that in.  

hoping you find some sort of useful pea in this comment.

[ADFinlayson]

How are you doing the radius? Are you taking the neck to a block or a block to the neck? If it's the latter, I'd bet you're putting too much pressure on and causing the neck to bow slightly in the middle as you're applying pressure and that is creating your hump because the heel and headstock are way stiffer. 

My preferred method now is to stick a long beam upside down in the vise and take the neck to it, that way I can target areas I need to while checking often.