Planing ramp in fretboard to correct high action

Construction and repair of Classical Guitar and related instruments
bullpuppy
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Planing ramp in fretboard to correct high action

Post by bullpuppy » Fri May 19, 2017 7:34 pm

I was wondering if there is a formula to determine the amount of saddle height you can gain in relation to the loss in fretboard height at the nut. For instance:
Current action height at 4.2 at the 12th. 660mm scale.
I need lower the action 1mm, I need 2mm at the saddle.
Based on that how much fretboard would be taken down at the nut. On this guitar I already did some bridge reshaping and have a good break angle but don't want to do any more on the bridge.

I have done neck resets and bridge reshaping but have not done fretboard planing to correct action height. I thought I would try it out one my classical guitars that needs a refret. I know there are different opinions on this approach. The fretboard is 6.7 mm thick at the nut with a taper from the 12th fret ending at 6.1 at the soundhole. It helps if I could visualize how thin the fretboard will end up after a 2mm adjustment.

If anyone would like to share their approach it would be most appreciated. Thanks

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Chris Sobel
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Re: Planing ramp in fretboard to correct high action

Post by Chris Sobel » Fri May 19, 2017 9:34 pm

Hello,

I've done this a couple times to get a little more saddle height and usually can't get much more than 2-4/10 mm, which would mean 1-2/10 mm action lower at the 12th fret. One thing to consider is that if the thinning of the fingerboard makes the neck more flexible, you will easily loose that 1-2/10ths in neck bow.. so you have to be confident that the neck can take it. If the neck has almost no relief I would feel good making that kind of modification, but if some of the tall action is actually caused by neck bow then I would avoid this approach.

Another angle is to veneer a .8mm ebony slat over the existing fingerboard and re-cut the slots through the veneer. The veneer with both strengthen the neck and lower the action considerably. I would do this before #1 if the neck already had a decent amount of relief.

EDIT: One good way to see how much you could gain by the re-ramp would be to draw it out to scale. Draw a new fingerboard ramp taking into account that the new ramp can only remove fingerboard material... then redraw the string line and measure the difference from the first string line. I'm guessing it wouldn't be drastic but hey, 2/10th difference in action can be the difference between playable and not. Especially if combined with a tall fret refret you might be in pretty comfortable territory.

Chris
CE Sobel Guitars

bullpuppy
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Re: Planing ramp in fretboard to correct high action

Post by bullpuppy » Fri May 19, 2017 10:22 pm

Thanks for the advice. I didn't think about the vaneer on top of the fretboard. I really don't need too much. When I reshaped the bridge I filled in the old (rather large) holes and redrilled them lower, deepen the saddle slot and converted to a 12 hole system. I was able to lower it down to 3.7 with 1.5 mm saddle left showing straight across with about 40 degree break angle. The string height above the soundboard is 10mm. I am more comfortable playing at 3.2 so I can get there now with .5 saddle left but rather not.

Funny you should mention about higher frets. I am putting on a set of Dunlop 6105 which are .055 high. After dressing them up they will be close to .050. I tried a nice classical with these frets and for me I could tell the difference in playiblity. I didn't take in account the effects on the action height. The current frets have been dressed before areturning somewhat low. So I may be close to where I want with minimal ramping and higher frets.

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mikfik
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Re: Planing ramp in fretboard to correct high action

Post by mikfik » Sat May 20, 2017 3:59 pm

Taller frets means a taller nut so I don't understand why this would help lower the action on a guitar.

bullpuppy
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Re: Planing ramp in fretboard to correct high action

Post by bullpuppy » Sat May 20, 2017 5:22 pm

I made a diagram and rechecked my measurements. I really only need 1.5 mm more at the saddle. It so the fretboard would have to be about 1.2 mm lower at the nut. My calculation is more probably complex than the had to be. I hope it's right but I'll find out soon.

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Eric Reid
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Re: Planing ramp in fretboard to correct high action

Post by Eric Reid » Sun May 21, 2017 4:46 am

The distance between two straight lines moving in two dimensions, how complicated can it be? Still, it's confusing to think about, and more difficult to explain.

Here are some reliable rules of thumb:

To raise the saddle 1mm, you need to plane the fingerboard down 2mm at the nut.

To lower the action 1mm at the 12th fret, you need to plane the fingerboard down 4mm at the nut.

What makes the geometry hard to visualize is the fact that the strings are pivoting from the fixed point of the saddle, while the surface of the fingerboard is pivoting from the sound hole.

Taller frets lower the action by 1/2 the increase in fret height--e.g. changing from .040" frets to .050" frets will lower the action .005" (.125mm).

Adding a veneer to the top of the fingerboard has the same effect as taller frets: a 1mm veneer will lower the action 1/2 mm.

On the other hand,

To lower the saddle 1mm, you need to plane the fingerboard down .67 mm at the sound hole.

To raise the action 1mm at the 12th fret, you need to plane the fingerboard down 1.33mm at the sound hole.

As you can see, it's much easier to raise the action than to lower it. For that reason, many builders aim for an action that's a little too low, and make adjustments by planing the fingerboard. You'll often see new guitars with fingerboards that are thinner at the sound hole.

To correct a guitar with very high action and a low saddle, I prefer to remove the fingerboard, pull the neck back to a better angle, glue on a new fingerboard to lock the angle in place, and plane the finger board flat in the new position. The neck joint is quite flexible with the fingerboard removed,
and this allows for a large adjustment without a significant change to the thickness of the fingerboard or the overall thickness of the neck.

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Chris Sobel
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Re: Planing ramp in fretboard to correct high action

Post by Chris Sobel » Sun May 21, 2017 6:34 am

Eric,

Thanks for swooping in and putting some numbers to everything... that's really helpful!

Chris
CE Sobel Guitars

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eno
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Re: Planing ramp in fretboard to correct high action

Post by eno » Sun May 21, 2017 10:20 pm

bullpuppy wrote:
Fri May 19, 2017 10:22 pm
Funny you should mention about higher frets. I am putting on a set of Dunlop 6105 which are .055 high. After dressing them up they will be close to .050. I tried a nice classical with these frets and for me I could tell the difference in playiblity. I didn't take in account the effects on the action height. The current frets have been dressed before areturning somewhat low. So I may be close to where I want with minimal ramping and higher frets.
Oh, that's a good idea.
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David Farmer
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Re: Planing ramp in fretboard to correct high action

Post by David Farmer » Thu May 25, 2017 4:08 am

If the numbers work out, you can sometimes avoid straying deep into re-finishing territory by planing below the frets on the old board and capping it with a new, thin, fretted one. Touchup on the edge of an ebony cap is much easier if fingerboard removal will get you into color matching. Another thing to keep an eye on when ramping or capping is string clearance to the tuners from the nut.
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