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PostPosted: Sat Jan 14, 2017 8:23 am 
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So I have a new acoustic project heating up. In the midst of it, I was talking to my 24-yr old son (who, unlike his father, CAN play guitar) and he said it was time for another Tele build for him. I've made a few of those, no probs. But now he has expressed a desire for it to have an all-maple neck with a 7.5" - 16" compound radius. I'm interested in doing that as I'd like to play around with that in some acoustic builds. I fully understand the geometry and how to make the neck with a adjustable truss rod, maple fretboard, no skunk stripe, etc. I am also familiar with different methods of getting the "compound" radius (completely by hand thru dedicated machinery). I am seeing being able to do very nice and accurate fretboard radius' (radii) as adding another dimension to enhancing the guitars' playability, which I am more interested than aesthetics. I am soliciting suggestions here based on this:

Let's assume budget is of no concern or consequence and that compound radii will be used throughout my guitar building lifespan.
I'll be slotting on a tablesaw/jig
Separate maple fretboard glued onto routed-for-trussrod maple neck
Trussrod adjusted at neck-to-body end, not headstock end

To reiterate, the money isn't a big deal, the outcome is. So what do I need to go buy? I hope you guys have a field day with this.

Thanks.....curtis

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PostPosted: Sat Jan 14, 2017 9:03 am 
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If money is absolutely not a factor, I suppose a CNC setup would do the most exact job.

For non-computerized solutions, lots of folks are going with the edge sander, and they rig up a swinging system where the axis of rotation is not parallel to the sanding belt, and can be adjusted. Others do something similar, but with a router and some sort of linear travel for the router.

I have experimented with mechanized ways of doing this, and I use power tools a lot in building guitars, but you know what seems to work best for me? A hand plane to establish the basic shape of the compound arch, and an absolutely flat steel sanding bar to establish an absolutely flat surface along the run of the strings after the planing.

The exact radius at any given point along the fingerboard is not what matters. 7" and 8" are not that different, 16" and 18" are not that different. What matters is that it is curved, the curve is tighter at the nut than at the other end, and the surface is flat under each string.


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PostPosted: Sat Jan 14, 2017 9:13 am 
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I believe you can buy compound radiused boards. Boom done! ;)

You can build a jig based on the Mario Proulx compound radius jig (just google that).

Otherwise get a set of sanding blocks in radius that step up from 7.5 to 16. You will also need a sanding beam (one of my favorite tools in my shop!). Use the blocks to get the radius as close as you can and then finish off with the sanding beam to get a perfect conical compound radius.

If you purchase a pre-compound board or even build a router jig then I'd still recommend getting a sanding beam to finish it off once glued down and you can even do your frets with it.


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PostPosted: Sat Jan 14, 2017 9:22 am 
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A set of radius gauges, leveling beam, and a really sharp block plane Maples cheap do a couple practice runs then knock it out of the park.


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PostPosted: Sat Jan 14, 2017 9:59 am 
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I built a Mario type compound jig out of mostly scrap materials. Works pretty good, but takes up a bit of space.

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PostPosted: Sat Jan 14, 2017 2:24 pm 
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Great info. I always keep hand planing in the back of my mind (I also build split bamboo fly rods so I am quite familiar with hand planing...have a stable of L-N planes). I'm also a gadget guy....so jigs are like toys. Gonna Google up the Mario jig and spend some quality time...before the Texans game comes on.....and stomps N.E....hehehehahh

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PostPosted: Sat Jan 14, 2017 3:37 pm 
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PostPosted: Sat Jan 14, 2017 3:55 pm 
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cwood3 wrote:
Great info. I always keep hand planing in the back of my mind (I also build split bamboo fly rods so I am quite familiar with hand planing...have a stable of L-N planes). I'm also a gadget guy....so jigs are like toys. Gonna Google up the Mario jig and spend some quality time...before the Texans game comes on.....and stomps N.E....hehehehahh


I often say the following: good judgment comes from experience, and experience comes from bad judgment. Like I mentioned above, I have tinkered with several ways of doing this with a machine. Here are some general thoughts, in the wake of those experiences.

The enemy is slop. Any spots where parts can move in any way other than the way you need them to move will cause problems. But, you need for things to be adjustable. This is where things get tricky and interesting. You want to be able to make jig arms shorter or longer, but you need them to not wiggle around or rack or do anything at all other than travel in the rotational direction you want. You need for sanding platens to be absolutely flat. You need for router tracks to not let the router jiggle around. You need for the router track to be absolutely flat and straight. Any play in any part of the system messes with you.

It can be done; people do it. I just got tired of messing with it. The Cumpiano hand plane method, followed by straight sanding along the run of each string, is what gives me what I want right now. Maybe I will revisit this someday.


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PostPosted: Sun Jan 15, 2017 6:55 pm 
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jfmckenna wrote:
Use the blocks to get the radius as close as you can and then finish off with the sanding beam to get a perfect conical compound radius.



NITPICK ALERT : a 7.5" - 16" board will indeed be a compound radius, but conical it will not be. Splitting hairs, I know , but nonetheless ...


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PostPosted: Sun Jan 15, 2017 9:09 pm 
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I've been radiusing to 16" straight and then when I level with the neck on the guitar I've been chalking the board and using a sanding beam in line with the string runs until the marks are gone. I level the frets the same way. I've been pretty happy with that approach.

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PostPosted: Mon Jan 16, 2017 9:10 am 
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murrmac wrote:
jfmckenna wrote:
Use the blocks to get the radius as close as you can and then finish off with the sanding beam to get a perfect conical compound radius.



NITPICK ALERT : a 7.5" - 16" board will indeed be a compound radius, but conical it will not be. Splitting hairs, I know , but nonetheless ...


Of course it will be conical. By definition a cone is a solid with a given length and varying radii along its length with zero at one end and r(n) at the end. A compound radius is simply a section of a cone of a particular radius and length, and thus a conical section.

Is CNC a great way to make a compound radius board? Sure. Is it necessary? Not in the least. Start with your board about .008" over your target thickness. Get 7.5" and 16" radius blocks, a set of radius gauges, a block plane and a medium sanding bar (18" or whatever Stew Mac's medium one is). Put three marks on the board -- a centerline, and a pencil line about a third of the way in from both sides of the board. Plane off some of the wings to rough in the cone, being careful that your plane is super shop to avoid chipout. Then use the 16" block to get the last 4-6" to 16" radius, and do the same at the nut. Now use the sanding bar to connect the dots. Use a single bulb long fluorescent to illuminate behind your straight edge and mark your high spots and sand those off with a smaller sanding block. Use the straight edge and a pencil to mark multiple lines following the cone to mark the high spots. Repeat until the board is true, using finer and finer sandpaper in the process. Use the radius gauges to check your work at both ends.


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PostPosted: Mon Jan 16, 2017 12:47 pm 
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jfmckenna wrote:
Use the blocks to get the radius as close as you can and then finish off with the sanding beam to get a perfect conical compound radius.

murrmac wrote:
NITPICK ALERT : a 7.5" - 16" board will indeed be a compound radius, but conical it will not be. Splitting hairs, I know , but nonetheless ...

dberkowitz wrote:
Of course it will be conical. By definition a cone is a solid with a given length and varying radii along its length with zero at one end and r(n) at the end. A compound radius is simply a section of a cone of a particular radius and length, and thus a conical section.

David, maybe you are being less precise in your definition of conical section than I am.

I am defining a conical section as a section of a cone on which a straight edge placed along either edge will contact the edge at all points. It is impossible to achieve this on a fretboard which goes from 7.5" radius to 16" radius, unless you have an impossibly wide fretboard width at the bridge end.

Assuming for the sake of argument that the nut width is 1.75", then for a truly conical section the width of the fretboard at the bridge end would be around 3.75".


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PostPosted: Mon Jan 16, 2017 3:18 pm 
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I start off with 12 at the nut and 20 at the end of the board then finish it up with the beam so that ultimately it comes out to be - what it comes out to be. I don't mean to imply that he will get 7.5 - 16, I didn't even bother to do the math, but rather just start there and finish off with the beam and you will have created a compound conical board of what ever dimensions it comes out to be.


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PostPosted: Mon Jan 16, 2017 4:04 pm 
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I will sometimes see folks (more players than builders, so not so much here on the OLF) post on the internet about how they want to have a compound radius fingerboard, and they want to be very precise about the exact radius at the nut, and the exact radius at the other end. I think this is probably not all that productive, for two reasons.

First, I think our hands aren't all that good at telling the difference between a 7" and a 7.5" radius across a 1.75" wide nut, or the difference between a 16" and a 20" radius across the 2.125" wide other end of the fingerboard.

Second, as Murray points out, the math might not come out right if they really do try to work with those exact radii and they don't fudge it here and there to actually make it work right.

I had mentioned above the following: I think the really important things are that the fingerboard be arched (at whatever radii), that it have a tighter arch at the nut than at the other end, and that the fingerboard be super flat along the run of each string. If you get all of those things at the same time, the fingerboard is a compound radius fingerboard. But it might not have the exact radii that some of these players talk about.


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PostPosted: Mon Jan 16, 2017 4:26 pm 
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doncaparker wrote:
... and that the fingerboard be super flat along the run of each string. If you get all of those things at the same time, the fingerboard is a compound radius fingerboard. But it might not have the exact radii that some of these players talk about.


Your post is helping me understand compound radiuses. I had always thought it was an electric guitar things, but when making my acoustic guitar fretboards I radius them to 16" or 20" and then use a Stewmac level beam to flatten the radius as it moved up the fret board following the string paths to make sure that the fret board was flat relative to each string. I started doing this after I was taught to level frets on a radiused Fretboard with a Stewmac Fret\Fingerboard Leveler, This seemed counter intuitive to me until it dawned on me the strings are straight and they are going across the radius. So I started doing it on the new fret boards as well. It was not until this thread that I have related what I have been doing to the specified compound radiuses I have been reading about.

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PostPosted: Mon Jan 16, 2017 6:35 pm 
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Here's a link to a Chris Paulick jig that is built over a belt sander

https://www.youtube.com/watch?v=qgfEM71sja0


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PostPosted: Mon Jan 16, 2017 9:13 pm 
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johnparchem wrote:
doncaparker wrote:
... and that the fingerboard be super flat along the run of each string. If you get all of those things at the same time, the fingerboard is a compound radius fingerboard. But it might not have the exact radii that some of these players talk about.


Your post is helping me understand compound radiuses. I had always thought it was an electric guitar things, but when making my acoustic guitar fretboards I radius them to 16" or 20" and then use a Stewmac level beam to flatten the radius as it moved up the fret board following the string paths to make sure that the fret board was flat relative to each string. I started doing this after I was taught to level frets on a radiused Fretboard with a Stewmac Fret\Fingerboard Leveler, This seemed counter intuitive to me until it dawned on me the strings are straight and they are going across the radius. So I started doing it on the new fret boards as well. It was not until this thread that I have related what I have been doing to the specified compound radiuses I have been reading about.


John--

The big thing to remember, and the reason we want conical, is because the strings flare out as they go from the nut to the bridge. The spaces between the strings get wider. If the strings stayed the same distance apart, a cylindrical shape (an arched fingerboard with a constant radius all the way up and down) would be ideal.

Even those who say they are fine working with a constant radius are probably grinding enough off the fret tops to make the end product pretty conical. Plus, once we get into to the flatter radii (16" and up), the differences are pretty tiny.


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PostPosted: Tue Jan 17, 2017 6:19 am 
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I am going to have one last crack at explaining the difference between "compound" and "conical".

Imagine you have a cone and at one point along the cone the diameter of the cone is 15" and 18" further along the diameter is 32" (the radii, obviously, are 7.5" and 16")

So you draw a circumferential line round the cone at both of these points. Then , on the smaller diameter, you mark two points 1.75" apart ... this is the nut width. You then draw two straight lines along the cone with both line origins at the apex of the cone and passing through the marked points. You have now drawn the outline of a conical fretboard, with a nut width of 1.75". The only problem is, the width at the other end is going to be 3 3/4".

Imagine that it is possible to cut this marked section out of the cone ...(and also that the material is somehow suitable for fretboards ). In order to get a usable fretboard, you would have to taper the cut out section so that the end measured approx 2 1/4" . The problem now is that the string paths do not lie flat along the board ... to make it usable you would have to sand or plane each string path flat , thereby losing the conical geometry. You now have a working compound radius fretboard ... what you do not have is a conical fretboard .

There are pairs of radii which yield a mathematically perfect conical surface as well as usable nut widths and fretboard end widths.... 14" and 18" will yield a perfect conical fretboard which is 1.75" at the nut and 2.25" at the wide end. I am sure there are others but that pair is the only pair I am aware of which yields a 1/2" width difference on a perfectly conical surface.

Basically, all I am ranting about is the use of the word "conical" to describe a surface which is not in fact conical... it is possible to take any two radii (within reason) and achieve a compound radius board which functions perfectly ... but that board will not be a "conical" board.



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PostPosted: Tue Jan 17, 2017 7:39 am 
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My apologies for the misuse of the term "conical," Murray. Not being a mathematician by profession, I use the term conceptually, not literally.



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PostPosted: Tue Jan 17, 2017 7:48 am 
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murrmac wrote:
I am going to have one last crack at explaining the difference between "compound" and "conical".
............... but that board will not be a "conical" board.

Thanks Murray, that made it clear, and highlights the effect of levelling along the string paths if using my type of radiusing jig (belt sander)

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PostPosted: Thu Jan 19, 2017 4:33 pm 
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murrmac wrote:
You have now drawn the outline of a conical fretboard, with a nut width of 1.75". The only problem is, the width at the other end is going to be 3 3/4".


But there's nothing stopping you from cutting it down to be 2.25 or whatever it is you like. It's still a ruled surface and can absolutely be conical. Just to be sure, I tried it out on my CAD software and it indeed works and can be conical. (Not necessarily conical but in the example I tried, I used your numbers with a 2.25 big end and it works just fine. Further investigation is required to make sure it's flat along the string paths though - too much for today tho but I'm pretty sure it still works.

I couldn't figure out how to make all the dimensions visible in one screenshot so I took two. The dark lines on the cone are the intersection of the cone and the "fretboard" with a 1.75" nut and a 2.25" end 18" down.


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PostPosted: Thu Jan 19, 2017 5:21 pm 
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Murray, wouldn't there be infinite combinations of nut radii to end of FB radii? Whatever the nut radius is would have a corresponding end radius that would fit on a cone for that same taper but he angle of the cone would change. I could be wrong as I am far from a mathematician but my thought experiments tell me that you can pick values for some variables and the rest would be determined from there. Variables being (but maybe not limited to) the nut width, width of the other end, length of the taper, nut radius, end radius and angle of the cone.

I'm looking at a cup on my desk that is a section of a cone. If I chose a narrow end width, a wide end width and a lenght that would fit on this cup, I could cut a section out using the narrow end of the cup for my narrow end measurement and cut out a section of my chosen length and wide width. Now if I did the same thing but starting at the wide end of the cup marking out my wide end width and used the same pre-determined length and narrow end with, I would end up with two sections of a cone with the same "nut" widht, lenght, and end width but the radii would be different. All of that is using a single cone angle. This would also work with a really acute cone or a really obtuse cone. Naturally extremes in any of the chosen variables would make for a fret board shape that is unfit for a guitar. . .

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PostPosted: Thu Jan 19, 2017 5:42 pm 
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Bryan--

I don't have it in front of me, and God knows I am not a master of geometry, but there a section of the StewMac book on fretwork that talks about this.

If there is a specific fingerboard length, and a specific fingerboard taper, and a specific radius at the nut, and we all assume that keeping the fingerboard flat under each string is a high priority, and we further assume that we want the lowest action possible, then there is a single specific radius that should exist at the other end of the fingerboard. It can be derived via the use of the formula in the book.

That's what I remember, anyway.

Like I have said above, I think we get into really small measurements with the kind of radii that acoustic guitarists like to feel (16" and up), so I think it all gets fudged out and made to work right with a decent fret job. But, if I were programming a CNC to do this job, I would want to know the math.



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PostPosted: Thu Jan 19, 2017 6:21 pm 
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It's easy to make a compound radius too complicated. The important thing is that the board/fret plane under each string is flat along its length from the nut to the last fret. How I do it:

1. Cut fret slots, then rip the fretboard taper with about 1/16" extra width. Make sure the center line is clearly marked.
2. Run the board through a drum sander, fret slots up, with a piece of ~ 1/4" wide binding lined up right under one edge (see attached picture). The binding effectively simulates a straight line under the fretboard right under the 1st and 6th strings. Switch the binding to the other edge, going back and forth and lowering the drum bit by bit after every other pass until you have a bevel that approximates the desired nut radius. Watch the center line... if you find you are removing material close to it before you have your desired radius you'll need to use thicker binding or a shim to get a tighter radius. Likewise if you have your radius and you've only cut a little way into the fretboard you need thinner binding to make a larger radius or you will spend a lot of time sanding/planing later. The radius at the end of the fretboard should be determined by the relative widths of the nut and the end of the fretboard, not some arbitrary number, so only pay attention to the radius at the nut at this point.
3. Take a sanding beam/plane and clean it up. It is pretty remarkable how little you need to shave off before you have a perfect ideal compound radius all the way down the board.

It took more time to type that out than it usually takes to actually mill the compound radius. It is surprisingly accurate and fast.


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PostPosted: Thu Jan 19, 2017 8:09 pm 
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doncaparker wrote:
If there is a specific fingerboard length, and a specific fingerboard taper, and a specific radius at the nut, and we all assume that keeping the fingerboard flat under each string is a high priority, and we further assume that we want the lowest action possible, then there is a single specific radius that should exist at the other end of the fingerboard.
.

Succinctly put, Don, and 100% correct.

I can see what Andy is getting at when he says
Quote:
It's still a ruled surface and can absolutely be conical.

Yes , the section is still by definition a section of a cone, the problem of course is that the string paths on such a section are not "normal" to the surface. [Using "normal" in the limited specialized geometrical meaning of "normal"] ... google "normal (geometry) "

To get the string paths normal to the surface requires an alteration to the surface which makes the surface non-conical.


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