Compound radius fretboard shaping jig
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Compound radius fretboard shaping jig
I hesitate to post this, as I'm not sure how clear it's going to be, but here goes.
The basic idea is: a router sits in a carriage and rides on two rails down the length of the board. The rails rest on arc templates with different radii, so as you move from string path to string path, the compound radius is cut.
The "fixed" part of the jig consists of a base (0.5" thick mdf x 10" x 36"), a platform (2.5" w x 2.75" h x 17" l) to stick the fretboard to, and two "stops" to attach the radius templates to.
The stop on the left (near the nut end of the board) is 5" from the platform, and is fixed to the board.
The stop on the right is moveable depending on the scale. In this case, it's clamped down at 24.75".
Here's the moveable stop at the bridge location.
Here are what I call the setup arcs. Two identical templates used to set the height and location of the rails. I'm cutting a 7.25 radius at the nut, and the rails will ride on an arc 1" above the fretboard, so they need to be an 8.25" radius.
I clamp the setup templates to the platform.
Oops, forgot the spacers, which hold the rails a fixed width apart.
More tomorrow.
It really does work, despite what it looks like so far.
The basic idea is: a router sits in a carriage and rides on two rails down the length of the board. The rails rest on arc templates with different radii, so as you move from string path to string path, the compound radius is cut.
The "fixed" part of the jig consists of a base (0.5" thick mdf x 10" x 36"), a platform (2.5" w x 2.75" h x 17" l) to stick the fretboard to, and two "stops" to attach the radius templates to.
The stop on the left (near the nut end of the board) is 5" from the platform, and is fixed to the board.
The stop on the right is moveable depending on the scale. In this case, it's clamped down at 24.75".
Here's the moveable stop at the bridge location.
Here are what I call the setup arcs. Two identical templates used to set the height and location of the rails. I'm cutting a 7.25 radius at the nut, and the rails will ride on an arc 1" above the fretboard, so they need to be an 8.25" radius.
I clamp the setup templates to the platform.
Oops, forgot the spacers, which hold the rails a fixed width apart.
More tomorrow.
It really does work, despite what it looks like so far.
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Re: Compound radius fretboard shaping jig
I had this exact same idea a little while ago but I wasn't sure if the router would actually follow the contour of the individual radius guides or just end up doing a radius that was somewhere in between them and consistent down the whole length. I intended to put bearings or rollers on the ends of the tubes to ride on the radius guides but in my mind at least it poses a problem. The tubes should be parallel so the router can follow a linear guide instead of a tapered one, and if that's the case the wheels would need to be located in such a way so the wheels can roll freely on both ends simultaneously, which may involve having some kind of hinge mount so the wheels can find their shared neutral point for rolling. I pretty much gave up on it and have decided when I have the time to make a rig similar to the grizzly compound radius jig for their edge sander but using a router in a table instead of a belt sander.
I kind of hope I'm wrong though and was overthinking it! Best of luck, I hope it works!
I kind of hope I'm wrong though and was overthinking it! Best of luck, I hope it works!
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Re: Compound radius fretboard shaping jig
I'm having difficultly following. Can you post some more shots with the jig in use?
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Re: Compound radius fretboard shaping jig
Here's one of the spacers (on the nut end) aligned to the center line (pencil lines) and clamped to the fixed stop.
At the bridge end I've just set the spacer in front of the stop. The rails get dropped into the slots in the spacers, and they rest on the setup arcs.
Here are the "cutting" arcs. Different radii. They get clamped to the stops.
Before I attach the cutting arcs I put some weight on the rails. The rails are resting on the setup arcs, and are kept a fixed width apart by the spacers. Next I'll slide the cutting templates between the spacers and stops, pulling them up tight against the rails (which is why I added the weight to the rails).
Here's the bridge arc pulled up tight against the rails and clamped temporarily.
At the bridge end I've just set the spacer in front of the stop. The rails get dropped into the slots in the spacers, and they rest on the setup arcs.
Here are the "cutting" arcs. Different radii. They get clamped to the stops.
Before I attach the cutting arcs I put some weight on the rails. The rails are resting on the setup arcs, and are kept a fixed width apart by the spacers. Next I'll slide the cutting templates between the spacers and stops, pulling them up tight against the rails (which is why I added the weight to the rails).
Here's the bridge arc pulled up tight against the rails and clamped temporarily.
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Re: Compound radius fretboard shaping jig
Then I clamp it up for keeps and clamp the spacer to the stop (not shown).
Same deal on the other end. Pull the cutting template up tight against the rails, clamp it to the stop, and clamp the spacer to the stop.
Now everything is locked in place, so I can remove the setup arcs. Now the rails are resting on the cutting arcs.
Now I double-stick the fretboard to the platform. Here I'm just using an mdf piece that I've already radiused, just for illustration.
This is the router carriage. Just a piece of 0.25" thick mdf, 5.25" x 5.25" with a couple of pieces of 1 x 2 glued on, and three knock-down bolts (which ride on the rails).
Same deal on the other end. Pull the cutting template up tight against the rails, clamp it to the stop, and clamp the spacer to the stop.
Now everything is locked in place, so I can remove the setup arcs. Now the rails are resting on the cutting arcs.
Now I double-stick the fretboard to the platform. Here I'm just using an mdf piece that I've already radiused, just for illustration.
This is the router carriage. Just a piece of 0.25" thick mdf, 5.25" x 5.25" with a couple of pieces of 1 x 2 glued on, and three knock-down bolts (which ride on the rails).
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Re: Compound radius fretboard shaping jig
Now you set the depth of the router bit, and align the spacers to the first string path. Clamp the spacers to the stops and cut the path. Move from path to path until they're all cut.
Wow. Unfocused. Here's the outermost path, which starts out (at the nut end) at a pretty good tilt.
So that's the easy part. Set it up relative to the radius you want at the nut, attach cutting arcs, use the string paths. Done.
But you also have to be sure that the cutting arcs are the correct radius. Or what's the point?
And that part I really do have to save for tomorrow.
Wow. Unfocused. Here's the outermost path, which starts out (at the nut end) at a pretty good tilt.
So that's the easy part. Set it up relative to the radius you want at the nut, attach cutting arcs, use the string paths. Done.
But you also have to be sure that the cutting arcs are the correct radius. Or what's the point?
And that part I really do have to save for tomorrow.
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Re: Compound radius fretboard shaping jig
Ok, I get it now. I thought the tubes were attached to the sled and the whole thing moved together. That would require some long tubes! I like how your main cuts are along the length of the board. Do you go back with crosswise passes to blend the facets?
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Re: Compound radius fretboard shaping jig
Jason: No, you don't cut crosswise. You can't because the spacers are clamped to the end stops. There IS a slight faceting of the board, but that can either be sanded out, or you can make an additional set of passes between string paths to fix that.
I thought that maybe a drawing or two might help visualize, so I'll try to post a few today before I explain how to get the correct radii for the cutting templates.
Here's an end view of some of the components:
I thought that maybe a drawing or two might help visualize, so I'll try to post a few today before I explain how to get the correct radii for the cutting templates.
Here's an end view of some of the components:
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Re: Compound radius fretboard shaping jig
If you put your spacers on wheels and had them run in a track that could be clamped so as to keep the spacers where you wanted them you could rout from side to side as well as end to end.
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Re: Compound radius fretboard shaping jig
Here's another view from the end with some dimensions. Unfortunately the drawing shows a 9" radius, but that's the only difference between it and the photos.
And here's the carriage width.
And some spacer dimensions.
On the last few drawings I included the cell references from a spreadsheet I use, but not sure I can upload an Excel file.
And here's the carriage width.
And some spacer dimensions.
On the last few drawings I included the cell references from a spreadsheet I use, but not sure I can upload an Excel file.
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Re: Compound radius fretboard shaping jig
This may be a good point to start over...
The basic idea is: a router sits in a carriage and rides on two rails above the fretboard. The rails rest on arc templates with different radii, so as you move from string path to string path, the compound radius is cut.
Here are the base components (all made from 0.5" thick mdf):
-base
-platform
-2 stops: one fixed (nut stop) and one that's moveable, depending on the scale (bridge stop)
And here's a side view:
You need to construct a simple carriage for the router to sit in (held in place with double-stick tape). Just a 0.25" thick piece of mdf, a couple of pieces of 1 x 2 (hardwood), and three knock-down bolts (1.75" long):
You need a couple of templates to setup the rails. Since I'm going to cut a 9" radius at the nut, and because my setup templates will suspend the rails 1" above the face of the fretboard, the setup templates need a 10" radius. Because the combined height of the platform (2.75") and the fretboard (0.25") = 3", the setup templates need to be 4" high:
The basic idea is: a router sits in a carriage and rides on two rails above the fretboard. The rails rest on arc templates with different radii, so as you move from string path to string path, the compound radius is cut.
Here are the base components (all made from 0.5" thick mdf):
-base
-platform
-2 stops: one fixed (nut stop) and one that's moveable, depending on the scale (bridge stop)
And here's a side view:
You need to construct a simple carriage for the router to sit in (held in place with double-stick tape). Just a 0.25" thick piece of mdf, a couple of pieces of 1 x 2 (hardwood), and three knock-down bolts (1.75" long):
You need a couple of templates to setup the rails. Since I'm going to cut a 9" radius at the nut, and because my setup templates will suspend the rails 1" above the face of the fretboard, the setup templates need a 10" radius. Because the combined height of the platform (2.75") and the fretboard (0.25") = 3", the setup templates need to be 4" high:
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Re: Compound radius fretboard shaping jig
Next, you need 2 spacers, which hold the rails a fixed width apart. The critical dimensions below are the spacer width and the slot width. The undercuts (below the slots) just keep the spacers from running into the clamps that hold the cutting templates to the stops. They should be 0.5" thick:
The rails are just 2 lengths of 0.85" dia pipe, 36" long:
And finally, you need the cutting templates (whose radii will be determined later). Note that they are 0.5" shorter than the setup templates:
The rails are just 2 lengths of 0.85" dia pipe, 36" long:
And finally, you need the cutting templates (whose radii will be determined later). Note that they are 0.5" shorter than the setup templates:
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Re: Compound radius fretboard shaping jig
Now we need to take these components and do the initial setup. Clamp the setup templates (with a long bar clamp) to the platform:
Use two spring clamps to tack a spacer to the nut stop (the white square is supposed to represent the point of attachment for the spring clamps in the following drawings). Just place the other spacer about an inch in front of the bridge stop. If your spacers are 0.5" thick it should stand up on its own without tipping over.
Now you can drop the rails into the spacer slots:
Next, apply some weights to the rails. I use two 10 lb weights. The reason for this is that you need to slip the cutting template between the spacer and bridge stop, and pull it up tight against the bottoms of the rails with one hand, and clamp it to the stop (spring clamps) with the other. You don't want the rails to move around while you're doing this.
I removed the rails from the inset at right (below) for clarity:
Use two spring clamps to tack a spacer to the nut stop (the white square is supposed to represent the point of attachment for the spring clamps in the following drawings). Just place the other spacer about an inch in front of the bridge stop. If your spacers are 0.5" thick it should stand up on its own without tipping over.
Now you can drop the rails into the spacer slots:
Next, apply some weights to the rails. I use two 10 lb weights. The reason for this is that you need to slip the cutting template between the spacer and bridge stop, and pull it up tight against the bottoms of the rails with one hand, and clamp it to the stop (spring clamps) with the other. You don't want the rails to move around while you're doing this.
I removed the rails from the inset at right (below) for clarity:
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Re: Compound radius fretboard shaping jig
Now that the template is tacked to the bridge stop with spring clamps (and you have both hands free), use bar clamps to securely attach the template to the bridge stop. (Bar clamp attachment points are depicted with gold squares in the following drawings):
Remove the spring clamps:
Now you can slide the spacer up against the stop and spring clamp it in place.
Attach the cutting template for the nut stop side of the jig using the same process used above for attaching the bridge stop template.
Once both cutting templates have been secured, the setup templates can be removed. Now the rails are resting on the cutting templates.
At this point, the cutting templates are attached and the spacers are aligned and clamped to the center line of the base, so you can double-stick the fretboard to the platform.
Place the carriage on the rails and adjust the router bit depth so that it just touches the face of the fretboard at the nut end. Then move the carriage to the other end of the fretboard and check the bit depth.
Ideally, the bit will just touch the face of the fretboard at both ends. In practice, I've found that the bridge end of the fretboard is slightly higher (like 0.01" or so), which is most easily remedied by taping a shim to the top edge of the bridge stop template. This will raise the rails enough to equalize the cutting depth of the bit.
Remove the spring clamps:
Now you can slide the spacer up against the stop and spring clamp it in place.
Attach the cutting template for the nut stop side of the jig using the same process used above for attaching the bridge stop template.
Once both cutting templates have been secured, the setup templates can be removed. Now the rails are resting on the cutting templates.
At this point, the cutting templates are attached and the spacers are aligned and clamped to the center line of the base, so you can double-stick the fretboard to the platform.
Place the carriage on the rails and adjust the router bit depth so that it just touches the face of the fretboard at the nut end. Then move the carriage to the other end of the fretboard and check the bit depth.
Ideally, the bit will just touch the face of the fretboard at both ends. In practice, I've found that the bridge end of the fretboard is slightly higher (like 0.01" or so), which is most easily remedied by taping a shim to the top edge of the bridge stop template. This will raise the rails enough to equalize the cutting depth of the bit.
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Re: Compound radius fretboard shaping jig
Here I want to take a moment to show how the cutting templates affect the positioning of the rails from string path to string path. Note that the spacers and the jig base are marked with center lines, and string path are also marked on the base.
First in the center position:
Notice how the rails are affected as the spacers are moved to the bass E string path:
This is why there are just three knock down bolts in the carriage. Four bolts would never all make contact with the rails given how the height of the rails varies from one end to the other. Note: It is especially important to apply downward pressure on the trailing of the two bolts, to keep the carriage from tipping forward as it's pushed along the rails.
Note also that because the bridge stop cutting template has a greater (flatter) radius, it sits closer to the base than the nut stop cutting template does. It is important to make these templates 0.5" shorter than the setup templates.
You can also see that the spacer cutouts allow the spacer to be moved beyond the outermost string paths without being interfered with by the clamps holding the cutting templates to the stops.
First in the center position:
Notice how the rails are affected as the spacers are moved to the bass E string path:
This is why there are just three knock down bolts in the carriage. Four bolts would never all make contact with the rails given how the height of the rails varies from one end to the other. Note: It is especially important to apply downward pressure on the trailing of the two bolts, to keep the carriage from tipping forward as it's pushed along the rails.
Note also that because the bridge stop cutting template has a greater (flatter) radius, it sits closer to the base than the nut stop cutting template does. It is important to make these templates 0.5" shorter than the setup templates.
You can also see that the spacer cutouts allow the spacer to be moved beyond the outermost string paths without being interfered with by the clamps holding the cutting templates to the stops.
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Re: Compound radius fretboard shaping jig
Okay, now comes the best part, figuring out the cutting template radii.
Bear in mind that a spreadsheet can be set up to do the grunt work that follows, which makes changing and updating scales, nut widths, fretboard radii, etc, all quick and easy.
To determine the cutting template radii, you first need to know the width of the fb at the nut (1.7") and at the 14th fret (2.125"), and the distance from the nut to 14th fret, which since I've got a 24.75" scale = 13.725".
The goal is to determine the width of the fb if it was extended to the nut stop on the one end, and to the bridge on the other.
So here's the math for the nut stop end:
fb width at nut stop = (((w14 - wNut) / distance to 14) x distance to nut stop) + wNut
= (((2.125 - 1.7) / 13.725) x -5.0) + 1.7
= ((0.425 / 13.725) x -5.0) + 1.7
= (0.031 x -5.0) + 1.7
= -0.1550 + 1.7
= 1.545 at nut stop
Do the same thing to determine the width of the fb at the bridge:
fb width at bridge = (((w14 - wNut) / distance to 14) x distance to bridge) + wNut
= (((2.125 - 1.7) / 13.725) x 24.75) + 1.7
= ((0.425 / 13.725) x 24.75) + 1.7
= (0.031 x 24.75) + 1.7
= 0.766 + 1.7
= 2.466 at bridge
The next thing we need to know is the sagitta (or "sag") of the 9" radius arc at the nut. The dictionary definition of sag = "the distance from the midpoint of an arc to the midpoint of its chord".
Math time:
sag = radius - sqrt(radius^2 - (chord/2)^2)
= 9 - sqrt(9^2 - (1.7/2)^2)
= 9 - sqrt(81 - (0.85)^2)
= 9 - sqrt(81 - 0.723)
= 9 - sqrt(80.277)
= 9 - 8.96
= 0.04
So the sag = 0.04
Now to find the radius of the fretboard at the nut stop position:
radius at the nut stop = (sag^2 + (width at nut stop/2)^2) / (2 x sag)
= (0.04^2 + (1.545/2)^2) / (2 x 0.04)
= (0.0016 + (0.773)^2) / (0.08)
= (0.0016 + 0.597) / (0.08)
= (0.599) / (0.08)
= 7.439” radius on the fb at the nut stop
Same math to find the radius of the fretboard at the bridge position:
radius at the bridge = (sag^2 + (width at bridge/2)^2) / (2 x sag)
= (0.04^2 + (2.466/2)^2) / (2 x 0.04)
= (0.0016 + (1.233)^2) / (0.08)
= (0.0016 + 1.521) / (0.08)
= (1.522) / (0.08)
= 18.921” radius on the fb at the bridge
Getting sleepy. More tomorrow.
Bear in mind that a spreadsheet can be set up to do the grunt work that follows, which makes changing and updating scales, nut widths, fretboard radii, etc, all quick and easy.
To determine the cutting template radii, you first need to know the width of the fb at the nut (1.7") and at the 14th fret (2.125"), and the distance from the nut to 14th fret, which since I've got a 24.75" scale = 13.725".
The goal is to determine the width of the fb if it was extended to the nut stop on the one end, and to the bridge on the other.
So here's the math for the nut stop end:
fb width at nut stop = (((w14 - wNut) / distance to 14) x distance to nut stop) + wNut
= (((2.125 - 1.7) / 13.725) x -5.0) + 1.7
= ((0.425 / 13.725) x -5.0) + 1.7
= (0.031 x -5.0) + 1.7
= -0.1550 + 1.7
= 1.545 at nut stop
Do the same thing to determine the width of the fb at the bridge:
fb width at bridge = (((w14 - wNut) / distance to 14) x distance to bridge) + wNut
= (((2.125 - 1.7) / 13.725) x 24.75) + 1.7
= ((0.425 / 13.725) x 24.75) + 1.7
= (0.031 x 24.75) + 1.7
= 0.766 + 1.7
= 2.466 at bridge
The next thing we need to know is the sagitta (or "sag") of the 9" radius arc at the nut. The dictionary definition of sag = "the distance from the midpoint of an arc to the midpoint of its chord".
Math time:
sag = radius - sqrt(radius^2 - (chord/2)^2)
= 9 - sqrt(9^2 - (1.7/2)^2)
= 9 - sqrt(81 - (0.85)^2)
= 9 - sqrt(81 - 0.723)
= 9 - sqrt(80.277)
= 9 - 8.96
= 0.04
So the sag = 0.04
Now to find the radius of the fretboard at the nut stop position:
radius at the nut stop = (sag^2 + (width at nut stop/2)^2) / (2 x sag)
= (0.04^2 + (1.545/2)^2) / (2 x 0.04)
= (0.0016 + (0.773)^2) / (0.08)
= (0.0016 + 0.597) / (0.08)
= (0.599) / (0.08)
= 7.439” radius on the fb at the nut stop
Same math to find the radius of the fretboard at the bridge position:
radius at the bridge = (sag^2 + (width at bridge/2)^2) / (2 x sag)
= (0.04^2 + (2.466/2)^2) / (2 x 0.04)
= (0.0016 + (1.233)^2) / (0.08)
= (0.0016 + 1.521) / (0.08)
= (1.522) / (0.08)
= 18.921” radius on the fb at the bridge
Getting sleepy. More tomorrow.
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Re: Compound radius fretboard shaping jig
Okay now for the last bit.
Getting the correct radii for the cutting templates had me stumped for awhile. Initially I thought that if I could find the fb radii at the nut stop and bridge, I could just add 1" to that, like I had with the setup templates.
That didn't work.
What I finally realized was that with respect to the cutting templates, a portion of that 1" setup offset is a fixed value, and adding that fixed value to the fb radius will help you solve for the template radius.
Here's an end view of the setup. I've shaded the area in question:
The setup template suspends the rails 1" above the face of the fb. The width of the rails (from center to center) is 6.15":
The fixed value is going to be the difference between the 1" offset height and the sag:
Back to the blackboard:
sag = radius - sqrt(radius^2 - (chord/2)^2)
= 10 - sqrt(10^2 - (6.15/2)^2)
= 10 - sqrt(100 - (3.075)^2)
= 10 - sqrt(100 - 9.456)
= 10 - sqrt(90.544)
= 10 - 9.515
= 0.485
So the sag = 0.485
And the fixed value is 1" - 0.485"
= 0.515"
NOW we can find the radii of the cutting templates.
First the nut stop template:
You just add the fb radius and the fixed value = the triangle's altitude (7.954), and we know the base is one-half of the chord (6.15/2) = 3.075.
a^2 + b^2 = c^2
7.954^2 + 3.075^2 = c^2
63.271 + 9.456 = c^2
72.727 = c^2
sqrt(72.727) = 8.528
8.528
Who knew?
8.528 is the radius of the nut stop cutting template.
Do the same for the bridge cutting template.
Again, you just add the fb radius and the fixed value = the triangle's altitude (19.437), and the base is still = 3.075.
a^2 + b^2 = c^2
19.437^2 + 3.075^2 = c^2
377.796 + 9.456 = c^2
387.252 = c^2
sqrt(387.252) = 19.679
19.679 is the radius of the bridge cutting template.
SEE? It's not that hard.
Actually, it's infinitely easier if you have all this in a spreadsheet.
Forum rules don't allow uploading Excel files, but I'd be happy to email the file to anyone who's interested.
(crickets)
Well, if you change your mind, send me a PM.
Getting the correct radii for the cutting templates had me stumped for awhile. Initially I thought that if I could find the fb radii at the nut stop and bridge, I could just add 1" to that, like I had with the setup templates.
That didn't work.
What I finally realized was that with respect to the cutting templates, a portion of that 1" setup offset is a fixed value, and adding that fixed value to the fb radius will help you solve for the template radius.
Here's an end view of the setup. I've shaded the area in question:
The setup template suspends the rails 1" above the face of the fb. The width of the rails (from center to center) is 6.15":
The fixed value is going to be the difference between the 1" offset height and the sag:
Back to the blackboard:
sag = radius - sqrt(radius^2 - (chord/2)^2)
= 10 - sqrt(10^2 - (6.15/2)^2)
= 10 - sqrt(100 - (3.075)^2)
= 10 - sqrt(100 - 9.456)
= 10 - sqrt(90.544)
= 10 - 9.515
= 0.485
So the sag = 0.485
And the fixed value is 1" - 0.485"
= 0.515"
NOW we can find the radii of the cutting templates.
First the nut stop template:
You just add the fb radius and the fixed value = the triangle's altitude (7.954), and we know the base is one-half of the chord (6.15/2) = 3.075.
a^2 + b^2 = c^2
7.954^2 + 3.075^2 = c^2
63.271 + 9.456 = c^2
72.727 = c^2
sqrt(72.727) = 8.528
8.528
Who knew?
8.528 is the radius of the nut stop cutting template.
Do the same for the bridge cutting template.
Again, you just add the fb radius and the fixed value = the triangle's altitude (19.437), and the base is still = 3.075.
a^2 + b^2 = c^2
19.437^2 + 3.075^2 = c^2
377.796 + 9.456 = c^2
387.252 = c^2
sqrt(387.252) = 19.679
19.679 is the radius of the bridge cutting template.
SEE? It's not that hard.
Actually, it's infinitely easier if you have all this in a spreadsheet.
Forum rules don't allow uploading Excel files, but I'd be happy to email the file to anyone who's interested.
(crickets)
Well, if you change your mind, send me a PM.
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Re: Compound radius fretboard shaping jig
Something I just noticed:
The drawing: setup 15.gif
The setup templates should still be in place in this drawing, as the nut stop cutting template has not been attached yet.
AFTER both sets of cutting templates are attached, THEN the setup templates can be removed.
The drawing: setup 15.gif
The setup templates should still be in place in this drawing, as the nut stop cutting template has not been attached yet.
AFTER both sets of cutting templates are attached, THEN the setup templates can be removed.
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Re: Compound radius fretboard shaping jig
Great illustrations and explanation Dennis. Thanks for taking the time to explain it all (and showing us your math work!)