Official Luthiers Forum!

Hi, I'm builidng a hollow body guitar. I left the neck pocket area untouched as I still yet to figure out how to calculate the perfect angle for the neck. As you can see the binding area is almost 1 cm from the top. Is there a way to make the break angle look nice without it being to extreme.

I’m not entirely sure what your question is. Generally you want to establish the break angle of the stings over the bridge. The neck angle is derived from that.

You want sufficient to drive the top and not let the strings slip on the saddle but generally as flat as possible. Allan Carruth did an experiment on how much was needed. I believe he found that you needed at least ten degrees, but adding much more didn’t improve the tone noticeably. Keeping things relatively flat helps with practical things, like fitting the guitar to a case.

You would establish you neck geometry off of the bridge you intend to use, your tailpiece, and a reasonable action within your bridge’s range of adjustment.

In my guitars, which are smaller than an L5, and my hand made bridges, I end up around 4.5° to 5° neck angle. But it is entirely dependent on your design.

I hope that helps.


Sent from my iPhone using Tapatalk Pro

I'm with rlrhett in not understanding your question. Break angle determines the amount of down force on the top and is at least partially determined by how much arching the top has.



RM Motolla has a wizard on his website to calculate the downforce, but its pretty straight forward high school geometry.

Neck angle (along with overstand) sets the geometry of the neck relative to the bridge which determines action and playability. Again, it depends on arching of the top and dimensions of your bridge. I like to set the neck angle so the fret plane just touches the tops of the saddles at their lowest adjustment.

This is one of the areas where it can be really beneficial to grab a pencil and draw it up, full size, to check the geometry works, before cutting any more wood.

I've gotten into the habit of doing that routinely. Nothing beats a full size drawing, especially on a new design, to get all of the geometry figured out before cutting any wood.

Dave

As is it doesn't look to extreme to me but you don't normally attach the bridge directly to the top like that.

As Freeman said, there are two things to think about here, the break angle of the strings over the saddle, and the overstand; the height of the neck above the top. They're related, of course. What you seem to be objecting to is the high overstand?

If you think of the strings as being an extension of the neck, you'll see that the whole business can rotate around the point at the top of the bridge. In the limit the end of the neck can be flush with the edge: zero overstand. Reducing the overstand actually increases the break angle, all else equal. Adjusting that is a matter of how you cut the neck pocket.

As rlrthett said, I did a rather involved experiment some years back looking at the relative effects of break angle and string height off the top in the sound of a Classical guitar. The conclusions would generalize pretty to steel string flat tops, but are less applicable in general to arch tops. However, the stuff about the minimal break angle certainly holds.

In my experiment, I used a mechanical 'plucker' to activate the strings. It always plucked them with the same force (within about 2%) at the same location and made the string vibrate 'vertically' with respect to the soundboard plane. The strings traveled 'up and down', with no 'crosswise' component. In that circumstance, a break angle of six degrees was 'enough'. What defines 'enough' is the fact that the string vibration stops at the top of the saddle. If you don't get enough break angle the strings can hop off the top of the saddle when they're pulling upward. If this happens more than minimally (one hop right at the start, say), you'll know it.

Real strings get pulled aside as they're plucked, and they can be dragged across the top of the saddle, or simply roll sideways. On a flat top you need more break angle to prevent this, but most archtops, in fact, notch the saddle specifically to prevent this. Thus an archtop can get away with something pretty close to six degrees of break (174 degrees on Freeman's digital protractor) without problems. On a flat top I suspect that you need closer to 12-15 degrees.

Again, the thing to keep in mind is that more than 'enough' break angle won't increase the output of the guitar. As long as the string stays in contact with the top of the saddle throughout it's whole vibration cycle without moving all of the energy that becomes sound will be transmitted to the top. In my experiment increasing the break angle from 6 degrees to 25 degrees didn't have any effect on the sound. Nobody could hear any difference in 'blind' comparisons of recordings of the sound made in very carefully controlled conditions, and there was no change in the amplitude or duration of the recorded plucks.

More break angle does increase the download on the top, which is a bad thing in general on arch top guitars. Since arch top generally don't use sound posts like violins do the 'cold creep' of the wood in the top can eventually allow it to sink quite a bit. There are ways to mitigate this with adjustments in the arch shape, particularly in the recurve area, and the graduations, but none of that will completely solve the issue. Minimizing the down force at least postpones any problems.

As to sound: at one point, years ago I did an experiment on that too. I had made an archtop with a 'hook' tailpiece: sort of an 'L' shape, where the short arm hooked over the end of the guitar and the strings attached at the top. The line of pull of the strings goes from the attachment point to the pivot point down at the bottom end of the 'L', so the strings can effectively go 'through' the top if the pivot is well below the edge, and produce a high break angle. The idea, of course, was to get more down force and thus more sound. It didn't work like that. Instead what I found was that when I moved the pivot point too far down the sound just died. This was quite reproducible; at a certain point the power, sustain, and clarity of the sound went away, if memory serves (it was some time back). At the time, before computers with sound cards, there was no way I could actually document what was happening, and, of course, I've learned some things since. I can't give you any objective data to back that up, and any model I could propose would be conjecture.

At any rate, since then I've been looking for ways to reduce the break angle on my archtops. I note that in his book, Benedetto shows about a six degree break. If it's good enough for him...

Exactly! Pick up your planned top profile, bridge height, and develop the fretboard and neck profile from there. Adjust for expected distortion of the top under string load to get the minimum and maximum extension of the bridge fixed.

A few years ago, I was 'cataloging' the shop's construction drawings (actually, rummaging around in piles of dusty paper), and came across the full-sized sketch for the gentlemen's near-twin guitar-shaped octave mandolins made back in 2012. These instruments were built for an online competition, and the first carved-top instruments for either builder. We had one of those instruments in for a pickup installation at the time, which led to a measurement session. The biggest variation was in top profile, where the bridge had been raised slightly to accommodate the flattening of the top by about 3/32". On closer inspection, one of the boys had sketched in a faint line about 1/16" beneath the top profile and noted the need to add this to total bridge adjustment to maintain the design break angle...not a bad guess for a first time.

In discussion, the key benefit of the full-sized layout was that the boys verified that a neck-adjusted truss rod (a repurposed Martin Size 5 double-action rod) could be fitted, with just the need for milling of an access channel in the neck extension over the body...no truss rod cover or adjustment nut excavation needed, and all but invisible.

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower

Thanks guys. Yes, I did draw out a quick diagram to help me get the correct break angle. It seems I need 9 degrees on the top to get zero overstand, but this will make the guitar unplayble because there is no action. To counter this, I need to make a 4 degree angle on the butt end of the neck. It sits a bit higher than I calculated maybe because my drawing wasn't so accurate. As of now, I'm happy because I can simply chisel away very little material to get the action I need, but I'll do this after I permanently fix the bridge posts into place.

I guess I could have been clearer when I said to 'think of the strings as an extension of the neck'. Think of the side view of the neck with the strings above it, and extending out to the bridge. You can pivot the neck around the fixed end point of the bridge top by raising or lowering the heel and the over stand. The strings coming off behind the bridge will point to the top of the lower saddle, or whatever you're using as the pivot of the tailpiece, and the angle between the back strings and the strings coming off the neck is the break angle. You need at least six degrees of break, or a little more. If the over stand is not enough then you'll need to either go to to a taller bridge or figure out some way to increase the break behind the bridge. Of it's more than you want with the over stand you like then you can raise the hinge point of the tailpiece above the edge by some means. The point is that including the strings as part of the neck automatically preserves the action height over the fingerboard as you make these decisions.

Yup, I know exactly where the pivot is. I'll make the the final adjustments after the nut arrives and the bridge posts are permanently fixed. I'm sure I'll get a nice action height by the end of this build.