doncaparker wrote:
Another fix: Don't do a tapered heel. Just make it a D shaped heel.
Which thankfully I use.
Thanks, everyone. I'm going to digest all this along with my lunch.
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| Accommodating upper bout curvature when fitting dovetails http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=53926 |
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| Author: | James Orr [ Fri Feb 05, 2021 2:52 am ] |
| Post subject: | Accommodating upper bout curvature when fitting dovetails |
I'm getting ready to use my LuthierTool neck angle jig for the first time, along with using a dovetail for the first time. I have a fairly good sense of the order of operations, but do have a specific question about a mental hurdle I'm running into when it comes to accounting for the curvature of the upper bout. I'm going to give Chris a call, but I'd love to see what the hive here has to say as well. Once you cut your tenon, you need to relieve the heel area a bit in order to account for the curvature of your upper bout (if there is any---mine has a 14' radius) to get a flush fit against the rim. But doing so also necessarily makes the dovetail narrower at the inner edges of the tenon (where I've circled in my crude illustration). Does this mess up that nicely fit dovetail? |
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| Author: | bluescreek [ Fri Feb 05, 2021 6:59 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
I flatten the area with a sanding block when pre setting here is my video on the process https://www.youtube.com/watch?v=FNcbA1P95KE the flossing the face with sand paper will make the curvature to make a perfect fit. |
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| Author: | Colin North [ Fri Feb 05, 2021 7:27 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Yea, I have this to look forward too and have been assessing geometry, etc. I'm coming to believe it's better to start with a mold flat in the area of the neck block. |
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| Author: | SteveSmith [ Fri Feb 05, 2021 7:53 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Keep us updated James, I'll be facing the same issue soon. That's assuming I get a break in setups/repairs so I can build a neck. Sent from my SM-G970U using Tapatalk |
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| Author: | Colin North [ Fri Feb 05, 2021 8:26 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
SteveSmith wrote: Keep us updated Chris, I'll be facing the same issue soon. That's assuming I get a break in setups/repairs so I can build a neck. Sent from my SM-G970U using Tapatalk Who's Chris? |
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| Author: | James Orr [ Fri Feb 05, 2021 9:34 am ] |
| Post subject: | Accommodating upper bout curvature when fitting dovetails |
My primary motivation for using the dovetail is efficiency. If we glue the fretboard extension anyway, and we’re tooled up to get a good fit off the jig, why not eliminate the extra steps of threaded inserts and bolts? That’s my thinking. I’d prefer not to have to change the body for it. If I had to weigh the two, I’d opt for sticking with a bolt on mortis/tenon. BUT I really want to use the dovetail, so I’m going to keep thinking through it. Sent from my iPhone using Tapatalk |
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| Author: | doncaparker [ Fri Feb 05, 2021 10:35 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
James-- Lord knows I'm no expert, but here are a few thoughts for you: I think it is important to distinguish between a few things, and what they do in this joint. First, other than the edges of the heel that show (which butt up against the body and define the neck angle and left/right leaning), all edges should be rounded off and gotten out of the way. It is the faces of the parts that do the work, not the edges. So, the parts that you have circled are not really important. In particular, you should round over the mortise outside edges that would otherwise fit into those inside edges you have circled. Also, the faces of the neck that are between those inside edges you have circled and the show edges of the heel do nothing in the joint. You want there to be open space between those edges, terminating in a relatively thin (1/8" to 3"16" wide) strip adjacent to the show edges of the heel. The joint is a sliding tapered dovetail. It only slides in the direction from the top toward the back. So, even if the body is rounded (and the show edges of the heel get a matching curve, by flossing later), the rest of the joint should have straight and flat faces that meet up with each other in a wedge fashion. The faces that do the work are the two side faces of the tenon and the two side faces of the mortise. Those pinch each other as the joint goes together, and they pull the neck toward the body as they pinch harder. Eventually, they come to be so tight against each other that no more pinching can happen, because the show edges of the heel are pushed as far as they can go against the body. This all works best if those tenon and mortise side faces are flat and straight (albeit at the angles defined by the dovetail bit you used and whatever carving you do afterward). The fitting is easier if, in addition to the side faces of the tenon and the side faces of the mortise, the body is flat and the show edges of the heel are flat. You can have the body be curved, but it means more fiddling around with the show edges of the heel later, to get them to match up with the curved body. I hope that helps. This is easier to talk about in person, with an example in hand. Even then, it is hard to visualize sometimes, because the working parts of the joint are never visible while the joint is in use. Good luck with the execution of the joint! I defer to anyone/everyone who knows more about this than I do. |
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| Author: | meddlingfool [ Fri Feb 05, 2021 11:35 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
I see the dilemma. It’s been a while since I messed with dovetails, but... What Don said is true, the outside corners of the mortise should be broken/rounded/chamfered. Anyway...this is what I -believe-... In your pic, as you relieve the space between the inside corner of the dovetail and the and the external part of the heel that becomes the outside visible joint, the relief cut will cause the touch point to move further along the radius away from centreline on each side. This will have the effect of loosening the joint. To account for this, I believe you need to adjust your tenon cutting jig so that once you cut the tenon, the whole neck sits proud by an amount above the surface of the body. Then, as you make your relief cut, the joint will slide down into final place. You just need to figure out how proud to start with so it ends where you like. Or be pragmatic and flatter your neck area. That being said, there won’t be a huge depth difference between the mortise corner and where the external corner of the heel meets the body, so you can probably make it work. Hope that makes sense... |
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| Author: | James Orr [ Fri Feb 05, 2021 11:36 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
doncaparker wrote: The faces that do the work are the two side faces of the tenon and the two side faces of the mortise. Those pinch each other as the joint goes together, and they pull the neck toward the body as they pinch harder. Thanks, Don. Conceptually this makes sense. The side faces of the mortis and tenon are what pull against each other when the joint is under tension. My thought was that by relieving the mortis, you make it narrower, but technically the parts of the sides you don't relieve stay never change and should still pull together. I'm going to watch John's video again this afternoon. I started this morning before leaving for work. I might also make a model because this geometry stuff is not my strength. |
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| Author: | meddlingfool [ Fri Feb 05, 2021 11:44 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Also, it’s harder to get the side to side angle right on a curved block as the neck blank can pivot on the centreline. A workaround would be to double stick a few binding offcuts on the outer edges of your neck blank so that it straddled the centreline instead of rocking on it. |
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| Author: | johnparchem [ Fri Feb 05, 2021 11:49 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
I do not do a lot of dovetails, but I do have smaller instruments with the upper rims radiused. With a classical heel shape, there is a pretty complicated curve on the heal cheeks as it follows the curved rims. I cut the tenon loose so that I can set the neck angle with a clean joint with the rims. I then shim the the tenon and fit it. This is less of an issue with heels that go straight back, but I still like to start with the dove tail loose enough to set the neck angle first then shim and fir the dovetail. |
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| Author: | doncaparker [ Fri Feb 05, 2021 11:51 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
James-- Let me springboard off of what Ed mentioned, because this may be the most important difference between having a flat body/show edges of heel, versus a round body (to which the show edges of the heel are conformed). If the body and heel are flat, then the fact that there is a width difference between the heel at the top and the heel at the back doesn't matter. It all sits on the same plane. However, if the body is curved, then the part of the heel that is near the top sits on a different plane from the part of the heel that is near the back. Stated another way: If you look at the face of the fingerboard at the neck/body joint (guitar standing straight up), the heel is all level when it sits against a flat body, but on a round body, the narrow part of the heel that is near the back is higher than the wider part of the heel that is near the top. Other than hand flossing, I don't have experience with any other fixes for this. I figure others might, and again, I defer to them. It sounds like Ed has experience with this. For sure, make some test joints. I found that cutting up a cheap 4x4 post to make test blocks works great. |
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| Author: | Clay S. [ Fri Feb 05, 2021 11:58 am ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
John and Glenn, I just finished watching another outstanding video. I would like to thank you for the videos you have made over the years to help people build better guitars. I have watched several of them, and they are always informative and professionally done. Hats off to you! |
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| Author: | doncaparker [ Fri Feb 05, 2021 12:02 pm ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Amen to Clay's thanks for John's videos, in particular the series on fitting a dovetail (relevant here). I have learned a lot from them. |
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| Author: | meddlingfool [ Fri Feb 05, 2021 12:23 pm ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Don, that’s a good point. With a tapered heel on a curved body, cutting the relief channel would have more effect at the FB than at the heel which would also bring the neck angle forward, and cause the joint to be loose at the top and tight on the bottom... The more I think, the more complex it gets. I would just flatten the block til you get the hang of dovetails, lol... |
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| Author: | doncaparker [ Fri Feb 05, 2021 1:02 pm ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Complex, indeed. This is why I have a little flat spot (2.5" wide) under the neck regardless of how the rest of the shoulders curve. Another fix: Don't do a tapered heel. Just make it a D shaped heel. |
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| Author: | James Orr [ Fri Feb 05, 2021 2:09 pm ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
doncaparker wrote: Another fix: Don't do a tapered heel. Just make it a D shaped heel. Which thankfully I use. Thanks, everyone. I'm going to digest all this along with my lunch. |
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| Author: | SteveSmith [ Fri Feb 05, 2021 8:01 pm ] |
| Post subject: | Re: Accommodating upper bout curvature when fitting dovetail |
Colin North wrote: SteveSmith wrote: Keep us updated Chris, I'll be facing the same issue soon. That's assuming I get a break in setups/repairs so I can build a neck. Sent from my SM-G970U using Tapatalk Who's Chris? Ha ha, thanks Colin, it would appear that I got my threads crossed.  Meant to address that comment to the OP, I fixed it. |
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