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Now that I have my CNC machine up and running, my first project is to make some routing templates for my O'Brien neck angle jig that will work for Benedetto-style dovetail joints. I've used a different style dovetail on my archtops before, but I thought I would try the Benedetto style now.

I confess his approach has me a little confused, and I'm hoping someone can help me make sense of it.

Benedetto uses a dovetail, but it does not have a v-shaped taper. Instead, he makes something that is an interesting cross between a mortise-and-tenon and a dovetail joint.

But here's the confusing part: On the neck dovetail, the narrow part of the mortise is 15/16". On the body, the top of the mortise is 1" at the narrow part, and then it tapers down to 15/16" at the bottom of the mortise. So, the top essentially has 1/16" of wiggle room. The way he describes it in the text of the book and his video suggest that the bottom of the mortise is actually narrower than the tenon, such that it squishes in, but the dimensions in the book suggest the opposite.

Does anyone here use his joint? Do you make it with a 1/16 gap at the top, as the dimensions in his book suggest? Or do you taper it so that the dovetail has to compress 1/16" at the bottom? Or do you skip the taper and just make it even all the way down?

Not sure how the double-post happened. Is there a way to delete this one?

My take after reading the book and watching the videos is that it is almost a sliding dovetail but with just a little taper so that it does wedge in place once you have the neck set. But I haven't made one and I could be misinterpreting. At any rate, it certainly doesn't have the pronounced V of the Martin style - almost the opposite, it's only very slightly tapered.

Which brings a question to my mind - what if you constructed the joint as an actual sliding dovetail, no taper or wedging at all, and used a single, possibly quite small bolt or even screw through the neck block to hold the alignment?

_________________
Jim Kirby
kirby@udel.edu

I made one using Benedetto's design. The bottom of the mortise is not smaller than the dovetail. You cannot compress wood like that. In fact, there has to be a little bit of wiggle room because when you put glue on everything swells a bit. The goal for the dry fit is that it should be loose at the top and as you slide the neck in it should go down to about 1/4" from the bottom before things get snug. When you put glue on it it will get snug when the neck is about half way down and you will have to use a clamp to get it the rest of the way.

Remember that the dovetail and mortise is not routed to final dimension. You use hand methods to refine the joint after it is routed. So your templates can be straight without incorporating the taper. The mortise can be easily opened to the taper with sanding sticks, files or chisels.

Thanks! Barry, that explanation makes sense to me, but the dimensions seem aggressive -- the dovetail is a full 1/16" narrower at the top than the mortise (1/32" on each side). That's a lot of swelling!

Jim,
I have a pretty good feel for dovetail joints and have reset quite a few necks with them so my take on putting a screw in a dovetail is that you would be completely defeating the purpose of the dovetail and might as well just make a M&T joint with bolts. The beauty of using the tapered dovetail is that it is a self locking joint and in most cases really doesn't even need to be glued. Although I always use glue on a dovetail when I reset a neck so it doesn't pop out if the guitar receives any shock or good bump to the neck. My biggest reason for gluing it is mainly as a lubricant when I set the neck all the way in. I do all of my adjustments within about a 1/32 of an inch to fitting all the way down into the joint so when I apply glue and clamp it it seats down that extra 1/32".

I have heard of people using a bolt through the headblock to lock the dovetail in place but they usually blunt the end so it just applies pressure to force the joint together but personally I still think that defeats the whole reason for a dovetail. In my opinion the dovetail is one of most effective joints in all of woodworking and it ideal for a guitar neck since it is self locking there is no need for any fasteners of any kind.

Of course this is just my opinion and preference and I am sure someone else will have a much different on than I do since it seems the consensus among most builders is the M&T is a better joint for guitars. So take my opinion for what it is worth.

Kelby, the slop at the top is to ensure that the joint does not bind up there. The place where it needs to be really tight is at the bottom.

Just build it as a straight joint that will fit tight the entire length. Then use a sanding stick to widen the mortise at the top so the joint starts sliding really easily. Don't overthink this until you make one and it is a good idea to make a mock up using your jig before you commit the good wood.

Thanks, Barry. Will do. When I've made archtops before, I've used the StewMac dovetail template, which creates a v-shaped dovetail. The Benedetto style is new, but I'll try it as you suggest.

One of the great things about having the CNC machine to make the templates is that I can try it one way, and if I want to tinker, it's not that big of a deal to modify the CAD file slightly and have the CNC spit out another template.

Maybe an odd question but since you have a CNC why wouldn't you just cut the joint with that rather than making a template to cut it with a router? It seems once you have the program set it would just be a matter of punching it in for a perfect joint every time.

Am I missing something?

Bob

Thanks for the comments. I get how a regular dovetail works - my speculation was related to a true sliding dovetail, which in principal wouldn't have any resistance to movement up or down. The screw would be to fix it's position. I wouldn't bother with that on a regular tapered dovetail.

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Jim Kirby
kirby@udel.edu

I like the way Rob described the dovetail and I am in complete agreement with it. The advantage to an MT joint is ease and resetting. But back to the Benedetto joint, he makes it clear that the maple squishes the spruce neck block so it is sort of self fitting. I think that is very clever but I dont know if it would work with a mahogany neck. That would be a good experiment.

Rob, excellent question on using CNC to cut the dovetail. The short answer is that my CNC skills are not at the point where designing and cutting the dovetail or body mortise on the CNC would be simpler or faster than cutting it with a jig and a handheld router. So for now, I'll use the CNC to improve my jigs and templates.

One day, I'll design a neck in CAD and set up the machining on my CNC. When I get there, I will certainly include the dovetail in the design and have the CNC cut it all at once. But I've got a ways to go before my CAD skills can handle neck geometry. I'll get there, though. In the meanwhile, I need a template and jig to do it with a router.

Having only done a handfull of dovetail resets I don't speak with much authority. However I have heard that non tapiered dovetails can be more of a problem to seperate due to the steam swelling the joint along it's intire length. As far as using a screw to secure the joint goes the first time l heard about it l was concerned. Tightening it would cause the joint to release some pressure. However if you think of the screw as a pin and install it with that in mind (dont over tighten it) you wont need steam to release the joint in the future...That's my understanding of how Charles Tauber aproaches it and he has been doing it for 30 years this way since being shown by Grit Laskin.

@Kelby, Cool, I wasn't sure if you had experience with a dovetail or not. As for a regular sliding dovetail it should be snug all the way down and not have any slop in it. I have done a few of them by hand on furniture projects so I know a little bit about them. I know doing them by hand is pretty tough to make them fit perfectly (and mine didn't come out as nice as I had hoped) but with your CNC and a template you should be able to get pretty close tolerances to get it to slide right in. I am assuming it is a glued joint? If so then you would want a slight gap for the glue and again the glue will act as a lubricant to help slide it together. As for a bolt to lock it in, I would think that wouldn't be necessary but could give you a tight secure joint if you didn't use glue. Otherwise between gluing the joint and the finger board extension it should be enough to hold it pretty tight. That is assuming I understand what the Benedetto dovetail is.

@jeff crisp, actually a bolt inserted through the headblock (with a threaded insert) would tighten up a dovetail. You would be pushing the tenon part on the neck out which would tighten up the joint. But you are correct that it would be more difficult to steam a sliding dovetail out as opposed to a tapered one.

Cheers,
Bob

A screw through the headblock, through the back of the (dovetailed) mortice with its final anchor point being the (dovetailed) tenon will loosen the joint if over tightened unless your dovetail joint doesn't have a gap at the back between the mortice and tenon, like nearly all dovetail joints on guitars have. It's the gap at the back left for steaming the joint apart (if a neck set is required) that would allow the the joint to loose it's tight fit from the pull from the screw on the tenon, if over tightened.

I think we may be talking about 2 different things here.

This is what I am referring to:

There is a threaded insert in the headblock which the bolt screws through, in turn pushing the tenon part of the dovetail out, locking the joint in place with no glue required.

Here is a quick little drawing to show what I mean.

Bob

Two different things, yep, a picture can speak a thousands words. That'll work. Thanks Bob.

Bob, that's a very interesting concept and your illustration explains it very well. My only concern would be that the bolt could push the neck forward far enough to open gaps between the heel and the guitar sides.

When I first read the start of this thread, I had to go check the dimensions of the dovetail in my Benedetto book. There is a little bit of confusion added if you watch the neck process on his Video /DVD series. I haven't watched it in a long time, but here's what I remember: He does use a straight, sliding dovetail on the neck. But I believe he routs in a very slight taper in the end block. He explains to his students that he used one of his very last mahogany end blocks on this particular guitar. He slides the neck in for a dry fit and presses it home, checking for alignment, etc. When he removes the neck, he points out small rub marks transferred to the bottom of the dovetail tenon by the darker mahogany when he pressed it in. The implication I drew from this is that the bottom half-inch or so locked very tight, compressing the two parts together at that critical area.

In any event, I cut my neck dovetail in tapered fashion by hand with a tenon saw, and fitted it into a tapered, routed mortise in the end block. Fitting for alignment was a somewhat tedious process for me, as it was only the second one I had ever done. But it wasn't particularly difficult. Just slow, tedious and time consuming. The necks I've made since (mandolins, a uke and a banjo) have all been bolt-ons. I would personally choose bolt on guitar necks from this point forward.

@cphanna,
That drawing is nothing I have actually seen before but a reference to what I think people have talked about when they say to put screws into a dovetail. It is the only way I could think to make it work and give you a tight secure joint with no glue.

I have never seen a Benedetto dovetail but from your description it sounds like a wide tapered dovetail because that is pretty much the explanation of a regular Gibson or Martin type joint. They fit quite loosely until they are almost seated and then the joint pulls itself together tightly the last 1/4".

I have to say I like the idea of the wide joint over the narrow one. It would seem to be easier to fit since you have more room to work. I may have to look into it more. Does anyone have any links where it shows it better? I could only find some general pictures but no details.

Bob

The first few dovetails I made back in the early nineties fitted like perfect. In fact I tapered them the last one I made I didn't bother gluing had to drive it in slightly. It's still holding after all these years.
However I just made one for a small parlor guitar and a baritone Uke. I must have lost my touch as they no longer fit so good no more. Plus I have to do them all with hand tools. I'm starting another baritone Uke this time I will be making a bolt on. As I've never done this before it should be exciting. Good luck with your CNC machine.


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