Neck tenon - how tight? (and the jig I made to do it)

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Neck tenon - how tight? (and the jig I made to do it)

Postby Steve Sawyer » Sun Jun 16, 2019 8:58 pm

I have a question, but my main purpose is to share how I cut this tenon. It's not my technique, but something that I picked up from some guy on YouTube a couple of years back, with a minor modification.

The question is, how much "windage" should I allow in the fit between the neck tenon and the body? As I snuck up on the final thickness, my thinking was that I don't want to have to do much "persuasion" to get the tenon inserted when dry. I'm expecting the glue (it will be fish glue in this instance) is going to cause the wood to swell a bit, so if it doesn't slide right in when dry, I might be trying to find some way to mechanically force the joint together once it's got glue spread over all the surfaces! Note that what is pictured below is a "test" neck - I didn't want to have any process suprises when I get to doing the actual neck which is just about ready for this step.

The largest feeler gauge I can force between the tenon and the side of the mortise is 0.010". I'm thinking that might be more than I need to address my concerns.

So anyway, this being a flat-top Lester, I calculated the proper neck angle to be 1 1/2°. The "floor" of the mortise was routed at this angle, so the tenon can be cut straight with no taper. However, I still need the shoulder to be cut at an angle so it meets the body properly.

I made a box just wide enough to accept the still-squared neck, and sliced off one end at exactly 1 1/2°. The idea is to create an angled surface against which to run a top-bearing router bit to get the tenon shoulders at exactly 1 1/2° The guy I saw do this then secured the neck within the box with wedges. I didn't like that (and my test neck is a bit twisted, which is why it's a "test neck"), so instead I bored three holes on each side and tapped them for 1/4-20 set screws and this worked super well.

Once this is all put together, I just run the two sides of the body-end of the neck across the router bit in the router table/extension. I had to change bits at one point to get enough length to do the job, but on the final cuts I kept cranking up the bit in tiny increments until the tenon was just narrow enough to fit into the mortise. I repeated the process for the bottom of the tenon. Because the bit is vertical, the angle on the end of the box does nothing for me when cutting the bottom of the tenon, hence note the tiny "step" visible between the heel and the body in the third picture below. When I do the "real" neck, I'll put a couple of layers of tape or maybe stick a piece of veneer on the bottom of the box so I can go back and do a bit of fine chiseling to clean that spot up. Also, note that while the angle is perfect, because the body starts to curve a bit, I may need to do a little bit of undercutting to get that joint to close up. I'll wait until I've band sawed and routed the neck to the final profile as that will reduce the size of that gap.

All in all, this technique worked extremely well. One thing I'll do before doing the next one is make some kind of pin or wedge to lever the neck against while running the router bit cross-grain across the width of the tenon. The little post that is made for the router plate I use (visible in a couple of the pictures) is much too short and much too close to the bit to work real well. I ended up putting a long bolt and a stack of nylon spacers into the T-track strip you can see in the first pic, and that worked better, but I can improve on it further. I could also use a miter gauge.

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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby Freeman Keller » Mon Jun 17, 2019 12:27 pm

Pretty clever, Steve. I have always just cut the neck heel angle by hand and everything else on my band saw, doing the final fit with chisels and sandpaper. The main gluing surface is the bottom of the tenon but I like a snug fit in the mortise - it slides in and out by hand but I could pick up the guitar by the neck and the body didn't fall off

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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby Steve Sawyer » Mon Jun 17, 2019 2:52 pm

Thanks, Freeman. That's a good indicator.

I'm going to use the band saw to rough out the bulk of the stock on the "production" neck, just to save wear & tear on the router bits, and to avoid having to change bits mid-process.
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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby Freeman Keller » Mon Jun 17, 2019 3:44 pm

I try to keep as many straight square sides as possible for as long as possible to reference against. The only thing I use a router for is the truss rod channel and the shape of the head. One trick I recently learned is to use my Safe-t-planer to do the back of the neck - by shimming at the nut end I was able to make that flat and straight.

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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby Bob Francis » Mon Jun 17, 2019 9:32 pm

That is very clever I need to break down and get one.
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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby Barry Daniels » Tue Jun 18, 2019 11:18 am

To answer your question about fit tightness. Yes, glue will swell the wood a bit, so you don't want a real tight fit to start with. However, it shouldn't be a sloppy fit either. And working quickly will also help. Have your clamping figured out beforehand by doing a couple of dry runs.
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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby Steve Sawyer » Tue Jun 18, 2019 11:46 am

As you can see, I too keep everything straight and square as long as possible. Good suggestion re tapering with the safe-t-planer. This neck only needs to taper about 1/16 from heel to nut, so I'm not sure I'll try that, but I do have two "test" necks to experiment with.
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Re: Neck tenon - how tight? (and the jig I made to do it)

Postby John Clifford » Fri Jun 21, 2019 12:26 am

I use this jig for both my dovetail and mortise and tenon neck joints. I can dial in whatever neck angle I want. For the dovetail joints, a router template fits over the top. For the m&t joints, the surface of the top plate is aligned with the shoulder of the neck joint. Then I use one of those Japanese saws lying flat on the top plate to cut all three sides of the shoulders on the same plane.

I agree with what others have said about the tightness of the dry joint. Being able to pick up the guitar by the neck is a good test.
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