The Mystery of the Curved Truss Rod Channel........Solved!

From Wikipedia:

A truss rod is a guitar part used to stabilize and adjust the lengthwise forward curvature (also called relief), of the neck. Usually it is a steel rod that runs inside the neck and has a bolt that can be used to adjust its tension. The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921, although the idea of "truss rod" can be encountered in patents as early as 1908.

If you didn't understand that I'll explain a bit more. When a guitar is strung up and tuned to pitch the strings exert quite a bit of pressure on the neck. Most of the time the pressure is so great the neck will bend forward. This forward bend must be corrected in order for the guitar to play properly. The truss rod is designed to correct the forward curvature cause by string tension. When the truss rod is tightened the neck straightens out. The truss rod is placed in a slot cut down the middle of the neck. The type of rod determines how deep the slot must be cut.

There are two types of truss rods I've considered for this build. The first is a vintage truss rod originally designed by Gibson. It consists of a long, round piece of 3/8" (approx) steel with an anchor on one end and a nut on the other. The nut is used to tighten the rod and straighten the neck. The other kind of rod is a dual action truss rod that consists of two round pieces of steel attached at both ends.

For this build I chose the vintage rod because that is what they used in old Les Pauls. Actually they still use them in current models. A vintage truss rod is a type of spring. When you tighten the nut the rod tries to straighten itself out. The straighter the rod gets the less effect tightening it will have. For this reason the channel the rod sits in must be cut with a curve in the bottom of the channel. The curve ensures that even if the neck is straight the truss rod still has a bit of tension in it. This tension allows the rod to put the neck in a backbow if necessary. This graphic (I believe drawn by John Catto) illustrates how a curved truss rod fits inside the neck:


After the slot is cut for the rod and the rod is placed inside the rod is covered by a strip of wood called a filet. In this case the filet will be maple. The bottom of the filet has the same curve as the bottom of the truss rod channel.

OK, so the big problem is how to cut the curved channel. There has to be some kind of jig to perform this function. Thinking about how to carve the curved channel almost led me to use the dual action truss rod instead of the vintage. The reason is the dual action rod requires a straight channel which can be easily cut with a table saw.

Finally after a ton of research I ran across this thread at the TGP forum. That thread is full of great jigs and advice offered by some of the best luthiers in the business including Roman Rist, Jack Briggs, Terry McInturff and Scott Lentz. In page five of the thread Scott Lentz has a jig that he uses for cutting a curved truss rod channel. One look at his jig convinced me that I could easily build something similar.

Now I had to build the jig for cutting the curved truss rod channel. Here's what the finished jig looks like with the neck mounted on it:


The construction is simple. You need a planed board about 30" long, 8-10" wide, and about 3/4" thick. I found some poplar at my local hardwoods store that fit the bill nicely. You mark where a few frets are for reference points. Then you take your truss rod and bend it a bit while you trace the shape of the bent rod into one side of the wood. I got the idea of tracing a bent rod from Roman Rist who I communicate with on another forum.

To shape the curve of the rod into the board you could use a band saw but I chose to use my oscillating spindle sander. It took me about ten minutes to rough out the curve with the spindle sander. I then finished it off sanding by hand.

I put the neck on the jig and used it to help place the clamps. I made it so the clamps touch the neck on parts that will eventually be cut away. I screwed the clamps into the maple and the jig was done.Here's a good top view where you can see the curve in the board in relation to the neck:The other piece of wood you need is something to mount the hold-down clamps. In this case I found some scrap maple at the wood store. It's perfect for the job. I got the clamps for five bucks each from Harbor Freight. I'm not a fan of Harbor Freight but for things like the clamps it's good. I screwed the maple onto the poplar like this:


I put the neck on the jig and used it to help place the clamps. I made it so the clamps touch the neck on parts that will eventually be cut away. I screwed the clamps into the maple and the jig was done.


If you look closely at the above picture you will be able to see the curve in the jig. By looking at the relation of the jig to the top of the neck you can see that the board is wider at the ends of the neck and virtually even with the neck in the middle.

One of the biggest pains of this project was finding an arbor long enough to mount a 3/16" slot cutter at the proper height. Most arbors are 3" long which isn't long enough. While I was at the tool store I eyeballed a router bit that was a little over four inches long and had room for two cutters separated by a bunch of spacers. I think these types of bits are used on moldings or something like that. I asked the salesperson the price of the bit and he told me it ran around $160. Yeeeech. It turns out I was able to buy the just the arbor and a bunch of spacers for around $30.

I bought a 3/16" slot cutter with the arbor and used the 7/8" bearing that came with it for the new arbor. When I set up the new arbor I found I didn't have enough spacers to reach the threads at the top of the arbor. Without more spacers there would be no way to tighten the locking nut of the cutter. I ended up buying another 7/8" bearing as a cheap way to add another spacer. Here's what the cutter looked like when I finished putting it together:


Here's how the whole thing works. The 3/16" slot cutter/arbor is mounted in the router table. The bearings follow along the edge of the jig that has the curve of the rod. As I get into the middle of the jig the curve of the jig gets deeper. At it's deepest point this setup should cut a channel of 1/2". I can manipulate the depth a bit by pulling the neck closer to the edge of the jig or pushing it back. you set the height of the cutter so that it hits the centerline of the neck.


I did some preliminary testing on this setup and found that the bit struggled when cutting the slot and burned the wood. Seriously as I was cutting the garage filled with smoke. Not long after I found I had mounted the cutter backwards and it was cutting the slot with the blade going backwards. DUH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I turned the cutter over and the blade cut perfectly. In fact it cut a perfect channel.

Here are some pics:





At it's deepest the channel is exactly 1/2" deep:


The rod fit perfectly in the channel:


Now I have to cut the maple filet that goes above the truss rod. I'll do that tomorrow and maybe even install the rod!

It goes without saying that I'm very relieved this went so well. I'm very happy that I now have a great way to carve a curved truss rod channel.

Going Overboard

The body thing still bugs me a bit although most of my annoyance is gone. However, a week or two ago I was still torn up by it. I decided to turn my attention towards the neck of the guitar.

Prior to the start of this project I ordered a neck blank from Stewart Mac and rough cut a neck out of it. When cutting out the blank I made rookie mistake. I figured I'd make my first cut with a table saw. This cut would be on the underneath portion of the neck and extend from the tenon all the way to the bottom of the headstock. I started cutting on the tenon side of the neck and stopped about an inch and a half from the back of the headstock. Then I took the neck over to the bandsaw to trim the rest.

As I trimmed out the back of the headstock with the bandsaw I noticed there was a big notch cut out of the back of the headstock. My big mistake when making the cut with the table saw was not stopping a few inches behind the headstock. The problem is I was seeing the blade of the table saw from the top which stopped well behind the back of the headstock. However the blade is circular and the part of the blade you can't see actually sticks out further. It was the part of the blade that I couldn't see that bored into the back of the headstock and left the notch.

All was not lost. I had just enough room on the neck blank to redraw the neck and cut it again. I just moved everything back about an inch. This gave the new back of the headstock just enough room to miss the notch. I had to compromise during the redraw to miss the notch so the headstock is about 1 1/8" thinner than is specified in the plans.

Fast forward to the middle of December. I have been struggling with some of the procedures I need to perform in order to build the neck. One of those is how to carve a curved truss rod channel. I'll cover how I'm going to do this in the next blog post. I'd settled on a plan and needed a maple board in order to make it work so I headed to my local hardwoods emporium. Just for kicks I asked the guy if he had any quartersawn mahogany around that was big enough to construct a neck blank. We went over to the mahogany and found a great board that was big enough to construct three necks. I bought it along with the maple. I went home and got to work and this is the result:



Yup, three new necks! The top neck is the original. Notice how the headstock (the short part that is angled) is thinner than the others. The neck is usable but not for the guitar I'm' building. I have something else in mind for the original neck.

So what am I going to do with the other three necks? One will go on the guitar I've been writing about. I haven't decided what the other two will go on but I'm thinking a Les Paul with a quilted maple translucent blue top and a Les Paul Custom with a Bigsby vibrato. Anyone have any ideas?

My camera ran out of batteries so I can't show the steps for roughing out the neck. I will give a brief description how it's done.

The first thing you want to do is true up the blank. This means making sure that it is square. I did this with a table saw and a jointer. I then inspected the blank so I could avoid any worm holes or other problems with the wood. Once I'd decided which side of the blank would be the fretboard side I began to draw the profile of the neck on the blank. This is done with careful measurements and a protractor. The protractor is used to draw the angle of the headstock and also the angle where the neck meets the body.


This time I rough cut all the necks with a bandsaw. Of course I can't cut perfectly straight with a bandsaw so the wood was a bit jagged along the cut. It occured to me that I could even out the top of the headstock with the jointer. In checking out the builds of others I've never seen it done this way so it was a risk. The risk paid off and I got a perfectly flat and smooth surface that didn't need any sanding. For any that may try this technique make sure you run the nut side of the headstock through the jointer first. If you run the tip through first you could experience tearout. Here's a pic of the top of one of the headstocks.


As noted in the picture I used an oscillating spindle sander to sand out the jagged stuff on the back of the neck. I finished up with a rotary sander.

Once the necks are roughed out you want to mark out the centerline and the places where the tenon will be cut.

Marking the centerline is easy. You just measure the width of the neck and then use a t-square to draw the line.


Marking the tenon is more difficult. You still remember what a tenon is right? The tenon is what fits into the mortice we already cut in the guitar body.

The tenon measurements must be exact or the neck will not fit properly into the body. The key is to make sure the centerlines of both the neck and the body match. For the top of the tenon you base all of your measurements off the centerline. The lines going across the width of the neck represent frets. The lines running the length of the neck represent the tenon and the wood outside of them will be removed.


The neck meets the body at the front of the sixteenth fret. There are two lines drawn here. The one on the left is the actual fret dead center. The one on the right represents the front edge of the fret which is where the neck meets the body.

This side view represents the profile of the tenon. Remember my routing the body four degrees to accomodate the angle of the neck? If you look at the profile lines of this neck you will notice that the same four degree angle is incorporated into the neck where it meets the body.


The line on bottom represents the bottom of the tenon and the line at the end represents where the tenon ends.

So why didn't I cut the tenon? It's simple. I need the side of the neck to be flat along its full length to cut the curved truss rod channel. Tomorrow I will build the jig to make this cut. I'm also waiting for a part I need for the router bit. The part should be here on Tuesday December 23rd.

I ordered a preslotted and radiused rosewood fretboard for this project a long time ago. I finally got it out to see what it looked like on top of a neck blank.


I'm also going to try and do the inlay (I've got the inlays and the tools for routing the fingerboard ready to go) this week as well.

Those of you that own Les Pauls know that the headstock isn't as wide as it should be. This is because on wings (additional strips of wood) are glued on to widen the headstock. Yes, this is how it is done on a real Les Paul. The reason for this is if you use a neck blank wide enough to make the full headstock you will waste a bunch of wood when you cut the taper in the neck. Remember that the necks are 1 11/16" at the nut and widen to slightly over 2" at the last fret. By making the neck blanks smaller and gluing on wings you can get more necks out of a billet. (A billet is a large board in its rough form before you cut neck blanks out of it.)

The next post will be about cutting the curved truss rod channel. This deserves its own post simply because it has caused the most stress. You will understand why when you read the submission.

If you're reading this blog please check in by making comments. I know a few who are following but I've heard there are a bunch of lurkers. Let me know what you think and give me some ideas for the next two Les Pauls.

Neck Mortice

Sorry I've been slow but I've been working some freaky hours and my sleep cycles are all screwed up.

The '59 Les Paul uses a mortice/tenon system for attaching the neck to the body of the guitar. One of the key differences between a vintage Les Paul and a modern version is the tenon. The vintage Les Paul uses a long tenon design that extends into the hole that's routed for the neck pickup. The new Les Pauls use a much shorter tenon. The long tenon is thought to sound better but the short tenon is cheaper to manufacture. Way to cater to the accountants Gibson.

The mortice an tenon have a male/female type of relationship. The tenon is the male and the mortice is the female. What happens is you slide the tenon of the neck into the....

Awe crap this is getting sexual. I'll just show you.

In order to carve the mortice in the body you need a template. This time I chose to make my template out of clear acrylic. I took a rectangular piece of acrylic and used a utility knife to score a line down the center. Then I measured out the length and width of the tenon and scored that with the knife as well. Then I took three small very straight pieces of acrylic and glued them on the lines I scored. These straight pieces of acrylic serve as the router guides for cutting the template.


The brown stuff on the bottom of the acrylic is actually a protective backing that you peel away.

Here's the piece routed with the backing still on:


And here's the finished template:


Remember the neck angle I cut in the body? The mortice template is attached to the body with double sided tape on the part where the angle is cut. The centerline I scored on the template lines up with the centerline of the body. The centerline ensures that the route is perfectly aligned. The mortice is 1.5 inches wide so I used a 1 3/8 inch forstner bit on my hand drill to hog out some of the material. This saves the router blades some stress.


To finish the cut out comes the router with a template cutting bit. You align the bearing on the template and proceed to cut out the mortice. I took out about an eighth of an inch worth of material with each pass.


After I got down far enough I was able to remove the acrylic mortice template and just use the actual mortice as the template. I did have one problem. The depth of the cut is 1.5 inches but my bit was only an half of an inch. Even with my router set at the lowest depth I couldn't route far enough down. The nearest place that sells router bits is Sears at Fashion Place mall. I know I know, Sears has a bit of a reputation. I picked up an inch long template router bit at Sears that actually worked very well and was just as good of quality as my more expensive bits. Yaaaaaay!

So I routed down to the depth of 1.5 inches. The mortice looked perfect but as I ran my hand along the sides at the spot where my smaller bit started to become inadequate I noticed some slight wavyness to the cut. The reason for the waves is I pulled the half inch bit as far out of the router collet (a collet is the part of the router that grips the bit and holds it in place) as I dared but in doing so the bit developed a slight wobble. Let this be a lesson to you would be woodworkers that you need to have more than a quarter inch of your bit's shank in the collet.

To fix the problem I put the smaller bit back in the router and started trueing up the route above the waves so the bearing would be able to ride on the perfectly routed upperThe portion of the mortice. This is when the router broke. Most routers have a dial that can be used to lower the bit when you are trying to increase the depth of cut. When I was tightening the bolt that secures this dial the assembly broke. It's ok because I also have access to a really nice DeWalt router.

The problem with switching routers at this time is that I was unfamiliar with all the adjustments of the DeWalt. I loaded the shorter bit in the DeWalt thinking I'd go down as far as I could with the shorter bit and then switch to the longer. Being unfamiliar with the DeWalt I accidently lowered the collet of the router lower than the actual router base. The result of this error was I started routing and then smelled something burning. I had a momentary brain fart and went down both sides of the mortice before I looked to see what exactly the problem was. The stupid collet (actually I shouldn't blame the collet I should blame myself) had dug into the top side of the mortice and made some ugly, burned looking, gouges in the top of the mortice.




I'm not even going to start on the words that I uttered when I saw the damage. In fact I'm pretty sure I invented some clever epithets as a result of my stupidity. As I inspected the rest of the mortice I noticed that when I'd pulled the router out I gouged one side a little bit about an inch down. This had officially become a bad day.

There is good news. The errors mean exactly nothing to how the finish product looks and plays. The burned gouges along the top edge will disappear under the fingerboard when it's glued on. I will correct the slight gouge in the inner part of the mortice by simply routing the mortice 1/32" wider on each side and making the tenon of the neck slightly wider to match. While this isn't perfect to the '59 Les Paul spec it is workable. Actually the specs of a '59 Les Paul vary slightly from guitar to guitar so it wouldn't surprise me if some of the mortice and tenons on those guitars were slightly wider than the norm.

When it's all said and done I can live with the result. The only disappointing thing is that everything had gone perfectly with the body up until this point and now it wasn't perfect. However if I sold you the guitar without you seeing how it was built you would never know that I'd made a mistake.

Neck Angle Nervousness

Today was the big day. I finally got to route the neck angle into my guitar body. I'll be honest, this particular operation has had me worried since I started exploring the idea of building a Les Paul guitar. It requires tooling that can't be bought and must be built. (Well they probably make some industrial gizmo that can do this operation but I don't know what it is and if I did I probably couldn't afford it.)

For those of you who aren't familiar with the construction of a Les Paul guitar I'd like to explain a bit about the neck angle. On a Les Paul the neck glues into the body on an angle somewhere between 3.5 and 4.5 degrees. The reason for the angle is the strings need a little height so they can properly align with the bridge of the guitar. I also suspect that playing comfort might figure in to the equation although I have had a number of guitars that use a straight angle that are comfortable to play. In fact most electric and almost all acoustic guitars have zero neck angle.

I can't remember exactly the angle that my plans specified but I decided to stick to a whole number and chose 4 degrees. I'm pretty sure this will work out fine.

In my past submission I showed some pics of my neck angle jig. While the jig is my own design I was heavily influenced by the jigs others have made for performing this task. Some of you are probably wondering what a jig is. The word has many meanings but in this case a jig is defined as:

Jig - A device used to maintain mechanically the correct positional relationship between a piece of work and the tool or between parts of work during assembly.

Thanks Websters...

My jig works by placing the guitar body inside being careful that the body sits square with the front of the jig. The back of the jig is elevated using blocks until the jig is sitting at the desired angle. The routing guide is placed on railes that run along the inside edge of the jig. The routing guide has a hole running most of its length that the router bit sticks through. If everything goes according to plan the angle of the box will be the angle of the cut in whatever material you are routing. The cool thing about my jig is it will also function as a thicknessing jig (poor mans planer) if you just use it laying flat.

Time for some pics

These are the blocks that I cut to hold up the back of the jig. The notch in these blocks is cut slightly over 1 1/4" high which, when considering the length of the bearing edges of the jig, put the box at an angle of exactly 4 degrees. I never thought I'd need trigonometry after my freshman year of college but it's necessary to figure out the heighth of these blocks. To keep things steady I used double stick tape to hold the blocks on the workbench.


This next pic shows how the box sits on the blocks. Also note that I also put some scrap wood along the each side and the front of the jig to keep it from moving. This scrap wood was also affixed to the workbench with double stick tape. These blocks did the trick and the jig didn't move.


This pic shows the front of the jig. You can see how I've used scrap wood with double stick tape to secure the front of the jig. It also shows why I made the front edge of the jig lower than the back. I do this so the router guide and the router bit can slide past the front edge of the jig to get in front of the material being cut. I guess I could have done the same with the back but I didn't. The guide the router sits on is made of MDF. I cut the slot in the guide using a table router with a fence. The place on the guitar body where the neck meets the body is the only straight part of the guitar body and is exactly 90 degrees in relation to the centerline of the guitar. I have to use this spot to align the guitar body with the front of the jig so everything is square. I had the bright idea to put the body as far to the left as I could in the jig while still keeping the body square in relation to the jig. The idea turned out not to be so bright. I'll explain later.


I decided to test the jig on some scrap wood before I began cutting into my guitar. I rigged a small piece of scrap mahogany into the jig and proceeded to cut. The jig worked ok but there were small marks from the router bit left in the wood. There are several reasons for this. The first was I was going to fast when trying to make the cut. Second was I didn't sand the rails on the jig well enough which made the router guide change height a bit as I moved it backand forth on the rails. Third was my MDF router guide was flexing a little bit under the weight of the router.

I couldn't make a new router guide but I did work on my routing technique and sanded the rails so they were perfectly flat. I then tested the jig again with a piece of scrap maple and while there were still router bit marks they were much more faint than the last test.

I then decided the jig was working well enough to put my guitar body inside and give cutting the neck angle a whirl. I got the body aligned, the router guide clamped, and placed the router in the jig only to find out that my router bit wasn't long enough. Grrrrrr. The nearest place that sold router bits is Sears so I went there and found a bit that was a half inch longer. Buying the router bit from Sears was a leap of faith. I've used enough different router bits on this project to know that not all bits are created equal. Sears tools are great for your average home mechanic or hobby woodworker but they aren't what most professionals use. As it turns out the Sears router bit was about eight bucks cheaper than its equivalent at my favorite industrial tooling store and performed just as well. To give Sears its due the hand router I've used for most of this project is an old Craftsman which is a Sears brand. I have two DeWalt routers available to me as well but chose the Craftsman for this project.(The others were set up for some other projects.) The Craftsman router has peformed like a champ and in my opinion is plenty good enough for a hobby builder. If I were to become a professional luthier I would probably go with a Porter Cable or a DeWalt simply because they are industrial grade which means they will last longer under heavy use.

So I set the height of the router bit, (The same height as the top of the binding channel) took a deep breath, and started cutting. I lucked out, the jig works! I took this pic looking down through the hole in the router guide.


Those marks in the hole of the router guide were made when I attempted to lift the router out of the guide before it stopped spinning. Oh well, I'd already decided to make a better router guide when I finished this operation.

I proceeded to slowly work my way down the guitar body by moving the router guide an inch or so and then cutting again. In this way I slowly cut the angle in the body. However, as I mentioned above there was one problem: By placing the guitar body in the jig as far left as I could I ended up not being able to cut all the way across the body because the router ran out of room. If you look at the top of the next pic you can see where I ran out of room to finish the cut. You can also see slight marks left by the router.


To correct the problem of not going far enough over I had to realign the body in the jig and then make the same cut out to the side. For whatever reason the bit cut at a slightly lower depth when I took routed the material on the side of the body. It really doesn't matter because this wood will be removed when the top is carved.

When I took the body out of the jig here's what it looked like:


In the above photo you can see some of the marks from the router guide flexing. This is nothing that couldn't be cured by two minutes of sanding. If you look at the left you can also see where the depth changed when I realigned the body in the jig.


There you have it. For the most part the jig worked as intended. I plan to rebuild the router guide using something that will not flex with the hope that the cut will be a little more accurate. I'm just happy things went as well as they did. Here are things ended up after a few minutes of sanding: (The body is discolored a bit because I rubbed naptha on it before taking these pics and it had partially evaporated by the time I took the pic.)




I will now breathe a huge sigh of relief.

Next up? Routing the neck pocket.