Truss rod slots: curved vs. straight

[Chris Richards]

Here's a link to an earlier post on this forum of a jig that I built to make a curved truss rod channel, works really well. http://www.mimf.com/phpbb/viewtopic.php?f=12&t=2480

[Jason Rodgers]

Hey, that's a great setup, Chris. It's really a one-stop-shopping kinda jig. I can't remember seeing that thread when it was originally up, so thanks for linking it for us here.

[Chris Richards]

Yes the jig works very well... Especially the metal blocks for lining-up and getting the angle correct for the anchor and adjusting nut. Before I used to do this by eye but the lack of consistency was causing real problems and in the worse case the truss rods weren't working no matter how much you tightened them.. Now though using the jig everything works smoothly, I think it's very important to get the filler strip to be a tight accurate fit to the truss rod and I can get that by using the curved router guides (in the jig) to mark the filler strip before cutting.

I wish I'd made the jig a bit better quality, as I didn't know if it would work, it's not made of great materials and could be a bit more "user friendly".

[Mark Wybierala]

I've received the long arbor and both a 1/4 and a 3/16 3-wing cutter. I'll get this set up and provide some feedback.

David, I hear you about Fender. I did a trussrod replacement on a biflex a few years ago. I went through about five new replacement nuts before I found one that actually had proper threads. I communicated directly with Fender about this and sent them the defective nuts. No comment from Fender but they did refund for the nuts. This was terrible especially considering the allen size is a small 1/8". I am not at all a fan of the biflex. For those clients I have who are assembling a Parts-O-Caster, I steer them towards necks made in Mexico if they really want a Fender neck. The only thing worse that I've come across is the older Rickenbacker split rods which are a bit of a joke

Does anyone have any information about trussrods used by PRS? They are delightfully easy to adjust and I've never come across one that seemed at all under stress -- the nut just turns so easy and the necks always adjust in a well behaved manner. The nut looks like an acorn. What type of design are they using? They employ the same trussrod in their import SE series -- always easy and smooth to adjust.

What do you folks like as a lubricant for trussrod nuts? My oversize tube of white lithium grease went missing one day and I've been using cork grease which seems to work okay. I figure that if its okay to use on cork, it shouldn't have an ill effect on wood. I also use it for nut slots with success. Plus, it comes in a very convenient chap-stick style dispenser and we always have it in stock.

[Jason Rodgers]

Cork grease, as in woodwind cork tenon grease? Yes, it's the consistency of chapstick, and is swiped on corks every so often to keep them from drying and splitting as joints are twisted together. On brand new cork, it mostly sits on the surface, but gets forced into the pores over time with repeated compressions of the joints. Very old cork can become completely saturated and compressed, looking like finished cork floors, and loses its sponginess, resulting in loose tenons.

As for truss rod nut thread lube, yes, unless it were to get very warm and run, it will probably stay where you put it and not soak into the surrounding wood.

[Steven Wilson]

Speaking of curved truus rods, can anyone expain the illustation on page 12 of this catalog?

http://www.acousticmusic.org/userfiles/ ... og%20Q.pdf

[David King]

Regarding PRS rods, I found this page on their site:
https://www.prsguitars.com/csc/trussrod.html
Seems like they used a single curved rod with a brass, Gibson style nut until mid 1992 and then changed over to a two-way rod with the nut fused to a threaded rod. I'll bet the PRS owners get their underwear all in a bunch thinking about that.

[Mark Swanson]

If I look at that I can easily see how the rod will compress the wood along the back of the neck. That's where it gets its action. It is strange to look at but it works.

[David King]

Steven,

The rod shown on page eleven is similar to the Gibson Patent granted to Thaddeus McHugh #1446758 from 1923.
http://www.google.com/patents/US1446758
It's hard to know how accurate the drawings were but this was a curved compression rod. The curve may have been inverted intentionally to throw off the competition but more likely it was just a bad design that worked in spite of itself. They kept the inverted curve in some models until the late 1930s or early 1940s.

[Steven Wilson]

Thanks
I guess it's hard to understand how a rod curved foward will bend back when tightened and a rod curved back will also bend back. I allways thought the forward curved rod put pressure on the curved filet when tightened forcing the neck to bend back. I have also allways wondered if when using a harder wood for the fret board then the neck made it act like a double action rod with the fret board being the stationary bar. Maybe we aren't supposed to understand every thing

Thanks
Steven

[Mark Swanson]

I think the key to it is where the rod is placed in the neck. With the rod in that picture, there is a lot of wood on the fingerboard side, and precious little on the back of the neck. With it positioned like that it can only do one thing....and for sure, if it were curved the other way it would certainly work better.

[Tom Sommerville]

I prefer single rods to the two way or double rods for two reasons:
1. they are lighter
2. the bending point of the rod and it's influence on the neck set can be precisely located.
3. Additionally--I think--the rod can stiffen the neck a little; at any rate I'm certain that the double rod
has very little effect.

Of course there are draw backs: it takes more time to mill the channel and filler strip for one thing.
Also, it's been claimed that the rod compresses the wood and caused the rod to operate less efficiently. It seems to me that
with a Fender-style trench--a dense filling strip would remedy this.

I wasn't aware of any convex-up layout like that in the Gibson Doc referenced above but it ought to work. You have to consider how the curved rod works: when the nut is tightened, it straightens and the concave side of the rod compresses the adjacent surface of the rod. The Gibson rod would push against the back , pushing the headstock backward. I think the Fender concave-up profile is better: it pushes directly against the fingerboard in the region of the neck that matters most, the area around the 7th fret. Straight rods, a third option, work like the guy wires of a ships mast, and probably exerts the greatest amount of stress on the adjusting nut.

By the way, it shouldn't be any major undertaking to create the curved channel: get a 2x board, saw or plane the curve into the top, and rip it into. If this is too much trouble you can carve it out with a chisel or rasp.

Don Teeter's book on repair, Volume one, gives instructions for making a router jig. There's also a production drawing of the Fender neck online with dimensions. Google will get you there.

Go now, and truss up your necks the proper way.

[David King]

Tom, one of the limitations with routing the curved slot is finding a 3/16" ball nosed, extra long cutter that will fit in a standard 1/4" router collet. They don't exist.
Apparently folks get by with a 1/4" dia cutter and call it good.
I found a 3/16" wide, 6" circular slotting saw blade for about $150 and had the teeth reground to a 45º bevel. That works very well for me but it's not something an occasional builder is going to want to invest in.
Music Man went a different route, slitting their necks down the middle and routing 1/2 depth, matching curved channels into each half and gluing the halves back together around the rod. The rods in that case would need to be carefully waxed to keep the glue from locking them up but it should be possible for someone to use this method at home with just the standard 3/16 box-core bit in a router and template guide.

[Barry Daniels]

I use a 1/4" curved bit, and then put heat shrink on the 3/13" rod to take up the extra space.

Also, you don't need to make a curved filler strip to fit a curved bottom slot. A straight filler strip easily flexes when gluing into place.

[Jason Rodgers]

Music Man went a different route, slitting their necks down the middle and routing 1/2 depth, matching curved channels into each half and gluing the halves back together around the rod. The rods in that case would need to be carefully waxed to keep the glue from locking them up but it should be possible for someone to use this method at home with just the standard 3/16 box-core bit in a router and template guide.

Say, that's doable. With a template, maybe with the ability to drill shallow holes for alignment pins, you could easily make the rod curve any way you want. Hmm, I'm going to think on this one.