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The idea was to create a measuring tool that wouldn't lie to me. Based on qualified comparison (not quantified data) the jig was created to shine a laser dot the full length of my shop allowing a direct comparison to steel truss rods.

The thinking goes like this...(and please feel free to challenge anything here).

When one of the common two way truss rods is taken to its ultimate yield point and the welds haven't broken, the round rod WILL break at the minor thread diameter when that rod is in tension. This represents the full potential of this design.

The jig is meant to show (via laser dot) the nominal position of the tool (no load) and then provide a record of the movement that has occurred as tension is applied to the point of failure.

I've broken many steel rods. I will show a full record of this process eventually. Not today though. The point is, the steel rods break predictably in the minor thread diameter...and they exert TREMENDOUS force when they get to that breaking point. Note the blue rod. This one was broken in a previous tool (this tool has gone through several revisions). The slot was all wood. Just prior to breaking it completely destroyed the jig. Note the NEW jig...with the steel reinforcement in that area.

My point here is that if one actually cranked down on a steel rod inside a guitar neck and broke this rod without breaking the welds first...he or she would have exerted many times more torque required to offset string tension (my best guess...between ten and twenty times more) and would probably destroy the truss rod groove in the process or pop off the fretboard. It would be bad one way or the other. In a battle with wood steel always wins.

This tool will be used for each truss rod that goes out the door. I've made comparisons to steel rods of course but I'm also going to know how many turns, and how much of laser move it takes to break my composite truss rod for each length...and then I'll have a quick testing operation for a pass/fail quality control inspection.

I will also know the difference between an average load required to offset string tension and what is required to break the rod. The thing about modern composites is that if one controls basic processing aspects, they yield a remarkably consistent result. So far, mine are breaking at precisely the same load every time. Pretty cool (to me )

The tool isolates two individual cross sections of Maple so they are in fairly perfect tension and compression and then, with the aluminum parts the rest of the rod is constrained snugly. The tool bends without restriction because of the tight tolerances on the aluminum parts.

The laser is focused on a wall 55 feet away. By taping a piece of paper on the wall and then marking the dot with a pencil, the zero load point and any load point can be recorded. The tool is calibrated each time by creating a zero point reference line and moving the paper to line up with the no load dot.

Anyway...I'm jabbering...I thought this might be interesting to some of you. I can't show everything yet but I can show you some of the thinking that went into this silly truss rod. I'll show all the molds and the pneumatic curing press later. The molding processes and this testing tool were conceived WAY back and it's a blast to finally get to it...and find out EVERY way in which my assumptions were completely wrong. It's nice to be able put some of the knowledge of composites to use for guitars and make something interesting and useful.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Stuart:
I wonder if you captured any torque reading during testing and if so was there any amount in variation before failure?
I would think all threads were cut and don't know if any one has tried rolled threads to ensure more consistent breakage values.
Tom

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A person who has never made a mistake has never made anything!!!

Thats very interesting Stuart,
I think Tom is on to something with the rolled threads.
Also J threads would be stronger.
Stuart, Will your threads be on steel or composite?

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Say what you do, Do what you say.

That's an interesting study Stuart. Way overload on a truss rod for sure. If your applied torque to the rod is destroying the containment fixture before breaking the rod, shouldn't that end all chance of an accurate reading? There are given failure ranges for all materials in ASME manuals, wondering how that relates to the values you are getting, and if different materials will fail at different times due to nothing else but the material composition.
The other failure that you did not mention, or maybe I missed, is the failure of the neck itself, or blowout, from over tightening a rod. In some ways that is shown in you fixture failure before the rod broke. I know most people talk about weld failures. I tested my rods similar to what you are doing, minus all the hi-tech measuring. My rods all broke before the welds. The other thing that was realized is that if someone is turning a rod that much there is definitely something else wrong.
Are you going to do the same test with different materials?

Tim

I use roll threads on mine which are 304L Stainless Steel.
There should not be any real difference in the strength between cut or rolled threads, notwithstanding a difference in the quality of the thread itself. Rolled are a smoother finish in ductile metals, and faster to make. There s a theory that because the material deforms to follow the thread shape the rolled threads are stronger. Maximum thread strength is achieved in 4 threads regardless.

Tim

I've seen three kinds of failure in double acting rods.

Cracked welds don't happen very often, but I have seen a few. Given the quality of many of the welds, I'm surprised that it's not more common. Many of the welds I've seen on new rods already had small cracks initiated from welding stresses. These don't seem to often progress when the rod is in use. Just to be sure, I put the rod in a piece of metal electrical conduit and apply a reasonable torque load to each rod.

Weld blow-through often fouls the threads and limits travel. If you try to turn the adjuster through its full range unsupported, it will kink near the threaded area (like the blue rod in your photo. Flex conduit supports this area, but allows for full travel to test the threads

The differential thread pitch models can just run out of travel before the neck is straight. The stiffer the neck is, the more this is an issue. I would be interested to learn how the adjustment ranges varied in your testing gizmo when comparing reverse thread models, and differential pitch models.

I haven't had a rod seize up completely from corrosion yet, but I've seen a few that were close. I think it's worth putting anti-seize compound on the threads, and using a non-corrosive silicone caulk if any during installation.

I'm looking forward to seeing your design. What does it weigh?

I have features machined into grade 8 steel bolt (genuine Holo-Krome) constrained in a composite housing. So...male threads on that. Then I mold female threads into a composite housing on the other bar. The true weight savings happens because of this molded thread.

I've seen analysis that puts a rolled thread at 7% stronger than a cut thread (mind you...this over the cross section of the entire bolt). Grade 8 steel is about 2.5 stronger than 1018 (typical truss rod material). So...the only metal I have in mine is a GREAT deal stronger than 1018 or comparable alloys. The rest is carbon fiber processed in five discreet molding operations. I'll explain in detail...after patents.

Torque AND tension breaks these steel rods in this jig. The torque is CLEARLY illustrated by the laser dot taking a rather pronounced curved path out on the wall. From about half load to full breaking load the curve begins...and it becomes much more pronounced...and then...SNAP. I love breaking stuff. A stress graph would obviously be an exponential looking curve.

Tim...the FIRST jig broke...and then it fell into the swamp...so I built another one...and it fell into the swamp. You are looking at revision...the last one.

Lol...way overload is right....but I needed somewhere to start and this shows me the full potential of a perfectly good steel truss rod. It's only a design reference.

It isn't necessary for me to test lots of different truss rods. I only need to get a practical, valid, visual reference of ANY truss rod acting to it full potential. These rods pictured measure .25" x .35". So does mine. I can break those, record the distance the laser traveled, stick mine in the SAME tool...and make a direct comparison.

Later this tool is used for QC.

It's easier to show you than explain but for now I can only say that having the ability to change shape along the rod allows for an interesting design. The truss rod delivers a PURE bending load to the neck and stores no torque along its length.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I've seen the ferrule weld break a few times...with virtually NO effort. Scary. So....test them before they go into a neck. That's for sure. Otheriwsie, you won't see too many broken welds until you're actually TRYING to break the rod...and then you'll lot more.



Look at the picture of the black rod that broke. See that gap? That gap is the ACTUAL displacement after the break. I didn't turn or adjust that after it broke...that's exactly how it came out of the jig. I honestly can't get my head around what happened there. But I think that represents your concern pretty clearly.



That's a valid concern but galvanic corrosion occurs as an electrolyte is present. In both place on this design where atmosphere could ingress the fit is so tight there would be little chance of having any electrolyte present. But grease is added to seal it anyway.



Now...THAT'S the fun part. 24.5 grams - 17" rod.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Nope. More like this.

For a bolt or threaded rod...under controlled circumstances.

I was referring to the nature of the stress accumulation in the tool though...not the steel of the truss rod at yield. The measuring system isn't doing anything more than recording force applied to the tool.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

For any design purposes, it's the first slope of that graph that maters. When you go beyond the yield strength, bad things happen. (Steel strings on guitars are routinely stressed beyond the yield strength to gain that slight rise in the graph before failure--hence the need to retune new strings, and the broken strings if you go a little too far.)

One problem with the design of most double-acting rods, is that they require the bolt to carry a torsion load. If you're trying to minimize weight, that's a significant issue. Old-school compression rod designs can be modified to look pretty good.