Necks, truss rods, CF etc - Musings

[jeffhigh]

Good Question Toejam,
I would consider an adhesive "structural" when it has sufficient bond strength to allow the two parts joined to act fully or partially as one.
In some cases the glass window panel adhered in place with silicone is a structural bracing member.
Contrasting to this is the non structural situation where plasterboard sheets are glued to timber studs with little dots of adhesive.
When the statement is made that the adhesive is stronger and stiffer than the timber it does not mean that the adhesive material by itself is particuraly strong but that the jonted item when tested to failure does not tend to fail by delaminating. Look at say polyester resin, a sheet cast out of it is not particularly impressive, but use it to bond fibreglass mat....

Back to the question posed..
If you are talking about vertical cuts and gluelines and vertical bending loads then the adhesive is not under any great load and it would not really matter if you used pva, Resinorcol or even Flour and water.
there would be a very slight stiffeneing effect due to the penetration of glue into the wood at the glueline but this will be very low and the glueline itself should be next to nothing and I would doubt that this would be measurable.
To use a common building analogy, floor joists spanning wall to wall 200 vertical 50 horizontal section. Need to double the strength? put two together side by side and wack in a few nails to hold them together.

If you were cutting and glueing horizontally then loading vertically then the situation changes. The glueline has to transfer shear loads between the two pieces and an adequare strength glue is required to restore the strength of the finished laminate to that of the original unsawn piece

[DarwinStrings]

Nice...thanks guys... I have never thought of laminated neck as being any stronger than a neck made of one piece. I have seen both laminated necks and one piece neck kink where the neck thickens into the heel. In my experience though laminated neck are more stable and don't usually twist like one piece types.

I have a Maton Fyrbyrd here at the moment for repair with a hellish twist and it is a one piece neck with a separate fretboard

I use laminated necks as well as one piece, not the type with a stripe just a bookmatched piece and find it easy to justify the extra work and glue. Also one recent example to justify laminated necks was that I had a piece of Rock Maple at a nice length for a neck, if I wanted to make a one piece neck out of it I would have only got one stable neck (that is a nicely backsawn piece) out of it and ended up with lots of waste so instead I cut it into four pieces and got two quatersawn laminated necks out of it, a much better use of my resources.

Jim

[jeffhigh]

Yeah the resistance to twisting is a very good reason to use a bookmatched two piece laminated neck.
And it gives you a readymarked centreline that you can't cut or sand away...

[Dave White]

All this theory makes my head hurt but as the egg heads are here assembled here's my question. Ripping the neck down the middle and reglueing releases stress and can make the neck more stable, so is there a difference in stability in a neck made from a strip with scarfe headstock and stacked heel compared with a one piece neck cut from a block?

[jeffhigh]

No
The scarf joint/ stacked heel has many advantages , but increased stability is not one .
And the rip in half and reglue only improves stability if you orient it so that the end grain is a mirror image across the joint not just putting it back together as it came apart.

[Kim]

An F8 beam is F8 beam for structural purposes whether it is solid or laminated from F8 graded strips.

Jeff,

All due respect to your qualification as an engineer, my own experience only extends to the construction side of things, however the "F" grading for for Glulam beams was abandoned by industry in 1997 in favour of the "GL" grading system.

"GL grades
Glulam (or glued laminated timber) products are manufactured products, made in accordance with AS/NZS 1328.

AS/NZS 1328 - 1998 defines methods for grading glulam products into GL grades which were introduced to the industry in 1997 and products complying with them have been included in Section 7 of AS 1720.1. These grades are performance targets which are independent of the species, milling procedure and manufacturing process and are based on a “standardâ€

[jeffhigh]

Well You learn something new each day, Thanks for pointing out the change in grading standards Kim, I left Engineering 15 years ago so I was not aware of this.

One of the important apects of what you have quoted is the ability of the manufacturers to use lower grade wood in the core of the beam where it is less stressed as opposed to the outer fibres (top and bottom edge layers) of the beam. with virtually no detriment to the overall strength.
Obviously you cannot do this with a natural timber beam other than selecting it to ensure it does not have any nasty big defects at these locations

I certainly agree that most of the wood in your beams is not high grade. when I used that term I was thinking more of the High rated hardwood laminated beams used for high load high span situations.

Anyhow, Interesting as Stack laminated beams are we dont use them in this orientation in luthiery
But thanks for an interesting discussion Kim.
regards
Jeff

[kiwigeo]

Im a Geologist so I like asking stupid questions after engineers have had their say.

Okay so youve managed to work out a way of building a neck that isnt going to bend or twist....what happens if you want to introduce a bit of relief into the neck???

[jeffhigh]

A very good question.
I believe the first moses graphite necks were built without truss rods, intending that they just remain straight under string tension, later ones had adjustable truss rods to meet customer demand.
I guess a two way rod would be able to dial in relief in any timber neck of practical density, or with a single action rod, add a bit of tension before leveling the neck/ fingerboard joint.