Starting out on J45

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Richardl
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Starting out on J45

Post by Richardl » Wed Feb 21, 2018 7:30 pm

Hi. Recently bought the excellent book (s) which I have been enjoying and have been busy making jigs and molds. I'm having a go at the J45. As a beginner, I have a couple of questions that i hope someone can answer (that probably show that I don't know what I'm doing)!

The first thing I noticed is how light the bracing is compared with other plans and the (only) other guitar I have made, all of which are 10 mm wide.... So presumably the instructions to make 20x10 mm brace blanks in 7-3 is a bit of an overkill in this case as they are planed to 6.3 mm.

I guess my (perhaps dumb) question is with this lighter bracing, is there more chance of failure of the body or is there more than enough meat in there for strength? I guess there must be as its a model that has been around for a while - just interested why the bracing is relatively thin.

Is there any advantage to notching the finger braces into the X-braces to avoid any liklihood of the finger braces coming adrift, or is Titebond so good that it is completely unnecessary?

As far as i can tell, the plan shows the bridge plate touching the bracing (and glued to the bracing?). Other plans I have seen have the bridge plate beveled at the edges with a 1 mm or so gap between the plate and the X-braces. Have I understaood this incorectly or doesn't it matter in terms of strength or acoustics?

Thanks
Richard

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Trevor Gore
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Re: Starting out on J45

Post by Trevor Gore » Wed Feb 21, 2018 11:48 pm

It's very easy to over-build a J45. The originals were made with less-than-best wood and retailed for $45 (i.e. cheap) compared to say the J200 which was originally $200. So if you choose good materials it's easy to end up with everything too stiff.

Having just re-read Section 7.1, I can see that it is not the clearest bit of writing! The dimensions given (20 x 10mm) are essentially back brace dimensions and the procedure described is primarily the back brace making procedure. You need to modify the dimensions to the X-braced plan you're using. The plan from the book can use brace blanks starting at ~15 x 7mm so you can finish at ~1/2" high at the X and a width of 1/4" (~13mm x 6.3mm), Section 11.2.2. The better your jigs and shop practices, the closer you can start to the finished brace dimensions, but the same procedure as for making back braces can be used, whilst leaving the un-gabled bit for the X as described in Section 7.1. These are the dimensions we use to get the resonant frequencies in the right target zone without risking undue distortion. Most published plans (usually taken from factory made guitars) are way over-braced when good materials and assembly practices are used.

There is no need to notch finger braces or face braces into the X braces. If you make good joints with fresh surfaces and glue with Titebond, the wood will fail long before the glue (at normal temperatures).

The bridge plate is beveled or rounded on the exposed top and bottom edges (Section 11.2.2.2) and is glued down first and then acts to locate the X-braces accurately when they are glued down. The X-braces are tightly butted up to the bridge plate edges, to which they are glued.

If you follow the procedures and plans, you should be pretty right. A good many people have been pleased with the results. However, if you do find anything else that requires clarification, just ask here.

Richardl
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Re: Starting out on J45

Post by Richardl » Thu Feb 22, 2018 5:23 am

Many thanks Trevor. That is very helpful. I'm sure other things will crop up so its great that you are willing to offer advice here.

I was a bit uncertain whether i should fork out for the books but i'm very glad I did and have learned a heap. Actually, I'm a trumpet player not a guitarist and am in the proces of upgrading my Bb instrument. It's fascinating that the sound of a collection of pipes that, in most respects look the same, can sound so different. A hand-built Trumpet by a craftsperson at the top of their game can yeild a far better instrument than many of the well known mass-producers...or so I'm told. Guess I'll find out when I order one! In that case, the differences in build are very subtle (e.g. where the braces are positioned to get the right rsonane from the metal). Dont know if the topic has had the same scientific treeatment so it may be mostly trial and error.

Cheers
Richard

Richardl
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Re: Starting out on J45

Post by Richardl » Thu Feb 22, 2018 6:46 pm

Just noticed the upper transverse brace in 7.3.5 says 10x20mm. I'm assuming that the plan is right and its 13 x 16mm, so maybe 15x20 stock. Or do I laminate 2 bits of 10x20?

Does the number of growth rings or growth ring density have any impact on the build? It varies a lot between 2 or 23 upto 10+.

actually, I think I have answered my own Q, the brace is discussed in 11.2.2.2

Cheers
Richard

Richardl
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Re: Starting out on J45

Post by Richardl » Sun Feb 25, 2018 5:38 am

Softwood species where the growth rings have a gradual transition from earlywood to latewood (e.g. the spruces and the firs). In these species growth of the denser latewood part of the growth ring is relatively constant irrespective of climate, whereas the earlywood part varies from year to year. This means that that a wider ring has more of the less dense earlywood and so density might decrease with increasing growth rate. The relationship is not consistent, however, as the density of a given ring width is affected by the genetic makeup and growing conditions of a tree, and can also be influenced by its stage of development. Therefore, particularly for timber of mixed origins, ring width alone is not an accurate predictor of wood density.

So I guess that might also be true of wood strength as density and strength are related (http://woodlot.novascotia.ca/content/le ... rties-wood).

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kiwigeo
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Re: Starting out on J45

Post by kiwigeo » Sun Feb 25, 2018 9:41 am

Richardl wrote:
Thu Feb 22, 2018 5:23 am
Actually, I'm a trumpet player not a guitarist....
Sell the trumpet and use the money to buy a drum sander :mrgreen:
Martin

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Trevor Gore
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Re: Starting out on J45

Post by Trevor Gore » Sun Feb 25, 2018 12:15 pm

Richardl wrote:
Thu Feb 22, 2018 6:46 pm
I think I have answered my own Q, the brace is discussed in 11.2.2.2
Yep. Chapter 7 is more about the techniques for making braces. Chapter 11 and the plans give specific sizes for the different types of guitar.
Richardl wrote:
Sun Feb 25, 2018 5:38 am
(http://woodlot.novascotia.ca/content/le ... rties-wood).
I hate it when people write about "strength". A far too ambiguous term. One thing to keep in mind is that there is a wide range of densities and stiffnesses within a species. So, for example, to say that Sitka is stronger than Engelmann is pretty meaningless, whereas to say that Sitka has, on average, a greater Young's modulus than Engelmann has more meaning. However, because the properties of Sitka and Engelmann overlap so much, if particular material properties are important to you, you need to measure the properties of the piece in your hand.

Richardl
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Re: Starting out on J45

Post by Richardl » Sun Mar 04, 2018 7:46 am

I guess I was just wondering whether the number of growth rings per cm had any influence on the likely failure of a brace and how I should choose the bit to use for the most critical braces as the number of growth rings can vary considerably. It seems maybe there isnt a clear relationship. Bought the trumpet. Plays well :D ! A decent drum sander would be nice though.

Cheers
Richard

Richardl
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Re: Starting out on J45 ...RH question

Post by Richardl » Sun Apr 15, 2018 5:47 pm

Hi - I've recently completed the soundboard, braced carefully at 45% RH (or a little below). It has been sitting at about 55% RH in the house for a day or two and has developed quite a convex curve (from the outside as shown in the book). I'm assuming that the rest of the glue-up will also need to be done at the same RH - i.e. the back bracing and the assembly - back to sides and soundboard to sides. I'm hoping when re-dried, the sounboard will come back to it's oringinal shape!

My question is, is there sufficient cross-grain distance in the sides, to necessitate bracing these at reduced RH as well?

I usually do woodwork in the garage at home so working at 45% RH requires moving into a room in the house and dehumidifying for a few days, so I'm interested to know the steps where RH control is essential.

Must say, last time, I only worried about RH with the soundboard but I guess a back (and possibly sides) can split just as easily. Is that true?

Thanks

Richard

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Trevor Gore
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Re: Starting out on J45

Post by Trevor Gore » Sun Apr 15, 2018 9:24 pm

I do all assembly at 45% RH.

The sides can be problematic because they tend to be heat and damp tortured and then ignored. They are often assembled at unknown RH and, mostly, people seem to get away with it. I tend to bend the sides early in the process and give them a week to normalise at 45% before I cut to length and glue in the end blocks, then linings and side stiffeners. The more you move away from "standard" woods (EIR, mahogany, blackwood) toward the less used local woods, the more I'd want to assemble everything at 45% RH, not just the top and back.

Richardl
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Re: Starting out on J45

Post by Richardl » Mon Apr 16, 2018 6:31 am

Hi Trevor
Thanks for your reply, that helps -I'll just have to take over a room in the house for a bit! I'm somewhat out of sequence in this build unfortunately as I still haven't sorted out a bending iron. I'll do it differently next time. The back and front will have to just sit around for a bit until the sides are ready. In this case, I'm using mahogany. I'm surprised how stiff it is compared with Blackwood.
Thanks
Richard

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