Classical Bridges using CF

[seeaxe]

Apologies if this is answered elsewhere but I did do a search on this forum and nothing came up. Questions aimed at those who have built composite wood/CF bridges for classical guitars, particularly @John Parchem and @kiwigeo and Trevor of course!

Dimensions
Fig 20-2 in the Build volume shows typical dimensions for the classical bridge with the height at 8mm

Section 20.2 discusses the make up of the bridge and calls for three wood plies of 2,3 and 4mm thickness with two layers of CF between these plies. I haven't got the CF yet but it presumably has some thickness. Assuming I can compress it when its being glued to 0.5mm each layer, that still adds up to 10mm (2+0.5+3+0.5+4=10)

Am I missing something? I cant see how the bridge can end up at 8mm. The margins for placement of the CF are pretty fine.

Anyone built one of these and what did you do?

Stiffness
Has anyone done any testing to see what increase in stiffness the CF provides as opposed to a plain wooden bridge? I was thinking of trying to do this but would be waste of time if someone else has already done it.

Weight
I'm in the process of building a blank to the dimensions in Fig 20-2 using a Tas Blackwood blank that I got from Mr Spittle. I'm not done yet but I think I should be able to get that down around the 16 mark - when Trevor quotes bridge weights in the book I have assumed he is including the bone saddle. Has anyone else use TB for a classical bridge?? How did it turn out?

Cheers
Richard

[Andos]

Hi Richard,

NZ grown Aussie Blackwood, 9mm high 13mm to the strings, under 20gms without the bone(from memory )The top is mangeao which is somewhat heavier than spruce. It was made to Josè Ramonillos design. I can't really say how good the bridge is being the only guitar I've made. I had it in at a guitar shop to get a more honest opinion than my mates gave. The shop owner was a bit disparaging about my workmanship (and fair enough too), but he liked the sound. Which was enough encouragement to start another.
Hope there is something useful in that wee ramble.

Andos

[seeaxe]

Nice, I like the look of the top.

I've made a blackwood bridge before too but I didn't weigh it at the time. But haven't tried laminating one yet.

Are you building another classical??

[Andos]

The next one is a steel string parlour. The 1st one was meant to be a parlour but we won't go there. After this one I'm thinking of a falcate. If I'm feeling brave enough!!

I'm a rank amateur at luthiery but you talking about bone, bridge weights and CF got me wondering if you could use CF as a saddle instead of bone. Bring the wood up to a peak at the saddle position and lay some CF along the top. Or laminate a vertical strip of CF at the saddle position -from the strings to the soundboard. Either way you lose the weight of the bone and there should be some excess wood to chop out. Just idle bedtime thoughts. If anyone thinks is worth trying, let's hear the results.

Ando

[kiwigeo]

I'm a rank amateur at luthiery but you talking about bone, bridge weights and CF got me wondering if you could use CF as a saddle instead of bone. Bring the wood up to a peak at the saddle position and lay some CF along the top. Or laminate a vertical strip of CF at the saddle position -from the strings to the soundboard. Either way you lose the weight of the bone and there should be some excess wood to chop out. Just idle bedtime thoughts. If anyone thinks is worth trying, let's hear the results.

Ando

My thoughts....

1) a saddle needs to be adjustable.
2) Filing bone is a lot easier than filing CF.
3) Reducing the weight of the bridge by laying up a wood/CF sandwich is IMO more productive than playing around with the weight of the saddle.

[Andos]

Cheers Martin, those are all good practical reasons, but if a CF saddle can act as well as bone it's worth a try. (My opinion only). First though, I need more experience. (I've yet to work with CF) Once I can build a 1/2 way decent guitar Ill revisit this.

To get back to original topic, Richard was asking how you got three layers wood and two of CF into an 8mm thick bridge.
Correct me if I'm wrong bit wasn't one of those layers the bridge plate with the CF sandwiched between it and the top?

[seeaxe]

I'd stick with bone for nuts and saddles. The saddle and the CF in the braces are there for different reasons. CF is used because of its high strength to weight ratio in bending or flexural elements hence its use in a very specific manner in the braces and bridge. The job of the saddle is to transfer the vibration of the strings to the top so its acting in compression.

You could argue that a layer of CF over the top of the bridge would be structurally efficient but it might be a bit confronting in appearance.

I think the lack of response to the layer question is a message to me to search a bit harder. I know i have seen posts about the bridges delaminating but im b*ggered if i can find them. Will look again tonight.

[Andos]

The answer to the bridge thickness is on page 20-8, second to last paragraph. "Sand material off the bottom laminate to get the required curve and thickness" but not quite put in those words.
Searching old threads is not my idea of fun, bit there is a wealth of info buried in them, so it's worth the effort.
We can give the old hands here a laugh while we nut our way stuff they sorted out long ago. Just gotta take the odd kick up the jaxie if we go to far off piste!
Seriously though, I can't thank the old hands enough. Without them I would've given this up a long time ago.

[seeaxe]

Nope.
For a 25 foot radius that accounts for less than half a mm, so still around 1.5mm of mystery left.
Not to worry, will all become clear in time no doubt.

[johnparchem]

I take some from the top and some from the bottom (including the radius) until I get the thickness that I want. I am considering simplifying my classical bridges. Like the steel string I will include a 1mm bridge patch and a layer of CF fabric under the the braces. When I make the bridge I will sandwich the second CF fabric under a 2 mm laminate on top of the bridge. The biggest problem I have had with the bridge with the 2 - 3 - 4 mm laminates plus the CF is hitting the CF when drilling the string holes and breaking a bit in the hole. Also I would like to angle the string holes to make it easier to string the guitar. I did do this on my 8 falcate string classical and it worked out well. I still get the advantage of 2 separated layers of CF fabric with not that much added work to the top.

[seeaxe]

Thanks John.

More searching last night didnt turn up anything on the bridge (possibly because the site wont let me search for "bridge" as its too common a term!!) But i did find the classical bracing plus cf thread which ill need to re read when i get that far. One thing i have picked up is that the tow i have bought is 24k while Trevor and others used 3k. I need to go back and read the design book again. Im working through it but the temptation to go and make sawdust is high. The tape i have uses 3k tow though so i think im ok.

I had been thinking about the machining of the bridge blank once i have created it. I have in the past cut my saddle slot on the bench saw or with a router. Having never worked with CF before im wondering how that might go. Any tips?

[johnparchem]

I machine the bridge blanks on a small micromill. The CF presented no problems. I wear a dust mask. I use 3K 5.7 oz plain weave fabric. (Sorry about the imperial measurement.)

[kiwigeo]

Ive never had any issues cutting saddle slots in my CF laminated bridges. The cutter might wear a little bit quicker but no major issues so far.

I machine the bridge blanks on a small micromill. The CF presented no problems. I wear a dust mask. I use 3K 5.7 oz plain weave fabric. (Sorry about the imperial measurement.)

[seeaxe]

Thanks guys, good to know!

[Trevor Gore]

...

Dimensions
Fig 20-2 in the Build volume shows typical dimensions for the classical bridge with the height at 8mm

Section 20.2 discusses the make up of the bridge and calls for three wood plies of 2,3 and 4mm thickness with two layers of CF between these plies. I haven't got the CF yet but it presumably has some thickness. Assuming I can compress it when its being glued to 0.5mm each layer, that still adds up to 10mm (2+0.5+3+0.5+4=10)

Am I missing something? I cant see how the bridge can end up at 8mm. The margins for placement of the CF are pretty fine.

Anyone built one of these and what did you do?

...

The CF adds almost nothing (at least the way I clamp!!), the base clean-up and arching takes off about 0.5mm, clean-up of the tie block after adding the wear strips accounts for about another 0.5mm, which gets me pretty close to 8mm.

[seeaxe]

Thanks for your response Trevor

I guess with enough pressure the CF will sink into the wood as the TB is fairly soft. I've got some fairly huge clamps that will do the job.

Interesting day yesterday trying to finish off that wooden bridge. All day to produce a 15g output!! I had forgotten how it feels to be working with sub millimetre accuracy in wood. Its easy to see how a mm can be lost on finishing.

Limited time in the shop today but hopefully over the weekend I'll get a bit further.

[seeaxe]

To close this thread out I have learnt:

a) the two outer plies can be adjusted after the blank is made. This will be easier for the thin 2mm ply.

b) the cf and glue only take up 0.3mm but because of a) doesnt matter much anyway.

c) greaseproof paper wrapped around the stops the blanks being glued to your workboard (mostly:))