Cepheid 5: Ripthorn Builds a Bass

[TerribleTim68]

Well, I pulled the clamps off and tested the bond between the aluminum and cherry. It pulled off with only moderate force, not what I want for a good instrument. I'm starting to think that I might need to ditch the aluminum. I figure I have at most one more shot at it before I leave it behind. It's a big bummer, because I'll be machining hardware from aluminum and it would tie together nicely...

 

[Reedt2000]

Well, I pulled the clamps off and tested the bond between the aluminum and cherry. It pulled off with only moderate force, not what I want for a good instrument. I'm starting to think that I might need to ditch the aluminum. I figure I have at most one more shot at it before I leave it behind. It's a big bummer, because I'll be machining hardware from aluminum and it would tie together nicely...

Seems a mechanical connection is needed to adhere the aluminum layer. What if you routed a channel for it, just the depth of the aluminum. Cut the center out of the aluminum so you have just the outline (which will sit in the channel). Attach the aluminum to the back with glue and screws, glue the top on, the bulk of which will be glued to the wood of the back? Might have to make relief spots in the top for the screws holding the aluminum.

 

[Jon Clegg]

Seems a mechanical connection is needed to adhere the aluminum layer. What if you routed a channel for it, just the depth of the aluminum. Cut the center out of the aluminum so you have just the outline (which will sit in the channel). Attach the aluminum to the back with glue and screws, glue the top on, the bulk of which will be glued to the wood of the back? Might have to make relief spots in the top for the screws holding the aluminum.

That's pretty much what I would suggest, as well.

 

[Beej]

My thought is similar... screw it down to the back piece, drill many holes through it, and glue with generous epoxy. It should let the top bond with the back right through all of the holes. Might produce a stronger bond?

 

[Jeff Siddall]

Agree with what others said, although I might also point out that, unless you plan to finish this with something that doesn't build a film, you could get cracking in your finish from the different expansion/contraction of the aluminum layer even if the adhesive itself remains in place. Gotta say I hope you find something that works. That aluminum layer would look awesome!

 

[Ripthorn]

I've thought similar as well. Mostly I just need to take time to process so that I don't rush anything and regret it. I have a traveling bit set, creating the channel would be easy, and most of the interior gap will never see the light of day. The bevel for the forearm carve is the only critical place...I would need to enlarge the rabbet there as well...hmmm...

Great suggestions, guys!

 

[Ripthorn]

Agree with what others said, although I might also point out that, unless you plan to finish this with something that doesn't build a film, you could get cracking in your finish from the different expansion/contraction of the aluminum layer even if the adhesive itself remains in place. Gotta say I hope you find something that works. That aluminum layer would look awesome!

I was planning an oil/ wax finish

 

[dwizum]

Doesn't Jerry Garcia's Rosebud guitar have a lot of brass inlay/binding on it? I wonder how that was done. I'm sure there have been a lot of copies of that guitar made, I wonder if you could get some ideas through some creative googling.

 

[Ripthorn]

Doesn't Jerry Garcia's Rosebud guitar have a lot of brass inlay/binding on it? I wonder how that was done. I'm sure there have been a lot of copies of that guitar made, I wonder if you could get some ideas through some creative googling.

From what I have read, aluminum is particularly challenging. I've done brass with regular epoxy with great success, but aluminum to wood has always been a bit of a crapshoot for me.

I have looked at what I have on hand and I think I can do the channel. I think it's a great idea, actually, so I plan to do that. I have screws that will require a .1" deep .25" diameter relief hole in the top, which shouldn't be a problem, as the top is a good .3" thick and the only place where I will need to be careful is the forearm bevel and the lower horn cutaway. I'm mentally mapping out where to put the screws so that I don't accidentally reveal them. That would be bad...

 

[Ripthorn]

Really crazy thought: what if I were to cut out the inner part of the aluminum, but not cut a channel in the back? I would still use screws, and the process would be to first dry fit the top/aluminum/body to ensure that everything lined up with screws and such. Then, apply epoxy to the cherry, apply the aluminum and put in screws, and then fill the center gap with epoxy and top the aluminum with epoxy, then apply the top, clamping it all together.

Reasons for crazy thought: 1) Titebond wouldn't glue the top to the aluminum, but epoxy likes a thicker layer than a typical wood to wood joint, 2) I don't have to worry about cutting a channel at all, 3) the thicker center layer of epoxy would act as an additional mechanical key for the aluminum, 4) I would need a larger batch of epoxy, which should result in more precise mixing ratios for optimal performance. There are all kinds of resin-casted instruments out there, so I'm not too worried about that.

So lay it on me, what am I missing with that train of thought? What type of epoxy should I go for? I have Z poxy finishing resin and I have loctite water clear 5 minute epoxy. I would probably prefer something a little slower setting, but I'm not sure what kind would be best...

 

[Jeff Siddall]

So lay it on me, what am I missing with that train of thought? What type of epoxy should I go for? I have Z poxy finishing resin and I have loctite water clear 5 minute epoxy. I would probably prefer something a little slower setting, but I'm not sure what kind would be best...

It sounds like that should work fine. You might want to put a small vent hole somewhere inconspicuous over the gap (ex: pickup cavity) to prevent any excess epoxy from having nowhere to go and causing hydraulic pressure on the lamination.

I am not confident in making any epoxy recommendations, but something slower setting than 5 minute would probably make the whole process less stressful. I have used carbon fiber laminating epoxy with a gel time of something like 20 minutes, but no idea if it would work well for your application. With most epoxies working time goes up considerably as temperature goes down so if you work somewhere cold it might give you more time.

 

[Ripthorn]

It sounds like that should work fine. You might want to put a small vent hole somewhere inconspicuous over the gap (ex: pickup cavity) to prevent any excess epoxy from having nowhere to go and causing hydraulic pressure on the lamination.

I am not confident in making any epoxy recommendations, but something slower setting than 5 minute would probably make the whole process less stressful. I have used carbon fiber laminating epoxy with a gel time of something like 20 minutes, but no idea if it would work well for your application. With most epoxies working time goes up considerably as temperature goes down so if you work somewhere cold it might give you more time.

Great thought on the vent hole. The neck pocket would be ideal, as I'm not 100% positive on pickup locations yet

After some reading, it looks like System Three T88 is available at my local woodcraft and has a one hour open time. I don't care if the open time is long, as long as it works. I've wasted 4 days trying to get the Weld Bond to work...

Also, I read a review that said a loose joint worked best for wood to wood applications with the T88, so the .019" thick aluminum as a gap for a "loose" wood to wood joint sounds like a plan.

 

[Ripthorn]

Just had another thought. Instead of screws, I could just do 1/4" holes in the aluminum. The epoxy would cure to form a peg of resin that glues the top and body woods together and keeps the aluminum captive, even if the joint between wood and aluminum is relatively thin. Then I wouldn't have to worry about accidentally exposing fastener heads.

 

[Reedt2000]

What about pins? Dowels glued into the top, glued into the back through holes in the aluminum. Could still use whatever glue holds to wood to aluminium best so the re e would be an element of adhesion there too

 

[Ripthorn]

What about pins? Dowels glued into the top, glued into the back through holes in the aluminum. Could still use whatever glue holds to wood to aluminium best so the re e would be an element of adhesion there too

I had this thought, but due to the fact that the aluminum is only .019" thick and the epoxy likes a little looser joint for maximum adhesion, I don't know if the extra effort is worth it. If necessary, I will totally go for it, but I'm not sure it's strictly necessary now. Thanks for the thoughts, though! All these thoughts are better than giving up on the aluminum, as it will look SO COOL!

 

[wraub]

Odd idea-
Cut out the middle of the aluminum, and route a matching shape into the top (and/or back), then glue as desired. The adhesive would be on wood touching wood, the aluminum would be captured between and be retained as well by the shape on the body... a keyhole kinda thing.

Might just be a dumb idea, but, I know it's odd.

 

[Ripthorn]

Odd idea-
Cut out the middle of the aluminum, and route a matching shape into the top (and/or back), then glue as desired. The adhesive would be on wood touching wood, the aluminum would be captured between and be retained as well by the shape on the body... a keyhole kinda thing.

Might just be a dumb idea, but, I know it's odd.

That's what was suggested a little earlier. I am thinking of just simplifying the whole idea by cutting out the middle of the aluminum and fill in with epoxy, thus making a keyhole from resin. The resin would also, at the same time, bond top and back woods, as the epoxy likes a little thicker joint for maximum strength. Seems like a two-for-1 scenario, and I figure that it's just wood and nothing ventured is nothing gained. I'll order up the epoxy and pick it up likely on Wednesday.

So to recap, it may be an odd idea, but that's how I roll!

 

[Ripthorn]

So I ran out to the shop and mocked this up really quick and rough:

I have ordered the epoxy from Woodcraft and will pick it up on Wednesday because it's on the way to Grainger, where I am getting the materials to make the bridge. So I guess today and tomorrow will be cutting out the aluminum interior, drilling the holes, and doing some neck work.

 

[Ripthorn]

Me and the jigsaw had some fun, along with the drill press and step drill bit:

You will also notice that I put in two locating screws in the neck pocket area, along with a vent path/vent hole for excess epoxy during glue up. It may not be the prettiest, but cutting the interior of .019" aluminum with minimum deformation is difficult. I used a 36 tpi blade and kept the cut line supported very closely with the edge of my workbench.

I also spent a good deal of time working out the compound scarf geometry. After getting the angles right on scrap, I decided that I needed to understand the problem better. Being a trained physicist, I started deriving a closed form equation that relates all the different angles to each other. I did figure it out, but I think I might post a separate thread to act as a tutorial for making a compound scarf where you don't use a square block for teh headstock, since my bocote neck blank is not long enough for ramp of the scarf and square headstock. I think I've got it all figured out, but I want to test on scrap using some other angles to confirm that the methodology works in general before detailing it. Once I have enough confidence there, then it will be time to cut the actual neck blank...

 

[Ripthorn]

Well, I confirmed that both my methodology and equation are correct for the compound scarf joint. I then cut a scrap piece of pine to the same width as my neck blank and tested again, just to make sure I had the process down pat. It now makes lots of sense to me. So, in preparation for the real deal, I straightened the edges of the neck blank, and then struck a center line with a marking knife. On dark woods, I do a marking knife and then rub chalk in it. A quick hit with 220 leaves me with a nice, crisp, fine center line:

Here it is with the scarf all cut. I realize now that this picture shows less than I thought it did. It is meant to show that I was able to get the headstock with enough material for my headstock design and the shaft with enough length for the fretboard, all from a board that would not have had enough material for a square headstock block and length for the shaft.

next up is to clean up the scarf joint surface and glue it up. I should also be able to get my epoxy along with bridge materials tomorrow, so I'm ready to roll! I've also got carbon fiber stiffening rods. Speaking of which, those of you who use them, how much of the neck length do you like to install them in? Just curious, as I've only ever done it once before and that was on a baritone and I just kind of made it up as I went.