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Double Bass Pics: top and back purfled, back glued

#5
Don Harris said:
Here's the latest of what I've gotten done. Now to graduate the top and install the bass bar. The neck is not glued in yet. I'll wait till after the top's on.
Click to expand...

I'm upset. I mean....Don Harris is just making basses left and right...and all I'm doing is thinking about it.

Between you and Mathew Tucker, I'm going to have to clean out my basement and start gouging wood.

Keep up the progress pics. Nice work.
 
#10
I put strips of masonite on top of the braces to keep from dinging them up, even though I was going to shape them afterwards. There was an awful lot of pressure from those pointy rods. For the fiberglass rods I used driveway markers or stakes from the hardware store. They were about 6', so I cut them to 2' lengths

I decided that I didn't want the hassle of fitting the sprung braces or having to fit a slightly curved back to the ribs. I really wanted to nail those joints on this bass and was a little worried about the curves. I made a sanding board from MDF and I really liked the idea that the front and back would fit flawlessly. I also thought that maybe a true flat back would have a lower resonant frequency without the bit of tension in the back. I realize that it wouldn't have been much tension, but it was a good rationalization anyway. Plus, I was thinking that old classic basses were probably truly flat backed. I'm guessing here.
 
#11
The idea behind the curved back is that it possibly deals better with wood movement, a bi like a roundback does; if the back wood shrinks, a truly flat back tends to cave inwards a bit and while this can be OK, it looks a bit odd. I've seen this effect on a number of old flatbacks. With a slight outward curve built in, a shrinking back will tend to go flat. And if the wood swells, a perfectly flat back might go either way, or else push on the ribs and maybe pop a seam, whereas one with a slight outward curve built in will tend to swell in an outward direction, just increasing the curve a bit.

Well, that's the theory, anyway.

Guitars are often made in a similar way, using a shaped (radiussed) backing plate under the go-deck, with the braces glued in forcing the back to retain a slight outward arch. It also helps for strength and rigidity.

Well, that's the theory, anyway.
 
#13
Nice progress Don! It is getting exciting at this point.

Matthew is right on with his explanation about the cross bracing on the back.

At this stage in building, it is critical to control the cross-grain gluing operation of the back. This is something that is not in any book on violin construction that I have read. It seems that you glued the back braces with your nice go-bar deck setup in the fall, right? What was the relative humidity in your shop? It should be around 30 - 45% to be optimal for this operation. If you had a higher RH, say 60%, you will get a bit of a cave-in in the winter when the furnace is lowering the room RH. Or worse yet a crack. If your RH was 30% when you glued the cross braces, you will be fine in winter. A curved brace helps in this situation and I recommend it for anyone else in this situation. Plus controlling RH.

Don, think about your situation and you might just want to go forward but you also might want to do it over. Removing the braces might seem like a big job now but I have done lots of things over, and chalked it up to "learning". Maybe you thought this out and glued it at low RH and I am preaching to the choir.
 
#14
I have a newly-glued-up flatback that I have yet to finish and put the braces on. After a few weeks leaning against the wall, it has developed a bit of a bow. Now I need to plane the thing flat on both sides. The easy way to flatten it for working is to spritz the concave surface with water and leave it for an hour. But it really illustrates the power of humidity!

I lightly sealed the inside of my bass with one coat of dliute shellac, to slow the uptake of moisture. I figured one surface would be effectively sealed, and so it may be a good idea to seal the other surface to slow things down a bit.

Here in Sydney we have a different climate to north USA, so I guess different strategies work in different places. I don't know what to down side of sealing the inside with shellac might be, so far i haven't come up with one.
 
#15
Thanks so much for the input. I did think a bit about the humidity, and here in Colorado the RH stays between 19 and 26 most of the time. The wood was well seasoned and the week I glued the braces was very dry. I'm hoping I'll have this bass for a long time and also that I'll be staying in Colorado. That's the nice thing about building here, that I won't have to mess with humidifying this bass. I am curious what will happen if it has to live somewhere wetter. My mentor who builds the smaller instruments said it's fine for them in a humid environment. No problems whatsoever. I don't know if the proportions of a bass would make that any different.
 
#16
Did you go with Chandler's dimensions for your back braces? Traeger says the the total thickness of the center brace, including the back, should be no more than 19mm I think. The plans call for thicker and I'm trying to decide whether to stick with the plans or go with Traeger. He also has a whole tap tone on the braces thing.
 
#17
Don,

Colorado (the dry west USA) is ideal for instrument making! You are in the right place and lucky. Your bass should be fine in a humid environment if you ever take it to the east coast.

The back needs to help the front vibrate, as it is connected to the top via the sound post. Try to think of that as you make the back as a whole. Tuning each brace is kinda silly, I think, if you don't consider the coupling.
 

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