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What would be a good final target thickness for a carpathian spruce top, for a small jumbo? It is quite stiff at .185" Thanks!

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

At .185 I would expect so.

There is a real problem with taking tops to a specific thickness, the problem being that the stiffness will be a random factor. Better to take the tops to a specific stiffness and let the thickness be a random factor. Because of the cubed rule of stiffness, even a few thousandths of an inch of thickness can vary the overall stiffness of a top by quite a bit.

I find deflection testing to be quite useful. Lots of info on the net, but in short...

The 'standard' setup is beams at 18" with a 5lb weight. Variations will work of course, the important thing is to take data and track your own work, and keep records.

As always, I recommend the Gore/Gilet books. Worth way more than anything else you could spend your money on in guitarmaking. Maybe not as sexy as a sweet set of guitar woods, but will definitely help you get the most tone out of your tone woods in short order.

As a start, .110 is the standard 'factory' thickness for spruce. It will still almost certainly be stiffer than it needs to be, but it's a place to start. I would build the deflection jig, measure the stiffness at that thickness, take notes, build the guitar, and assess it carefully, hopefully amongst peers, trying to decide if you could out more bass or if it's too bassy already. That type of thing. Of course, many other factors come into play such as your brace mass/height/stiffness and the stiffness of the back as well.

Which is why quantification is such a valuable tool. The how's and why's of which can be found in the books...

At .185 I would expect so.

There is a real problem with taking tops to a specific thickness, the problem being that the stiffness will be a random factor. Better to take the tops to a specific stiffness and let the thickness be a random factor. Because of the cubed rule of stiffness, even a few thousandths of an inch of thickness can vary the overall stiffness of a top by quite a bit.

I find deflection testing to be quite useful. Lots of info on the net, but in short...

The 'standard' setup is beams at 18" with a 5lb weight. Variations will work of course, the important thing is to take data and track your own work, and keep records.

As always, I recommend the Gore/Gilet books. Worth way more than anything else you could spend your money on in guitarmaking. Maybe not as sexy as a sweet set of guitar woods, but will definitely help you get the most tone out of your tone woods in short order.

As a start, .110 is the standard 'factory' thickness for spruce. It will still almost certainly be stiffer than it needs to be, but it's a place to start. I would build the deflection jig, measure the stiffness at that thickness, take notes, build the guitar, and assess it carefully, hopefully amongst peers, trying to decide if you could out more bass or if it's too bassy already. That type of thing. Of course, many other factors come into play such as your brace mass/height/stiffness and the stiffness of the back as well.

Which is why quantification is such a valuable tool. The how's and why's of which can be found in the books...

Righto, double post for whatever reason. The blue face is supposed to be the word

E
E
K

...

Here is what I use.

It's 18" between the risers and allows the dial indicator to hit the center of a 17" wide plate. For accuracy I also cut all of my plates to 17" wide, that being as wide an instrument as I'm likely to build. I also like the brass hex bar stock. Doesn't roll around...

Cheers

Deflection................yes ..............but no one is telling Alex how much deflection.
Tom

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A person who has never made a mistake has never made anything!!!

And no one really can, as we all do things differently, and my numbers might not work for him or his work methods.

Hence testing, evaluating, recording, and repeating in order to find his own numbers.

He's been give a safe thickness number to start his evaluations at, and directions to the Fount of Wisdom.

The rest is up to him.

Alex:
I use 18" spread on the rests and look for .200 deflection with 5 lbs. of weight. As Ed says the rest is up to you but this works for me and what I do.
Tom

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A person who has never made a mistake has never made anything!!!

Thanks Ed and Tom. Gore/Gilet is the only thing on my Christmas list, this year! I thinned the top to .110", and measured .165" deflection at 18" and 5lbs. I've entered the info in my build log, and will check again after I do a little sanding.

Alex

_________________
"Indecision is the key to flexibility" .... Bumper sticker

Alex:
Carpathian spruce (European spruce) always seems to be quite stiff to me. If it was me, doubt if I would go below .100". One can always work the braces more. Also I like to thin the edges of the top a bit. The how much of this also depends on guitar type, finger style gets the least stiffness and a dread more. Another good reference is the Kent Everett video on top voicing. I even work a bit of that into the mix. You can't go wrong on Trevor's books, best overall reference that I have.
Tom

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A person who has never made a mistake has never made anything!!!

Thanks, Tom. I'm working from the OLF SJ plans, and the bracing looks to be on the heavy side, so I'll lighten it up. I knew that I wasn't going to get a definitive answer because of all of the variables involved, so a ball park figure goes a long way to getting me in the right direction.

Alex

_________________
"Indecision is the key to flexibility" .... Bumper sticker

Alex,

I think that's a little stiff still. Try doing what Tom suggested...

I'll whittle it down a bit tomorrow, and see what happens.

Alex

_________________
"Indecision is the key to flexibility" .... Bumper sticker

Alex,

Not sure if you're aware, going from a deflection of .165 to .200 happens really quick! My deflection target before final sanding is .180 with a 4.5 pound weight, an uncut sound hole, and 18" centers on the test rig.

Also Alex, you're right about the bracing. On the plans they are truly epic. I think about 3/4" in height? Much too much, think more like .550 height...and that still leaves room for work.

Todd Stock stuck his neck out and posted suggested bracing sizes for an OM or OLF SJ to help newbies, but we have lost all his posts.
I wrote notes of these and will pass them on if it helps.
X Braces - 0.250 x 0.550"
Tonebars - 0.250 x 0.500"
Finger braces - 0.250 x 0.575"
UTB - 7/16 x 5/8
Bridgeplate - 0.100 rift sawn hard maple.
No guarantees here, just an indication.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

OM's and up you can have "telegraphing" with 1/4 bracing, of course it depends on your top

I built the deflection jig. How much, if any, diffence is there between say, a jumbo top and a parlor??

I'm guessing this a typo on the finger braces. That's awfully tall for a finger brace.

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Jim Watts
http://jameswattsguitars.com

That's what I thought too, but that's what I wrote - I went with 0.525 or 0.500 I believe on the SJ I made, well peaked.
Englemann soundboard at around 0.1" before finish sanded, so quite thin, hence double X-braces on this.
But a pretty lively guitar, reasonably good bass to treble balance, loud, and yes, braces telegraphed though a bit.
First member of the public to try it bought it on the spot, although he'd only come in to buy strings and picks.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

well ok then. Just checking.

_________________
Jim Watts
http://jameswattsguitars.com

Deflection was 2.5 mm, 5lbs, 18" across and same roller spacing on that Englemann top.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

For what it's worth, I made notes on what I think may have been the same post by Todd, and I have the finger brace dimensions as 0.250" x 0.375".