block angles, radii, sticks, whatnot

[MuddyFox]

Alrite... I'm probably waaaaaay overthinking this but it's left such a confusion in my head that I need clarification.

Regarding neck/end blocks and the angle they need to be sanded to (along with kerfing) to accept a radiused top/back.
Wherever I turn people speak of 1.5/5deg. Now that just can't be right and universal for all top/back radii?
If I'm building an OM with a 28ft top and a 15ft back, what block angles do I need?

I've considered building a StewMac thingy
http://web.mac.com/magads/Site/Stewmac_ ... _Back.html
http://www.nichebooks.com/kits/sander.html

and I've considered making a radius stick of appropriate radius
http://web.mac.com/magads/Site/LMI_OM_K ... Edges.html
http://www.kennethmichaelguitars.com/ba ... ouro3.html

and I've considered doing the hinge thingy
http://www.kitguitarsforum.com/archives ... 04_0_6_0_C
http://members.ozemail.com.au/~scraptechs/page2.htm

Now, with my vast array of nonexistent tools, I will have pretty much equally hard time fabricating either of these.
Radius sticks seem hardest to make (with proper radius and squared as well) but they could double as a caul for gluing x-braces without a gobar deck.
Hinge thingy could be manufactured through mostly trial and error to get it close to desired angles.
StewMac stick just looks strange and the wedges pretty hard to make with any precision given my situation.

I'll take any advice I can get on this topic, but first things first, before I lose whatever little sanity I still have left... please, how does one decide/calculate/pull-out-of-the-hat the angles needed for a specific build?

[kencierp]

Here's the first thing every style is the same -- no angles are pulled out of the hat! The neck is set at 1.5 +/- degree sloped from the edge of the sound hole toward the neck block so that is the dimension you to sand on the top/sound-board edge of the rim. You can either use 15' or 20' radius braces on the back -- so you need to make or purchase a sanding bar with one of those two dimensions. The Stew Mac idea is way out there in my opinion and if you try and make sense out of it no doubt you'll pull most of your hair in frustration. Don't over think this -- get that neck correct and you'll be fine.

[MuddyFox]

Thanks for chiming in, Ken!
As you rightly pointed out, it was indeed the SM solution that started this confusion in my head in the first place.
Since the back is 15ft, I'll try to stick to that. I realize that the back angles don't participate in neck alignment and are therefore not as crucial, but can you tell me whether that 5deg is anywhere near the required angle?

I've considered eyeballing it with a braced top/back as a guide. Probably doable but I'd probably make a mess of it so I'm looking for a safer solution.

[kencierp]

Well the slope created on the edges of the rim likely are 5 degrees but that is not the issue -- the entire back of the rim is spherical therefore the inner most curve of the waist is the highest point of the back edge of the rim if you don't get that and the inner edge of the neck block correct your fit and subsequent appearance will not look very good -- even Kinkead has this wrong in his instruction publication. -- Make sense? PM me we'll get you a 15' radius bar for cheap $$.

[deadedith]

It helps me to think of it this way:

The word 'radius' is used in different ways:

1. to describe the amount of 'dome' in a top or back. Radius=dome
2. to describe the angle that we sand in the kerfing/sides to make gluing surfaces match. Radius=gluing surface on kerfing/sides

Blocks:
Think of them as extensions of the kerfing.
So, the blocks have to have the same gluing surface as the kerfing.

For both the kerfing and the blocks, therefore, I use a single radius bar - simple, inexpensive, perfect results.


As to the 1.5 degree angle: it works. It is used by Martin and others. It is also easy to do using the flat side of the radius bar (or complicated methods if that is your choice) No sweat.

[tippie53]

http://www.youtube.com/watch?v=QYHPCeVRUA4
this is a video on how we do it but all that matters is you get it done is some controllable fashion. A flat board with sandpaper will do it . Don't over think it , it isn't that hard .

[MuddyFox]

Thank you all for your thoughts. I'll put this on a back burner and see what kind of a plan I can come up with once the dust has settled.

[tippie53]

I can tell you from personal experience that stew mac instructions don't translate well to a martin kit . The body rim geometry is a bit different . Like Ken says, don't overthink it.