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i have the back fitted nicely to the sides so that there are no gaps during dry fitting. but here is the problem, there is a gap between the back and the neckblock becuase due to my inexperience i sanded down the neckblock flush with the side height. i didnt realize that the back was going to angle out that much. so i am thinking i could make a wedge out of scrap koa from the back ,glue it to the neckblock and sand it down till it has the angle of the back. i know that sanding the sides down further to get the angle would bring my sides down way too much. i would say the gap is just under an 1/8th in. any other suggestions to correct this problem? thanks for the help everyone.   

I would add the shim to the head block and would use the same wood. I would also use a couple of dowels when glueing the shim to the block for two reasons. One, it would line up the new block to the old one and the second reason is more important in my thinking. You will most likly be gluing to end grain which will not hold up as well as gluing to side grain. The dowels will help strengthen the new glue joint.

Take a look at This post about 2/3rds down at Paul Woolson's picture of his neck block, you could add this to yours and than shape it to fit the back, this will also help with the end grain glue joint.

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Add a shim to the headblock. I did the same thing on my first, joint's invisible.

Aaha!! kamaK
I Knew I was right. I was affraid of that!! But not a biggie. I too would suggest a mahogany laminate graft. Before I would made the sides shorter, I would laminate the extra piece on the block and re-sand. Don't feel too silly, you are not the first to make this mistake...don't ask (forgive the exuberance of the opening. I don't know why but I just felt that this was one of the issues you were having from statments in your previous post)

Now!!!! if you did this, I am also betting you sanded with a flat sanding board along the linings. If so you have no bevel or taper in your linings to match the dome either. This means that only the edges of the lining is in free contact with the back. You can if you wish depend on the clamping pressure and glue joint to hold the back down to full lining contact, but the stesses on the glue joint will be high. If this is the case and I hope I am wrong here, if I were in your shoes, I would be sure to sand the dome bevel into the linings as well. this will give you a much stonger glue joint due to less stress on the joint and a greater glued surface area. Let me also suggest that you read my post on cheap alternitive to sanding bowls, where I show a drawing of a simple sanding board and how to use it to produce a dome contour on your blocks and linings. I posted it yesterday after reading your post a couple times. Cost of this board is scrap 1x?x24 hardwood and 2 pieces of sandpaper.

Be sure to note this mistake in red in your journal. If you don't keep a journal, Start. It will be the most valuble tool in your shop

"Those that do not study history's mistakes are domed to repeat them"

I know.... I sound like a teacher. My wife tells me that all the time MichaelP38746.0375

Michael Payne (and others),

There is one thing about this domed back, rim taper, 15' sanding dish approach that's confusing me & I'm hoping you or someone else can help clarify this.

In tapering the ribs (either before or after bending) I have been using Cumpiano's basic method of starting the side taper at 2" behind the waist & continuing the taper towards the neck block for the minimum rib height (which is from 3/4" to 1" less tall than at the lower bout maximum).

Then, I smooth that taper transition point with a flat sanding board so that it is more gradual tail to head. I have a 24" 2x4 with a 15' arch cut into one long edge & that arch is lined with 60 grit sandpaper. I choose a visual center of the body & manually rotate the sanding arch around that visual center - not unlike sanding with a sanding dish "section". After this treatment has freshly scratched all of the linings, I fit the back plate.

The back seems to fit the linings OK (especially around the lower bout) but there is still the need to bend the upper third of the back down to meet the head block about 5/8", putting quite a bit of tension on the subsequent glue join for the entire back to rim attachment. The back is only ladder-braced by the way, so the back is only arched from side to side ... barrel-like. Even using a 15' arched dish to glue the arched braces on, it still is only arched side to side.

Is this bending back tension accepted as just part of the stresses the box is under? Is it better to taper the rims beginning further back from the waist & make the taper form more of a curve to smooth this transition further? I'm not sure how best to deal with this.

Also, when you use an arched dish to sand the rim to back join, are you using it centered at the head-to-tail arithmetic center or instead more towards the lower bout as a "weighted" center point. For that matter, do you angle the sanding dish at 5 degrees down toward the neck or is it spun parallel to the "flat" horizontal top plane?

Sorry to take this thread in a slightly different direction, but I bet this clarification will help KamaK & others as well. Thanks in advance for any sage advice or even just opinions!

Skip
Skip Beach38746.5810300926

If you used a concave sanding board ( lets say 15' radius) then the bottom of your block should never be greater/lesser than the radius. I agree you could just add on to the block, but who says you can't just take the sides down and make it ( the neck block end) a little thinner than your plans? As Long as you don't have the kerf lining on yet... so what if you sand another 1/8th of an inch or so off? Remember all this stuff isn't rocket science. Who's going to complain if you change things a little? I believe our friend Cumpiano doesn't use a mold anymore( I bet that really flips a lot of people out!). Who needs a "stinkin mold" anyway !!!

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Since I have gone to a sanding bowl I have changed the way I handel the transition from upper to lower bout on my backs I have a home made barrel nut with a 3/4" all-thread, my sanding bowls have a 13/16 hole on center for this all-thread I have a locating fixture for my mold that puts the barrel nut assembly in center of the box as measured from end to end. I adjust the assembly so that the bowl touches my rims at the trasition point 2" past the lower bout side of the waist, then adjust the angle so that I have equal seperation between the heal block, neck block and the bowl. I lock this angel and use this as my sanding axis. by doing this I have a pure dome shape sanded into my rim and linings. Therfore my backs drop directly into place and fit because the sanded dome is pure only the axis of the is tilted to meat the dome at heal and neck height.

To me this pure dome shape relievs the srtress you put in to a plate by draw fitting.

PS I leave enough material on the blocks, rims and linings to allow me to sand them all to the dome profile by the time I get to the desired neck end and heal height. depend ing on thickness of blocks and size of guitat that is about 3/32 to an 1/8" of sanding (mostly the neck block) required for me based on my neck and heal block thickness.MichaelP38746.7040277778

others do this by putting the sanding rim on a motorrized shaft and adding blocks to the rim to set this angle. but i sand by and by rotating the bowl around the 3/4" rod.