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Hi there...This if my first go at this on the OLF. I have been an on looker here for a bit, trying to pick up bits & pieces of what it takes to try and put an acoustic together. I've wanted to do this for quite awhile, and this last Xmas, my wife surprised me w/ a kit guitar. I've been woking on electrics for the last 10 years, but this my first attempt at an acoustic build.

I have the sides, blocks, kerfing complete, and the Maple wedge at the tail is all done. The back has been glued up w/ the backstrip, and the cross-grained spruce piece is in on the other side. I want to begin fitting the back braces in...

Here's where the problem lies. I dry-fitted the back and noticed that there is a gap (.1") where the heel block and the back are supposed to meet up. It sits up about as high as the inside back strip. I know what it's from, besides my stupidty...my over zealous attempt to grind a bit off the back of the block before I glued it in. Somewhere I got the notion that if I gave it a little slope, the back wouldn't show a bulge later, when it was put on...apparently I gave it too much slope or slop! I hope there's a fix for this?

It's a Martin Deep-Body OM kit, w/ a Dovetail joint. It's a real sweet kit w/ some great upgrades that came from John at Blues Creek, and I'm trying my best not to mess it up...Help!

Rich, welcome to the OLF.

Well, I can say, you've picked a very good vender for your kit (or your wife did). John is very helpful and will be here shortly to answer your question.

I am wondering if you are using a radius dish (a form used to help glue braces to the back and top in ensure a proper arch) for your build. If you are, than what you will want to do is cover the dish with 100 grit sandpaper and grind the edges of the rim till there are no gaps. This will close the gap at the heel block and ensure a nice tight fit all around.

If you don't have one, there is are a couple of recent posts about this very thing, cylindrical or spherical and Cheap option for sanding bowls

Hope this helps.

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You can glue a slice of similar wood on the end block and reshape to fit.

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[QUOTE=Rod True]

I am wondering if you are using a radius dish (a form used to help glue braces to the back and top in ensure a proper arch) for your build. If you are, than what you will want to do is cover the dish with 100 grit sandpaper and grind the edges of the rim till there are no gaps. This will close the gap at the heel block and ensure a nice tight fit all around.

.[/QUOTE]

Thanks Rod...

No, I don't have any dishes yet. I have the sanding discs for them, but I reluctanly didn't get the radius dishes. I thought that this might be where the problem might lie. I thought the the kit was pre-radiused and I did not need them, hence, I held back on getting them. I don't quite yet have the wherewithall to make my own.

It seems that after all the gluing, fitting & sanding of all the different parts, that it would be only logical, that one would need to radius the body to be sure everything is trued up prior to gluing the back on...correct?

When I first saw the gap, a shim was what first came to mind for the fix. Now, I am not so sure that's the remedy. I'd rather not have to shim it, if it only needs radiusing to pull it in.

Thanks...Rich

[QUOTE=RichB]It seems that after all the gluing, fitting & sanding of all the different parts, that it would be only logical, that one would need to radius the body to be sure everything is trued up prior to gluing the back on...correct?[/QUOTE]

Well if your good, And I meen really good (I'm definatly not this good) you may be able to profile the sides before bending and get away with it, but there is the difference in tail and head block heights, the linings which will not be arched on the gluing surface and the fact that the tail and head block are not arched as well, so I realy doubt that anyone would be able to not use the sanding dish or some other sort of device (like the one in Michael P's post) to "true" up the rims before gluing the back on. Also, if you use the same sanding dish to profile the back braces and use the same dish as a realy big gluing caul (don't know how you glued the braces onto the back) than you're sure to have a consistant radius and the stress on the back is greatly if not totally reduced.

Get the dishes, I'll bet this won't be your last build and I'm sure you'll use the dishes on your next builds.

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If you can't justify buying radius dishes at this time, You could do what a lot of us did in the olden days... Take a chunk of good straight 2X4 (or similar) about 30" long & radius it lengthwise to suit the arch of your back & top. Usually that'll be a 20' & 25' arch. You'll need one each. Stick on some 80 grit & carefully sand the rims. Check as you go by placing the top or back in position on the rim & press it into place to see where you need to sand more.
I found it worked best to keep changing the direction of the sanding motion, sometimes smoothing a localized area by holding the sanding block still at one end & taking short swinging stokes on the oposite side of the rim.
Does any of this make sense???
It's nowhere as easy as using a dish, but it does work (with care). Using this method, it's easy to oversand the head & tail blocks, & you may have to glue a slip of mahogany on top & sand to shape again.
Folks built instruments for centuries without radius dishes, & it can still be done... It's just more difficult.
If it was me, I'd buy, make, or borrow (or steal) a set of dishes.

Or, y'know, make your own dishes. It's really, really not that difficult. Take all of 4 hours if you go nice and slow, and most of that time's spent getting the arched guides 'right', honestly.

[QUOTE=Mattia Valente] make your own dishes. It's really, really not that difficult. [/QUOTE]

I agree, it's really not difficult, however the biggest mess I ever made in my shop was from making a dish. Never again. Some things are just worth paying for, and to me, this is one of them. Man, thinking about it brings back dusty memories...

John

[QUOTE=John Elshaw] [QUOTE=Mattia Valente] make your own dishes. It's really, really not that difficult. [/QUOTE]

I agree, it's really not difficult, however the biggest mess I ever made in my shop was from making a dish. Never again. Some things are just worth paying for, and to me, this is one of them. Man, thinking about it brings back dusty memories...

John[/QUOTE]

Oh, I did mine outside. No way I ever machine MDF indoors. Just..no. Nasty.

Well...I broke down & ordered some radius dishes. I will need them for future builds... I am not going to attempt to try and make my own and suffer the wrath of inhaling MDF. Getting back to the gap between the heel and the back though, when I pull the back in by hand, it's a tight fit, all the way around. w/o any daylight shining through. Maybe a little showing at the heel. I measured the distance needed to fill it the gap w/ some shim stock, and came up w/ .085" (or 2.75/32") of a gap. I can't believe I ground---+ that much off.

I took a couple of Pics. The lighting isn't ideal:

Hey Rich;
If it's any comfort, you ain't alone. I've had to put a shim in exactly the same place twice... Slow learner!

I've done the same thing, and I just used the same wood as the heel block with the same grain orientation, and once sanded, the shim became invisible.

Al

the other option is measuring an existing guitar to get the depth of the sides all the way round at say 1/2" intervals, just assume the top has no radius, markit on the rim with pencil, get your plane out, and trim the back, always checking for symmetry in the two sides. this is how i did it for a while, and once you get the shape in your head it is actually very fast... plus you can have a tighter radius across the guitar, and a larger radius lengthwise.... IMHO this looks a lot better than trying to dome the whole back to one radius, as it doesn't end up looking so "pregnant"

paddy

If I understand you correctly the block doesnt touch the plate. If that is what your situation is it might be interesting to you to know that many Martin's are done like that on purpose at the factory.

Even though I am new, I am somewhat proud of the workboard/mold/radiusing set up I built.The workboard is 3/4 birch ply with a centerline and the outline of the guitar body incised. The mold fastens to the board with inserts and the knobbed threaded rods. The center block and axle for the dishes fastens to the board the same way.There is a swappable steel insert for the soundhole cutter/rosette rout set on the centerline.Maybe someone, most likely another rookie, can get some use from it

Mike, Why do you have the rigid center post? I like the freedom to move the radius dish around loose using more of the surface area of the sand paper, and this does not affect the accuracy of the radius dish surfacing.

Hi Barry, I do it like you for the same reasons. One thing I don't like about it however is the tendency my fingers have to hit the rims or mold and skin my knuckles. That's one of the reasons I want to build one of those motorized affairs. I guess when I put my fingers in the wrong place on that set up I will REALLY feel it, huh?

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The center post makes quicker work of it, I use a similar deal. I put a crank handle on the dish and just spin it round and round while the weight of the dish does the work.

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The center post just makes it easy. The downside is, of course, that only one area of the sandpaper is used.On the other hand, I got my sanding paper from John H.-very low price. If I get to where I am doing enough guitars that it makes an economic difference-well, I'd be so darn happy it would be a small price to pay.Another side note. The workboard is also marked for the Martin slope dread shape using the same centerline- the soundhole for it is centered on the same steel insert for the rosette.The work board has short legs at each corner and maple 3/4 by 2" stiffeners on the underside to keep it flat. I clamp the whole mess to my bench in use.

Thanks for all the input guys. I went ahead and put a shim in. It only took me 5 tries to get it right. I overshimmed it, and I will radius the back when the dishes arrive. Hopefully, it will all fall into place w/o any discernable gap(s).

I found that if I cut some strips of paper, and pass them under the vacant areas, I could measure the thickness needed to bring the gap (.006) flush to the back after I shimmed it. It seems that it was the height/depth of the upper bouts on the backside that were preventing it from going down any further! A good night's sleep also helped.

Down load Stewmac's Kit instruction or see my post on cheap alt. to dishes. The sanding block show in Stewmac's kit instructions is basically the same idea as what I was taught in the beginning. The trick to using it affectively is maintaining the center point of the arch as you sand around the guitar. To do this the pivot end or support end of the sanding board moves in the opposite direction of the sanding end as you work you way around the guitar, while keeping the center of the arch basically in the same point on the centerline of the body. The arc is formed by a wedge at the sanding end that formes an angle close to the relitve cord of the arc. a 15' arc is very close in rise to a 5 deg wedge, 25' = 2 deg wedge. A sanding board will not give you a perfect arch but darn close. It will at the lease double the gluing area achieved as compared to flat sanding the rim, on both the lining and the blocks. MichaelP38755.7440509259

It appears from the above...I looked very quickly at it, that you haven't glued your top or back on yet. Since you have not, the fix is simple...sand the upper bout sides down in your radius dish until the blocks meet as they are supposed to. No need for shims or anything...yes your upper bout will be a little thinner...perhaps a 1/4" max...so what? What's the problem? Maybe I should read this over again...

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remember...

"If it doesn't play in tune...it's just pretty wood"

You do need either a slope, or really a radiused rise sanded into both the end blocks and the lining. It should match the slope or radius rise of the dome created in the back by the arched braces. This is not a big slope 5 deg for a back with a 15' radius 2 deg for a top with a 28' radius. If the tail blocks inner most edge is in contact with the back before the outer edge of the rim then you have too step of an angle sanded into them if the other way around then naturaly too shallow. I am not sure you understand that this angle or slope needs to be continious around the intire rim. What you are trying to accomplish is to create a ledge all the way around the rim that mates and matches the slope (radiused rise) of the backs (or tops) dome. This gives the largest possible glue surface with the least possible stress imparted to create the bond. In the perfect world 100% of the linings and blocls surface area should be in free contact with the dome of the back or top when it is just laid down on the rim. If the back or top requires pressure to be exserted in order to bring the lining or blocks into full contact with the back or top, then that force or stress is being built into you assembly. Your back would then be constantly trying to seperate from the rim.

I agree with what Michael said, but it should be noted that a ladder braced back will not have the full compound arch built into it. One still needs to press the back into contact with the linings. (Even an X braced back will usually need a nudge at the heel block.)
Gentle pressure at the tail & neck blocks should bring it into full contact. (the perfect world scenario again...)

Yep! That is why I said in the perfect world.

The closer the fit the better! The less force required to make full contact, the less stress is on the glue joint. The larger the contact surface area, the greater stress the joint can bare.