Dovetail joint fit need help with correction

[ken cierp]

Need the heel cheek angle -- but there are a couple of contradictions out of line with the norm -- BTW to make an assessment/diagnosis I set up a simple CAD model to evaluate how the dims and angles stack up. It seems that the neck block may have been glued to the sides with the back edge tipping outboard -- with the SM assembly system this can happen sometimes because they machine a slope on the finger-board side of the neck block which can tend to tilt the NB.

[blnote]

Thanks to your articles I have a better undrstanding how my mistakes with the rim/neck block (actually starting with making my mold) are coming back to bite me. I do not have the tool you use in the link you provided. I took a few pictures measuring the gap with a square and using a level that shows the angle. The angle is different on either side. The heel is 4" exactly and yes it does drop flush with the top, this is with no shims and the dovetail barel getting any bite.

In case you cant read the display one side is 86.5 the other is 85.7. I hope this is the information you requested. Thanks, Dave

IMG_20120612_075838.jpg (45.53 KiB) Viewed 2577 times

IMG_20120612_080059.jpg (33.74 KiB) Viewed 2577 times

IMG_20120612_080402.jpg (41.02 KiB) Viewed 2577 times

IMG_20120612_080446.jpg (41.26 KiB) Viewed 2577 times

[blnote]

If I let the neck sit where it wants, it appears that the straight edge is close to where it should be, just brushing the bridge. Is it possible that removing material from the bottom of cheecks will pull the neck in to mate with the body and keep the angle? Not trying to jump ahead, just curious. I can't believe how far off all this is, taking these measurements have gone along way toward giving me a better understanding.

IMG_20120610_084411.jpg (26.34 KiB) Viewed 2576 times

IMG_20120610_084452.jpg (29.51 KiB) Viewed 2576 times

IMG_20120610_084506.jpg (32.42 KiB) Viewed 2576 times

IMG_20120610_085432.jpg (31.71 KiB) Viewed 2576 times

[ken cierp]

Let me think about this a bit before I give you a (at least my thoughts of) workable action plan. Note that using the finger-board other then getting its thickness is not a good idea -- it has its own set of variable (bows,curves) which can be miss leading. The angles of the cheek fit that I came up with on the CAD model, match your readings so that is not a surprise. The contradictions that need to be resolved are that the neck heel and rim are way out of whack too steep, but the slope on the sound-board is about a .5 degree too shallow, that's why your straight end does not clear the bridge by a .06" or as it "MUST."

[ken cierp]

I am still hesitant about making a recommendation – as you can imagine even with pixs having the instrument in hand is the better situation. What needs to be done is to get the relationship of the sound-board finger-board extension area in the perfect plane relationship to the bridge area. That slope seems to be too shallow since in your photo the straight edge does not pass over the bridge – it should just clear, there is wiggle room since the bridge thickness and saddle height are easily adjustable.

After this is corrected the neck heel angle can be addressed and lastly the joint itself can be made to do it's job I believe you have fallen victim to the idea (poor instructions) that by making the DT fit tight all the problems are solved where in fact this premise is what causes all the problems.

I believe you need to start by fabricating a fingerboard simulator – make sure it is “perfectly flat” Described here:

http://www.kennethmichaelguitars.com/NECKSET.html

[ken cierp]

There has been a lot of views but there is no more interest in this thread -- the bottom line is when fitting the neck you need a true postion to start so you "know where you are going" and if for some reason you get lost you must re-establish a known location point and/or angle. Otherwise its impossible to forumlate a logical appraoch --- trust me, trial and error, shaving, carving, shimming over and over again often leads to an abandon project.

[Tim Benware]

From the original poster?

As I have been reading, what I would have done (if it was me and I wasn't asking anyone, which is probably how I would have started out) was measure the body angle and the heel angle. They need to be 180*. So the adjustments have to be to the heel or the top where the extension goes. Getting the heel angle right seems to be he answer (of course I say that not knowing the angles, I think one pic had it at 86.5*. Material would come off the heel from the bottom angled up to the point where the heel and bottom of the fret board meet. Slowly until the correct angle is achieved. Then shim it up, adjust the center line and drop the DT into the joint.