Top Slope Math Recheck

[allenb]

On my current OM build I'm using the masterwork mold mounted to the hinged base and rotating abrasive bar. I'm at the stage of producing a 1.3 degree angle (neck block to waist area of top. It's been stated that the amount of material removed from the front edge of the neck block should end up being around 1/16" when the slope extends from neck block to waist. But, by eye, it looks like it will be in excess of 1/8".
At the 22.5" mark from the hinge end I'm at .510" as directed.

For verification I made some right triangle calculations to see what the rise would be with a right triangle base of 6.5" (neck block to center of waist) and angle of 1.3 degrees. The distance of rise comes out to .149" which would seem to be way off compared to all of the threads I've read on the subject.

Looking to see if this is in the ballpark or if my setup is out of wack?

Thanks,

Allen

[tippie53]

sounds like you may have taken too much
I use solid molds and dishes, they allow more accurate work. The key here is to get the neck angle so you will have about .094 above the top at the point of the saddle. Having the sides locked into position is key the less movement the better.
when you check this angle what are you seeing off the tail block? Also what top radius are you using? You may need to ask the supplier of the mold what you should see . Top radius will play a part on the angle.

[allenb]

Hi John. I'm using the one designed by Ken and is pretty much identical in all aspects. I haven't created the angle on the guitar yet as I've jumped in before without verifying and cost me a few days of heartache! This will be the first build using this system. I've done a couple of previous builds and just used a sanding board to arrive at the 1.3 or so degrees and they worked out very well with very little tweaking necessary. Top radius is 40'.

What I would like to see is if someone would also make the calculation to verify if this angle made on a perfectly flat rim, including top of neck block, causes more than 1/8" of drop at the front edge of the neck block (when angling the area from front of neck block to center of waist).

BTW, I watched your video on setting up the rim (very nice video!) and I do use the method you showed in checking the gap at the tail when placing a flat surface down on the upper bout top angle and is a great way to check progress.

Allen

[johnnparchem]

I do not use Ken's method of sanding in an angle to the rim, but as I read through your analysis I do not see calculations for where the saddle would be because of the dome from the 40' radius. It is not flat so the right angle analysis does not help that much. What you would be looking for is the difference between where the saddle is on the 40' arc and where a straight line is at a 1.3 angle from the top plane. To visualize: a straight line between the top of the neck and heel block, a 40' radius arc between those same point, a straight line at 1.3 degrees from the first line from the neck block. Place the saddle the correct distance on that line. The difference between the arc and the angled line at that point is the measure you are looking for.

[johnnparchem]

If you want a flat rim, the calculation is just rise over run as an input .125 in 12.8 (depending on you scale length). You can draw this and measure. I am lazy and use a calculator http://www.1728.org/gradient.htmr. The angle you need is close to .56 degrees. So 1.3 is way to steep as you found.

You can use the rise over run as a fraction as well to set the height of your jig 22.5" from the hing by multiplying that ratio by 22.5.

[Ben-Had]

I don't want to sound condescending but did you follow the instructions exactly? You have your line in the right spot on the mold to measure the height to raise the board? the you do the following:

0 Level the top edge of the rim to the base of the mold. Use an adjustable square to
pick off the high point from the tail block.

0 Raise the neck block to the exact height as the tail block.

0 Insert a shim to maintain that elevation (a stack of Popsicle sticks and piece veneer).

0 Secure the rim to the mold with the internal clamp system

0 Add several more shims under the upper bout for stability (stacks of Popsicle sticks
or tapered wedges).

0 Slide a ¾” stop collar on the MM spindle.

0 Test for level with the flat side of the abrasive bar

0 Raise the mold base the required height (.510" for a neck angle of 1.3*)

0 Measure the gap at the inside edge of the neck block with feeler gauges.

0 Set the stop collar at the same level as the gap measured at the inside edge of
the neck block. Use feeler gauges to transfer the gap. Place the gauge on the edge
of the abrasive bar bearing then pinch it with the collar and tighten the setscrew.

0 Chalk the top of the neck block to check sanding progress From top of sound hole to neck block)

0 Swing the abrasive bar back and forth over the neck block and upper bout rim until
the chalk has been removed from the neck block and the bar bearing hits the stop
collar.

I have had about perfect results with this method every time.

[allenb]

but as I read through your analysis I do not see calculations for where the saddle would be because of the dome from the 40' radius

On my last jumbo acoustic build (not using Ken's mold and rotating abrasive bar), using a 40' top radius came out very close to the 1/10" rise Ken mentions in the section titled "Neck Set Angle Myth, Magic or Mathematics?" For that build I found using 1.3 degrees for top slope along with a neck angle of 88.7 worked perfectly so I want to use that again on this build. Height to top of saddle was 1/2" from soundboard and had great action and around 3/32" clearance from low E to 12th fret.

It is not flat so the right angle analysis does not help that much.

The reason for the right triangle analysis was to discuss how much drop in elevation a 1.3 degree angled line would fall in the distance of 6.5" which is the distance from the center of the waist curve to the outside edge of the neck block which I found by calculation to be around .149". It wasn't meant to be used in determining a geometrical layout for neck and bridge.

I'm guessing that Ken's stating the drop to be around 1/16" is probably based on using a 1 degree slope and only sloping from outer edge of neckblock to top of soundhole instead of going all the way to the waist curve. I think my confusion came from reading Ken's text and then reading where he quoted people at Martin stating other angles and lengths of slope.

I don't want to sound condescending but did you follow the instructions exactly? You have your line in the right spot on the mold to measure the height to raise the board? the you do the following:

No worries, you don't sound condescending. Yes, I've read everything written in the tutorials and tips enough to recite them without notes. Not really but I'm fully understanding all the intricacies in setting up the adjustable angle base with mold and abrasive bar. I also verified the angle using a good reliable angle finder. Top of rim was perfectly parallel before changing the angle of the base.

Thanks for the input guys. I'll most likely just stop the sloping when I reach the 4" mark from the neck block which is all one should really need in the first place to make sure the fingerboard has a landing place all the way to it's end without a gap.

Allen

[Ben-Had]

Thanks for the input guys. I'll most likely just stop the sloping when I reach the 4" mark from the neck block which is all one should really need in the first place to make sure the fingerboard has a landing place all the way to it's end without a gap.

Allen

Allen, if you've set the gap under the abrasive bar with the stop correctly, you stop when you've reached it. In fact, unless you force the bar down it can't sand any further because of the stop. That is the beaty of the beast it's a no brainer. If you've sanded to much you've either set the gap wrong or forced the abrasive bar down out of position. I really shouldn't matter if you use a 25', 28', 40' or 52' radius on the top they will all be so close there will be little adjustment (if any) left to do. I use 25' radius and have been just about dead on for at least a dozen guitars of more now.

[allenb]

Tim, I just read and re-read an older thread where you, Ken and Dave Bagwill really went around a few times on the subject. Talk about a chicken and egg dilemma this can become. http://www.acousticguitarconstructionfo ... ?f=18&t=86

So, to make sure I understand, starting out with the top of the neck block level with the rest of the rim, sand down to the point where you just nearly obliterate the chalked mark on the inside edge of the neck block and you're there? I had mistakenly thought if you stopped there, you would not have sloped the rim far enough back to make it to the top of the soundhole. I understand your point on not forcing the bar which will flex if you're not careful. After re-reading lots here, I'm going to go for a 1 degree top slope and 89 neck angle with my 40' top since it appears to all complement together. I'm not sure why the 1.3 worked on my last build but maybe my aging eyes weren't seeing the protractor very well.

Let me know if I'm still off in my take on this. (it always takes me a while!)

Allen

[Ben-Had]

Yes, that was a great discussion and what ultimately lead to my construction method. Here is one of the key take aways:

"but in reality the brace pattern by its very nature is not going to be transferred exactly to the sound-board. The top will most likely be significantly flatter after glue up -- and even further flattened with the bridge plate."

Because of this variable I now do my slope, finish construction & finishing and the last thing I do is make my bridge. That way I can get the exact thickness I need so the FB projection goes over the bridge at the correct height. Even using this same method EVERY time there are small variances.

The light bulb really went on when I went to the Martin factory and saw their final fitting of the bridge, they didn't have 3 different sizes they had about 15 different bins of bridges. Now I think some were different type woods and styles but the bins were labeled with a bridge thickness and after they measured the FB projection height they would grab a bridge from the bin marked with that thickness.