Building for playability

Discussion in 'Tele Home Depot' started by Freeman Keller, Dec 4, 2019 at 8:21 PM.

  1. Freeman Keller

    Freeman Keller Friend of Leo's

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    Geometry 101

    I’ve been thinking about doing this thread for quite a long time but several recent threads tell me this might be the time.

    If any of you have followed my ranting, er posting, you know that I am quite focused on the geometry of the guitar. I frankly don’t care what your guitar looks like or sounds like or what fancy pickup you put in it or how many coats of whatever you put on it – if it doesn’t play easily it is not a good guitar. Geometry is simply all those parameters that define whether or not you can make the guitar play easily. Basically if you have good geometry you can adjust the action to play well and easily, without buzzes, and have enough adjustment to change the action as needed.

    Geometry is not the same as action – action is the height of each string above each fret, geometry is the thing that allows you to adjust the action. Another way to say that is geometry is the relationship between the ends of the strings (nut and saddle) and the fretboard. I’m going to define some terms so I can use them later – these are my definitions and there is nothing sacred about them.

    - Fret plane. The tops of all of the frets on the center line of the neck. This assumes that the frets have been leveled and at least for now, there is no relief in the neck. If you lay a straightedge on the frets between the 3rd and 4th strings it follows the fret plane. The fret plane can be extended to the bridge

    - Neck angle. The angle between the fret plane and the body of the guitar, usually measured relative to the side next to the neck heel. Neck angle can be positive or negative, zero is an acceptable angle. I will show measuring the neck angle with a protractor that will read somewhere around 90 degrees, the actual angle is 90 minus that reading.

    - Overstand. This is a term from the bowed instrument world (violins) that indicates how much the fretboard is raise above the body of the instrument. It normally does not include the thickness of the fretboard itself. It may be angled or wedge shaped, in that case it is usually taken at the neck/body joint. It will make more sense when you see some pictures.

    - Arching. Some guitars are flat on top, some are not. The amount that the top is arched or domed is important when we talk about geometry. I will assume that the outside rim (top of binding if the guitar is bound) is in a level plane, by laying a straightedge across the top behind the bridge we can measure the difference at the rim, assuming the gap is the same on both sides.

    - Action. We have already mentioned action but my formal definition is the height of the strings above the first and twelfth frets. Action is a very personal thing – different players like totally different feeling guitars. Action often varies with different styles of guitar and sometimes style of playing (a Flamenco player or bluegrass flat picker might like a completely different action than a heavy metal shredder or an acoustic finger picker). For the sake of his discussion I am going to use a very average “medium low” action that I happen to prefer (and many people who play my guitars seem to agree). However this is one of the things that YOU should change in the following discussion to fit your preferences

    - Action values for this discussion are first fret clearance between 15 and 20 thousands of an inch and 12th fret clearance between 60 and 100 thousands. Note that I am going to use decimal inches in all of this – sorry, that’s the way I think the best. Most players prefer action rising slightly across the neck – I might set the high E at 0.014 (1st fret) and 0.060 (12th) and the low E (0.018 and 0.090). I’ve done a whole ‘nother thread on setups and this is discussed in lots of detail. For most of this discussion we will assume that the relief is so small as to be negligible. Relief however is a factor in action and needs to be considered when you actually set it.

    - Electric vs acoustic vs other instrument. The following discussion works equally for both acoustic and electric guitars, and interestingly, I find I like the same range of action values for both. My electric and acoustic guitars have a completely different feel when playing, but that comes from my choice of string tension and not action.

    - Fretboard radius. I’m going to make all of my measurements on the center line of the fretboard, with the assumption that the radius is consistent (no compound radius here) and that the saddles are the same radius (or can be adjusted to be the same).

    - Future fudge factors. I think it is very important to have some fudge room in any adjustment. I don’t like things at their maximum or minimum setting. Guitars change over time (particularly acoustics), playing style change, I want to be able to change my action accordingly.

    So my refined definition of geometry is those parameters (neck angle, overstand, thickness) that allows me to have playable action (0.015 at the first fret, 0.075 at 12) and some adjustment in case something changes. There are an infinite ways to get there.

    The last part of this little preamble is my observations that if I make the fret plane just hit the tops of the saddles at their lowest possible adjustment for most guitars (and bridges). It works for most guitars, most bridges and most players but it is up to you that it is going to work for yours.

    OK, what I propose to do is design the geometry for two guitars, working backwards. I’ll start with a few assumptions about what I want to build and playable action. From that I’ll try to derive the geometry, with luck that will be recognizable and then I’ll show the actual guitar. Being that I am a total retro grouch I’m going to do this with pencil and paper (and some colored markers to make it easier for all of you to see). Obviously a nice computer aided design software would be faster and more accurate (particularly the “offset” command) but pencil will be accurate enough. Besides, I can’t cut wood to three decimal places…
     
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  2. Freeman Keller

    Freeman Keller Friend of Leo's

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    Here are the parameters for the first guitar. As simple as possible, slab body with a flat top. I’m going to use a commercial bridge designed for slab bodied flat topped guitars. I want to do everything with a band saw and a router, and I frankly don’t want to futz around with fancy angles. I don’t mind the neck standing proud of the body, I just don’t want an angle. I’ve got a nice piece of rosewood ¼ inch thick for the fretboard and I’ve got a ¾ inch thick hunk of wood for the neck. Sound familiar?

    First thing is to draw the side view of the neck cut down its center line that’s the green thing. Mark the nut, 1st, 12th, 16th and 22nd fret locations and the scale length. Draw some little 0.040 bump to represent frets and draw the fret plane in red.

    IMG_5804.JPG

    Take the bridge and screw the 4th string saddle all the way down. In fact I took one screw out so I could get my calipers in there. At its lowest setting the saddle is 0.249 over the mounting plate which is 0.119 thick. That’s 0.368 which is pretty darn close to 3/8 of an inch.

    IMG_5805.JPG

    Draw my acceptable action of 0.020 (1st fret) and 0.070 (12th), connect them with a blue line and extend it to the scale length. That represents the D string over the fretboard at a nice reasonable action. Measure down from the red line (fret plane) the height of the bridge and make a black line parallel to the fret plane. That represents the top of the guitar.

    If I measure from the red line to the blue line I get right at 0.125 – that is the amount that I need to raise the D string saddle to get my playable action. That is a very reasonable number – it puts me about the middle of the height adjusting screws. That satisfies my criteria for fudge factor.

    IMG_5807.JPG

    Since my neck heel will be 1 inch thick I measure down from the top green line and that establishes the bottom of the heel or the depth of the neck cavity, measuring gives me 0.625. The distance from the bottom green line to the black one is the overstand, 0.125.

    IMG_5806.JPG

    Pulling out my Tdowns telecaster plans I see that the neck pocket is 0.625 and flat on the bottom. Success so far.

    So we build this critter with a 5/8 inch deep neck pocket

    IMG_4687.JPG

    And here it is all done. 1/8 inch overstand, action at 0.075 for the D string and saddles nicely centered in their screw travel

    IMG_5813.JPG

    IMG_5815.JPG
     
  3. Freeman Keller

    Freeman Keller Friend of Leo's

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    Parameters for the second guitar. I want an elegant carved body with graceful curves from the top down to the bound rim, the top 3/8 higher than the rim. I want the fretboard to sit on the carve top without any overstand. I’ve got the new style tune-o-matic bridge. I’ll glue the neck into the pocket so it better be right. Once again, sound familiar?

    Basically the same game as before. Draw the green fretboard, put some fret bumps on it and the red fret plane. Draw the blue “acceptable action” line. Measure the total height of the ToM – is 0.4570 for the bridge, 0.1500 for the studs and knurled adjuster, minus 0.073 for the little recesses at the end. Total height is 0.534 – a skitch over a half inch. That establishes the top under the bridge.

    IMG_5817.JPG

    Add the thickness in the center (the amount of the arching) and that establishes the rim, which meets the fretboard at the 16th fret (no overstand). The top will be planed flat under the fretboard but at an angle, then the top gently arches up to the bridge. The whole thing looks like this.

    IMG_5825.JPG

    Here is an important measurement – this is the angle between the neck and the sides at the heel – subtract from 90 and you have a neck angle of right at 4 degrees. Checking my StewMac plans they call out that angle as 85.6 degrees – I’ll accept that.

    IMG_5816.JPG

    Lets build it. Do the beautiful elegant arching of the top and angle the upper bout at 4 degrees.

    IMG_0653.JPG

    Here it is, all set up, action is right at 0.075. The bass side bridge post is a bit above the calculated 0.125 because its on the outside and I want a hair more action on the low E.

    IMG_5818.JPG

    IMG_5819.JPG

    So, totally different guitar, totally different parameters yielded totally different geometry. The action are almost identical and they play very similarly. The rest of the ergonometrics are different due to weight, shape, scale length and the angle of the neck relative to the body, but the fretboards feel the same.
     
  4. bender66

    bender66 Poster Extraordinaire

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    Thank you for taking the time to post the thread Freeman. I've seen a few of those recent threads. They're entertaining & enlightening.

    I'm lucky to know a great local luthier/retired math teacher. He's all about numbers & geometry.
     
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  5. RickyRicardo

    RickyRicardo Friend of Leo's

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    Very nice Freeman. Thanks.
     
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  6. Bergy

    Bergy Tele-Holic

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    I always learn a lot from yer posts, thanks for ranting!
     
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  7. Freeman Keller

    Freeman Keller Friend of Leo's

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    Here are a few random pictures of geometries that work.

    My hollow body jazz guitar. 3/8 arch, floating ToM, 3 degree neck angle, small wedge under the f/b tapering to zero at the joint

    IMG_5830.JPG

    IMG_5831.JPG

    Here it is under construction

    IMG_3520.JPG

    The little wedge is really funky when you look at it this way
    IMG_3522.JPG

    Here is another hollow body, an old Guild X-150. 3/8 arch, 5/16 overstand, 4 degree neck angle, very tall bridge. These big wedges are pretty common on archtops

    IMG_5820.JPG

    IMG_5822.JPG

    Here is another old Guild. Almost 1/2 inch of arch, 5/16 overstand, and a negative 1 degree of angle.

    IMG_5800.JPG

    IMG_5799.JPG
     
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  8. Freeman Keller

    Freeman Keller Friend of Leo's

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    And we can talk all day about neck angles on acoustics - their geometry changes with age and it becomes necessary to take them apart and fix it. However this one is a year old, 1/8 inch of arch, one degree of angle

    IMG_5832.JPG

    IMG_5833.JPG
     
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  9. hopdybob

    hopdybob Tele-Afflicted

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    may i ad another aspect that has to do with play-ability?
    body shape.
    my first homebuild solid body guitar had the body size of a Spanish acoustic with a strat scale neck.
    i only play guitar sitting on a chair.
    it sounded good but somehow the proportions were not right.
    so the position where the guitar rest on you knee and were the nut is from that position does a lot to, i think?
     
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