Bridge Position on Scratch Guitar Design.

A 'humble engineer' by day and a 'want to be amateur luthier' by night I have always dreamed of building my own guitar from scratch and am putting the details down on paper. Well technically a screen... but you get the idea. I have access to CAD software and several thousand hours in it. I am basically building a semi-hollow single cutaway body. Think if a tele and an ES-335 had a baby.

My main question relates to the bridge. I want to use a bigsby B70 (or maybe a B5) if it works out and a tune-o-matic style bridge. I was looking a this setup here:
http://www.stewmac.com/shop/Bridges,_tailpieces/Electric_guitar,_Tune-o-matic_bridges/Locking_Roller_Bridge_with_Bigsby_B70_Vibrato.html ?tab=Pictures

Now I am using a standard tele neck CAD file I got off the net for the layout and the fret spacing (that's a task to tackle another day). I want to be able to build this guitar with a pre-slotted fretboard.

the question I have is how do I know where to position the bridge? Are there plans or guidelines anywhere I know some people kick it off at an angle for playability (i think?). I know the final tension on these strings is going to be a function of where I drill the holes for the bridge. So before I screw up.... please wise TDPRI crowd bestow your wisdom upon thee!

Also does anyone have a CAD model/drawing of any bigsby? Or the dimensions of the hole pattern for the B5?

Much Thanks in Advance!

A tunematic on a fender..no problem. Adjust the saddles forward all the way and then back a hair ( for wiggle room) and position the breaking edge of the top of the saddle on the scale length measurement which in this case would be 25.5" from the nut end of the fretboard. I like to use the center web of the casting between the d and g saddles as the centerline of the bridge. You might want to rotate the bridge slightly counterclockwise if you are using an abr- 1 version as opposed to the nashville one to gain extra intonation space . You'll notice on on old LP's the bridge is tilted this way. That is because the saddles needed to go back farther than they normally could be positioned.

You may also want to design a deeper neck cavity to allow for shimming or a 2 degree angle in there to account for the extra height of the bridge. Some people opt to rout the bridge down into the body some instead... but not me.

There is less room for error on these bridges so you need to be accurate when drilling and locating. Somebody mentioned using a trapeze tailpiece to position it in another thread a week or so ago, which sounds logical if you are not confident about this.

Thanks for your reply. I have never seen a trapeze tailpiece in person so . So to expand upon this 25.5 is the scale but what is the tolerance? Is +/- 1/16" tolerable or are we down to 1/32" or 1/64"? Or is it the travel distance of the saddles? Could I scale the body up so it was a 26" scale? How would this affect the fret spacing?

In all honesty, I was planning on having the main body core CNC routed from a nearby company which is why I am trying to lay everything out in the PC realm. I know it was key and I want to get it right.

I'm not an expert, but I have learned a few things about scale length and bridge position the hard way.

Theoretically, scale length is the distance from the break of the nut to the break of the saddle for a .000 gauge string. Obviously, your real life string is thicker than that, so aim to get your high E saddle at the scale length +1/32 to 1/16". Your other strings will need the saddle to be progressively farther back, about 1/10 to 1/8" by the time you get to the low E.

guitarbuilder is right on with the trapeze tailpiece idea, it's just a substitute for a TOM or in your case a bigsby tailpiece. In fact, I think you could use your bigsby, just don't screw into the face of the guitar yet, mount it by the strap button. Put your TOM bridge on top of a washer or two, put a high & low E string on. Move the bridge around until you find the spot that gives you some room for adjustment, then mark for your bridge post holes.

Remember that heavier strings will need to have more compensation, the thicker the string, the longer the actual scale length will need to be.

You can't scale up to 26"... the frets will be spaced wrong on a precut fretboard intended for 25.5".

The formula for fret spacing is some constants, and variables arranged around the 12th root of 2. So that after 12 frets, you've halved (or doubled) the string length.

Best plan is to buy the Bigby and mike it out. I do my guitars in CAD as well. The drawings for these parts SUCK... don't trust anything but measurements from a part in hand.

The actual distance from nut to bridge will never be less than 25.5. In actual practice, the saddles are moved farther from the nut to compensate for the pitch shift that occurs when fretting hinger notes (increasing the string tension).

Follow GB's advice above and you can nail it. Stated another way: adjust the saddles until they are as far toward the nut as they can go... then set the apex of the saddles at 25.44" from the near side of the side of the nut.