Wrap around bridge placement

[Maurice Lavergne]

Just found out about project guitar from Gray Bramwell at Graph Tech Guitar Labs. I am currently building a Les Paul junior single cut.  I could really use some help in determining where to place the 24 5/8” scale line. The neck pocket is flat and I have put a 1.5 degree angle in the heel. Unlike using a hard tail bridge where I just measure the scale length from the nut I was told that because of the neck angle that the scale line would be a little further back than in a hard tail set up. Could some one please let me know how and where to make the scale line? 
 

My second question is the placement of the studs for the Graph Tech ResoMax Sonic 1 bridge that I will be installing on this guitar. Unlike their other wrap around bridges this one does not have individual intonation screws for each saddle. It is a compensated bridge like the one on PRS guitars, it only has the grub screws on either end of the bridge to intonation the low E and high E. Once the scale line is established I am confused about where to install the stud post inserts,l could really use some help to properly place the posts. Should the be right on the scale line or the high E side ahead of the scale line and the low E behind the scale line? I would greatly appreciate some feedback on this subject before trying it on the actual body and putting them in the wrong place. 

[Bizman62]

Hi and welcome to the addiction!

Coincidentally I've built an LP Jr with a somewhat similar bridge, link to some pictures here: https://www.projectguitar.com/forums/topic/49819-guitar-of-the-month-july-2019/?do=findComment&comment=579304. The most accurate tool I used for measuring the scale was a 1 metre steel ruler with 1 mm increments which did the job accurately enough.

As far as I know there's nothing too different in measuring the scale length on a guitar with a neck break angle, the angle is so subtle that it doesn't significantly shorten the scale. A simple way to visualize that is to take a tape measure locked to 25" and lift it by half an inch (the height of your bridge) at the 12.5" mark. You really can't see the end move, that's how much it affects.

As the bridge has adjustment backwards you'd want it to be as front as possible.

If you want to get the intonation as good as possible, you can make a trapeze sort of jig of something like a clothes hanger or other stiff wire. Bend it to a V with eyelets for the ball ends of both E strings and hook it to the end pin hole. Tighten the E strings and lay the bridge with makeshift spacers where you think it should be. Measure the intonation and move accordingly, then mark the place. Something like that, don't forget protective padding:

 

[henrim]

4 hours ago, Maurice Lavergne said:

scale line would be a little further back

Mathematically it would be.

At 1,5° the change maybe negligible, or at least you can compensate the error with the bridge adjustment screws. Of course it's always good idea to get it right if you don't have stud holes there already. Btw, how did you end up tilting the neck 1,5 degrees? Is there math behind it or is it just a guess? To get an optimal angle, I'd either do the trigonometry or use my preferred method; draw the side profile in 1:1 and measure from there. That way you get the correct angle and intonation line too. Double check the scale length first, though.

Place the studs so that you can adjust the bridge further back for intonation adjustment. If the high E is on spot you will never need to adjust the bridge towards the fretboard. I like to leave just a little bit for adjustment to that direction too. To be able to compensate for drilling inaccuracies and so on.

Also the LP bridge is tilted a few degrees. I don't know how much exactly, but you need to check that to get a sufficient adjustment range. 
 

[Andyjr1515]

With reference to the bridge position, it will usually to be around 1mm longer than scale for the top string and then progressively up to 5mm longer than the scale for the bottom string.  It will never be shorter than the scale length.

What I do is:

- Never do anything until I have the actual bridge in my posession

- I then check the full movement of the saddles.  Does it have at least 6mm between fully forwards to fully back?  If yes, then the bridge can be set square to the guitar centre line.  If not, then the bridge will have to be angled to allow that extra distance added to scale length to be able to intonate the bottom strings properly.

- I then wind the top string saddle 1mm short of fully forwards

- I position the bridge so that the top string saddle is at scale length.  That 1mm is a safety factor for the vagaries of measuring, accuracy of hole drilling and also wood not always behaving as we humans would wish 

 

[ADFinlayson]

I have made about 30 guitars with a break angle and a wraparound bridge and never once increased scale length to compensate for the angle. 

Re stud hole position - place the studs on the bridge and look at the bridge and the centre of the studs relative to where the top E string will finish, if the centre of the stud is level with the scale line, then the stud hole goes on the scale line. But on quite a few bridges the studs have to go further back (I just eyeball it). But a good example is the Schaller signum bridge, the studs sit about 5mm behind the scale line. On a Golden age bridge, the studs sit on the scale line. See if you can find a spec drawing for the bridge you have and you will be able to tell.

Unrelated but important: 1.5º may not be enough to give you good string action unless your fretboard sits pretty high off the body. You might want to consider tweaking your neck tenon so the angle is a bit steeper, I normally go between 2.5º - 3.5 for a typical wraparound bridge.

 

[Bizman62]

Looking at the picture @henrim posted made me think of a known issue with wraparound bridges, i.e. the stud holes potentially becoming oval at some stage. It's a minor issue and may not become serious during the life span of an instrument but it does exist.

Now if the bridge is at a 90 deg angle to the strings and there's a steep neck break angle, wouldn't that pose a risk of the posts being slowly pulled out? Again, a minor issue but obvious when looking at the lowest exaggerated sketch.

But if the studs were vertical to the body, wouldn't that mean that they should be closer to the nut? The strings meeting the bridge at 87 or so degrees may not be any issue as it's the edge that matters.