[Closed] Scale length and Bridge/Saddle position
The drawing I am working to shows the scale length (25.4”) being measured to the B string position on the saddle thus making the top E shorter than the scale length. This seems odd to me as surely even the top E has some degree of null length before the node. Is this normal as other references mention the G string position or the centre of the saddle. Confused!
Thanks Mark, I was watching it last night but I couldn't tell exactly which point you were measuring to on the saddle. Top E, B or somewhere in between I think. Is this correct?
The plan I have is from Georgia Luthier Supply for their Grand Symphony Guitar. It comes as 5 separate editable PDF's which great for me as I can open them in Adobe Illustrator and redraw any sections that need to be adjusted and then take the same outline file straight into the software that drives my router (VCarve Pro) and hot wire cutter (GMFC Pro) which is used for cutting the vacuum press moulds. Illustrator is much under rated as a CAD package and, whilst not cheap, is much less expensive and easier to use than Autocad, AlphaCam etc. I was at an exhibition in London a few weeks ago and a group of Architecture students from Bournemouth University (!!) said that they are only taught to design buildings with straight walls as "Curves are really difficult to do in CAD". I may still write to the Head of Faculty and let them know what I think!
I wonder if this might shine some light on this one...
The correct way to measure the scale length:
measure from the front (string take off side) of the nut to the centre of the 12th fret
multiply by two = scale length
try that on your drawing and see if it makes any more sense?
Measure twice, cut once...
Nope. No more sense but I am more than happy to concede that the drawing is wrong. The scale length is stated as being 645.2mm ie 25.4" but this is shown as being from the nut to centre of the centre of the saddle which works out as being to the string take off position of the A string. Clearly an FU but they're not answering emails at present for some reason! Nut to centre of 12th fret is correct but the saddle position certainly isn't.
+2.5mm at top E is then! Just to add another variable into mix, would this change with string gauge and is this why manufacturers often recommend the gauge of strings to use on any particular guitar? I would usually stick lights (strings, not indicators!) on an acoustic but I feel there could be a nice physics project in this for someone to come up with the definitive spreadsheet for string gauge/scale length intonation compensation. Maybe it already exists! In fact, yes it does!! https://www.liutaiomottola.com/formulae/compensation.htm
This is a seriously useful page if you have the required variables to hand. Even just using the defaults for action etc it gives the interesting results. Their default settings give top E as +2.16mm and bottom E as +5.7mm ie the saddle should be at an angle to give +3.545mm from top E to bottom E. The ones I have machined are about +3mm but I won't let that spoil my day as I can always machine them out a little bit to increase the angle should it be necessary or just glue the bridge on at a very small angle. I am not even sure how likely it is that +/- 0.5mm would produce an audible difference.
Ok. Have just been out string shopping and picked up a set of Ernie Ball Earthwood lights and extra lights. Without changing the action settings from the defaults these give the following compensation figures. Diameters in inches and relate to the core diameter of wound strings. Compensation for 25.4" scale length
Lights - Top E - 0.011" + 1.8mm, B - 0.015" +3.637, G - 0.012" +2.027, D - 0.0155" +3.632, A - 0.016 +4.206, E - 0.018 +6.396, Total Offset from top to bottom E = 4.582mm
Extra Lights - Top E - 0.01" +1.5mm, B - .014" +3.17, G - 0.0105" +1.552mm, D - 0.0149" +3.358mm, A - 0.0164" +4.42mm, E - 0.0177" +6.185mm, Total Offset from top to bottom E = 4.685mm
I have just called D'Addario and their techie is going to contact me on Monday and will hopefully be able to give me a list of the core wire diameters for their wound strings. With a bit of playing around you can also use the calculator in reverse to work out the gauge of strings best suited to match the offset of any given saddle. I guess the danger here would be, if going to the extremes, then the string tensions may be a bit weird. Hey, this is fun!
Darren - I believe you may be overthinking this...
My method advises to add .10" to the scale length along the top e
simple as that - the other strings take care of themselves (providing you have a standard bridge and glue it on in the right place)
I realise that this is an average but as you can see it is only .001" different from Ernie Ball lights and if you can mark out that accurately then you are welcome to try! You can always make small adjustments on the saddle.
I am trying to make things easy folks and faced with a list of figures like that most folk would think it is really difficult...which it isn't.
Measure twice, cut once...
I am sorry if I am making things appear harder than they are. This really wasn’t my intention and I have spent a large part of my working life trying to open up seemingly complicated and mysterious woodworking processes to as many people as possible from first year DT students upwards. I think that main thing that these calculations reveal is that the compensation offset from top E to bottom E, and therefore the angle of the saddle, is pretty much consistent and I am guessing that this isn’t an accident ie string companies match sets of strings to the most common geometry of saddle on mass market guitars to achieve the best average intonation. The problem I had was I couldn’t find the figures for the ‘common’ saddle geometry given anywhere and certainly nothing giving different saddle geometries for different scale lengths which has to be a thing right? So, whilst I am perfectly happy to add 0.1” to the top E, I wasn’t able to find the corresponding offset figures for bottom E and especially for the B string. I fully understand that this isn’t a consideration if buying a bridge and preshaped saddle but I just wanted to understand the maths/physics behind the accepted practice as I am planning to make these parts myself.
Having said all this, yes, I am probably over thinking it.
Sorry to keep pestering but I am still not clear if you are saying that the saddle offset by 1/8" over 3" or the top and bottom E strings are offset by 1/8" over the normal spacing of around 54.5mm or doesn't this matter?
It's cool - sorry for any confusion... trust me it gets easier after the first one...
Yes the angle is offset by 1/8" over 3" - its in this video if you missed it
Measure along the High E to the front of the saddle
The distance should be: Scale Length plus intonation adjustment
Once you have the position of the high E then the other strings take care of themselves - the low E is (I should have said) 'roughly' 1/8 further back.
Just noticed I said in a previous comment add .01" to the scale length - I should have said .1" (edited now) I also noticed that on the bridge drawing it says add 2mm...
TBH: either one is close enough (within .5mm? that is nothing between friends...)
To clarify I would add (roughly (as accurate as I can))
- +1.5mm for 10 gauge strings
- +2mm for 11 gauge
- +2.5mm for 12 gauge (approx .1")
I don't want to sound like a PITA either but my point is that when the time comes to mark out and locate the bridge most people will be using a 1m rule - and .5mm accuracy over this distance is about as good as most people can manage and its fine. You can easily make small adjustments like that on the saddle.
I am sure you want to make a pattern for this with your CNC and that is why you need exact figures...unfortunately the are no absolute numbers as there are too many variables string gauge/height/material/type, fret height even, and final set up of the guitar. The figures I have given work pretty spot on for most standard set ups and leave a little room for adjustment on the saddle.
If you are still worried then I strongly recommend finishing one guitar at a time - if you want to get them all painted at the same time then you still have the option to leave the masking and locate the bridges later.
hope that helps a bit and makes more sense now?
Guitar making is probably more of an art then a science - go for it!!!
I'm getting excited now!!! 🙂
Measure twice, cut once...
Ha, all that just to get a machine to drill three holes in a piece of 6mm thick laminate! I really don’t have a problem with +/- 0.5mm so long as I am fairly confident of where the 0.0mm position should be.
Anyway, the jig has been made so that the 3mm ground steel pins pushed through the holes locate against nut end of the fretboard in the top E string position and in the extremities of the saddle rebate of the bridge. As the template is the same shape as the neck it also gives the central alignment to the neck without having to mark the sound board. As it turns out, the bridge is less than 0.5m off centre anyway so I think I can live with that. Having established the bridge position I drilled through the two E string peg holes and pushed 4mm steel pins through to maintain the position whilst I cut around the masking tape. I have put the first coat of grain filler on the ziracote guitar using a Morells product and will do the same to to the massur birch guitar in the next couple of days using Aquacoat. I’ll let you know how it goes.
Thank you s much for your patience
I have built the acoustic guitar and if you follow Mark's instructions, the saddle position is spot on (within 1%)