1st build and misunderstood bridge location measurement

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Bob Howell
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1st build and misunderstood bridge location measurement

Post by Bob Howell »

Glued the bridge 7 hrs ago with yellow glue, and now realize I was off 2mls. I measured distance from 12th fret to front of bridge rather than center of saddle. So I guess I take a hot knife and remove the bridge.

Gordon Bellerose
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Re: 1st build and misunderstood bridge location measurement

Post by Gordon Bellerose »

Unfortunately, that is what it looks like.
I need your help. I can't possibly make all the mistakes myself!

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Barry Daniels
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Re: 1st build and misunderstood bridge location measurement

Post by Barry Daniels »

Wait a minute. Are you aware of string compensation? You need to add about 0.15" to the scale length at the center of the saddle (assuming a scale length of 25.5"). So depending on how far it is from the front of the bridge to the middle of the saddle, you may be fine.
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Gordon Bellerose
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Re: 1st build and misunderstood bridge location measurement

Post by Gordon Bellerose »

You're correct of course Barry.
A scale of 25.5 actually measures 25 9/16 to the treble E string.
I need your help. I can't possibly make all the mistakes myself!

Bob Howell
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Re: 1st build and misunderstood bridge location measurement

Post by Bob Howell »

I had added 1/4" compensation and needed only 2ml.
I got it off ok. Now must refinish.

I put this in wrong forum. Its an OM acoustic.

Bob Howell
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Re: 1st build and misunderstood bridge location measurement

Post by Bob Howell »

I am using two books as guides to build my OM. Cumpaino and Alex Willis books. Everything has been easy to understand till I got to set up. Now I see Willis adds 2mm compensation but Cumpaino adds none. Then Willis shows a higher saddle; 15/32 and Compaino, 5/16. Reading around I see string weight is a big factor but have not seen it mentioned yet. Perhaps it is buried somewhere ahead.

Brian Evans
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Re: 1st build and misunderstood bridge location measurement

Post by Brian Evans »

There are compensation calculators on-line that take everything into account and tell you exactly where to place the saddle for each string. A typical acoustic straight bridge hugely compromises exact compensation in favor of simplicity. Here is the calculator I use: http://www.liutaiomottola.com/formulae/compensation.htm It includes a rather complete description of the what and why of compensation, including string gauge (core wire gauge in the case of wound strings). One thing it ignores is overall string length, which has a small but measurable effect on need for compensation.

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Barry Daniels
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Re: 1st build and misunderstood bridge location measurement

Post by Barry Daniels »

Cumpaino definitely adds compensation. He adds 0.150" to the middle of the saddle and slants it appropriately. It is sometimes a bit difficult to find things in the book, but it is there.

I am not familiar with Willis' book but I can tell you that 15/32" is WAY TOO TALL for a saddle. That is nearly a half inch. Maybe he has a bunch of that saddle buried in the bridge but that is still excessive.

Stew-mac has a good fret calculator on their website that includes compensation.
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Freeman Keller
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Re: 1st build and misunderstood bridge location measurement

Post by Freeman Keller »

Bob Howell wrote:I had added 1/4" compensation and needed only 2ml.
I got it off ok. Now must refinish.

I put this in wrong forum. Its an OM acoustic.
I would have considered filling the slot and reslotting it but it sounds like it might be too late. That's done all the time on old Martins where they put the slot in the wrong place.

Bob Howell
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Re: 1st build and misunderstood bridge location measurement

Post by Bob Howell »

I found where Cumpiano included compensation. .15 " or 4 mils as I see it. Willis called for 2 mils.
I made the saddle per Cumpaino's instructions. With a 12" radius it is only 1/16" or so on the ends.

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Barry Daniels
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Re: 1st build and misunderstood bridge location measurement

Post by Barry Daniels »

Do you mean it only protrudes about the bridge by 1/16" on the ends? That is too low. Where will that place your action at. I think you may need to make a new saddle.
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Gordon Bellerose
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Re: 1st build and misunderstood bridge location measurement

Post by Gordon Bellerose »

I use this tool to help me set the placement and compensation of a bridge saddle.
I find it very simple to use.

http://www.stewmac.com/Luthier_Tools/To ... matic.html

As for bridge plate height, if you place a straightedge on the fret board and extend it over the bridge, there should be approx 1/16 of an inch of clearance with NO saddle in place.
Others may correct me, but that is very close.

If that is not the case, you could have either a bridge plate thickness issue, or a neck angle problem.
I need your help. I can't possibly make all the mistakes myself!

Freeman Keller
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Re: 1st build and misunderstood bridge location measurement

Post by Freeman Keller »

Gordon, that is my rule of thumb also - a straightedge on the frets should just be a hair over the top of the bridge. Normally a bridge will be about 3/8 thick, with an additional saddle height of around 1/8 inch and a bit of relief I end up with 12th fret action of 70 to 90 thousands.

As far as the location of the saddle (slot), again, I've had good luck with adding 1/16 to the scale length for the high E and 1/8 to 3/16th for the low. You can finesse the break point on the saddle, normally the high E and G gets pushed to the front, B and low E to the back and the others form a line from G to E. I've use the trick up putting a tiny piece of wire on top of the saddle to get the break point but the "standard" method is usually good enough. Btw - I've used the wire trick to compensate each string of a 12 string - seems to help.

I had an interesting case the other day - I was setting up an electric for a very good player and he asked me to make the intonation about five cents flat. He said he could sharpen a note as he plays it but he can't flatten it - its all in how much fretting pressure.

Bob Howell
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Re: 1st build and misunderstood bridge location measurement

Post by Bob Howell »

The issues discussed here about compensation and saddle height have got me digging in to design. I see Willis has provided plans for a light weight OM, using ultra light strings designed for a beginner. This is very different from Cumpaino's design. Switching back and forth between books is getting me in trouble.

I don't think 2mm compensation per Willis goes with the saddle Cumpaino shows. The guitar has light weigh bracing so I should stick with light strings; he says ultra light for easy play for a beginner. Now I must settle on compensation and saddle height. I have a 3/8" bridge shaped as Cumpaino describes; so what saddle height at the center?
I'm thinking 1/8" is too low for ultra light strings.

2mm compensation seems lower than anyone else uses.

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Barry Daniels
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Re: 1st build and misunderstood bridge location measurement

Post by Barry Daniels »

Yeah, going back and forth between two books will mess you up. You can't go wrong with Cumpiano (except for the neck joint shown in the book; he has upgraded to barrel bolts)

You should have a minimum of 3/16" of saddle protruding about the bridge.
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Gordon Bellerose
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Re: 1st build and misunderstood bridge location measurement

Post by Gordon Bellerose »

Barry. When you say a minimum of 3/16 of saddle protruding, is that measurement at the center of the saddle, or the ends?

I have done set ups on older guitars that needed the action lowered, and ended up with maybe 1/16 at the ends.
As long as the strings do not touch the bridge while being played, it shouldn't matter all that much.
I have also had to plane, or sand the plate down to get the action low enough.

In the build stage though, I aim for a minimum of 3/16 at the ends, and up to 5/16 at the center.
I do not like more than that, as the saddle is more prone to leaning forward.
I think that this gives either me, or some future tech, the ability to lower the action just by sanding the saddle down.
I need your help. I can't possibly make all the mistakes myself!

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Barry Daniels
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Re: 1st build and misunderstood bridge location measurement

Post by Barry Daniels »

3/16" at center. I have also taken old guitar saddles down to 1/16" at the ends but you usually have to ramp the bridge's string slots to get that to work. At this point the guitar needs a neck reset but many guitars are not valuable enough or the client is not willing to pay for that amount of work. Also, most import guitars are not worth doing a reset due to questionable neck joints and glue.

When you said "I have had to plane or sand the plate down", are you talking about sanding the bridge? I did that a few times back in the 70's but will not do that today. I think it is bad form as it permanently affects the integrity of the bridge and is not reversible unless the bridge is replaced. Unless we are again talking about an import guitar.

I think we are pretty much on the same page though.
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Bob Howell
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Re: 1st build and misunderstood bridge location measurement

Post by Bob Howell »

Did I misunderstand Cumpaino? I read it as 1/8" above bridge and then radiused to match the fretboard. That's how I got so low.

Freeman Keller
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Re: 1st build and misunderstood bridge location measurement

Post by Freeman Keller »

Bob Howell wrote:Did I misunderstand Cumpaino? I read it as 1/8" above bridge and then radiused to match the fretboard. That's how I got so low.
That is a good rule of thumb. If the fretboard plane (a straightedge laying on the frets) touches the top of the bridge and you have about 0.125 of saddle sticking out of the slot, then neglecting nut action and relief you would have an action of 0.063. If you have 15 thou of nut height that would move it up to 0.080. Add a couple of thou of relief and you've got a very playable fingerstyle action of 65 to 85 or 90 thousands.

In fact, one of the rules of thumb for determining if you have the proper neck angle (besides the straightedge on the frets hitting the top of the bridge) is IF you have acceptable action AND there is at least 1/8 inch of saddle sticking out of the bridge THEN your neck set is acceptable.

Having the 1/8 inch of saddle will allow you to lower it as the neck block rotates and sometime in the future, when the little boolian above fails it will be time to reset the neck. If he was a bluegrasser I would have left the saddle a bit higher.

Here was an interesting guitar that I worked on the other day. With no strings on it the fretboard plane is about 0.020 over the bridge (bad picture, take my word). I adjusted the relief to 0.004 with no tension

Image

With strings on the fretboard plane now hits the top of the bridge (actually might be a hair below it) and the relief went up to 0.008.

Image

Adjusted the relief back down to 0.005 (frets aren't perfect, customer didn't want me to work on them) and sanded the saddle to give 0.065 at the high E and 0.085 at the low - nice playable buzz free action. Plenty of saddle to work with in the future.

Bob Howell
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Re: 1st build and misunderstood bridge location measurement

Post by Bob Howell »

I think I have it close. I will have the finish repaired this week and Then try what I have Made so far.

Thanks for the help.

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