Nut Slot Thousanths

Mar 11, 2011
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I like to measure, and I also like nut slots that are as low as possible without buzzing. With a tensioned string, I use my finger between frets 2 and 3 to press the string against fret 2, then measure between the bottom of the string and fret 1 using automotive feeler gauges. This takes a LOT of time but it works. I have done well at cutting slots so far and I have another one on the bench and a question:

How many thousanths can I go? Does anyone else use this method? I have always stopped at around 0.004", scared to go further. Should I do that again, or can I carefully go lower?
 
I like to measure, and I also like nut slots that are as low as possible without buzzing. With a tensioned string, I use my finger between frets 2 and 3 to press the string against fret 2, then measure between the bottom of the string and fret 1 using automotive feeler gauges. This takes a LOT of time but it works. I have done well at cutting slots so far and I have another one on the bench and a question:

How many thousanths can I go? Does anyone else use this method? I have always stopped at around 0.004", scared to go further. Should I do that again, or can I carefully go lower?

Hello Old Fart;

What you are doing is the classic method of checking the nut slot height; pressing the string down on top of the 2nd fret and checking the clearance between the underside of the string and the 1st fret. A better way to measure using that same technique is to use a thin 6" steel ruler up on edge. Set the edge of the ruler in the nut slot and on the 2nd fret. Then check the gap between the ruler and the 1st fret with a feeler gauge. You can get a better reading because the ruler won't flex like the string.

Using that method of measuring, you should cut the nut slot depth to get a gap of 0.003" to 0.004", under the ruler at the 1st fret. That's the standard number for electric basses, regardless of action height, string gauge or string type. Lower than 0.003" and the string will buzz too easily when played open. Higher than 0.004" and it will become stiffer to press the string down at the 1st than at the 2nd and beyond. That range of 0.003" to 0.004" puts the 1st fret pressure about equal to the others.

If you've read through some of the many threads around here about setting nut slots, and height of Zero frets, some of us will talk about a height of 0.010", the height of the nut slot (or Zero fret) above the 1st fret. But don't get confused, because that's using a different measuring method. Using a longer straightedge set on top of all the frets, the nut slot should be 0.010" higher than the bottom of the straightedge.

That 0.010" number gets you to the same place as the 0.003"-0.004" number, just measured in a different way. The 0.010" number applies to a straight line across the tops of all the frets, while the 0.003" - 0.004" number is for the gap at the 1st when measuring with the short tilted ruler touching the 2nd. Don't get confused by those two numbers. It's a common point of argument in any thread about nut slots.

The reason why some of us use that 0.010" number is because of a Secret Pro Trick method for filing nut slots quickly and accurately:
  • Measure the height of the 1st fret above the fingerboard surface, after the frets have been leveled. Say it's 0.035". You want the bottoms of the nut slots to be exactly 0.010" higher than that, which would be 0.045" above the fingerboard surface.
  • Find several small rectangles of thin aluminum sheet, around 2" x 1/2", which measure 0.045" thick total when stacked up. Hand bend them slightly to approximately fit the radius of the fingerboard.
  • Set the stack of aluminum shims on the fingerboard, right up against the nut. Hold them there.
  • File the nut slot, holding the file so the vertical angle is aiming toward the tuner post, creating the break angle in the slot.
  • File the slot down until the file just touches the edge of the aluminum shim stack. You see a tiny shiny spot at the edge against the nut. That point sets the front edge of the bottom of the slot at 0.045" above the fingerboard and 0.010" above the 1st fret. It will also measure 0.003"-0.004" at the 1st using the tilted ruler method.
  • Don't use steel feeler gauges up against the nut for this, because they will dull your precious nut files. Use aluminum or brass for the shim stack.
That's the pro trick for filing all the slots accurately without all the trial and measure stuff with the feeler gauges.

When I'm trimming the height of Zero frets or fretless Zero Blocks, I use a special tool that I made, a dial indicator on a beam. That's also where that 0.010" number comes in.

IMG_0633B.jpg


IMG_0494B.jpg
 
I did not imagine to learn this much from must posting one question. As you might imagine, answers improve my understanding but also raise more questions, usually. In the morning I plan to study this further along with the findings of Mr. Bales. For now, I am enjoying some scotch and canning some mighty spicy and delicious salsa from my garden. In the meantime I will leave a wonder from my mind this afternoon: Is there a Double-Secret Luthier Secret related to cutting certain frets a wee bit lower than others, to accommodate the surely 'organic' shape of a wiggling string? Inquiiring minds want to know. Guessing at your love of physics and objective data, I assume this question has occurred to you.

Much appreciated, Bruce and everyone!

salsa cannings.jpg
 
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Double-Secret Luthier Secret related to cutting certain frets a wee bit lower than others, to accommodate the surely 'organic' shape of a wiggling string? Inquiiring minds want to know. Guessing at your love of physics and objective data, I assume this question has occurred to you.

That's what the relief curve is for; clearance around the shape of the string as it's wiggling. So you can set the string as low as possible without buzzing. We trim the tops of the frets to flat during the leveling process, strings off and truss rod half tight. Then, when we string it up, the neck pulls forward into a shallow curvature called the Relief Curve. The neck is bending from the tension load of the strings, and the truss rod is trying to hold it back.

The depth of the relief curve, called the Relief, is adjusted by a small amount of movement of the truss rod; tightening it to reduce Relief and loosening it to increase Relief. The reason why we do the pre-loading of the truss rod during leveling is to make sure the truss rod is able to adjust the Relief within a workable range.

The shape of the Relief Curve is built into the neck in its construction and dimensions. How the neck bends into a curve as you hold the heel and pull up on the headstock. Designing the neck as a beam that tapers in stiffness down its length. That's a complicated subject in itself, which mostly applies to designing and building a new neck. The thickness of the neck at various points, the geometry of the truss rod installation, and any other internal reinforcements. Things that you can't easily change on an existing production neck.

The ideal Relief Curve isn't a simple radius. It's actually 1/4 of a long shallow ellipse; the radius is almost flat at the heel and gets smaller as you get closer to the nut. A correctly built and machined neck should go to that curve when you pull on the headstock, regardless of the tightness of the truss rod. The whole purpose of the truss rod is to finely adjust the depth of the Relief Curve. And to add stability to the neck by being a steel strap on the back side of the neck to balance out against the steel strings on the front.

That's the Double-Secret Luthier Secret. I'm passing you this knowledge in the hope that you will use it to go forward and build some fine basses in your retirement. We don't give this information out to just anybody.
 
Hello Old Fart;

What you are doing is the classic method of checking the nut slot height; pressing the string down on top of the 2nd fret and checking the clearance between the underside of the string and the 1st fret. A better way to measure using that same technique is to use a thin 6" steel ruler up on edge. Set the edge of the ruler in the nut slot and on the 2nd fret. Then check the gap between the ruler and the 1st fret with a feeler gauge. You can get a better reading because the ruler won't flex like the string.

Using that method of measuring, you should cut the nut slot depth to get a gap of 0.003" to 0.004", under the ruler at the 1st fret. That's the standard number for electric basses, regardless of action height, string gauge or string type. Lower than 0.003" and the string will buzz too easily when played open. Higher than 0.004" and it will become stiffer to press the string down at the 1st than at the 2nd and beyond. That range of 0.003" to 0.004" puts the 1st fret pressure about equal to the others.

If you've read through some of the many threads around here about setting nut slots, and height of Zero frets, some of us will talk about a height of 0.010", the height of the nut slot (or Zero fret) above the 1st fret. But don't get confused, because that's using a different measuring method. Using a longer straightedge set on top of all the frets, the nut slot should be 0.010" higher than the bottom of the straightedge.

That 0.010" number gets you to the same place as the 0.003"-0.004" number, just measured in a different way. The 0.010" number applies to a straight line across the tops of all the frets, while the 0.003" - 0.004" number is for the gap at the 1st when measuring with the short tilted ruler touching the 2nd. Don't get confused by those two numbers. It's a common point of argument in any thread about nut slots.

The reason why some of us use that 0.010" number is because of a Secret Pro Trick method for filing nut slots quickly and accurately:
  • Measure the height of the 1st fret above the fingerboard surface, after the frets have been leveled. Say it's 0.035". You want the bottoms of the nut slots to be exactly 0.010" higher than that, which would be 0.045" above the fingerboard surface.
  • Find several small rectangles of thin aluminum sheet, around 2" x 1/2", which measure 0.045" thick total when stacked up. Hand bend them slightly to approximately fit the radius of the fingerboard.
  • Set the stack of aluminum shims on the fingerboard, right up against the nut. Hold them there.
  • File the nut slot, holding the file so the vertical angle is aiming toward the tuner post, creating the break angle in the slot.
  • File the slot down until the file just touches the edge of the aluminum shim stack. You see a tiny shiny spot at the edge against the nut. That point sets the front edge of the bottom of the slot at 0.045" above the fingerboard and 0.010" above the 1st fret. It will also measure 0.003"-0.004" at the 1st using the tilted ruler method.
  • Don't use steel feeler gauges up against the nut for this, because they will dull your precious nut files. Use aluminum or brass for the shim stack.
That's the pro trick for filing all the slots accurately without all the trial and measure stuff with the feeler gauges.

When I'm trimming the height of Zero frets or fretless Zero Blocks, I use a special tool that I made, a dial indicator on a beam. That's also where that 0.010" number comes in.

View attachment 7045965

View attachment 7045967
Rather than the foil, if you were shooting for .045" from bottom of nut slot to fretboard surface, could you use one of these as a 'soft' material gauge
Screenshot_20241005_175752_Chrome.jpg
 
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Rather than the foil, if you were shooting for .045" from bottom of nut slot to fretboard surface, could you use one of these as a 'soft' material gauge
View attachment 7046180

I suppose you could. I'd file the top edge into a straight line, that you could seat right up against the nut. A little clumsy to hold in place, but it would work.
 
I'm going to study and benefit from the answers to my questions here which will raise more questions, some of which will be answered and others perhaps not, and I've come to grips with that. Such as:

* Irresistable force/immovable object?

* Chicken/egg?

* How in the world did this Fender neck end up with fully 0.030" of relief with the truss rod all the way loose? It certainly is not memory, since this neck has had roughly 0.010" of relief for the past 10+ years. I suppose Fender must have milled the fingerboard with the truss rod fairly tight.

* Huh? Why in the world would a nut slot need to sit 0.010 above the height of the first fret? Why could it not be exactly as high as the first fret? A zero fret, or nut slot height, matching the height of the first fret would, physically and mechanically be the same as using a capo. If the nut slot has to be substantially higher than the frets, then a capo would not work. Fretting notes with out fingers would not work either. Hm....

I don't actually need answers to these questions. I accept the help and instruction I see here, with gratitude, and I accept the world as it is. Because I have no choice. And I'm okay with that.

....go forward and build some fine basses....


Indubitably. Probably obsessively.
 
I'm going to study and benefit from the answers to my questions here which will raise more questions, some of which will be answered and others perhaps not, and I've come to grips with that. Such as:

* Irresistable force/immovable object?

* Chicken/egg?

* How in the world did this Fender neck end up with fully 0.030" of relief with the truss rod all the way loose? It certainly is not memory, since this neck has had roughly 0.010" of relief for the past 10+ years. I suppose Fender must have milled the fingerboard with the truss rod fairly tight.

* Huh? Why in the world would a nut slot need to sit 0.010 above the height of the first fret? Why could it not be exactly as high as the first fret? A zero fret, or nut slot height, matching the height of the first fret would, physically and mechanically be the same as using a capo. If the nut slot has to be substantially higher than the frets, then a capo would not work. Fretting notes with out fingers would not work either. Hm....

I don't actually need answers to these questions. I accept the help and instruction I see here, with gratitude, and I accept the world as it is. Because I have no choice. And I'm okay with that.




Indubitably. Probably obsessively.
You might add a couple of thou more than the fret height to compensate for some wear in the nut. But I maintain that the bottom of the nut slots should be equal to the height of the frets. Many other pro luthiers and techs disagree with me, but I remain unconvinced by their reasoning. I see the nut as having two functions, creating the string spacing and setting the height. In the latter function, the nut is the equivalent of a zero fret, and if the first fret is high enough to allow string clearance, then the bottom of the nut slot should be the same as the top of the first fret (allowing a little extra for wear as suggested). There's nothing different about the string vibration or the neck geometry that would require a zero fret or the nut to be higher than the first fret.
 
* How in the world did this Fender neck end up with fully 0.030" of relief with the truss rod all the way loose? It certainly is not memory, since this neck has had roughly 0.010" of relief for the past 10+ years. I suppose Fender must have milled the fingerboard with the truss rod fairly tight.

That's mostly warpage. The neck moved into a small forward bow as it slowly dried out internally and released its internal stresses. Long thin sticks of wood do that. Usually, most of the internal stresses will relieve themselves in the first year after the board is machined into a neck, but sometimes it can take 10 years for it to stop moving. That's why we Luthiers go to great pains to prepare the wood strips before making them into necks. And we do things like make multi-lam necks with the rings oriented in a particular way. All to minimize the movement of the wood over time.

Last I knew, Fender does pre-tighten the truss rods to some degree in their manufacturing process, as they make the necks. They install the truss rod nut and tighten it down until it is at least snug before passing the neck through the shaper that cuts the radiused fingerboard surface. I don't think they tighten the nut as much as I recommend, but they make sure it's seated and not loose.
 
Interesting. On this Warmoth neck on my bench, which will get frets next week, Warmoth apparently did not tighten the truss rod nut much before shaping/milling. I measure only about 0.011". I hope fret installation does not reduce this relief/bow very much, as I imagine it is good to have a decent amount of tension on the truss rod while fighting string tension. For purposes of stability, as you explained. But I am told not to worry my little head about that, so I won't.
 
* Huh? Why in the world would a nut slot need to sit 0.010 above the height of the first fret? Why could it not be exactly as high as the first fret? A zero fret, or nut slot height, matching the height of the first fret would, physically and mechanically be the same as using a capo. If the nut slot has to be substantially higher than the frets, then a capo would not work. Fretting notes with out fingers would not work either. Hm....

This is another one of those technical topics that has been argued out at regular intervals around here. A quick summary answer:
  • If you adjusted the neck dead flat, with no relief curve, then yes, the nut slots (or Zero fret) should theoretically be the same height as all the other frets. It's just another fret, right? But when you string up a neck and set it up with the neck dead flat, the space between the string and the frets becomes a long shallow triangle. The action is very tiny at the 1st fret and becomes larger and larger out to the heel. And if you set the action high enough that it doesn't buzz at the 1st, the action at the heel will be uncomfortably high. Back in the old days, many basses were set up like that, with the neck flat and high action at the heel. Because bassists didn't play that much up high, and they were okay with higher action.
  • The Relief and the Relief Curve are there to correct that. Allowing the neck to pull forward into a shallow curve gives clearance for the string as it swings around. This allows the Action to be lowered significantly at the heel end. So, the Action at the 20th fret isn't that much higher than the Action at the 5th fret. This technical innovation allowed Bassists to play faster and higher!
  • The reason why the nut slots (or Zero fret) need to be slightly higher than the other frets, when the neck is pulled into a Relief Curve, is because of the way the neck bends under the load of the strings. On most necks, the zone between the nut and the 1st fret gets thicker and wider as the neck shape transitions to the headstock. Thicker and wider means it's stiffer and doesn't bend as easily as the rest of the neck from the 1st to the 10th. When the strings pull up on the headstock, the neck will pull up into a nice shallow curve........except there's a flat spot in the curve between the nut and the 1st fret. Because the neck is stiffer right there. And that's what the extra 0.010" height of the nut slots and Zero fret is for. It compensates for that flat spot in the relief curve. Which is caused by the uneven stiffness profile of the neck.
  • And that's why you theoretically don't need that extra height if the neck is set up dead flat with no Relief Curve. But practically, you'll seldom do that, because the action will be unreasonably high at the heel.
  • And, you theoretically don't need the extra height if the neck is pulled into a good true relief curve. But it hardly ever is. Almost all necks will have that flat spot in the relief curve out near the nut.
  • You see this in practice. If you cut the nut slots to be even with the frets, you'll get much more buzz at the 1st fret when the strings are played open, and less buzz when fingering up the board. That's why we recommend that the slots be cut higher than the frets; 0.010" measuring straight like I do, or the 0.003"-0.004" measuring with the tilted ruler at the 1st. It's to correct for that little kink in the neck.
 
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..... When the strings pull up on the headstock, the neck will pull up into a nice shallow curve........except there's a flat spot in the curve between the nut and the 1st fret. Because the neck is stiffer right there. And that's what the extra 0.010" height of the nut slots and Zero fret is for. ....


Ah! Hah! This is the part I was missing. Thank you. That's just one less thing I don't understand. Many still remain, of course.
 
This is another one of those technical topics that has been argued out at regular intervals around here. A quick summary answer:
  • If you adjusted the neck dead flat, with no relief curve, then yes, the nut slots (or Zero fret) should theoretically be the same height as all the other frets. It's just another fret, right? But when you string up a neck and set it up with the neck dead flat, the space between the string and the frets becomes a long shallow triangle. The action is very tiny at the 1st fret and becomes larger and larger out to the heel. And if you set the action high enough that it doesn't buzz at the 1st, the action at the heel will be uncomfortably high. Back in the old days, many basses were set up like that, with the neck flat and high action at the heel. Because bassists didn't play that much up high, and they were okay with higher action.
  • The Relief and the Relief Curve are there to correct that. Allowing the neck to pull forward into a shallow curve gives clearance for the string as it swings around. This allows the Action to be lowered significantly at the heel end. So, the Action at the 20th fret isn't that much higher than the Action at the 5th fret. This technical innovation allowed Bassists to play faster and higher!
  • The reason why the nut slots (or Zero fret) need to be slightly higher than the other frets, when the neck is pulled into a Relief Curve, is because of the way the neck bends under the load of the strings. On most necks, the zone between the nut and the 1st fret gets thicker and wider as the neck shape transitions to the headstock. Thicker and wider means it's stiffer and doesn't bend as easily as the rest of the neck from the 1st to the 10th. When the strings pull up on the headstock, the neck will pull up into a nice shallow curve........except there's a flat spot in the curve between the nut and the 1st fret. Because the neck is stiffer right there. And that's what the extra 0.010" height of the nut slots and Zero fret is for. It compensates for that flat spot in the relief curve. Which is caused by the uneven stiffness profile of the neck.
  • And that's why you theoretically don't need that extra height if the neck is set up dead flat with no Relief Curve. But practically, you'll seldom do that, because the action will be unreasonably high at the heel.
  • And, you theoretically don't need the extra height if the neck is pulled into a good true relief curve. But it hardly ever is. Almost all necks will have that flat spot in the relief curve out near the nut.
  • You see this in practice. If you cut the nut slots to be even with the frets, you'll get much more buzz at the 1st fret when the strings are played open, and less buzz when fingering up the board. That's why we recommend that the slots be cut higher than the frets; 0.010" measuring straight like I do, or the 0.003"-0.004" measuring with the tilted ruler at the 1st. It's to correct for that little kink in the neck.
I largely agree with this, but I maintain that the angular force of the string over the nut overcomes the slight added stiffness at the first fret. So on all of my basses the nut is cut about 2 thou higher than the first fret, and I get no more fret buzz on the open string than I do when fretting at the first.

I think I posted in the Pro Bench area a thread about relief where I measured and plotted the string clearance at each fret on a dozen or so basses where all basses were set with the same amount of relief. The relief profile varies quite a bit. Higher quality basses tended to have more even relief curves that more budget-minded models. In my tests, the most even was a NYC Sadowsky, but both USA Fenders and USA Laklands showed smooth relief curves. The worst was a Korean Squier, with a MIJ Fender near the middle, along with a Music Man Stingray. The wood in the more uneven necks was less rigid generally causing more bend to occur in the centre of the neck than at either end.