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precision in location of fret slots
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=32788		 Page 1 of 1
Author:  Stefan [ Thu Jul 07, 2011 3:35 pm ]
Post subject:  precision in location of fret slots
Hi folks,

What do you consider to be an appropriate degree of precision for fret slots? Cumpiano's book demonstrates using a ruler marked in 1/64" to position frets.

I recently used a miter box and stewmac template to do an ebony board, I've now taken my fancy ruler graduated in 1/64"s and measured the frets. Seems some of them are off by 2/64ths or so.

Maybe time to scrap the board :roll: . Too bad it is my only ebony board!

Related question: If I order another ebony W. African ebony board from Lmi, how long do I have to let it acclimatize before I can use it?

Cheers,

Stefan
Author:  Corky Long [ Thu Jul 07, 2011 3:59 pm ]
Post subject:  Re: precision in location of fret slots
I don't think that's accurate enough. Fret cutting is one area where I measure many times, then cut once. Tough enough to get intonation as close as possible. With frets measurably off, I think you'll have some intonation problems.
Author:  Kent Chasson [ Thu Jul 07, 2011 4:02 pm ]
Post subject:  Re: precision in location of fret slots
I would be uncomfortable if any of my slots were off by even .010", less than a third of 2/64".

A friend of mine did some calculations many years ago to answer this question and if I remember correctly, he was surprised to find that .003" - .005" could make a noticeable difference on the higher frets. By the way, the higher up the fingerboard (or shorter the scale), the greater the effect.

For your second question, you would probably do better to ask LMI. Only they know what RH their boards are stored at.
Author:  Chris Pile [ Thu Jul 07, 2011 7:13 pm ]
Post subject:  Re: precision in location of fret slots
I'd want it dead on if at all possible.
Author:  Markus Schmid [ Thu Jul 07, 2011 8:34 pm ]
Post subject:  Re: precision in location of fret slots
Stefan wrote:
Maybe time to scrap the board :roll: . Too bad it is my only ebony board!

No need to scrap it, but order another one anyway - for your next guitar!

Fill the offending slots with ebony dust and super glue, re-cut slots.
If this second attempt fails too fill all slots, flip the fingerboard over and cut the slots there.

If you are doing the cuts without template scribe the fret positions (or draw with a sharpened 0.5 mm or 0.7 mm pencil - use sandpaper or normal paper to get a bevel on it) as lines which determine the border of the slot (not it's center) and as a guide, clamp a block of wood flush with the line (on the correct side!). Then, cut the slot to the correct depth. It's almost foolproof but takes a bit more time than using one of those fancy templates. 8-)

Good luck!
Author:  Stefan [ Fri Jul 08, 2011 2:18 pm ]
Post subject:  Re: precision in location of fret slots
Hi All,

Thanks for the feedback

I've contacted LMI about the boards, they said their wood is seasoned to withing 5-7%, they recommended letting it acclimatize for a few days before building.

I'm still wary about building with wood that hasn't been sitting around the shop for a couple of months. So I'm going to try to repair the board.

My current idea is to use this technique http://luthiersforum.com/forum/viewtopic.php?f=10101&t=32766 to fill every single slot in the board. I'll used the ebony from a couple of bridge blanks, glued side by side to do this. Instead of a 1/8" router bit, I might use my 1/16" dremmel bit. The idea would be to create a series of those ebony pieces, tap them in and wick in thin CA glue.

Or I could fill with ebony dust, wick in thin CA glue. I'm not sure about this as then the fret tang will be paritially sitting in dust/CA glue. How big an issue is that?

Time to get a stew mac miterbox, I thought I was good at this but am now humbled.

Thanks for all your insight,

Stefan
Author:  Kent Chasson [ Fri Jul 08, 2011 2:49 pm ]
Post subject:  Re: precision in location of fret slots
If it's one or two frets, fill them with snug fitting ebony shims, wick CA, and re-cut. If it's more than that, I'd wait for a new board.

Ebony dust and CA is not a good choice. The fingerboard is a structural element and the top surface is under compression. CA and dust is not strong under compression.
Author:  Markus Schmid [ Fri Jul 08, 2011 5:03 pm ]
Post subject:  Re: precision in location of fret slots
Kent Chasson wrote:
CA and dust is not strong under compression.

Any numbers?
Author:  Kent Chasson [ Fri Jul 08, 2011 5:26 pm ]
Post subject:  Re: precision in location of fret slots
Markus Schmid wrote:
Kent Chasson wrote:
CA and dust is not strong under compression.

Any numbers?


No but a thumbnail test is pretty conclusive.
Author:  cphanna [ Fri Jul 08, 2011 5:41 pm ]
Post subject:  Re: precision in location of fret slots
No intention of hijacking this thread, but the whole concept of accurately measuring and marking fret locations has left me scratching my head, too. I'm a hobby builder, so I can buy the few pre-slotted boards that I need for my purposes. But, seriously, how do you guys measure these things down to a thousanth of an inch? I know about the fret calculators available on the web, but how do you translate that information into something you can actually measure to that degree of accuracy and mark on a board? And even if you can measure it, how do you mark it accurately? I'm a graphic designer, and I have a number of software programs that will accurately draw a pattern for me (of course, then I have to print it to accurate scale) but this still leaves me with drawn lines or marks on the pattern. And the drawn mark itself is wider than the degree of accuracy we are discussing. So....how do you guys do it? I'll guarantee you that Stefan and I are not the only ones wondering!

Best to all,
Patrick
Author:  Michael.N. [ Fri Jul 08, 2011 6:16 pm ]
Post subject:  Re: precision in location of fret slots
Only one way that I know how - very accurate engineers rule, magnification and knife marking.
To be honest within 0.2mm (whatever that is in imperial) is good enough for me. 0.3 mm and I might doctor the fret. 0.4 mm and I'm worried.
Author:  Rick Davis [ Fri Jul 08, 2011 6:52 pm ]
Post subject:  Re: precision in location of fret slots
.2mm=.008" (approx.)

I have mine CNC'd to a tolerance of +/- .0005". My supplier will even taper and radius the board, holding the edge thickness constant. Makes life simple and intonation better.

There are enough suppliers and machinists who can do this for a reasonable charge that it's a no-brainer for me. Oddball scales are a problem, of course, but are fairly rare even in the wide world of repair.
Author:  acoustic12 [ Fri Jul 08, 2011 8:11 pm ]
Post subject:  Re: precision in location of fret slots
Rick Davis wrote:
I have mine CNC'd to a tolerance of +/- .0005".

You mean over the entire length.. or fret to fret? And if over the entire length, can you give us the company name..


cphanna wrote:
But, seriously, how do you guys measure these things down to a thousanth of an inch?

You could build a small routing jig using these:

http://cgi.ebay.com/28-Horizontal-Elect ... 5ae130279e

and these:

http://cgi.ebay.com/THK-RSR-12VM-LM-Gui ... 565wt_1142
Author:  Clay S. [ Mon Jul 11, 2011 7:37 am ]
Post subject:  Re: precision in location of fret slots
For a slot that is off 1/32 of an inch you could possibly shape the the top of the fret to get it close enough. With even tempered tuning there is a fair amount of "slop" in the tones as it is, and different keys will have different amounts of error between the intervals. It is good to be accurate and precise, but recutting a few slots and doctoring a few frets should get you there.
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