Neck pullup shim size formula?

[polkat]

Hi guys! It's been a few years since I've done a pullup (shim between the neck end and the top plate). I can't remember if there is a formula to indicate how thick the shim (full size violin) should be for a certain amount of lift at the end of the fingerboard. Anyone know? Or is it just a matter of experimentation?

Thanks!

[Brad Dorsey]

I do it by experimentation. I start off with a shim that's too thick, try it and take plane shavings off it until it's right.

I'm sure that you could come up with a theoretical formula based on ratios and proportional triangles and so on. But once you cut the shim to the formula you would want to try it to make sure it's right and adjust it if necessary. So it seems easier to forget about the formula and just try it.

[Michael Richwine]

IIRC,it's roughly 6 to 1. That is, the neck projection at the bridge rises approximately 6mm for each 1mm of shim. That's a pretty rough approximation. I cut a shim with a "handle" on it so I can slip it in and out easily, start a little fat, and scrape it to fit. I check it with strings fairly tight, and when I glue the shim in, I use string tension as a clamp. Been doing a lot of them lately on old fiddles I bought for resale.

[polkat]

Thanks guys! Nonado, I figured it was somewhere in that neighborhood. I like the idea of the little handle to help slip it in and out! I can take it from here.

[COB3]

I just pull the neck to the right projection and look underneath to see what size shim I need-- that gets me in the ballpark on the first try.

[duane88]

I use fiber as the shim, the black fiber that Core and others sell fom making purfling and such. It comes in .2, .3, .4,.5, ect thickness. 0.5 is a huge shim for a pull-up, to me, althopugh I once fit 0.8 in the space.

I use the fiber because: 1-It doesn't compress as much as wood, 2-It is a known thickness for the whole sheet/strip, 3-It is much more malleable and agreeable than a brittle piece of maple veneer.

[Janito]

Confused by the fuzz: Neck-fingerboard wedge versus neck-top block shim. Which?

[COB3]

A "neck pull-up" implies a shim between the neck heel and the meck block...at least that is what I thought we were talking about...

(edit) Of course the block moves with the neck...what was I thinking? But I understood the above question to be asking about a wedge under the FB, which is what I thought we were NOT talking about. The wedge only goes as far as the block. Just a filler, really.

I have had to do a couple of these, in my own instruments, usually. Also one where the neck ended up just a tiny bit crooked, laterally. I moved the head sideways after loosening the block from both plates. It was only a couple of tenths of a mm, but it straightened the neck out beautifully... required my reshaping the button a tiny bit, but it was a good move.

[Brad Dorsey]

A "neck pull-up" implies a shim between the neck heel and the meck [sic] block...

No. It implies a shim between the neck heel and the top. The neck remains solidly attached to the block.

[luthierwannabe]

No. It implies a shim between the neck heel and the top. The neck remains solidly attached to the block.

In order to do this, I assume that you have to free the top plate from the top block and top of the ribs for a distance on each side of the neck. By inserting the shim between the neck heel and the top, you are moving the neck and top of the ribs to the north. Thereby elevating the south end of fingerboard. Is that right?

[COB3]

Yes. See my edit above--thanks, Brad, for catching my brain-cramp. :-)

[Doug Marples]

In order to do this, I assume that you have to free the top plate from the top block and top of the ribs for a distance on each side of the neck. By inserting the shim between the neck heel and the top, you are moving the neck and top of the ribs to the north. Thereby elevating the south end of fingerboard. Is that right?

Right, the top is loosened down to upper corners.

[robertdo]

Does it mean the edge overhang is reduced in the top part of the upper bout area by the width of the shim and the ribs are slightly distorted too?

[Brad Dorsey]

Does it mean the edge overhang is reduced in the top part of the upper bout area by the width of the shim and the ribs are slightly distorted too?

Yes.

[polkat]

No, I was not talking about a fingerboard shim (I always thought they looked obvious and (to me) somewhat ugly). yes, the overhang refuces roughly the same amount of the shim size, but if you have good overhang already, a .5mm shim shouldn't be that obvious. duane88, how much lift at the board end did you get from that 0.5mm shim?

[Wolfjk]

Hi guys! It's been a few years since I've done a pullup (shim between the neck end and the top plate). I can't remember if there is a formula to indicate how thick the shim (full size violin) should be for a certain amount of lift at the end of the fingerboard. Anyone know? Or is it just a matter of experimentation?

Thanks!

Hi Polecat,

There is a mathematical formula! It works out about the same as "Nonado" given! Perhaps a little less - 1:5.95, depending on how far the neck is morticed into the top.

[Brad Dorsey]

....how much lift at the board end did you get from that 0.5mm shim?

Since the end of the neck is roughly at the center of the fingerboard, a shim a half millimeter thick at the end of the neck would raise the end of the fingerboard one millimeter, as a very rough approximation. It's a simple proportional relationship.

Erratum: This is wrong. What I said here applies to a shim between the neck and the fingerboard, which is not what this discussion is about.

[COB3]

If the shim were vertical, yes, but that shim is in the longitudinal dimension, changing the set of the neck.

The shim changes the angle at a point where the whole height is only 30mm tall, say, or less, in some cases-- so, in terms of proportion, the value would be 30/.5=(length of FB)/(change in height at end of FB)...in other words, the ratio between the height of the neck mortise and the shim should be the same as the ratio between the length of the FB and the change in height at the end of the FB. (right?)--pretty close to a six-to-one ratio, I would expect.

[David Burgess]

--pretty close to a six-to-one ratio, I would expect.

Yes. (sorry for the couple of edits, I hadn't cleared the memory on the calculator)

If you glue with string tension, I wouldn't use very much. It can bow the top a little, causing a small separation at the inside of top/block gluing surface. Even if filled with glue, it can make this area more vulnerable to separation down the road, and this can cause some sound problems which are difficult to diagnose and fix.

[Michael Richwine]

No, I was not talking about a fingerboard shim (I always thought they looked obvious and (to me) somewhat ugly). yes, the overhang refuces roughly the same amount of the shim size, but if you have good overhang already, a .5mm shim shouldn't be that obvious. duane88, how much lift at the board end did you get from that 0.5mm shim?

I do fingerboard shims out of ebony. They're very unobtrusive. I agree that maple looks bad.

[polkat]

Well, let me approach it from this angle. I want to raise the end of the fingerboard 3 to 3.5mm. According to what Wolfjk says (and what I think the rest of you are saying)is that if I start with a shim around 1/2mm thick, that will get me in the ballpark. This is a violin I made about 10 years ago. At the time all measurements were correct, but I used this particular fiddle for playing bluegrass, and kept it stringed with heavy strings. Over the decade the fingerboard dropped and I associate that with the heavy strings. Would you folks associate the strings with this problem? Other violins I made at that time don't show this problem, but were strung with medium strings.

[Brad Dorsey]

....Would you folks associate the strings with this problem?....

Yes, because if you hadn't put strings on the violin, it is unlikely that the fingerboard would have dropped. String tension tends to make the fingerboard drop on any violin, and more tension will be more likely to lower the fingerboard. But heavier strings do not necessarily exert more tension. Besides tension, other factors in whether or not the fingerboard drops include the shape of the arching, the stiffness and thickness of the top wood, the humidity and the use or non-use of an arching protector.

Erratum: After thinking about it, I realize that I was wrong when I said, "heavier strings do not necessarily exert more tension" in the preceding paragraph. Given two strings of equal pitch and length, the heavier one (heavier in the sense of more mass per unit length) will be under more tension.

[polkat]

I see no visual change in the arching (although that doesn't mean there isn't any), but having made only 10 violins in as many years, I'm no pro. In fact, this is the first I've ever heard of an arching protector! Can you explain what that is?

[Brad Dorsey]

....this is the first I've ever heard of an arching protector! Can you explain what that is?

String tension tends to rotate the upper block and squeeze the top shorter, causing the top arching to rise and the fingerboard to fall. An arching protector is a custom-fitted shim inserted between the top and the fingerboard overhang, when an instrument is not being played, to help stabilize the top arching and prevent the fingerboard from falling. Arching protectors are usually made from strips of cardboard to fit the spot where the top and the fingerboard overhang are closest. If you do internet searches for "arching protector" and "arch protector," you will find them mentioned on some violin dealers' websites.

[Janito]

Some simple diagrams would help this discussion enormously.

ps I don't agree that a carefully matched maple shim under the fingerboard necessarily looks bad. A butcher will make any wood look bad.