Welcome to my blog! I thought that I would try my hand at writing about various aspects of mandolin making on a more or less monthly basis. I say 'more or less' because new submissions will largely be based on wether or not, I have the time and inspiration. New entries will be posted here with the latest one at the top so you can quickly see if there has been any activity since your last visit.
Wow! I can't beleive that I'm into my third year of blogging here! In order to keep things from becoming too unruly, I will begin seperating each year of blogs onto its own page. Just click on the links below in order to read my past postings.
February 1, 2018
A well made string nut with accurately spaced string slots that are also filed to a depth which provides the lowest action possible is one of the pillars of a professional set-up. I can see no reason to have an action height at the nut, that is any higher than necessary to prevent string buzz. Some players may prefer to raise the bridge up a little more than necessary to prevent buzzing just because they like the feel of (as Bill Monroe would say) a "manly" action. Personally, I like a low action at both ends of the string. How low you can go will depend on three factors, how well the frets are dressed, the string guages used and the individual player's attack. A player who strikes the strings aggressively will require a taller bridge height in order to avoid buzzing than someone with a lighter touch. String height at the nut should be the same or slightly higher than the height of the first fret whether you play hard or soft. Excess string height, especially at the nut will make fretting a note more difficult than it need be and even worse, it can cause intonation issues as the string needs to be excessively stretched to make good contact with the fret, thus raising its pitch.
Good string to string spacing as well as good spacing between the courses also contributes to a good overall set-up. Although I have heard other schools of thought on this, I contend that the gaps between the four courses should all be equal as well as the gaps between the string pairs themselves. To achieve this requires some tedious but basic math to layout the slot locations on the nut. Simply using a divider to lay-out the centers of the string pairs equidistant will not do. This would result in less of a gap between the courses of the heavier strings than the lighter ones due to their varying diameters.
In order that I did not have to do the tedious math every time I needed a different nut width and/or string guages, I came up with an Excel© spread sheet that would do the calculations for me just by entering the known and desired variables. I have included a working version of this nut slot spacing calculator below.
To use the calculator, double click on any of the buff colored boxes and enter the value you would like. The white boxes will be automatically re-calibrated when you "off click" in a blank area of the web page. The calculator is presently filled out with metric values but you can use decimal inch values as long as all of the values in the buff boxes are changed to decimal inch. In other words, all of the values in the buff colored boxes must be in the same unit of measure.
Once I have the measurements that I want for the nut slot locations, I enter them into a special Illustrator© file and print off a very accurate template.
Thanks for reading,
January 5, 2018
Happy New Year everyone!
On top of the holidays being very busy, I came down with a nasty cough/cold/flu that I just couldn't seem to shake. After 4 weeks I was still under the weather and losing productivity in the shop and so I finally saw my doctor who seems to have whatever it was on the run now. I am presently well behind schedule in my mandolin making and I'm grateful for my customer's patients and understanding. I apologize to my readers for re-posting this blog from July 9, 2016. I just could not find the time to create an original post this month. I consider the following to be one of the most important of all my blog posts and I think it's worth re-reading.
July 9, 2016
In today's blog, I'm going to do my best to demystify the relation of wood to humidity. I'll start by describing what we mean by terms like humidity and relative humidity. Humidity is a term for the amount of bound water molecules in the air. If you measured the amount of water in a defined volume of air, say a cubic meter, without regard to temperature, that would be the absolute humidity. (usually expressed as grains per cubic meter) In the real world, what is important is the relative humidity and not the absolute humidity. Relative humidity refers to the amount of water molecules that can be bound in the air at a given temperature.
The warmer air gets, the more water it can absorb. Relative humidity means, the amount of actual water in the air relative to the maximum amount of water that the air could hold at that temperature. So for example, if we have a volume of air with an amount of bound water in it that is equal to 50% of how much water the air could hold at that temperature, it is said to be at 50% relative humidity or, commonly abbreviated to, 50%RH.
Now, if we take that same volume of air and heat it up, it now has the ability to carry more grains of water but if the absolute amount of bound water remains the same, then the relative humidity will drop because it now has less than 50% of the water that it potentially can hold. This is exactly what happens in the winter months. The outside air is cold and say, at 50% relative humidity then we heat it up with our central heating and unless we are adding moisture to the inside air, the inside air drops in relative humidity. It is not uncommon for heated homes and apartments to get down to 15%RH (worse than desert conditions) if humidity is not added to the inside air.
Now here's how this relates to your wooden instrument. Wood, all wood, is hygroscopic which is a fancy term for saying that it picks up or releases water depending on the relative humidity of its surroundings. If the relative humidity stays steady at a given amount then eventually the wood will reach an equilibrium moisture content (EMC) with its environs. The exact moisture content (MC) of wood in equilibrium with a given RH will vary somewhat from species to species. The various spruces used in making stringed instrument soundboards will generally have an actual moisture content by weight of 7% when in equilibrium with a relative humidity of 37-40%RH. Problems arise because as wood takes on water it actually increases in size and as it gives up water or MC, it actually shrinks. This is a property of wood and remains a property of the wood as long as the wood is still wood. We cannot change that. What we can do is learn how to deal with that fact.
Imagine a spruce mandolin soundboard being glued to a rim in an environment of 40%RH. The soundboard is measured at say 250mm across at its widest point. Now the instrument is completed and is sold to a customer who takes it home in the winter time to his heated home. If that home is not being humidified and has gotten down to say 15%RH the soundboard will now start to release moisture which will then cause the soundboard to shrink. Going from the 40%RH at which it was built to 15%RH in a heated home will cause an actual shrinkage of the soundboard in excess of 1 full millimeter. The problem is, the rim does not change in diameter, in any significant way, which means that the shrinking soundboard, which is glued to the rim, feels itself under tension and somethings 'gotta give. Either a crack develops in the wood to relieve the stress or a seam lets go.
In the summer, when the humidity goes up beyond the 40%RH that the instrument was glued together in, the wood swells in size. This time, though the rim is holding things in place, the soundboard does have somewhere to go. It can bulge up causing the string action to rise. Although the soundboard does have some room to maneuver, there is a limit and if the RH gets too high, the stress caused by the expanding wood will typically lead to seams letting go. Since high humidity is very often associated with high temperatures, these conditions will also weaken glue joints while putting the stress of the expanding wood on them. In actual numbers, a spruce mandolin soundboard that was glued up at 40%RH will grow by nearly 3mm when at an equilibrium with an environment of 80%RH
The key to avoiding problems caused by excesses and extremes of humidity is to keep your instrument from being exposed to these extremes. Solutions for excessive dryness include using a whole home humidifier on your furnace, using a room sized humidifier in the room where you store your instrument or using a humidifying device in a case in which you store your instrument. My preference is in using a whole home humidifier. It requires the least amount of ongoing maintenance and is good for the human occupants of the home as well. A room size humidifier can do a good job if correctly sized and maintained. Using a device in your case is my last choice. I find that these require a lot of maintenance and monitoring. It is difficult to insure that your are adding the right amount of moisture and not overdoing it.
Here comes one of my real pet peeves and that is that the typical hygrometers you buy at the hardware store are very often very badly calibrated. It's OK for me, I have sling psychrometers to calibrate my hygrometers because it is my business to maintain proper humidity levels but it is not so good for the average person. Proper calibration of your hygrometer is essential in maintaining proper humidity levels in your home. Other wise you could be lead to believe that you are maintaining the the proper level, which by the way is 40 - 50%RH, when in fact you are not. I wish I had a good answer for this. Here in the Toronto area, I know of one music store, The Twelfth Fret, that will calibrate your hygrometer for free if you bring it in. Maybe there are other reputable shops in your area that will do the same for you.
So the main takeaway is, keep your instrument at a comfortable humidity level, avoid extremes and you will be able to pass it along to your grandchildren in perfect working order.
Here is a handy chart that I drew up. It plots the equilibrium moisture content of a typical spruce species vs relative humidity and also shows the actual dimensional change that occurs across a 250mm wide mandolin soundboard. Keep in mind that the maples used for the back actually have a larger coefficient of expansion than the spruce top. This is even more so in rift-cut and slab-cut backs which are popular for their figure.
Click on the image below for a PDF version sized for printing on 8.5 X 11 paper.
Thanks for reading,
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