Wednesday, 4 December 2013

Update On My Portable Vise Design...

A few weeks ago I posted an article about a portable vise I had designed. Before it was posted I had already placed an order with a local wood supplier for enough Beech to build it, but hit a snag a few days afterwards. The yard had problems coming up with Beech in the thickness I wanted, which was 1½". I called around to a few other local suppliers and heard the same story; nothing thicker than 1". I found that I could get the stock I needed from a supplier in the States, but was shocked at the cost to ship it up to me. When I stated such, the supplier said something that made me rethink my choice of Beech for this project, which was; "Beech isn't exactly the lightest species of wood." Given this is supposed to be a "portable" vise, naturally this comment caught my attention. I ain't gettin' any younger here, so not wanting to have to slug around something that weighed a ton, I thought I should do some more research.

Almost everything about workbenches that I came across stated that Beech was the material of choice, mainly due to its "density". Why everything to do with woodworking has to be so bloody technical, I'll never know, but I decided to see if there were other species of wood that came close to Beech's "density", but weighed less. I had seen Density Values listed on many different wood suppliers' websites but I really didn't have a clue what it really meant, so I decided the first up on my agenda should be figuring out exactly what the hell "density" meant.

Huge mistake.

I really have to be honest here; I am so far removed from an analytical thought process, I don't even fully understand what that term really means, so trying to decipher pretty much all of the definitions for "Density" that I found was as impossible for me as making cream from sand. Why do those that post definitions on the web think everyone is as like-minded as they are? Hell, if I could think like them I wouldn't be reading their damned definitions in the first place, but instead, I would be posting my own, which, I must admit, is exactly what I am going to try to do here, but one I came up with that, hopefully, rewrites the gobbledygook in laymen's terms.

den·si·ty (dĕn'sĭ-tē)

  1. Basically, the amount of wood in a piece of wood.
  2. The more "fibers" (read that as "wood") a wood has per square-whatever, the stronger it is.
  3. Density of wood is defined by a decimal value.
  4. The more dense the wood, the higher the decimal value.
  5. The higher the density value, the stronger the wood is, which means the less effect wailing on it has.
  6. There are as many different ways to calculate a wood's density as there are species of the damned stuff, which causes me to believe that anyone in their right mind should stay the hell away from the subject whenever possible. (which, in turn, explains why I didn't)
  7. The standard way to measure the density of wood, or at least the way I gleaned it to be from reading about 100 different articles on the subject, is by taking the weight of a specific volume of wood which has been properly dried and dividing it by the weight of the same measurement of the same species of wood which hasn't, or is still "green".
  8. Once you understand the principles of calculating Density Values, you realize that to calculate it, you first have to find the weight of the particular species of wood you want to know about in both its green and dry configurations, and if you can do that, you can also find a Density Value list somewhere, so for God's sake, use it, and forget about going through all this other shit!

So putting all rational thought aside, I took my new-found knowledge for a test drive and learned...

  • A cubic foot of dried Beech has a weight of 45-pounds and by dividing that value by the weight of a cubic foot of green Beech, which is 54-pounds, the density factor of dried Beech is 0.8333, or at least that is the value I am going to apply to it, despite any critics.
  • Comparing that value with one for Poplar, for which a cubic foot of dried has a weight of 28-pounds and a cubic foot of green is stated to be 39-pounds, means that Poplar has a density value of 0.737, much less than Beech.
  • With those two values as a base for "good" and "bad" densities for workbench production, I checked out Black Maple, which is one of the harder species and one that weighs in at 40-pounds dried and 54-pounds green; giving it has a density value of 0.740, meaning it isn't much better as a material to build a workbench from than Poplar, which was a bit of a surprise.
  • Checking out the Mahogany that I have had hanging around in my stash for over 10-years now and the material was thinking of using for this portable vise, I found it has a density value of 0.735, which tells me, as a workbench, it would really suck.
  • So I did the same for the Red Oak that I also have in my stash, and found that its density value of 0.688, which means it will suck even more than the mahogany.

One of the main things I learned through this exercise was that I wasn't as smart an ass as I thought I was, so I'm going to have to head back to the drawing board and come up with a better design for this portable bench that is lighter, but just as strong. I also learned that it will have to be made from...go figure...Beech, and more specifically, Beech in 1" thicknesses.

Peace,

Mitchell


3 comments:

Anonymous said...

When contructing a portable device, you are not interested in the density of the material nor in its specific strength but you want the material which offer the best ratio strength/density. Density is not equal to strength as some dense material are also fragile (glass).
If you compare beech (density 0.64) and balsam poplar(density 0.34), you would see that you would need, to achieve the same strength in bending, a poplar thickness 2.19 times as great (it is in fact a bit more complicated)and finaly the poplar vise would wheigth approximately 1.16 the wheight of the beech vise (2.19X0.34= 1.16X0.64)for the same strength.
On the other side, if you compare balsam polar (density 0.34) with black cotton wood (density 0.35),for nearly the same thickness and wheigth, you get 25% more strength (in bending) with the black cotton wood.
You can find mechanical properties of various woods here :
http://www.woodweb.com/Resources/wood_eng_handbook/Ch04.pdf
Sylvain

Mitchell said...

There you go, Sylvain, talkin' that analytical thought stuff - ;0)

I really appreciate your comments, which were very helpful to me, but they do leave me with a question...

Isn't density tied to strength? I am not talking torsion strength or bending strength here, but how a particular species of wood would take the knocks, drops and general abuse of something that sees heavy use and encounters misuse when it is moved around, as a portable vise would be.

Somehow I got it in my head that a denser wood would take more abuse than one with sparser fibers.

I get you point about having to balance strength with weight, but I think building it from a material that will last, something that can absorb years of beating on it.

Again, thank you very much for your thoughts. Great stuff,

Mitchell

Anonymous said...

If I had oak, I would use it.It has properties not far away from beech.
In my experience mahogany is more brittle.
Sylvain