What effects will grain have on MOE/Stiffness, crush strength, etc

[don dsrekab]

This is kind of a thought exercise more than anything, but I was reading about a Moxon vise and someone asked if they could use walnut instead of maple for the front jaw. This got me thinking about what wood would be the best for that application (based strictly on strength its greenheart, but who'd want to use a piece of that on a shop tool?) and I started reading about the Modulus of Elasticity (MOE) of different woods, which relates the stiffness. Think the amount of weight a shelf can bear before deflecting, or the distance between the twin screws on a Moxon vise before the wood deflects too much to hold the workpiece. So my question is, all other things being equal, would the way a piece of wood was cut (flatsawn, quartersawn, riftsawn) matter at all to it's MOE? Secondly, if you had one big piece with no glue joints (I've heard modern glues are strong enough that the wood fails before the glue anyway), would it be stiffest oriented with the face toward the workpiece, the end grain toward the workpiece or the side toward the work piece (face up and down, end grain left and right)? Again, this is more of a thought exercise for understanding wood better.

Thanks for any thoughts,
Don

[Jim Belair]

The radial MOE is greater than the tangential MOE by 40% (douglas fir) to 100% (poplar) and as high as 200% (balsa). And both are only 5-10% of the longitudinal MOE. This according the The Encyclopedia of Wood (table 4-1). So if I'm not mistaken, end grain against the workpiece is stiffest, although tough to accomplish!

[Adam Cruea]

My question is why would you want a really stiff wood for a vise to begin with? (Asks the guy with a hickory twin-screw end vise).

IMHO, you want you vise face to crush before you want the wood of your workpiece to crush, yes? This is why I want leather on my vise; to keep from marring workpieces accidentally because my vise is so hard.

I'm not totally sure if end grain would be the stiffest because the grain fibers would have a tendency to separate rather easily. If you think about it, it's easier to tear a thin piece of wood with the grain as opposed to across the grain; granted this isn't "crush" strength, but it's probably failure before the ability to crush has been reached. Hope that makes sense.

[don dsrekab]

I was more just thinking about the structure of a piece of wood, the vise just started the thought process. Though I would think a stiffer vise would allow you have a greater distance between screws.

[David Weaver]

I wouldn't worry too much about it on a moxon vise. You could make a perfectly serviceable vise out of walnut, maple, birch, beech, etc, and other than the screws, I'd imagine you could do it with softwood if you didn't mind it being bulky.

[Jim Matthews]

I would be more concerned about choosing a piece that has even grain at the top and bottom faces,
where the most amount of rough handling will take place.

Chamfer the facing edges, in any case.

I made mine out of 8/4 oak, because it was the widest board I could find at 10" across.
The board was reassembled with the same "face" direction - so that seasonal cupping will be parallel.

I'm with Adam, a little flex in the front chop is a good thing.
Not sure I would use Balsa, but you could certainly make this from plywood.

[Frank Drew]

Don,

I get that you're not really choosing wood for a vise; I think Jim supplied the answers to your thought experiment.

[Pedro Reyes]

The radial MOE is greater than the tangential MOE by 40% (douglas fir) to 100% (poplar) and as high as 200% (balsa). And both are only 5-10% of the longitudinal MOE. This according the The Encyclopedia of Wood (table 4-1). So if I'm not mistaken, end grain against the workpiece is stiffest, although tough to accomplish!

Your numbers seem correct, but the conclusion is flipped. Longitudinal refers to along the grain in this case, so the MOE when you try to stretch a board end to end, which is what happens to the outside fibers of a Moxon jaw (face grain). Whether it is quarter sawn or not seems a minor point to consider since at best it is an order of magnitude smaller than the MOE along the grain. But if we are sticklers, radial would be trying to separate (at the) growth rings, so this would say that a flat sawn board would be marginall better, since the force would be acting perpendicular to the rings.

All in all, it does not matter, even poplar can hold a board for DTailing, assuming a sharp saw is used ;-)

/p

[Jim Koepke]

Just an added note here, some advocate planing the front of the Moxon to be slightly proud in the center to counter act any bowing effect due to a piece held in the center when the ends are tightened.

jtk