Optimizing laminations?

[willie]

 

When you say edge grain I assume you are talking about quarter sawn with no bias?

yes, and some bias is probably Ok too.  If you say "quarter sawn" to the guy at the specialty hardwood store he might show you a very expensive board.  avoid ring lines parallel to the gluelines, the more so if early/late ring densities vary much.

black poplar is cottonwood?  I have seen some that is very soft.

I would be curious to know if many trilam builders find a compelling benefit for optimizing the middle lam over building a with just two lams?

some guys get by with minimal tillering by carefully tapering the middle lam while controlling lam tolerances with thickness sanders.

[mmattockx]

yes, and some bias is probably Ok too.  If you say "quarter sawn" to the guy at the specialty hardwood store he might show you a very expensive board.  avoid ring lines parallel to the gluelines, the more so if early/late ring densities vary much.

I asked because I have a good supply of decent lodgepole pine and would be cutting the pieces myself. I would never dare to specify something like that at a lumber store because of the price tag, as you note.

I would be curious to know if many trilam builders find a compelling benefit for optimizing the middle lam over building a with just two lams?

I expect there is not a significant (or even measurable) performance benefit to using different wood for the core. There may be a cost savings and aesthetic benefit of being able to match contrasting woods and/or grain, etc. I can see a construction benefit of easier bending multiple thinner lams onto a form and a structural benefit in being able to run the belly lams up onto the handle like fibreglass bows do to avoid having the handle pop loose when scabbed onto the back of the lams.


Mark

[Dances with squirrels]

My most trusted, durable, and favored trilams are bamboo/osage/osage. But sometimes I like to mix it up a little.

Bamboo/yew/yew is excellent when it survives, but I've had a couple of compression failures with what appeared to be superb, clear, flat and quartersawn yew in trilams where I didn't think it should have been an issue. ...when I've had more highly stressed yew trilams survive just fine. I don't yet understand why.

Bamboo/yew/osage is a really good combo, but others work too.

I taper the core lam, and leave the belly lam parallel to allow wood for tillering, weight reduction, shaping, etc. That said, my goal is to get as close as possible to the needed stack height, with a little bit of working room, so that there won't be a need to remove much wood. Good tapering means little removal of material for tillering, and good stack height means little removal to hit weight. Less wood removal overall helps avoid backtracking and some possible hurdles. The bulk of the work with them is best done in preparation.

Of course it also depends on width, length, cross section, and other design features, but osage core and belly lams of .200 to .220 can make a 55-60# bow easy enough. That's with the core lam tapered .003 to .0035 per inch.