Boomilever B/C

[flyingwatermelon]

I know that this may seem like a trivial question, but I have to ask. Recently, my boomilever broke at the base vertically along the centerline of the hole.

When my partner and I put the bolt in at competition, it was slightly wider than the bolt I had used and tested at home. I believe that this may have caused excessive strain on the base or put microcracks into the base as we inserted the bolt. However, an alternative theory would be that the base itself was also weak. (My base consists of a 1/8" basswood laminated (grain running up and down) on about 1/4" balsa (grain running horizontally)). The base broke so that the basswood was split and the grains of the balsa were torn apart.

I'm not sure if I should increase the base dimensions, modify my base design or just make the hole slightly bigger for next time? Thoughts?

[_HenryHscioly_]

I would switch the grain direction for bass and balsa.
Make sure your tension members are not far out from the edge of the washer.
1/8"bass, is that on the back side and front side?
Sounds very thick, 1/16"bass on both sides with horizontal grain + 1/4"balsa core vertical grain

Better might be: 1/32"bass horizontal + end grain balsa core

you should read Balsaman's post from earlier this season about how to make a base.

[Balsa Man]

Henry's understanding things correctly.
I would think 1/32nd bass on both sides should work fine. I actually think (and had suggested before) 1/64th birch plywood, which at least around here, Ace hardware carries, would be even better. With the weight saved by that vs. the 1/8th" of bass, I'd also suggest thickening up the balsa - and you do want it end-grain oriented) so the total thickness is 1/2 inch. That additional thickness gives you more glue area on the tension memberss, which means less chance of a shear failure in the tension member to block connection.

[flyingwatermelon]

Henry's understanding things correctly.
I would think 1/32nd bass on both sides should work fine. I actually think (and had suggested before) 1/64th birch plywood, which at least around here, Ace hardware carries, would be even better. With the weight saved by that vs. the 1/8th" of bass, I'd also suggest thickening up the balsa - and you do want it end-grain oriented) so the total thickness is 1/2 inch. That additional thickness gives you more glue area on the tension memberss, which means less chance of a shear failure in the tension member to block connection.

So just to clarify a few things. Horizontal grain for the bass/birch and end-grain oriented means that the grains are running perpendicular to the face of the testing wall correct?

Awesome. The failure wasn't in the connection, it was just in the base which basically snapped in half.

Thank you guys!

[Balsa Man]

Correct on grain orientation.
Something I forgot to comment on; your saying the bolt they provided was too big. The rules call for a 1/4" bolt. If you had a 1/4" hole, what they provided should have fit. The next size up is 5/16ths, which is a lot bigger. Was it a 1/4?

[iwonder]

One thing I've noticed is that in wood a 1/4" hole and a 1/4" bolt don't quite fit together, I end up having to turn the bolt and kinda thread it in to the hole. So that may be the issue? A 5/16" bolt probably wouldn't fit through a 1/4" hole without putting noticeable cracks in the balsa.

[jander14indoor]

Actually, a 1/4 inch hole in wood will slide over a 1/4 inch bolt without problem.

The problem is, a 1/4 inch twist drill (which most people have) doesn't drill a 1/4 inch hole in wood, first pass. See, twist drills aren't really optimised to cut wood, they are designed for metal. As a result, they tend to push the wood fibers at the edge of the hole out of the way instead of cutting them and when the drill is withdrawn these fibers spring back into the hole leaving it undersize.

So, what to do.
One trick is to run the drill into and out of the wood multiple times. Sort of works. If you are on a drill press generally nothing bad happens, but you are a little undersize still. If hand drilling, you are likely to get an ovalized, non-straight hole.
Alternatively, you can use SLIGHTLY larger twist drills, say 9/64 or a number 30 (0.1285 in) or 29 (0.136 in).
Best is to use a drill designed for wood. These cut the sides of the hole BEFORE the center ensuring it is true to size. Brad point bits, auger bits and forstner bits all do this.

Hope that helps.

Jeff Anderson
Livonia, MI

[iwonder]

I think you mean something more like a 17/64" or maybe an F(.257) or G(.266)

And thanks for the correction, I guess I've been doing to much metal work to realize the woods deformation is significant

[Balsa Man]

I think you mean something more like a 17/64" or maybe an F(.257) or G(.266)

And thanks for the correction, I guess I've been doing to much metal work to realize the woods deformation is significant

Aside from the dimensions, Jeff's advice is right-on (of course). Right tools/techniques matter. One other option is a 1/4" rat tail file

[jander14indoor]

I think you mean something more like a 17/64" or maybe an F(.257) or G(.266)

And thanks for the correction, I guess I've been doing to much metal work to realize the woods deformation is significant

Dang, probably too late to go back and edit the note to correct 1/4 inch dimensions instead of 1/8 inch.

Wait, I'm a super user with super powers. All I have to do is delete iwonder's note, edit mine, edit Balsa Man's and NO ONE WILL EVER KNOW about my mental slip!!

Right, like that'll work.

Wait, what was I talking about...

Jeff Anderson
Livonia, MI