Balsa block: how to fill

[prfesser]

Okay, I'm thinking about using this block to make a transition, 3" aft end, 1.9" forward end, about 7" long. There is a small problem...


...that ends up being a deep problem...


The ruler goes into the narrow crack almost 3" deep. Crack will be exposed as I file/sand, regardless of which end of the block is used for forward/aft end.

Possible solutions:

• Fill with epoxy before turning.
  • I can do this--thin epoxy with slow curative will work, I'm sure---but turning the transition will be a bumpy ride since epoxy is far harder than balsa.
• Mix q-cells/microballoons with epoxy, fill the crack.
  • It'll be quite difficult to make a mix that cures about as soft as balsa, but thin enough to pour into a 1/16" wide crack. I won't know if it's filled the crack until after I turn the piece.
  • Attempting to 'knife' a putty-like mix into the crack is going to fail. No way can I get a stiff mix to the bottom of that crack.
• Turn the piece, fill the crack after turning to shape.
  • I expect that would also leave quite a bump where the sandpaper or file hits the crack. Symmetry will take a hit.

Suggestions? Comments? Physical threats?

 

[David_Stack]

Admittedly I've never used it so have no idea how viscous it is, but is 2-part expanding foam (like the product used to foam nose cones and fin cans) thin enough that you could pour it into the crack and then allow it to expand and fill the void?

Perhaps quickly 'cap off' the end in order to force it to expand into the recess? (hopefully not expanding to the point that it splits your block)

 

[neil_w]

Normally pouring in epoxy would be the approach, but you're right that it would make turning the piece somewhere between difficult and impossible. But I don't think there's anything you can fill it with that will have the same physical characteristics of balsa.

You could fill it with thinned wood filler (just thin enough to pour in), which isn't *too* much harder than balsa, but it also isn't very strong.

If you have some pieces of scrap you could experiment. Make a hole, fill it, and then try to turn it and see what happens.

 

[jmasterj]

Could you fill it with something soft and water-soluble (maybe wood filler like @neil_w suggested), do the turning, then wash out the initial fill and replace it with epoxy?

 

[NTP2]

A epoxy wood glue mix? It would be soft and thin I think?

 

[dhbarr]

Polyurethane glue with a syringe, gorilla or similar? Shouldn't be as hard to turn as wood glue or epoxy.

 

[David Schwantz]

Do not turn without glue I will catch and split. Whatever glue u use, clamp it tight to close crack.

 

[lakeroadster]

Turn the piece, fill the crack after turning to shape.

Balsa is so soft....

 

[bguffer]

Can you saw the block along the crack, then wood glue back together such that the crack sides are on the outside of the glued block?

I assume wood glue would file/sand easier then epoxy.

Bob

 

[K'Tesh]

Can you saw the block along the crack, then wood glue back together such that the crack sides are on the outside of the glued block?

I assume wood glue would file/sand easier then epoxy.

Bob

I was just about to suggest this exact thing.

 

[augendoc]

Gluing and clamping that void will be nigh on impossible without crushing the balsa.

also, anything you pour into the crack is going to soak into the balsa to some extent and even if you add microbaloons or balsa dust, some of the binder (glue) is going to soak in and make turning a challenge.

I agree with bguffer to cut the block in half on a bandsaw and reglue with the good outside faces now inside. The glue line will not be in the center, however, and may still telegraph through to the final surface.

Personally, i would buy a new piece of balsa for your project and cut the block with the crack into maybe five pieces (two big and three small) to make other smaller nose cones or transitions. More rocket parts is always a good thing.

 

[neil_w]

Can you saw the block along the crack, then wood glue back together such that the crack sides are on the outside of the glued block?

I assume wood glue would file/sand easier then epoxy.

Bob

 

[prfesser]

Can you saw the block along the crack, then wood glue back together such that the crack sides are on the outside of the glued block?

I assume wood glue would file/sand easier then epoxy.

Bob

I'm going to give this a try. Thanks for the idea!

 

[jqavins]

I had a couple of good(?) ideas, but they're out the window thanks to @bguffer.

 

[Alan15578]

I'd try splitting the block along the crack first. If it does not want to split, saw it along the crack, Then decide how to glue it back together. I'd try one of those sandable glues.

 

[David Schwantz]

Buy another block