Friday, February 13, 2009

Blind bolts

My centerboard is a heavy piece of wood and lead about two feet by four feet, the size of a small coffee table. For a variety of reasons too awesome and whip-crackingly adventurous to go into, it's made from a stack of 22 small, flat strips of wood glued face to face. Kind of like a jumbo-sized pack of Juicy Fruit.

Great, right? I know!

Because wood is always a little flexible (compared to, say, steel ... which is also a little flexible, just less so than wood), in clamping up a two-foot wide assembly of wood strips to glue, the outer strips compressed a tiny bit more than the inner strips. Multiply that by 22 strips and you (me) end up with less clamping pressure in the middle of the stack.

And what that means is that much later, hairline seams appear between some of those middle pieces, where there just wasn't enough clamping pressure to squeeze them together.

Blast and death and raggedy old hell, quoth I, smiting my brow, this must be fixed.

But how to squeeze those pieces together? Re-clamping is no good because the same thing will happen again: insufficient pressure in the middle of the stack. I can't eye-drop glue into the cracks because it's too thick, like maple syrup or mayonnaise, and the cracks are too narrow. I can't put screws in because the cracks are six inches from the edge of the centerboard. Which, by the way, I had shaped into a graceful and thin curve not much thicker than a dictionary.

Then I had it: blind bolts! This is an arcane and complicated technique that lets you (me) attach things I can't reach.

Step one: take 3/16" x 2" stainless machine screws (I retract my frustration at having to buy these in quantities of 100!) and lay them atop the centerboard to determine ideal position, way inside the board. This is like taking a little pine needle and laying it across the narrow edges of the stack of gum.

Draw lines on the (curved) wood representing the end of the bolt (inner end), the top (its head), and the depth of hole required to get it that far into the centerboard. Note that the head is exactly 3/8" in diameter.

Clamp a straightedge to the line, chuck a 3/8" brad point bit ('cause you're drilling into to a curved surface: the narrow edge of the centerboard) into the drill, and drill away to a depth of a few inches (exact depth has been marked on the drill bit with white tape).

Switch to the super-long 3/16" bit and continue the hole, dead straight, to the appropriate depth for the actual bolt.

Now you have a 6-inch hole. The first half (from the outside of the flat face to about 3 inches in) is big enough for the entire bolt to slide into. The second, deeper half, starting about 3 inches in, is 3/16", just the size of the bolt shaft but too small for the head to fit through. It's a homemade countersink!

Here's the best part. Take a 3/4" Forstner bit, which drills smooth-sided and flat-bottom holes (not all bits do this), center it on the pencil marks on the top of the wood showing where the bottom of the hole SHOULD be, and drill straight down.

If the first two holes were like inserting a needle into the side of the gumpack, passing through five or six pieces, then this last, vertical hole goes straight down to meet -- you hope -- the bottom of the other holes.

When this happens the 3/16" hole connects at right angles to the vertical 3/4" hole. Push the bolt into the first, horizontal hole, press it on through with a screwdriver, and wait for the tip to appear in the side of the vertical hole. With needlenose pliers pinching the tiny washer and the nut in one hand, and the other on the screwdriver deep in the hole, thread the washer and nut onto the bolt like a surgeon.


Tighten and fill the holes with glue. Result: an absolutely secure bolt, directly through the cracks and pulling both pieces together, six inches deep in the center of the board.

It took almost as long to do it as to explain it, too!


Babs said...

Well heck...I could have told you THAT! Ok, well, maybe not the part about the maple syrup/mayonnnaise consistency of glue.

Shawn Spencer said...

Thanks for the post. A video would be nice to really understand how blind bolts work, because I think it might help me with a project but I still can't quite grasp how they work. Pangs of being a visual learner...

Liam Manning said...

Great info in your post, thank you for sharing! My friend keeps telling me to find out more about blind bolts and I'm glad he did. Do you have any advice for someone just starting out with them? I'd appreciate any help you have to offer, thanks!

Mia Hart said...

Great post. This stuff you posted was kind of genius. But do you happen to have any recommendations for where I can buy blind bolts? Please let me know, thanks.

Jayden Eden said...

These instructions are very helpful. Thank you for helping me with blind bolts it made my project a lot easier.