Joinery Experiment

I just tried both the methods listed below (Joinery 2 & 3). For the hanger-bolt joint (joinery 2), I omitted the epoxy, since I hoped to reuse the materials. For the mortise-and-tenon joint (joinery 3) I used a piece of 1/8" ply cut in a modified rectangle for the tenon and bamboo skewers for pins. The modification was essentially a tail, an extention of the rectangle at a width slightly less, so that it could slide inside the cavity of the crosspole, giving more pinning length while requiring the bamboo to be cut less.

Hanger-Bolt Joint
Joining caused splitting of the hardwood dowel, and subsequent stress and splitting of the crosspole. Joint was not square. No problems with compression of the bamboo from the bolt. There was minimal wiggling in the joint, but when I applied pressure the screw of the hanger-bolt pulled out of the dowel, due to its split.

The non-squareness of the joint was likely due to the difficulty I had centering the hole for the screw-side of the hanger-bolt, and making the hole perpendicular. Also, I did not perfect the curvature of the crosspole cut to fit the main pole. The splitting dowel was encouraged because of the difficulty getting a snug fit between the dowel and the bamboo cavity.

Also, screwing in the hanger-bolt is challenging.

Mortise and Tenon Joint
Fitting the mortise-and-tenon to each other was easily done with a dremel. Getting a tight fit was not difficult (the hanger bolt pulls the parts together in the other joint, in this joint this must be done manually). Pins held without glue. The resulting joint was square with minimal wiggling. However, when force was applied, the wiggling gradually increased. Eventually, pins cracked, one failed entirely, and the tenon tail split off.

Conclusions
I like the mortise-and-tenon joint, despite its failures. 1/4" ply and larger diameter pins (bamboo chopsticks? wood dowels?) will improve strength. Epoxy in the joint will not only give adhesion, put will also fill gaps and minimize the wiggling. Triangulating joints will also minimize wiggling. If wiggling can be eliminated, the progressive growth in wiggling should also disappear.

Most important, of course, was the ease with which I was able to get a good-looking joint. Precision is not a strength of mine, so any method that doesn't require me to hold my drill perfectly vertical is a better method.

Bamboo Joinery 3

Here is an alternative joinery technique I just found on the web. It has a number of advantages over the technique I planned in the previous post:

• It doesn't rely on glue anywhere
• It doesn't use expensive hardware (hanger bolts, at 50¢ each, add quickly)
• It doesn't compress the bamboo as a bolt does, something the literature says is not good for bamboo.
• It doesn't require one to make a precise fit between a dowel and the bamboo interior.

On the other hand, it does require one to make a careful joining tenon, to drill the bamboo four times rather than one (which has proven tricky for me), and to fit pins precisely. Here it is:

1. Use a dremel cutting wheel or similar tool to cut a slot in the main pole, the dimensions of the tenon you will make. Make another cut in the end of the cross-pole the length of the tenon.
2. Make a tenon out of plywood the thickness of the slots (mortises) you have just cut. I show above a triangular tenon, which would give more support for bending of the cross-pole, but requires the slot on the main pole to be slightly longer than the joint, leaving it visible. A rectangular tenon, the width of cross-pole, would not have this defect. Insert this tenon into the two mortises and close the joint.
3. Drill four holes through the bamboo and the tenon, making sure that the joint is very tightly closed as you do so. Two holes lengthwise on the crosspole, and two lengthwise on the main pole.
4. Cut four pins and insert into the holes. You may use a weatherproof glue to hold the pins in place. This glue does not hold the joint: no force applied to the joint is resisted by the glue, but it does prevent the pins, that hold the joint, from working loose. What to use for the pins? Smaller-diameter bamboo is recommended. In some applications, bamboo skewers from grocery aisles are all that is needed, and have the advantage of being a consistent size. I think they are too small for my purposes, however.

I neglected to include in this illustration that you ought to first cut the crosspole end into the shape of the bamboo mainpole (see Bamboo Joinery 2, below).

Bamboo Joinery 2

Making a T joint:

1. Cut the end of the side pole into a curve closely fitting the cross-pole, using a coping saw.
2. Take a 1" length of a hardwood dowel that fits snugly into the bamboo cavity, apply a strong, weatherproof glue such as Weldbond Outdoor wood glue, and insert into the end of the side pole.
3. Bore a hole into the dowel in the side pole (use a bit the diameter of the shank of your hanger bolt screw-side) and another through the side of the cross-pole (use a larger bit, the diameter of the threads of the hanger bolt bolt-side).
4. Drive the screw end of the hanger bolt into the dowel, and insert the bolt end into the cross-pole.
5. Secure the bolt with a washer and nut.

Bamboo Joinery

The official way I read about doing bamboo joinery is with bins and hooks and bolts. Say, to make a T-joint, you would put a pin through the stem of the T, bend one end of a threaded rod into a hook, insert the hook into the end of the stem and hook the pin, then put the other end of the threaded rod through a hole bored in the cross bar and secure with a nut.



The problem is, my bamboo is so narrow there is no hook can be inserted into the hollow.

My solution: cut 1" pieces of 1/2" dowel (or other size that fits snuggly inside the bamboo cavity. Glue one piece into each end of the bamboo with a strong, weather-proof adhesive, such as Weldbond's outdoor wood glue. Bore a hole into the butt end of the dowel, and insert a hanger bolt (one of those bolts with a screw on one end and the bolt on the other. Bore a hole through the piece you want to join to it, put the bolt through, and secure with a nut.

This method should be equal to the official in sheer or compression. If pulled apart, the joint is dependent on the strength of the glue. I think I might use an epoxy between the two part, such as JB Weld, to help. This would make two distinct glue joints which each support each other: this chain is as strong as its strongest link.

Simplify

Okay, on second thought, the hole saw doesn't work. Spent $25 on a new hole saw the right size (1"), just rip the bamboo apart.

New approach: the coping saw. I use one of my successful hole saw bamboos as a template, trace it, and cut the next one out by hand. It goes really fast, doesn't require pre-drilling or extension cords, and NEVER splits the bamboo.

Working Bamboo

This is my insightful discovery: bamboo is different than wood.

Bamboo is very strong in terms of bending and load bearing. But let something sharp strike it across the grain, and WATCH OUT!

Bamboo handles like a bundle of silk threads running the length of the pole. It is prone to splitting.

• Don't use a speed bore on bamboo. The moment that spinning blade touches the bamboo, POW! split if not shatter.
• Be careful using a large-bore (3/16"+) drill bit. A very gentle touch works, but push too hard and it splits.
• Predrilling with small bore bits helps. Ironically, the spear point on a speed bore has never caused trouble for me, and I use it to pre-drill. Just have to stop before the blade touches.
• Use small-tooth saws. I used a mitre saw.
• I wouldn't dare nail it.

Don't know if this will work, or if I'm off on the wrong direction. We'll check in again.