Here I show how to make telescoping tube clamp joints which allow extremely strong, light, and adjustable length and rotation connection between two tubes.

Tubes can be made telescoping when one fits over the other with only small clearance. if you can squeeze the larger tube, you can make a very, very strong linkage which is able to be adjusted both in length, and in rotation. There are vast numbers of uses for this. The most familiar example is making bicycle seat height adjustable, but use your imagination and you'll have a dozen uses within minutes.

I'm using these telescoping clamps to make rigid but removable side walls for a lightweight bicycle trailer that will carry my german shepherd when I want to take him farther distances than he can run alongside me (I like to bike-tour). The trailer can then be reconfigured, when needed, as a flatbed, and as a rigid mounting platform for kinetic art and other whimsical engineering.

What you'll need

Two steel tubes of complementary size.

approx. 1/2" length of 1/2" square bar-stock steel per clamp

And endmill of the same diameter as the larger tube's outer diameter (and a milling machine to use it in).

A drill and thread tap suitable for the size screw you use to close the clamp (I used #8-32 tap and 0.140" drill)

A drill to enlarge the screw hole to a clearance diameter (I used 0.180" for #8-32).

An angle grinder with a thin cutting wheel (recommended) or hacksaw or bandsaw.

Silver Brazing Wire (I got "Harris Safety Silv-45" at my local welding shop. This should not be confused with silver-bearing electronics solder: if it's intended for electronics or use with an iron as opposed to a hot flame, it's not what we want.)

Brazing flux ("white brazing fluz")

An oxy-fuel torch, like oxypropane or oxyacetylene.

A sandblaster really helps clean the flux off after brazing, but isn't strictly necessary.

The plan

We're basically making a screw-tightenable belt buckle for the tubing.

We'll braze on a metal tab, then cut that tab in half changing the "O" cross section to a "C" cross section.

Threading one (half of the original) tab for a screw, and drilling the other (half of the original) tab for a clearance hole completes the locking clamp. The two halves can now be pulled together with a screw, clamping down on an inserted tube firmly.

This locking tube clamp can now be used as a part in larger assemblies. I use fillet brazing to attach these clamps to my bicycle trailer frame.

Step 1: Get Tubing That Fits Closely Together!

While you can make telescoping joints out of anything (and carbon fiber tubing is exquisite) I'm concerned with metal here.

Telescoping joints require tubing that is

smooth and round on the inner and outer surfaces, and
consistent measurements along the length

Because of these constraints, the cheapest kind of tubing won't work: it has a weld seam standing proud on inner or outer surfaces, the whole length. It would snag and jam. What we want instead is "Round Seamless" tubing, also often called "Drawn Over Mandrel" or "DOM" tubing, or "Aircraft Grade Tubing". It is more expensive, sometimes a couple of bucks per foot, because it has gone through additional steps to make it smooth inside and out, and to make it consistent. But it inspires projects just being there: It is glorious to behold. If you're not inspired by a length of thin-walled DOM tubing, something is wrong. Silky smooth, blemish free, strong, light, resilient, and straight as an arrow. So many uses.

What *Kind* of Metal to get:

I usually get "chromoly" steel, 4130 Chrome Molybdenum alloy; it is just great stuff. It is easily machined, can be hardened, is easily welded, and has fantastic yield strength and ductility well above other grades of steel (it can stretch or bend farther than other steels and still spring back to it's original form unscathed, while not being too brittle). Because of these attributes it is the go-to alloy for builders of lightweight strong frames, be they aircraft or bicycles or anything between (or beyond!). However, you need to educate yourself about working with it, since it can be hardened inadvertently by too fast cooling. Generally, let it cool as slowly as possible after getting it red hot.

NOTE: the lightness of frames achievable with this alloy does NOT come from any particular lightness of the alloy itself: It is just as dense as any other kind of steel. The reason why frames made with this alloy are lighter is that the alloy is stronger, so you can make tubes with thinner walls out of it, that use less metal.

What *Diameter* Tubing to get (and where to get it):

You're limited by what sizes of seamless tubing manufacturers already produce, so get a table of tubing sizes available, and start looking for sizes which closely fit: the inner diameter of the larger tube should be just a few thousandths of an inch larger than the outer diameter of the smaller tube. Too little of a gap, and it'll be super hard to insert; too much of a gap, and it'll not clamp as well, and the joint won't be very rigid either.

Amongst the wide array of 4130 seamless tubing sizes available at Aircraft Spruce, I have found that getting tubes in 1/8th inch outer diameter steps works great, when the outer tube's wall thickness is 0.058". This is because 2 of the wall thicknesses add up to 0.116", which leaves 0.009" gap around the next smaller 1/8th inch increment of tube. A little less gap would be preferable, but this works fine for my application.

The wall thickness of the innermost tube is completely up to you. It does not need to be of any particular wall thickness other than what your strength requirements dictate. I'm using 5/8" OD, 0.035" wall thickness, tubular posts for the walls of my trailer, for which the tube clamps are 3/4" 0.058" wall.

If you wanted to make a long telescoping metal arm, you could use numerous diameters all nesting together, provided they were in 1/8th inch increments with 0.058" wall, or other complementary pairings. They sell tubing in 1/8th inch increments all the way up to 2", so you could have a large number of linkages!

Step 2: Prepare the Clamp: Cut a ~1/2 Cube of Metal

The metal tab will be brazed onto the tube, drilled and tapped, and cut in half to make two tabs that can be drawn together by a screw.

The first step is to cut a chunk of bar stock off, leaving an approximately 1/2" cube. I used a bandsaw. Here, I've made ten of the blocks at once.

I tidied up the ends of the blocks in the mill, but that's purely aesthetic.

Step 3: Mill the End of the Bar-stock Piece So It Conforms to the Curvature of the Outer Tube

Ultimately we're going to silver solder this metal tab onto the tubing, and for that to work, the two pieces (the tab and the tube) need to conform almost perfectly to each other. (silver solder, when molten, will be drawn in between tightly fitting faces, but will not draw in to large irregular gaps caused by an imperfect fit.) Use an end-mill with the same diameter as the outer (clamp) tube, in my case 3/4", to make the bar-stock piece fit perfectly onto the tube.

Step 4: Drill the Tabs

Decide what size screw you are going to use to pull the tube tight, and drill holes in the tabs, perpendicular to the axis of the curved tab face . For the #8-32 screws I use, I drill a 0.140" hole. Drill the hole as close to the curved face of the tab as possible, so that the tension of the screw does not act on much of a lever-arm: we don't want to bend the tab, but to use it to apply tension to the tube wall.

Your drill will last a lot longer if you use a center-drill to start the holes. Your drills will also last a lot longer if you use coolant/lubricant.

We will thread only half of the hole, so don't be afraid to plunge that center-drill down to the shoulder, a little more than 0.25" (half of the 0.5" tab width). This half of the length of the hole, which will not be threaded, will be drilled larger later, to make clearance for the screw to pass.

Step 5: Cut the Clamping / Outer Tube to Length

I am using these telescoping clamps as post-holders for the walls of a bicycle trailer, and so only need about 2.5 inches total length for the clamp. Cut the tube clamps to whatever length you require, and clean up the ends with a de-burring tool or sander.

Step 6: Braze the Tabs to the Tube

Brazing is an intermediate temperature process, in which a lower-melting-point alloy is used to "hot glue" two pieces of metal together. It is preferable to welding in applications like this, because the base metal pieces will not change shape. That's critical for making nesting, close-fitting things like these telescoping tube joints, because if the metal changed shape or a weld puddle penetrated to the inner diameter of a tube, the tubes wouldn't fit together smoothly anymore. Brazing is also incredibly strong, approaching the strength of the base metal. While the principles of its use are similar to electronics solder, it differs in that it is much stronger, and it melts at a much higher temperature. Your metal will be red-hot when the braze is flowing.

To make a good braze joint, keep these things in mind:

molten braze metal flows under capillary tension, and so likes to flow into tight crevices, not big open gaps. so make your parts fit like a glove before brazing.
molten braze metal only sticks to clean metal, so clean all parts very well beforehand, and only braze shiny clean metal.
use flux on the parts where you want the metal to go, since otherwise, red-hot metal will quicky form an oxide layer in the heat, and no longer be clean enough for the braze to stick. Flux will also protect nearby metal from oxidation, so feel free to coat the whole area liberally.
get the metal hot enough, and pay attention to whether parts are heated evenly. A light, thin part will heat up much faster than a heavy thick part, so play the flame more over the heavy parts to equalize the temperature. Braze metal will flow towards heat, and this can be used to make it go where you want.

There are many kinds of alloys used for brazing. Some are more fluid, some are more viscous. Here, I'm using "silver brazing alloy" which is 45% silver content ("Safety-Silv 45" by Harris welding supplies). It flows like water once hot enough, which is what is needed to be drawn by capillary tension into the tight crevice between the tube and the tab. Silver bearing solder which is almost half-silver can be expensive -I think I paid $45 for this 1 troy-ounce container - but it takes only very small amounts to fill the gap. (I bought this 1 oz. coil of silver brazing wire about two years ago and have plenty left). For each tab, you'll need less than 1/2" length of the brazing wire.

Other brazing alloys exist, and bronze and brass are popular. Those are more often used for "fillet brazing" where a large fillet of metal is built up, like caulk, at the junction of two pieces.

Step 7: Cut a Strain Relief Line Just Below the Tab.

When the clamp is drawn closed, we don't want it to close more at the top of the tab than at the bottom of the tab (orienting terms here presume tube axis is vertical). That kind of closing, which I'll call "conical", would pinch the clamped tube in a small area, tending to crimp / dent it at lower clamping forces than it could withstand if we clamped over a larger area. Cutting the line shown frees the clamp to close without becoming conical.

Tip: don't cut too far! A cut line which severs one quarter to one third of the circumference , centered on the middle of the tab, is plenty. If you cut too far, the clamp will be weakened excessively.

Step 8: Cut the Tab in Half

Carefully use an angle grinder with a thin cutting wheel (< 1/8th inch wide), or a saw, to cut a groove down the middle of the tab, leaving approximately equal widths to either side. Don't cut through the backside of the tubing opposite the tab, but do make sure that you cut fully through the tab, and all the way down to the strain relief cut you made in the previous step, so the two cuts meet at a "T".

Step 9: Thread One Tab, Drill Out the Other

Drill one half of the tab out to a clearance diameter for your chosen screw; for #8-32 screws I'm using, I drill a 0.180" hole. To avoid accidentally drilling out the second half of the tab, be careful and prepared to stop with the drill as you feel it fall the short gap between the tabs. Alternatively, put a piece of sheetmetal into the slot between tabs, to stop the drill from happily over-penetrating into the part you want to thread.

Then, thread the other tab, guiding the thread tap through the clearance hole you just made to keep the two holes aligned.

Clean up any burrs, insert a screw, test-clamp the sliding tube, and you're done!

At this point, I fillet-braze these clamps onto my trailer frame, to hold the wall support posts.