Bending And Joining Tubes

You will use tubing for many jobs around the home; follow the correct techniques since mistakes can be hard to rectify.

Although plumbing installations are the one obvious example of where tubing has to be bent and joined, there are many other uses to which tubing can be put; these include towel rails, fence and guard rails, storage racks, garden furniture, bicycles and children’s swings.

Types of tubing

Steel, aluminium, brass and copper tubing will be held in stock by most metal suppliers, in a wide range of diameters and thicknesses; you can also buy rectangular section. Stock size diameters are 3-150mm (0.118-6in), each being available in a range of wall (metal) thicknesses. It is sometimes possible to obtain shaped tubing, such as fluted or spirally twisted, but these types are not easily bent and joined.

Square section in various metals and finishes, with patent fittings to make right-angle joints, is available for a variety of structures. With this type of section you require little more than a mallet and hacksaw to make items such as a frame for a coffee table or a hi-fl rack. You can also use plumbing fittings to make structures, but you may need pipe-threading tools to thread the ends of the pipes before assembly and the end product may not be very elegant. Copper tubing can be used with soldered capillary joints and fittings to make a towel rail, which you could connect to your hot water system.

Bending tubing

Small bore and light gauge tubing up to about 22mm (or tin) diameter can be bent by hand, but the most accurate and satisfactory bending method is to use a bending machine and bend around formers. Machines are available from hire shops; some can also be used to bend heavier gauge and larger diameter tubing.

Bend radius

If you are not using a bending machine with formers and you wish to bend a heavy gauge tube, the radius to which the tube can be bent can be measured from the centre of the bend arc to the axis of the tube. Bend radius depends on the diameter of the tube, its wall thickness and the bending method you employ. If the diameter of the tube is not more than eight times the wall thickness of the tube, the following guides to the bend radius apply:

If the tube is bent without internal support, the minimum radius (to the centre line of the tube) should not be less than four times the tube diameter. If the tube is supported internally with a filler, the minimum bend radius should not be less than one and a half times the tube diameter.

Large radius bends are easier to form, but tight bends will cause some thinning of the metal on the heel (or outside) of the bend; they will also increase resistance to flow, if the tubing is used to convey a fluid.

Bend allowance

The approximate length of tubing taken up in making a bend will depend upon the required angle of bend. It can be calculated from an approximate formula when you know the required radius and angle of bend; multiply the bend radius by the bend angle and divide by 60 (or more accurately 57.3). The bend allowance should be marked out on the work prior to heating and bending. It is never a good idea to cut pipe to its calculated final length before you make the bend, since errors in bending could leave you with a pipe which is too short. It is easier to trim the straight portion to length once the bending has been completed.

Annealing metal

Bending is easier if the metal is annealed beforehand. Mark out the bend allowance and heat this region with a blowtorch until the metal reaches a cherry red colour. Steel tubes should then be allowed to cool in the air, while you can cool brass and copper tubes by immersing them in water. You may find it easier to bend the metal while it is still hot; however manipulating red hot tubing can be difficult and dangerous and, if you have filled the tube with sand, your blowtorch may not be able to supply enough heat to raise the metal to annealing temperature. An alternative to a blowtorch is a gas ring; place a steel sheet over it and put the tube on this. Anneal aluminium by heating it in boiling water for five minutes, allowing it to cool in air; don’t heat aluminium directly.

Cutting tubes

Use a pipe cutter to cut copper tubing; other types of tube may be cut with a fine-tooth hacksaw. After cutting tubing, remove any burr from the inside of the tube with a rat’s-tail file and clean the outside of the cut with emery cloth.

Using filler Filler can be used in light gauge tubing to prevent it thinning while being bent. Pack the tube firmly with dry silver sand or resin and plug the ends to prevent losing the filler during bending.

Bending methods

When bending, the metal should be firmly supported around the area of the bend so it cannot expand sideways and cause the tube to kink. There are three basic bending methods.

Using a former

The simplest former you have is your knee; you can easily bend light gauge tubing around your knee, if the bend radius is not critical. (Copper tube made to British Standard 2871 should not be bent by this method since it tends to kink.) For more precise bending, use circular formers which are made to fit particular pipe diameters and to produce a range of bend radii. These are normally used with bending machines, although you may be able to improvise a circular former from a car fan pulley or from the drive pulley of an old spin dryer; you can then simply design all the bend radii to suit your former. If you can obtain one, an old stepped pulley will give a wider range of radii for bending.

Using a bending block

You can make this from a piece of ash, one of the toughest hardwoods. The timber should be about 1200mm (or 48in) long and about 50mm (2in) thick; drill holes 150mm (6in) from each end — one 16mm in diameter and the other about 20mm (or tin) Chamfer both holes on one side of the block. The tubing is inserted in the drilled holes and bent in stages.

For the best results, make a full size drawing of the bend required; at each stage of bending compare the tube with the template for accuracy. The tube should be marked with chalk at 20mm (or Sin) intervals and bent at each chalk mark before being compared with the template. This method is most suitable for mild steel tubing, although it may be used to bend softer materials, if an internal filler is used.

Using a bending spring

Small bore tubing may be bent without kinking, if you use a bending spring; again BS 2871 copper tube may kink on tight bends. The greased spring, the same size as the internal diameter of the tube, is inserted in the tube before it is bent by hand. The spring is removed after bending by twisting it until its diameter is reduced, so it may be slid out of the tube. The spring will come out more easily if you slightly overbend the tube then bring it back to the correct angle. Make sure you use the correct diameter spring for the tubing you wish to bend.

Using a bending machine

You can hire a standard pipe-bending machine to bend tubing of 15mm (or lin) and over. The machine consists of a former mounted on a bar and is held in a vice; a pivoted arm forces a roller over the tube, which is placed in the groove of the former. The machine is suitable for light gauge copper and aluminium tubing and should produce a curve without distorting the tube.

A hydraulic bender is used to bend heavier gauge and large diameter tubing. It comprises a hand-pumped ram which bends the tube as it is held between two pivoted ‘dollies’ (or grooved metal blocks). A former of the correct radius and groove size is placed along the centre of the tube, opposite the dollies. If hiring this machine, check the formers and dollies supplied are suitable for the job.

These machines can also be used to bend metal bars and angle and tee-section material; always grease the metal before bending it, since there is a tendency for the metal to stick in the former.

Joining tubing

Tubes may be joined in a variety of ways; but if rigidity is required, it will usually be necessary to weld or braze them. To join tubes of equal diameter, cut the end of one to a vee-notch and file the notch out so it fits neatly against the second tube; then weld or braze the joint together. Branch joints are suitable for both equal and unequal diameter pipes; a hollow is shaped into one pipe and the other pipe is shaped to fit it.

Tubes can be joined mechanically by making a spigot to slide inside the two tubes. Use a plumber’s steel drift to enlarge the diameters of the tubes, if this is necessary. Clamp the spigot and one of the tubes and drill through them both so you can insert a screw or rivet; then drill the second tube placed over the other end of the spigot. Alternatively you can make up a sleeve to pass over the joint instead of inserting the spigot.

A third method of joining tubing is to heat the metal and hammer it flat so a nut and bolt can be placed in the joint. Mark the centre of the joint carefully on each of the tubes; taking each tube in turn, heat to cherry red in the area of the punch mark and flatten with a hammer. You can then drill holes through the marked centres of each tube and join them with a nut and bolt. Take care not to crack the tubing when you hammer it flat; it is not necessary to flatten the tubes completely, as long as there is a flat surface for the nut and bolt to grip the metal without turning inside the work.

Warning

When working with tubing you may find it necessary to hold the work inside a vice; take care not to scratch or squeeze the tubing. A cloth placed over the tubing at the gripping point can help avoid this problem; alternatively you can groove two pieces of softwood with a chisel to accept your tube and use these as clamps.

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