Working With Tubular Frames

This article was first published in 2010.

Two articles that we have previously run on working with ultra lightweight tubular frame vehicles have proved to be very popular. In Building Ultra Light-Weight Tubular Frame Vehicles, Part 1 we covered spaceframe design, selecting the best tube diameter and wall thickness, joining tubes and finishing. In Building Ultra Light-Weight Tubular Frame Vehicles, Part 2 we looked at lots of tips including mounting ball bearings, making gussets, feeding loads into tubes and integrating threaded fasteners. This time we take a different tack, making use of a book published in 1943. Called ‘Welding in the Construction and Maintenance of Aircraft’, it is based on tube-frame structures, most often fuselages. The small book contains a wealth of hands-on practical tips for welding (gas welding is used in the book, but the same ideas apply to any type of welding), tube end treatments, controlling distortion and sizing tubes. It’s one of the best books on the topic I’ve ever seen, and builds directly on the previous articles.

Round Tube Strut Ends

Various types of ends are fitted to tubes so that they can be assembled by bolting. A number of these types are given in the following data. While there are many variations of the types shown here, these are a representative group. Whenever tubes are squeezed in a vice, copper jaws should be employed to avoid scratches.

The simplest form of end to place on a strut or radius rod is out  lined here.

This shows the tube squeezed-up in a vice.

This illustrates the same tube replaced in the vice and re-squeezed on one side.

This is repeated on the other side and the corners rounded off, a hole drilled, and the edges welded together.

A similar type of end is shown here, but this end is reinforced with a sleeve. It will be noticed that the end of the sleeve, which is inserted inside the radius rod, is cut to a fish-mouth or simple mitre of 45 degrees, the latter being preferable. This minimises stress con­centration at the end of the sleeve. The sleeve, which is 2.5 tube diameters long, may be placed inside or outside the member. When placed outside, the practice is to weld around the mitred or fish-tailed end of the sleeve, joining it to the tube.

A different type of end is illustrated here. A small pilot hole is first drilled in the tube and a larger one super-imposed. The section is then removed by cutting with a hacksaw.

A mandrel is then inserted into the slot, and this, when hammered down, forms a shoulder on each side of the tube.

The next operation is to squeeze the tube in the vice to flatten the end, giving the contour shown here.

The mandrel is then removed...

...and a piece of sheet shaped into a U is fitted into the slot.

This show the corners rounded and the edges welded.

The mandrel is shown here.

Marking Tubes for Cutting Sometimes difficulty is experienced when cutting the end of a lateral or diagonal tube to fit neatly on to a longitudinal member. A simple way to find the profile of the member is as follows. First, mount the tube, which has had the end cut square, on a similar type of tube to that to which it is required to be fitted, and at the correct angle. Now, take a piece of flat steel cut to the shape shown here, with an arrow head stamped 25 – 50mm from the point. By holding this guide (A) alongside the diagonal member (B) and moving it around that member while keeping the point in constant contact with the longitudinal member (C), it will be found that the arrow has traced the profile that will fit the tube (C). If the guide (A) is moved in a series of progressions and a pencil mark made of the position of the arrow each time, these points can be joined to give the exact profile.

Elliptical Tube Strut Ends

Here an end suitable for elliptical tubes is shown.

Two small holes are drilled in the body of the tube and hack- saw cuts are made from its edge down to the holes.

Two pieces of plate are in­serted into these slots, the plates being cut wider than the tube to permit a fillet weld to be made.

A plate bent into a U is inserted between the projecting plates and other plates are also cut and bent to continue round the outside. The three plates, partially welded, are shown here.

This diagram illustrates a simple end for stream­lined tubing. Tapered plates [note use of taper to avoid a stress concentration] are welded to each side and a U plate inserted between them. Finally, the edges are welded together. The U plate should be shaped to cover the entire top of the streamlined tube and be welded to it.

A yoked or forked end is shown here. This method can be used where a very wide yoke is necessary. The tube is cut and dressed square at the end and a sheet metal U is welded to the tube with a fillet weld. Reinforcing plates, cut to shape, are then tacked in position, heated to a dull red and forged around the assembly with a ball pein hammer. The plates are then welded to the tube and U plate.

Clamps

A commonly used clip is the "Fokker" type, which is illus­trated here. There are several variations of this clip, and they are very easy to make. Depending on the use to which they are put, sizes vary a good deal, but the simplest form consists of a small diameter tube, just large enough to give clearance to the bolt which will provide the clamping action, welded at right angles to the axis of a larger tube which will fit around the member of the fuselage. A saw cut in line with the axis of the larger tube then bisects the smaller tube at right angles to its axis, and parts the longer tube.

To facilitate placing and removal, the use of two bolts, one on either side of the larger tube, and the splitting of the unit into two separate pieces, is sometimes employed. If a long clip is necessary where, say, it is also fitted with a lug or lugs, then two or more bolt tubes are welded to equalise the strain along the clip.

Welding for Reduced Distortion

Fillet Welding Tube to Plate

The effect of procedure on a weld is illustrated by a simple fillet weld uniting a piece of tubing to a piece of plate.

If the fillet weld is com­menced at point A and continued completely around the circumference of the tube, the general contraction of the weld will accumulate at A where, the weld finishes, with the result that the tube will have a tendency to lean in that direction.

If, however, the weld is started at A and continued to D, then recommenced at B and continued at C before the first weld has had a chance to cool and allow the major contraction to occur, it will be found that these two welds counteract each other.

The next weld should then be commenced at A and continued to C, and the final operation commenced at B and continued to D. These two latter welds do not affect the original position.

The stress under this procedure has been gathered to the points C and D, which counter each other, and there is not an accumulation of total stress at any one point.

Cluster joint

In welding a cluster joint as shown here, several factors need to be considered.

If the longitudinal member is welded to the cluster first, the total stress of the subsequent welds B and C will overcome the resistance of the cold longitudinal member with resultant distortion. If welds B and C are completed first, the contraction will not pull the unattached longitudinal out of alignment and the total contraction stress affecting that member will be that of welds A and D. If A and D are welded first, the stress against the longitudinal is the total contraction of A+B+C+D welds. If A and D are welded last, the stress on the longitudinal member is the contraction of A+D welds only, and this may be countered by contra-heating on the diametrically opposite side of the longitudinal before welding.

Again, if the weld joining the cluster to the longitudinal is commenced at X and continued around the joint, past W until X is reached again, X will have an accumulation of lateral weld contraction tending to cause more distortion at that point than at W. Any stress concentration should be avoided, particularly at the groin of a cluster where direction changes. This may be said of the other two welds also, if com­menced at U and V. In this case, the weld should be started at X and continued to a point halfway between U and V, then restarted at W and welded towards U and V, thus com­pleting the weld from X to W in two passes: this sequence should then be repeated on the opposite side. By this method, the original concentration of stress is divided at points midway between U and V on each side.

The correct sequence for welding the complete cluster is as follows:

1. Contra-heat at back of tube (ie use heat to distort the member such that when the welding is completed, it will be pulled straight again).

2. Weld from Y to U, then from Z to V.

3. If possible repeat these welds on other side of joint. If this is cumbersome, then weld X to P and W to P.

4. Repeat on other side.

Butt Welding of Tubes

When it is necessary to butt weld tubing, the procedure is different to those already described. Having aligned the tubing with the correct spacing between edges, a tack is placed on the seam and allowed to cool. The tube is rotated (if possible) and tacks are placed at three equidistant points around its circumference.

As there is no resistance, it will be found that the first tack will pull the tube out of alignment more so than the two succeeding tacks, as it draws the edges close together. The amount of contraction decreases with each of the two succeeding tacks, so that the seam is open widest at the third tack and is closest at the first tack. Welding should be commenced at this first tack and continued around the tube until the commence­ment of the weld is reached.

The tube will then be found to be free from distortion, as the contraction of the seam, when first tacked, is countered by the accumulation of contraction at the third tack when welded.

Right-angled Tubular Joint

In making a right-angled joint in tubing, it will be found that the outside corner (A) should be welded first, commencing on the acute point and welding to the neut­ral line (XX) on each side respectively. The inside corner is next welded, com­mencing in the groin and con­cluding at the neutral line XX on each side respectively. The major contractions are thus gathered at the neutral line on each side of the joint as well as being distributed in two places. They are least trouble­some at these points and little stress exists in the groin of the joint where the tension load is greatest.

Right-angled Tubular joint With Gusset

If a gusset plate is to be welded into the right-angled joint, it is advisable to cut the right-angled corner of the plate so that the flame need not approach a pocket, providing for plenty of escape for the secondary phase of combustion. Welding in such a pocket would cause flame instability with a deleterious effect on the weld, due to an excess of either gas. This should be avoided, if possible, by cutting the corner of the gusset as shown here.

If the fillet welds uniting the tube to the plate are commenced in the inside of the joint and proceeded with to the outside, a stress accumulation is formed at A and B, where members offer little resistance to distortion, with the result that there will be a permanent deformation.

The method which minimises this distortion is as follows : Commence the weld at the outside of the joint, A and B, so that contraction is accumulated at the points C and D, where the members offer great structural resistance and where stress is of less importance than at A and B.

Right-angled Joint With Diagonal Tube and Gusset

When a slotted tube is required to be fitted over the gusset, the procedure for welding the gusset to the tube is: the welds joining the slotted diagonal to the plate then made, working from the outside of the joint into the corner.

Buckling of Gussets

In many cases, the longitudinal shrinkage along the welded edges of the gusset causes the hypotenuse edge to become buckled. This is easily overcome by heating wedge-shaped portions one at a time and lightly tapping them down level with the flat face of a hammer while they are still hot. This consolidates the metal, and if carried over the "loose" areas will tighten them, thus removing the buckle.

Overall Welding Tips If contraction stresses must be concentrated, then con­centrate them at a neutral point in the joint where they will cause least trouble, viz., on the side of a cluster, NOT in the groin. Divide contraction stresses wherever possible, so that they are not gathered at any one point, but spread over several points. Weld in such a sequence that the total weld contraction is not directed against the member or members offering least structural resistance, but so that such contraction takes place before that member becomes part of the joint.