Metal-working Tips

Tubing diameters - Internal and External

I tend to buy all my tubing from B&Q Wharehouse simply because they sell tubing and other stock metal shapes in sizes that are ideal for DIY projects. The only issue that I have found with them (apart from the hobbyist pricing) is that the tubing can be quite variable in internal diameter and wall thickness.

In theory the nominal wall thickness for much of the tubing should be 1mm but in practice it can vary in either direction. Likewise the external diameter can be a bit variable. The result is that sometimes I can buy an 8mm tube that will fit inside a 10mm tube without any drilling and sometimes I cannot.

For making tent poles you will generally want tubing with a wall thickness that is greater than 1mm. This allows you to drill out part of the tube to the requisite diameter and still have enough of the interior wall left to form a healthy lip that will act as a stop for the tube that will be inserted. If the wall is too thin you will have to create an internal stop yourself (see below).

Drilling tubing to an 'accurate' diameter

This is a little tricky! The tubing that I buy might have a nominal internal diameter of 8mm. In reality it might be off by 0.1 or o.2mm in either direction. That is enough to prevent tubes nesting. It then needs to be drilled out using an 8mm drill.

The tubing needs to be clamped in a V-clamp on a drill-press and then drilled out to 8mm by use of careful alignment of the tubing and the drill and by suitable lubrication.

If the tube is clamped too tightly it distorts and tube-wall thickness is lost. If it is clamped too loosely it spins.

Drilling tubing to wider than the nominal drill diameter

To make a good tent pole requires that the nested pole fit loosely but not too loosely. One way to do this is to find an imperial drill size that is mid-way between two metric drill-sizes. Often this is not practical. Instead the way that I use is outlined below.

I am only using a cheap drill press and I am aligning everything by eye. In reality this means that it is almost impossible for me to perfectly align everything. This means that the first drilling of the tube will quite likely be a fraction of a mm off-centre.

What I do is drill to the chosen depth a couple of times - to make a clean cut. I then rotate the tube in the V-clamp by 45 degrees along the axis. This means that the tube will now be slightly off-centre but in a different direction. I re-drill in this position. I repeat this twice more until the tube has been drilled out 4 times with the tubing at 90,180,270 and 0 degrees offset to the start. The result is a hole that is about 0.2mm wider in diameter than the nominal drill size. This is just about perfect for providing a loose fit that is not too loose. It is well worth the extra effort.

Fitting internal stops to tubing

If the tubing wall is relatively thin it may well be that drilling the tube will not provide a sufficient lip on the remaing internal wall to form a good stop. This manifests itself as the thinner tube jamming when inserted into the thicker tube. The way to prevent this is to insert an internal stop. The way to do this is as follows:

For this example I will assume that the larger tube is 10mm and the smaller tube is 8mm and that you have drilled the larger 10mm tube to a depth of 5cm such that for this depth the internal diameter is around 8.2mm.

1. Cut a 5mm slice off some scrap 8mm tubing
2. Insert into the 10mm tubing
3. Tap into place using a (scarp) piece of 8mm tubing until the 5mm slice jams.
4. If the fit is very loose you may need to coate the 5mm slice with some araldite first but then you MUST clean the interior of the tube to remove all surplus glue. You can do this by redrilling once the glue has set or by applying heat to the tube to burn the glue out.