When a plain band 
needs to be enlarged slightly it is often possible to achieve this through mechanical stretching. The number and variety of tools made to achieve this give some indication of how common a repair this is! Stretching is faster and less aggressive than the alternative of inserting an extra piece of metal and should always be considered.
One device uses an attachment for a rolling mill to redistribute the metal in such a way that the ring is made larger. If the shank is made thinner, the metal that use to be seen as “thickness” has gone somewhere. Where? It has been pushed outwards, or converted to length, making the ring slightly larger.
Another alternate is to anneal the ring, then slide it onto a lightly oiled steel ring mandrel. Tap the ring gently with a wooden, leather or plastic mallet as shown in figure 13.7a to stretch it. Because of the tapered nature of the mandrel it is important to take the ring off periodically, invert it, and repeat the process.
A variation on this that avoids the possibility of marring a ring with mallet blows is the use of a tool called the Schwaan Ring Stretcher. The tool consists of a tapered mandrel with a hollow core that has been split into four vertical segments (figure 13.7b). The ring is put onto this rod and a smaller solid steel taper is driven into the first unit with a mallet. This has the effect of stretching the petals (and therefore the ring), though there is no direct contact with the ring itself. This has to be done slowly with great care so that the ring doesn’t break.
Figure 2 shows a stretching machine developed by the Fisher Company in Pforzheim. The machine uses a internal bushing of the intended size to prevent the stretching process from going too far, and includes a copper ring that protects the jewelry piece from being damaged.
No matter how sophisticated the tool, any stretching will stress the metal and there is always a risk of the ring snapping. More than that, there is a limit to how far a ring can be stretched. As mentioned, it’s always worth considering but there are many cases where a more radical technique will be needed. Have your wedding ring safely enlarged by one of our professional jewelers.