Every single component of the Sherwood Router Table range has been engineered to perform and to never let you down. The insert rings are no exception.
Rather than taking the easy option and injection moulding simple plastic inserts that are brittle, prone to break if ever coming into contact with the bit and will eventually break - the Sherwood insert rings are milled from dense, stabilized phenolic.
High quality phenolic can be machined just like steel to mechanical engineering standards. The twist-lock design ensures perfect mating with the aluminium mounting plate to be perfectly flat with the surrounding surface. If it ever comes into contact with the bit, it will engage the cutting edge of the bit so that the bit won't be damaged and a nick will be taken from the ring, which can be bored out and used again in a larger size.
Blank insert rings are available so you can drill whatever size you need - from 1mm to 90mm. You can also load the ring into the mount plate and raise the bit through the ring to create a perfect zero-clearance insert - this is not commonly available in most other mount plate systems.
Also available as a set of four rings that are pre-drilled with 6.75, 12.9, 30.0 & 38.2mm holes - slightly bigger than standard sizes like 6.4mm (1/4in) 12.7mm (1/2in) etc found in router bits for maximum support close to the bit.
Sure these rings seem expensive compared to other brands but is indicative of the no-compromise attitude to developing the Sherwood Router Table Range.