This is a vitrified grinding wheel. It has been trued about five times. It's almost like new. Set of 2. 200mm/8 inch metal grinding disc, Safe speed up to 3600 R.P.M. - Type: 8" Metal Grinding Disc - Size: 8" x 1" x 5/8" (200 x 25 x 15.88mm) - Maximum Safe Speed: 3600 R.P.M (38 M/S) 【Vitrified】 This is a manufacturing method for grinding wheels, where abrasive grains are mixed with a binder such as feldspar or clay and fired at a high temperature of 1000°C or higher. This firing process vitrifies the binder, resulting in a very hard wheel with excellent abrasive grain retention and shape stability, making it suitable for high-precision and long-life grinding (also known as ceramic grinding wheels).