The Forney 71992 4-inch type 29 high Density Blue Zirconia flap disc cuts fast and outlast sanding discs 10 to 1. Great flexibility. Ideal for rough or fine finishing. High Density discs are ideal for the extra tough jobs. 4-Inch (10.16 cm) by 5/8-inch (15.88 mm) Arbor. ZA60 with max 15, 200 RPM. Typical spec example a24r-bf: a=used for steel, 24=grit (coarse), medium to hard BOND. The first series of letters indicates grain type; aluminum oxide (used in metal applications) Silicon carbide (Used for masonry applications) ZA= alumina zirconium used for (stainless steel and ferrous metal applications) ta=ceramic treated aluminum Oxide (used for metal applications) The second series of letters indicates grain size or grit; 8-24=coarse, 30-60=medium, 70-180=fine and 220-up=hard. The third Series of letters indicates BOND hardness; a-n=soft, o-s=medium and t-z=hard. The fourth Series of letters indicates type of BOND; bf=retinoid fiberglass reinforced (double bonded), e=shellac, rubber and v=vitrified. Wheel types; type 1=flat wheel for cutting, type Depressed center wheel, Type 28=hat shaped for grinding and type 29=flap disc for grinding and finishing. Note: harder wheel have a longer life where a softer wheel will cut faster. Fine grain=long life=slower cutting and grinding. Coarse grain=short life=fast cutting or grinding. Claims for warranty must be made within 90 days of the date of purchase. Proof of purchase required. This warranty does not cover any product that has been subject to misuse, neglect, negligence or accident, or that has been used in any way contrary to the purpose intended.