A method aiming to reduce chatter in a rotary cutting tool comprising: providing an end mill (10, 110) including a generally cylindrical body having a cutting portion and a shank portion; a plurality of pairs of diametrically-opposed, symmetrical helical flutes (30, 32, 34) formed in the cutting portion of the body, where adjacent helical flutes (30, 32, 34) form an index angle (γ, ε) therebetween in each radial plane along the axial length of the flutes (30, 32, 34); and a plurality of peripheral cutting edges (40, 42, 44) formed along an intersection of a circumferential surface of the cutting portion of the body and a portion of an inner surface of a respective one of the helical flutes (30, 32, 34) facing in a direction of rotation of the body, each peripheral cutting edge (40, 42, 44) having a radial rake angle (α, δ, σ, ρ, λ, ψ, ω); forming the index angle (γ, ε) between a first pair of adjacent helical flutes (30, 32, 34) different from the index angle (γ, ε) between an other pair of adjacent helical flutes (30, 32, 34) in at least one of the radial planes, where the index angles (γ, ε) are non-ninety degree angles; characterised by forming the radial rake angle (α, δ, σ, ρ, λ, ψ, ω) of one peripheral cutting edge (40, 42, 44) of a first helical flute (30, 32, 34) different from the radial rake angle (α, δ, σ, ρ, λ, ψ, ω) of another peripheral cutting edge (40, 42, 44) of an adjacent helical flute (30, 32, 34), where in a cutting end view a) one of the radial rake angles (δ) is seven degrees positive and the other radial rake angle (α) is three degrees positive, or is zero degrees, or is seven degree negative, or b) one of the radial rake angles (δ, λ) is three degrees positive and the other radial rake angle (α, ω, σ) is eight degrees positive, or is nine degrees positive. providing an end mill (10, 110) including a generally cylindrical body having a cutting portion and a shank portion; a plurality of pairs of diametrically-opposed, symmetrical helical flutes (30, 32, 34) formed in the cutting portion of the body, where adjacent helical flutes (30, 32, 34) form an index angle (γ, ε) therebetween in each radial plane along the axial length of the flutes (30, 32, 34); and a plurality of peripheral cutting edges (40, 42, 44) formed along an intersection of a circumferential surface of the cutting portion of the body and a portion of an inner surface of a respective one of the helical flutes (30, 32, 34) facing in a direction of rotation of the body, each peripheral cutting edge (40, 42, 44) having a radial rake angle (α, δ, σ, ρ, λ, ψ, ω); forming the index angle (γ, ε) between a first pair of adjacent helical flutes (30, 32, 34) different from the index angle (γ, ε) between an other pair of adjacent helical flutes (30, 32, 34) in at least one of the radial planes, where the index angles (γ, ε) are non-ninety degree angles; characterised by forming the radial rake angle (α, δ, σ, ρ, λ, ψ, ω) of one peripheral cutting edge (40, 42, 44) of a first helical flute (30, 32, 34) different from the radial rake angle (α, δ, σ, ρ, λ, ψ, ω) of another peripheral cutting edge (40, 42, 44) of an adjacent helical flute (30, 32, 34), where in a cutting end view a) one of the radial rake angles (δ) is seven degrees positive and the other radial rake angle (α) is three degrees positive, or is zero degrees, or is seven degree negative, or b) one of the radial rake angles (δ, λ) is three degrees positive and the other radial rake angle (α, ω, σ) is eight degrees positive, or is nine degrees positive.