Tham khảo tài liệu 'metal machining - theory and applications episode 2 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Process models 273 Fig. The effective radial depth of cut in milling concave and convex surfaces Fy with a helical end mill is always positive irrespective of up- or down-milling except for up-milling with a small effective radial depth of cut. Hence down-milling gives rise to undercut and up-milling to overcut unless the radial depth is small - in which case anyway the deflection is small. An additional factor of practical importance must be considered when end milling a curved surface. Other things being equal the deflection in milling a concave surface is greater than in milling a convex one. Figure shows two surfaces of constant curvature one concave one convex both being end milled to a radius rw by a cutter of radius R or diameter D by removing a radial depth dR. The effective radial depth of cut de as defined previously is greater than dR for the concave surface and less than dR for the convex one. According to equations for the same values off and dA the force and hence the tool deflection will be larger for milling the concave than for milling the convex surface. The size of this effect is conveniently estimated after introducing a radial depth ratio cr equal to de dR. From the geometry of Figure for a concave surface for a convex survace rw- dR 2 - rw- de 2 R2 - R - de 2 rw dR 2- rw de 2 R2- R - de 2 Hence de 2rw - dR for a concave surface cr ------ dR 2rw - D __ _ de 2rw dR for a convex surface c -------- dR 2rw D Since dR D cr 1 for a concave surface cr 1 for a convex surface and cr 1 for slotting dR D or for a flat surface rw tc . It often happens in practical operations that the radius of curvature rw decreases to the value of the end mill diameter D. Then the ratio cr can increase up to a value of around 2Ì4 Process selection improvement and control two. The consequent force change depends on the appropriate regression equation such as equation . Another way of explaining this effect is to note that the stock .
