Abstract
In this paper, contact conditions between a tool and a workpiece material for wear-simulating turning by a cutter with a sharp-cornered edge and with a rounded cutting edge are analysed. The results of the experimental study of specific contact load distribution over the artificial flank wear-land of the cutter in free orthogonal turning of the disk from titanium alloy (Ti6Al2Mo2Cr), ductile (63Cu) and brittle (57Cu1Al3Mn) brasses are described. Investigations were carried out by the method of ‘split cutter’ and by the method of the artificial flank-land of variable width. The experiments with a variable feed rate and a cutting speed show that in titanium alloy machining with a sharp-cornered cutting edge the highest normal contact load (σh max = 3400…2200 MPa) is observed immediately at the cutting edge, and the curve has a horizontal region with the length of 0.2… 0.6 mm. At a distance from the cutting edge, the value of specific normal contact load is dramatically reduced to 1100…500 MPa. The character of normal contact load for a rounded cutting edge is different -it is uniform, and its value is approximately 2 times smaller compared to machining with a sharp-cornered cutting edge. In author’s opinion it is connected with generation of a seizure zone in a chip formation region and explains the capacity of highly worn-out cutting tools for titanium alloys machining. The paper analyses the distribution of tangential contact loads over the flank land, which pattern differs considerably for machining with a sharp-cornered edge and with a rounded cutting edge.Abbreviation and symbols: m/s - meter per second (cutting speed v); mm/r - millimeter per revolution (feed rate f); MPa - mega Pascal (specific contact load as a stress σ or τ); hf - the width of the flank wear land (chamfer) of the cutting tool, flank wear land can be natural or artificial like the one in this paper [mm]; xh - distance from the cutting edge on the surface of the flank-land [mm]; σh - normal specific contact load on the flank land [MPa]; τh - tangential (shear) specific contact load on the flank land [MPa]; HSS - high speed steel (material of cutting tool); Py - radial component of cutting force [N]; Py r - radial component of cutting force on the rake face [N]; Pz - tangential component of cutting force [N]; γ - rake angle of the cutting tool [°]; α - clearance angle of the sharp cutting tool [°]; αh - clearance angle of the flank wear land [°]; ρ - rounding off radius of the cutting edge [mm]; b - width of the machined disk [mm].
Highlights
Cutting tool wear causes a change in its geometry – a wear-land with width hf forms on the flank surface, the cutting edge rounds with radius ρ, a wear crater forms on the tool face
During discontinuous chip formation at the moment when formed chip elements are separated from the workpiece, the radial component of the cutting force on face Py r quickly decreases [3], which leads to elastic recovery of the transient surface and its pressure upon the cutting tool flank surface
In machining of the ductile brass (63Cu), which forms continuous chip, the cutting edge rounding leads to pattern change of the contact loads distribution over the flank-land – it becomes uniform. It is explained by dragging of additional material under the rounded section of the cutting edge, which leads to a smaller influence of the transient surface sag
Summary
Cutting tool wear causes a change in its geometry – a wear-land with width hf forms on the flank surface, the cutting edge rounds with radius ρ, a wear crater forms on the tool face. Contact loads vary on the face and flank surfaces of the cutting tool, as well as the temperature in the chip formation zone. In machining of difficult-to-machine titanium alloys the width of wear-land hf reaches 2.5...5 mm, and cutting edge rounding radius reaches ρ = 0.3...5 mm [1, 2], but even with this big value, cutting tool is still capable of working, which is obscure. It is necessary to know distribution of contact loads and temperature over the surfaces of the cutting tool for estimation of cutting wedge strength at various stages of the wear process
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