Investigation of Forces and Contact Area for Modelling Turning Processes

This thesis is focused on an investigation of forces andcontact area for modelling turning processes. The primary taskin short term research has been to take a first steptowards animproved model of the cutting process. The ultimate goal is tofind predictive models and thereby reduce the number of cuttingtests necessary to perform before an“unknown”tool-material combination can be considered for machining.Today, lot of time and money is spent to gather appropriatecutting data and material parameters. This is, nevertheless,necessary to understand and characterise the fundamentalbehaviour of a workpiece, when it is machined. To describe the complicated nature of the deformationprocess in cutting, analytical shear plane models are oftenemployed. In this thesis, a review of some classic models ispresented. The validity of a proposed model can be confirmed ifthe values generated can be correlated to experimental results.A difficulty is often to find a method that gives the desiredexperimental parameters. An important source for information isthe samples generated through Quick Stop tests. A reliableQuick Stop Device (QSD) has therefore been developed and ispresented in the thesis. Since the focus for this investigation has been to propose atentative model for cutting forces in turning, a number offorce measurements have been performed. Two different steelgrades have been evaluated in orthogonal machining, AISI-1045and AISI-12L13. Cutting force measurements (feed- and maincutting force), were conducted with a high accuracydynamometer. To investigate contact area conditions, Quick Stopspecimens, chips and inserts were examined. The evaluation wasperformed both in an Optical Light Microscope and a ScanningElectron Microscope. Three different models can be used tocharacterise the contact area. According to the contactconditions these models refer to sticking friction, flow-zoneand sliding friction region. Results from model simulations were fitted to those ofexperimental cutting investigations in order to evaluate theaccuracy of the cutting force and contact area approach. Thetest results are also used as reference data for a proposedsimple tentative model presented in this thesis. The model isrestricted to orthogonal machining. The proposed model consistsof several steps. The first step is to determine the maincutting force Fc. Expressions that describe Fcas a function of the cutting velocity (vc) for different feed (f) values have been derived.To be able to evaluate the feed force, the contact length lchas first to be determined. Relationships where lcis expressed as a function of vcfor different feeds have been derived. Tocalculate the feed force, a formula for Ff/Fcas a function of lcis applied. Equations for determiningΛ, hchandΦ have also been derived. In short,the functions found are: Fc= f(vc; f), lc= f (vc; f), Ff= f(Fc; lc),Λ = f(lc; f) andΦ = f(vc; f). This research work shows that it is of utmost significance,in order to generate realistic values, that a predictive modelis supported by adequate reference data. It is also shown thatthe contact area is an important parameter that has to beconsidered in modelling. Keywords:Modelling, turning processes, cutting forces,contact length, Quick Stop Device, QSD, simulation programs,FEM, predictive models.