14.02.2022

A gun drill mechanics model analysis
Gun drilling is a process used to produce small-diameter holes at a high depth-to-diameter ratio, beyond what is capable using conventional tooling, especially for difficult-to-machine superalloys such as titanium, Inconel, Monel, etc. Presently, it often incapacitates by low productivity, rapid tool wear, frequent tool breakages, and straightness deviation. This chapter addresses the challenges of tackling these problems by employing game-changing approaches and technologies. Existing research in the advanced gun drilling technologies tends to focus on the choice of drilling parameters. There is little literature available for the cutting mechanics and workpiece deformation, tool geometry, wear and failure mechanism, especially in the deep hole drilling process for superalloys. Consequently, the aim of this chapter is to provide an overview of how the game-changing approaches and technologies for advanced gun drill tool can be explored and utilized.
The drilling of deep holes by means of deep hole gun drill is investigated so as to establish the parameters of the guide hole and its production. The rate of change in the load on the tool is proportional to the active length of the primary cutting edge. Assessment of the smoothness of tool insertion on the basis of the rate of change in the load on the tool is proposed.
Referencing special energy model, a new gun drill mechanics model was established and the influence of the cutting parameters on axial force and torque was based on 15-5PH solid solution stainless steel through theoretical analysis and cutting tests. On the basis of the existing model, the model coefficients were modified under various cutting conditions and the integral area of the cutting edge was enlarged. The eccentricity at the bottom of the drill groove was taken into account in the integral area of the cutting edge. In the meanwhile, the torque calculation was simplified reasonably according to the theoretical analysis. The feasibility of the model was verified by experiments and the influence of cutting parameters on the axial force and torque was analyzed. The experiment data had shown that the relative error between the calculated values and the experimental values were within the acceptable range. The axial force and torque increased with the increase in cutting speed and feed rate.
Drilling mechanics model has always been the key and difficult point in the research field of solid carbide gun drill. In this paper, through theoretical analysis and processing experiments, the gun drilling mechanics model of Ti6Al4V titanium alloy is studied. On the one hand, based on the Oxley cutting model and the Johnson-Cook flow stress model, this paper takes Ti6Al4V titanium alloy as the research object and use the “microelement” method to establish the mechanical model of gun drilling, which includes cutting parameters, tool geometric parameters and material mechanical properties. On the other hand, the drilling model considers the influence of process damping and verified by experiments. The results show the calculated value of the model is consistent with the experimental value and the error is within the acceptable range. The model provides a theoretical basis for the prediction of drilling force, tool analysis and straightness error analysis.
The dynamics of the gun drilling process is analyzed in this paper. The tool shank is modeled as long straight beam vibrating in transverse direction under action of cutting forces. Axial force component is expressed as proportional to cutting thickness, which is determined as nonlinear function of beam transverse deflection with time delay. Nonlinear equations of motion of the drilling shank are derived. The stability diagram of the system dynamics was determined. The bifurcation analysis of nonlinear differential delay equations by means of multiple scale method was performed. The obtained results were verified by numerical integration of nonlinear equations. The influence of cutting conditions on system stability and chatter amplitude was observed.
Long straight holes are usually produced by means of a special drilling tool. This efficient process is widely used in the automotive industry to drill deep holes in cylinder heads, crankshafts, fuel pump housings, turbine blades and etc. Gundrilling is the mostly used method of deep small hole machining. Gun drill has asymmetrical single edge tool design with long straight tube of asymmetrical cross-section with a typical reachable diameter range of 0.5 mm up to 40 mm and length-to-diameter- ratios up to / = 400 (in special applications even / = 900 [1]). The method is widely applied in machining small deep holes as it provides a good straightness and high quality of machined surface due to its self-guiding action [2]. Optimal drill performance in gundrilling is achieved when the combination of the cutting speed, feed rate, tool geometry, carbide grade, and coolant parameters are selected properly depending upon the work material, deep-hole tool machine conditions, and the quality requirements to the drilled holes. Due to low flexural stiffness of gun drill shank lateral vibrations of high magnitude could be excited during the machining. Excessive vibrations are detrimental to finish surface quality and may damage the tool. Therefore, it is important to predict in advance regimes with chatter vibrations. The regenerative mechanism is the main source of chatter vibrations. And it requires that time-delayed terms in model equations should be taken into account. The same mechanism emerges not only in drilling [3-5], but in milling [6], boring etc. The comprehensive review of present state of deep hole drilling modeling was given in [1]. Most authors modeled drill shank using the reduced single degree of freedom system. In this insert gun drill is considered as flexible continuous beam loaded with eccentrically applied cutting force. The new approach allows considering the influence of lateral vibrations on the dynamics of the gun drilling system. The multiple scale method is applied for nonlinear vibrations analysis. Stability diagram was constructed and bifurcation diagrams were obtained by multi-scale expansion. The nonlinear behavior of system in vicinity of stability borders was analyzed by using numerical integration of nonlinear equation.
Gun drill shank is modeled as long slender beam using Euler-Bernoulli beam theory. The beam axial line is supposed to be straight line in undeformed state. Schematic of gun drilling process is presented on Fig. 1. Usually gun drill rests upon intermediate supports, but we suppose that clearance in these supports is large enough, so that these supports are not taken into consideration. Therefore, for the sake of simplicity we assume, that gun drilling shank as simply supported beam.
In this paper nonlinear model of chatter vibrations of gundrilling tool is presented. Stability diagram of the linearized system was obtained. The relative cutting speed is the main critical parameter which effects on process stability. Post-critical behavior after stability loss was analyzed by means of method of multiple scales. The bifurcation diagrams of the dimensionless cutting speed (speed of detail rotation) influence on chatter vibration amplitude were determined. The results of numerical simulation and the nonlinear equation asymptotic solution showed good agreement. Presented model could be used for stability analysis and for prediction of amplitudes of chatter in unstable zone. The results of modeling can be exploited in industry for optimal cutting conditions determination.
The IMSA MF1000C is equipped with an ISO40 spindle (13 kW, 6000 rpm) used both for drilling and for the milling. Gun drills are used for deep drilling and are guided along the entire length by steady rests.
One strong point is the "Swing on Top System" developed by IMSA, which enables a smooth transition from deep drilling to milling and vice versa in a short time. The entire drilling unit, including the chip box and the steady rests, swings upwards leaving the spindle free in order to perform the milling. The manual disassembly of components of the machine by the operator is not necessary. In the milling configuration, the spindle is positioned on the front part of the machining unit, toward the workpiece. From the five places in the tool store, the appropriate tool is loaded onto the spindle (cutter, twist drill, thread cutter,…) and processing can start immediately.
The MF1250/2FL does not require continuous attendance by an operator during operation, and it is capable of drilling many meters before the diamond gun drill needs to be re-sharpened. This gun drilling and milling machine is designed and manufactured to make moulds weighing up to 6 ton, with a diagonal of up to 1900 mm (diameter in rotation within the structure of the machine). This model is equipped with a rotary/tilting table as a standard. This makes it possible to drill complex cooling circuits with compound angle drilling, thanks to the combination of table rotation 360,000 pos/rev and the ±22,5° tilting movement in infinite position.

The dual-spindle headstock configuration allows fully automatic switchover in a few seconds, with no intervention required by the operator. The MF1250/2FL indeed has two spindles: a gun drilling head and an ISO40 machining spindle, positioned on the same headstock above the gun drilling head. Both spindles are liquid-cooled.
The machine is equipped with an ISO tool changer magazine for 12 tools as a standard; however, magazines holding 24 to 40 tools are also available in option.
As regards axis positioning, the MF1250/2FL uses optical scales. The machine is increasingly used to perform pocket milling and precision guides, or for ejector drilling. The precision offered by a positioning system with optical scales for these tasks is significant.
Our EVO Series, that currently consists in 3 models, is the evolution of our top performing BB Series.
The MF1350EVO gun drilling and milling machine is designed and manufactured to drill moulds up to 12 tonnes, with a diagonal of up to 2.600 mm as diameter in rotation within the machine structure. The structure rigidity that comes from the vertical gantry column gives as a result the same high performance in drilling large diameters regardless of the vertical position.
Compound-angle deep holes are executed by combining the table rotation and the headstock tilting movement (from -20° to +20°). Long transversal movements increase the approach to workpiece, so that the workpiece is only set up once on the table centre.
The headstock accommodates the two spindles: gun drilling spindle and auxiliary spindle, now both liquid-cooled.
IMSA technical team performed a complete revision of the projects from our previous BB Series, with the new EVO Series as a result. The increased stresses during machining, caused by the higher-performance new spindles, are distributed on a machine structure that has been reengineered.
This high-tech concept includes, in particular, transmissions by planetary gear boxes, optical measuring systems, liquid-cooled spindle motors, oil management by inverter and cnc.

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