13 th INTERNATIONAL SCIENTIFIC CONFERENCE ON PRODUCTION ENGINEERING –CIM2011 Croatian Association of Production Engineering, Zagreb 2011 THE PROBLEM OF TOOL SELECTION FOR MILLING LARGE INTERNAL THREADS Mladen Bošnjaković Dragomir Moškun Marko Jerković M.Sc. Mladen Bošnjaković, Slavonski Brod University of Applied Science, Dr. M. Budaka 1, 35000 Sl. Brod B.Sc. Dragomir Moškun; Đuro Đaković Strojna obrada d.o.o, Dr. M. Budaka 1, 35000 Slavonski Brod Marko Jerković ; Đuro Đaković Strojna obrada d.o.o., Dr. M. Budaka 1, 35000 Slavonski Brod Keywords: Thread-milling, tool selection, technological and economic constraints Abstract Internal thread-milling in large bores is becoming increasingly important alternative to tapping, especially in cases where the diameter and depth of threaded holes exceed the standard size of taps. Problems of choosing tools for thread milling on a CNC machining center are presented on a practical example according to the technological-economic constraints that arise in small batch production. The factors of choice of tools are featured in order to achieve satisfactory quality of threads with minimum tool costs. At the end, the obtained quality of machining is discussed as a result of of tool selection for thread-milling operation. 1. INTRODUCTION Thread milling in large holes is necessary when you are machining large work pieces that cannot be rotated on the lathe because of the size or asymmetrical shape (thread is not in the axis of rotation which would lead to imbalance and low- speed cutting), in the case of small batches for which are not worth ordering special taps or the power needed to drive large tap is greater than the power of the machine. The main prerequisite for thread milling is CNC machining center that has the possibility of moving the tool in helical path. In addition, the advantages of milling of large internal threads in relation to tapping are following: Complete operation in one clamping Threading of large diameters requires less power than threading by taps No upper limits to bore diameter Chips are short Creating threads in blind holes is possible without the need for machining relief groove Thread relief groove unnecessary One holder used for both internal and external threads Interchangeable inserts One tool used for both right hand and left hand thread Tooling inventory can be reduced to a minimum as small range of tooling covers a wide range of thread profiles Suitable for machining of materials range from soft, non-ferrous alloys to heat-treated steels, or tough alloys such inconel and titanium, where tap breakage often occurs Threads have a high surface finish Allows for correction of tool diameter and length Interrupted cuts easily machined A better thread quality in soft materials where taps normally tear the material Short machining time due to high cutting speed and rapid feed rates Small cutting forces allow machining of parts with thin walls Tolerance adjustments can be made using exact calculations No danger of chip jamming - better chip evacuation due to small milling chips Lower tool costs per work piece The same cutter can be used for many diameters, as long as the pitch is the same Ability to use the machine with less power Efficient repair of large internal threads (in-situ) using portable CNC machines with helical interpolation, etc. Enables dry machining No reversal of the spindle required Successful selection of thread milling tools involves the definition of thread milling operation by taking into account all factors that affect the final quality of the thread, to achieve minimum processing costs. Manufacturers today provide software tools that allow step by step input in order to obtain a machining program for previously selected tool.