2010 Full-Line Drill Catalog - Desanto

Machining technologyDry machining and minimal quantity lubrication (MQL)Dry machining and minimal quantity lubrication (MQL) areimportant current technologies with the aim of reducingproduction costs. Guhring has invested heavily in thesetechnologies and developed tools as well as tool holderswith optimal geometries for this type of machining. Anobservation of the thermal conditions at the tool and theworkpiece was therefore extremely important.Basic observationsBecause with dry and MQL machining, any generated heatis not dissipated via coolant like with conventional wetmachining, the design of the optimised tool must ensurethat(i.e. through sharp cutting edges and a positive rake anglewhilst increasing the cutting parameters), the leading margins in comparison to the wet tool andincreasing the back taper of the tool),heat insulating hard coatings and polished tool surfaces toreduce the friction between chip and face),through improved chip evacuation from the hole or fromthe workpiece surface respectively).Influence of rake angle on temperatureThe thermal imaging of the tool point show a clearconnection between the rake angle and the heat generated.A positive rake angle resulted in a clearly lower temperaturebeing generated in the shear zone of the chip, because witha tool with a 30º helix the chip only requires deflecting by60º (reduced shearing action), whilst the chip deflection for astraight-fluted tool is 90º (increased shearing action).The heat generated in the shear zone directly enters theprocess as cutting heat. A shorter chip transfers less frictionalheat to the tool due to a smaller contact area on the flutesurface resulting in improved temperature conditionsIn addition, the chipflow was recorded using a high-speedcamera. With the cutting parameters v c = 300 m/min and f =0.35 mm/rev., distinct differences were apparent regardingthe chip evacuation and the process heat. Chip evacuation,i.e. the continuous transportation of chips from the hole,improved when the helix angle of the tool was increased.This is primarily due to a positive geometry and the resultingimproved chip fracture, providing a shorter shearing chipthat, due to its improved surface-volume-relationship, canbe evacuated from the hole with greater ease and is lessprone to jamming in the flute.TechnicalTo examine this parameter, Guhring produced three drillingtest tools in 10 mm diameter for a drilling depth of 100mm. The tools were geometrically the same, however, thetools had different spirals and subsequently different rakeangles. The test tools had 0º (i.e. straight-fluted), 15º and 30º0° rake anglefentrypoint of drill15° rake angle30° rake angleD = 10 mm, AlSi7v c = 300 m/min,f u = 0.35 mm°C555045403530252015rake angles. The internal coolantduct diameter of the tools wasidentical.Using a thermal imaging camera,the heat generated during themachining of a hole in Al-alloyAlSi7 was taken in real time anddocumented. The sheets appliedfor the test had a thickness of14.00 mm and were drilled onthe face so that the remainingresidual wall between the holeand thermo-graphic examinationof the sheet surface was 2.00 mm.Using the above test layout it waspossible to make a qualitativeanalysis of the heat generated bythe individual tools.Thanks to considerably improved chip evacuation andcomparatively lower process temperatures, spiral-flutedtools play an important role in the increased processreliability in dry machining and MQL applications.However, the application of straight-fluted drills can be ofadvantage for the machining of aluminium and cast ironmaterials, where the demand for hole quality (improvedroundness and reduced run-out) is high. This is becausestraight-fluted tools generally possess four leading margins.In addition, the temperature profile of straight-fluted drillingtools can be reduced by an optimised, geometric design ofthe coolant ducts to an extent that the thermal disadvantagein comparison to spiral drilling tools is compensated to alarge degree.308