Makita HB350 Instruction Manual - Page 7

EN MATERIAL AND CUTTING SPEEDS • The ease with which material can be drilled is dependent on several factors including tensile strength and abrasion resistance. Whilst hardness and/or strength is the usual criterion, wide variations in machinability can exist among material showing similar physical properties. • The cutting conditions can be dependent upon requirements for tool life and surface finish and further restricted by the rigidity of the tool and work piece, lubrication, and machine power available. • The harder the material the lower the cutting speed. Some materials of low hardness contain abrasive constituents leading to rapid cutting-edge wear at high speeds. Feed rates are governed by rigidity of set up, volume of material to be removed, surface finish and available machine power. • It is preferable to set and maintain a constant surface speed (RPM) for a given material and vary the feed rate within defined limits. • Machine feed is measured in inches or millimeters per minute and is the product of RPM x number of teeth in the cutter x feed per tooth. Too light or excessively high feed rates will both cause premature cutter failure. Heavy feeds on hard materials will cause chipping of the cutting edge and excessive heat generation. • Slender and long shanked cutters are restricted in feed rate due to deflection, and wherever possible the largest and most robust tool must be used. This is important for harder materials. Steel up to 400 HB is the potential limit for conventional M2 HSS tools. Above 300 HB, cobalt alloy cutters should be considered for increased tool life. In softer grades of material, cobalt alloy cutters may give increased output by increasing speeds and feed rates by up to 50%. Tungsten Carbide cutters permit surface speeds and feed rates up to those for standard cutters.