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The ease with which material can be milled is dependant on several factors including tensile strength and abrasion resistance. These may be assessed from the hardness tests, material composition and known heat treatment. Whilst hardness and / or strength is the usual criterion, wide variations in machinability can exist among materials showing very similar physical properties.
The cutting conditions used can be dependant upon requirements for tool life and surface finish and further restrictions, such as, tool rigidity, workpiece, lubrication and machine power available.
In general the harder the material the lower the cutting speed, but some materials of relatively low hardness contain abrasive constituents leading to rapid cutting edge wear at high speeds. Feed rates are governed by rigidity of set up, width of cut, i.e. volume of metal removed, surface finish required, and the available power. Conventional or climb milling can also effect the
life and finish from the cutter.
Taking the above into account, therefore no one set of speeds and feeds is necessarily correct for a given material. It is usually preferable to set and maintain a constant surface speed for a given material and vary the feed rate within defined limits to obtain the desired life and finish.
Machine feed is measured in mm per minute and is the product of RPM x the number of teeth in the cutter x feed per tooth.
All machine feed should be worked back to the recommended feed per tooth. Too light a cut may fail to penetrate work hardening materials and cause edge breakdown, too heavy feeds will cause chipping and excessive heat generation. Slender and long shanked cutters are restricted in feed rate due to deflection of the cutter, where ever possible the largest and most robust tool should be used. This is particularly important to material over 250 Hb. Over 300Hb then Cobalt HSSE-Co8 or carbide cutters should be used. For softer materials Cobalt & Carbide cutters may give increased output by increasing speeds and Feeds up to 50%.
Hints On Milling
As in most machining operations optimum performance is controlled by the following:
(1) Rigidity of set up, use the most robust tool. Where this cannot be done, feeds and speeds must be reduced. Avoid large overhang of shank cutters. Adequately clamp cutter for both movement and chatter.
(2) Speeds, feeds and lubrication, consult the charts and adapt to suit workshop conditions, no coolant requires a reduction to the speed by up to 50%.
(3) Tool maintenance. Check tool shanks, machine chucks and arbor for dirt. Scoring or damage before mounting the tool. Tools with these faults will run eccentrically giving poor finish, inaccuracy, and poor tool life.