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HOME Milling Tools Watermelon Mill Taper Mill Tubing Mill Concave Cone Buster Mill Cement Mill Basic Calculation (RPM) (SFM) (IPR) (IPT) (IPM) (IPM) Machining Time Calculation |
Processes - Milling The process of milling is, at its most basic the removal of material by a spinning multi toothed tool to give a flat surface, channel, groove or hole in the work piece. There are two basic types of milling machine: vertical and horizontal. Both are similar in their holding and movement of the work piece, the difference lies in the positioning of the cutter. On a horizontal miller the tool is mounted horizontally, so its rotation is perpendicular to the ground. On a vertical miller the cutter is mounted vertically and hence its rotation is in the same plane as the ground. Of these two sorts of miller, the vertical miller is considered to be the more versatile, (and is probably more common) as it is capable of milling profiles. As with most machine tools, millers are available in a range of sizes and capabilities; from the small manually operated hobby type machines to the very expensive and specialised CNC controlled industrial units. The most common type is the vertical or universal mill.  DRO (Digital Read Out) fitted to increase speed and accuracy. The Basic Cutting Tool Unlike drill bits which are held in a universal Jacobs＊ chuck, milling cutters are held in collets. A collet has a tapered body at the end of a parallel section, which is female threaded at the opposite end.The tapered section is divided along its length into three equal segments. The threaded part of the collet screws onto the draw bar (which runs down the central length of the spindle) as the collet is pulled up into the spindle the tapered section engages into a matching taper in the spindle of the machine, and due to the longitudinal cuts closes up on the cutter which is located inside the hole 每 therefore holding it securely. Although likened to a pillar drill earlier, the cutters used in vertical milling are considerably different to standard drill bits. There are two main sorts of cutters commonly used for vertical milling, these are slot drills and end mills: Slot drills have two cutting edges similar to normal drill bits, but the cutting edges are unequal so that holes are cut across the centre. are shorter and the tips are square as opposed to being angled in relation to the length of the bit. Slot drills are used for cutting slots and holes, they have cutting edges that are unequal in length, this enables them to produce flat bottomed holes. End mills usually have more cutting edges than slot drills (typically 4 or 6) and the blades end square at the tip. End mills are used for machining flats and grooves, as the greater amount of cutting surfaces and flat end gives a better finish on the work. Other Cutting Tools As well as the commonly used slot drills and end mills, there is a range of other cutting tools that are used for tasks for which these are not suitable or less efficient. Face mills can be thought of as large diameter, shallow end mills; they are used to cut large flat surfaces on materials as due to their large diameter disk shape, give a more accurate and clean finish. If an oddly sized hole is required in a piece of work, a boring head can be used 每 this is basically a chuck that is capable of holding a range of different sized single point cutters, offset to the central axis of the miller spindle. This causes them to spin off centre - the distance of the cutter from the central axis can be finely adjusted, allowing it to create a hole of virtually any size. When using the boring head a hole must first be made in the required place with a conventional slot drill, the boring bar is then used to gradually increase the diameter of the hole until it reaches the desired size. Some different milling tools Watermelon Mill Taper Mill Tubing Mill Concave Cone Buster Mill Cement Mill Milling Speeds As with turning and to some extent drilling described earlier, both the speed at which the cutter spins and how quickly the work is fed into the cutter is critical to the preservation of the tools, quality of the work and general safety. As described earlier, slower speeds and feeds are used when cutting harder materials, and faster speeds are used on softer materials. Also in common with drilling, the larger the cutter being used, the slower it must be used. The use of correct lubrication while milling is especially important, as the removal of large amounts of material generates considerable heat and friction. Basic calculation for milling REVOLUTIONS PER MINUTE (RPM) SURFACE FEET PER MINUTE (SFM) INCHES PER REVOLUTION FEED RATE (IPR) INCHES PER TOOTH FEED RATE (IPT) INCHES PER MINUTE FEED RATE (IPM) INCHES PER MINUTE FEED RATE (IPM) Machining Time Calculation Workholding The basis of workholding on the milling machine is the table - if a large piece of work is to be machined, it can be bolted directly to the table using G-clamps or similar. If large cylindrical objects are to be machined along their length they can be held in V 每 blocks as well as being bolted to the table, to ensure that they do not move. If smaller work is to be machined it can be held in a machine vice which bolts into several "T" section slots running along the length of the table. To ensure that the work sits perfectly horizontal in the vice, a set of parallels is used 每 these are a pair of matching precision ground rectangular plates which are placed at each end of the vice. The work is placed onto the parallels and tapped down with a soft hammer to make sure it is fully seated. The vice is then fully tightened up and the parallels removed if necessary. A rotary table can be used if work needs to be milled accurately in a circular configuration 每this allows the piece to be held securely while it is rotated around a point by winding a handle either clockwise or anticlockwise to rotate it accordingly. An example of work machined on a rotary table can be seen in the photo gallery. |