CNC milling

Defining precision cutters

A very small cutter may not be included in a CAM parts library. MeshCAM requires definition of the overall length, shaft diameter, "flute diameter" (cutting tip diameter), flute length, taper angle, spindle speed

Glossary

  • 2FL - 2-flute, indicating the number of flutes on a cutter.
  • bleeder board - a type of spoilboard that allows air to flow ("bleed") to the surface of the stock.
  • ballnose mill - a cutter whose end forms a half-sphere shape when viewed from the side. Good for milling curved surfaces.
  • bullnose mill - see corner radius end mill
  • chipload - the thickness of the chip removed by a tooth each rotation. Chipload is proportional to cutting speed and flute count. Too large a chipload will result in a rough surface and possibly chips becoming jammed and breaking the cutter. Too little will result in chips being pressed under the tooth instead of shearing off, increasing friction on the tooth - "rubbing" or "burnishing". This will prevent cutting progress through the material, cause tool hardening, and eventually break the cutter. The recommended chipload for a tool varies by material. More on chipload phenomena.
  • climb milling - rotation of the spindle so that the teeth cut into the stock "from behind" (in effect moving more slowly relative to the surface). This is recommended to reduce wear on the spindle. Conventional milling has other advantages.
  • CNC - computer numerical control, also commonly refers to a CNC mill.
  • collet - an adjustible part of the mill used to grip the shank of the cutter.
  • computer numerical control - digital control of an industrial machine, whether factory-scale or desktop. A design is produced by a user via computer assisted design (CAD) and the series of tool operations is planned with computer assisted manufacturing (CAM). Human involvement in actual production is minimal, perhaps consisting of monitoring and changing tools. Output from CAM is commonly in the form of G-Code. CNC is used in subtractive technologies such as machining, and additive technologies such as 3D printing.
  • conventional milling - rotation of the spindle so that the teeth cut into the stock in a direction opposing the motion of the bit relative to the stock. This increases the chip load, but is recommended for a good finish. Contrast climb milling.
  • corner radius end mill - a cross between a ballnose mill and an end mill, this is a cutter whose end shows rounded corners when viewed in profile. The degree of rounding is determined by the radius of each corner. A radius of zero would result in a perfect end mill, while a radius of half the cutter diameter would result in a ballnose.
  • collet - the part of the rotating assembly that firmly grips the shank of a cutter or drill bit and forces it to rotate at the same speed as the machine's spindle.
  • cutter - a replaceable, rotating, metal tool end, sharpened at one end. Like a drill bit, but capable to cutting material by moving sideways. The cutter must be much stronger than the stock - carbide metals and high-speed steel are the usual choices for cutter material.
  • cutter diameter - the diameter of the cutting end of the cutter, including teeth. Contributes to the width of cut.
  • cutting speed - the speed at which the teeth move relative to the stock, averaged around the cutter (i.e., ignoring the effect of feedrate). This is the linearized equivalent of the spindle speed: π * spindle speed * cutter diameter (more on speeds). A smaller cutting tip will have lower cutting speed at the same spindle speed, which will limit how much material its teeth can cut in a given time. Since the cutter must clear material as fast as the feedrate delivers it (or else risk damage), a finer spindle requires either a proportionally greater spindle speed to compensate for reduced diameter, or a proportionally reduced feedrate. Optimizing cutting speed
  • deflection - bending of the cutting bit away from the intended path, generally due to resistance from the material. Minor deflection results in a cut along an unintended path. Excessive deflection in the tip will cause it to break.
  • depth of cut - how deep the cutter is below the local surface of the stock. The maximum depth is limited by the cutter's length of cut, how quickly it can remove swarf of a given material, and how much stress the cutting tip can withstand.
  • downcut - a cutter orientation where the flutes spiral so that swarf is pushed downwards and compacted in the bottom of the cut. This removes less material than upcut milling, but does not damage the surface finish. More on downcut bits
  • drill bit - the replaceable tool end held by a drill and used to make holes in a material. Only designed to cut along its axial direction. Mills use a cutter.
  • end mill - cutter, typically refers to a flat end mill when used without a modifier such as ballnose or corner radius.
  • feedrate - the speed at which the cutter advances through the stock, often expressed in inches per minute (IPM). A single pass of 5 inches at 10 IPM will take 30 seconds.
  • flat end mill - cutter whose end forms a profile with two right angles when viewed from the side. Good for milling indentations with right-angled corners.
  • flute - a helical valley on the cutter, through which chips of material cut off by the teeth are pushed up and out of the way. Cutters often have 2-4, but one can find flute counts from one to six. More flutes can remove more chips, but their size relative to the cutter must decrease as the count increases. A larger flute can remove bigger chips and is thus good for large-chip materials such as aluminum.
  • G code - a set of codes used in computer numerical control to control the operation of machines. Each line of code begins with a letter and number (generally G, preparatory commands, or M, miscellaneous) followed by other letter-value pairs defining parameters such as coordinates in various axes (X, Y, Z, A, B), feed rate (F), tool (T), or other (P). Most letters, though having meanings, are unused in practice. Though standards exist, the implementation of different commands differs among technologies and machine models. More on G-code
  • helix angle - a measure of how steeply the flutes and teeth ascend up a cutter while circling. An angle of 0 would correspond to flutes going straight along its length, while 90° would mean a circle around the cutter - neither would remove much swarf. The optimal helix angle typically increases as the stock becomes harder.
  • HSS - high speed steel, a common material in cutters. Not as strong as carbide.
  • IPM - inches per minute, a US measure of cutting speedfeedrate, and plunge rate.
  • LOC - length of cut
  • Length of cut - a cutter parameter describing how far the teeth and flutes ascend up the cutter from the tip. Since these structures weaken the cutter, the length of cut (LOC) tends to decrease with cutter diameter. The LOC places an upper limit on the depth of cut.
  • MRR - material removal rate
  • Material removal rate - the volume of material removed per minute by the cutter. MRR = Depth of Cut * Feed Rate * Width of Cut (courtesy of CNC etc)
  • OAL - overall length
  • Overall length - the total length of a cutter from shank to tip.
  • Plunge rate - the feedrate of a cutter along its axis - essentially, how quickly it is inserted into the stock like a drill.
  • rapid motion - motion at the greatest speed allowed by the CNC motors.
  • RPM - rotations per minute
  • Rotations per minute - A parameter of a mill or drill referring to how fast the spindle, and thus the cutter, can be rotated. For a low-end mill, the maximum value may be a few thousand, while high-speed spindles may reach into the many tens of thousands. Gre
  • Roughing - faster milling that removes a larger amount of material at the cost of leaving a rougher surface than finishing.
  • Runout - The side-to-side wobble of a mounted cutter being rotated. Runout results from imperfections in the spindle assembly, the collet, the way the cutter is mounted in the collet, and the cutter itself (not straight). Excessive runout is a major cause of cutter breakage. Much of the difficulty of precision milling is due to the decrease in acceptable runout with decreasing cutter diameter.
  • SC - solid carbide, refers to a common cutter material.
  • scalloping - the extra material left behind as a result of making multiple parallel passes through an area of stock material using a rounded cutter. The center of the cutter may pass at the same desired height each time, but the rounded ends will not reach as low, and material may thus remain behind, contributing to roughness. Scalloping can be reduced by decreasing the stepover, increasing cutter diameter, or polishing the surface after milling. More on scalloping
  • SE - square end, refers to a flat end mill.
  • SFM - surface feet per minute, a larger US measure than IPM, more common for larger tools.
  • shaft diameter - the diameter of a rotating part used to transmit torque; for a cutter, refers to the diameter of the shank.
  • shank - the part of the cutter that is held by the collet. For a fine-tipped cutter, the shank may be much thicker than the cutting end so that a standard collet is able to grip it.
  • spindle - the core of the mill that transmits rotation from the motor to the cutter; also used to refer to the whole rotating assembly. Spindle
  • spindle speed - the speed at which the cutter rotates, in rotations per minute. Together with the cutter diameter, it determines the cutting speed.
  • spoilboard - soft material (e.g. wood) placed under or around the stock material to hold it in place while letting the cutter operate around the edges of the stock without risk of collision with something harder, such as a vise or the platform.
  • stepover - the cutter moves in lines relative to the stock, and when it is not wide enough to remove a section in one pass, this programmed parameter determines how far it will move to the side on each successive pass. A stepover of 0 would correspond to repeating the same pass over and over. A small stepover results in greater overlap between neighboring cutter paths, reducing scalloping but increasing cutting time.
  • stock - the material from which a part is cut with a cutter. May be plastic, metal, wax, wood, composite, or any other material the mill can handle. Generally has a highly regular inner structure and precise edges, and is more expensive than one would expect if buying the material based on quantity alone.
  • swarf - the chips and debris removed from the stock by the teeth, which must be removed from the vicinity of the cutter to prevent clogging. Swarf
  • surface speed - see cutting speed.
  • tooth - the mountain crag to a flute's valley, this is the part that actually separates material from the stock. The tooth is the sharpened edge rather than the whole of the helical thread. If the thread between flutes is shaped like a narrow plateau rather than a mountain range, there can be two sharpened edges, and thus two teeth per flute. More teeth generally corresponds to faster cutting.
  • upcut - cutter orientation where the flutes spiral so that swarf is pushed up and out of the cut. This removes more material than downcut milling, allowing for use in more materials and tighter cuts. The top edges of surface finish may be left ragged. More on upcut bits
  • waste board - see spoilboard.
  • Width of cut - the width of material removed by a single pass of the cutter. It may differ slightly from the cutter diameter: runout will cause the cutter to encounter a wider cross-section, and chips may tear off additional material when being removed by teeth (both increasing width of cut); rubbing may force softer material past the teeth so that the width of cut is reduced.
Subpages (1): Nomad 883 Pro
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