Bit Dimensions and Definitions
Your CAM software needs to know some things about the bit to generate a toolpath. The amount of detail depends on the specific CAM software you are using. Things like the diameter of the bit are very important so it can work out how to cut out the shapes and contours you want. The shape of the bit is also important so it can calculate how deep it needs to be to achieve the desired contours, can it cut a vertical edge...
Shank or Shaft
The round piece at the top of the bit that goes into the collet; the collet size must match the shank diameter.
Overall Length
Length of the bit from top to tip.
Stickout (AKA Length Below Holder)
How far the bit tip sticks out from the end of the tool holder. The maximum length that the collet will clamp is 18mm, and the length that can go into the tool holder is ~25mm depending on the mounting. The stickout and the positioning of the spindle motor in the holder needs to be long enough to reach the bottom of the cut with the Z axis at its lowest and still to clear the top of the stock with the Z axis at its highest.
Keeping the stickout as short as necessary helps reduce flexing of the bit, increasing accuracy, reducing wear, and the chances of bit failure.
Shoulder Length
Especially in the case of small diameter bits, the shaft diameter may be larger than the cutting diameter. The shoulder length is the distance from the tip where this change starts. Effectively, this is the maximum depth a bit can go vertically into the stock material, not as far as it can cut effectively though.
Flute Length
This is the maximum height of the working range of the flutes from the tip of the bit. This is not the same as the length of the spirals from the tip as these normally continue up a bit more than they can effectively work.
Maximum Depth of Cut (DOC)
Simply how far down the bit can effectively cut; this is normally the smallest of the flute length and shoulder length for a straight-edged bit. If you are using a V-bit, for example, this would normally be the maximum depth at which it will cut a V before just starting to leave a round hole. DOC is also used to describe the distance the bit is actually instructed to cut in a single pass, but for a bit definition, it’s the maximum depth it can cut.
Flutes (AKA Teeth)
The number of cutting surfaces on the bit. Used in speed and calculations to determine things like how big the chips are going to be, and it will affect surface finish, speeds.....
Diameter
The diameter of the cut or slot the bit will make. Well, not so simple in practice. What is the diameter of a V Bit? It changes from the tip to the top! This depends for some bits on what CAM software you are using.
Corner Radius
Normally applicable to bull nose bits, the radius of the corner between the tip and the side of the bit.
Taper and Other Angles
Just angles, but depending on the type of bit and CAM software, the angles can be expressed in different ways! Normally a V bit angle is measured by the angle of the groove it will make from edge to edge, often referred to as the included angle. But for a tapered bit, it is normally measured between one edge and the vertical or the half angle.
Spirals
The spirals on a bit have a simple job: to remove the chips away from where it is cutting so the bit does not get clogged. Something like a V bit doesn’t have spirals, as the chips can just move away sideways or upwards by themselves.
But they may also come into contact with the side of the stock, so while they are not exactly cutting surfaces, they still rub and so have an effect.
Corncob
Where the spirals are shaped to provide extra cutting edges or surfaces to break up the chips more, or both. Think of a cob of corn but with sharp kernels arranged in spirals rather than straight lines.
Upcut, Downcut and Compression
This is all about the direction of the spirals on the edges of the bit. The edges of a straight bit will create friction from the rotating spirals, and the movement of the chips and direction of the spirals can cause splintering on the edges of the cut, especially on laminated materials such as plywood or veneers and others.
Upcut
The standard bit type, just like a drill, the spirals move the chips UP away from the cutting edge. If a bit description doesn’t mention any cut type, it’s probably an upcut. Can cause a bad finish at the top of the stock.
Downcut
The spirals move the chips DOWN towards the cutting edge, meaning that the action of the spirals on the edge of the material is downwards, which can help prevent splintering or the creation of a lip at the top. However, if you are cutting all the way through a material, it can transfer the splintering/lip to the bottom surface.
There is a major consideration when using these in that the chips still need to be moved away from the cutting edges to prevent clogging. Having them leave at a side is the only option, so don’t use them to plunge or drill into the stock!
Compression
Where you have a cut that using an upcut bit leaves the top ragged, but when using a downcut bit leaves the bottom ragged. Use a compression bit, upcut at the bottom and downcut at the top!
The disadvantage is that all the chips meet somewhere in the middle, hence the compression name, so make sure there is some way for them to get out. Normally, these will have a short upcut section at the bottom and a longer downcut section at the top rather than a meet-in-the-middle design, as this allows much more flexibility in machining different thicknesses of stock.
Material
What the bit is made of, not what you are cutting. A basic rule of thumb is that the bit must be a lot tougher than the material you are cutting, but there are a lot of other factors such as rigidity, wear resistance, and don’t forget cost.
I took a course in metallurgy a long time ago, but nowadays the nuances of the metal blend escape me.
HSS
High Speed Steel; this is pretty much a generic description for a modern steel blend designed mainly for high-speed tools.
Carbide / Tungsten Carbide
Carbide is a compound of carbon and a metal. Carbide tools are normally formed using tungsten as the metal. This has a very high wear resistance, keeping sharp for longer, but it is also expensive. Often a tool will be made from HSS with just the cutting surfaces made from carbide. Tungsten carbide is harder, stiffer, and will wear less than HSS.
Coatings
A coating can be applied to the bit to enhance mainly its resistance to wear, reduce friction heat transfer, and improve finish quality.
Nano Blue Coat
“A high-performance nanograin (very small particles) carbide and silicon coating giving 2-3 times better tool life than a micrograin carbide coating.”
Titanium
A titanium nitride coating to enhance cutting performance and wear. This has a gold color.
Others
There are a myriad of other options including no coating; the differences between them are beyond my comprehension and best left to materials scientists and professional machinists!
Milling and Routing Terms
Not really about bits, but everything in CNC is related somehow! This may help if you are just starting out!
Splintering
Mainly for laminated materials, this refers to the action of the bit as it cuts, causing the top (or bottom) surfaces to separate slightly, resulting in splinters on the surface. Other materials can do this as well; normally hardwoods and softwoods can have ‘strings’ left on the edges of a cut, which is basically the same.
Climb vs Conventional Milling
Any router bit will be turning clockwise; that is traditional, and small router controllers will only turn the motor in a single direction, which is clockwise. In very simple terms, the bit is being moved into the stock at a constant speed while rotating clockwise. Each flute will carve out a chip from the stock as shown by the dotted line due to the movement and rotation of the flute as it turns and moves.
Conventional Milling
The chip will start off small and get thicker as the bit moves and rotates.
Climb Milling
The chip will start out thicker and get smaller as the bit moves and rotates. Climb milling will normally leave a cleaner finish with less splintering at the top than conventional milling.
As far as I can tell, climb milling gets its name as the bit will try to climb out of the stock unless firmly held, but please don’t quote me on that.
Spindle Speed
The rotational speed of the bit in revolutions per minute, often just referred to as speed.
Spindle Power
The power of the spindle motor, measured in horsepower (HP) or watts. The ‘standard’ power of the ‘standard’ 775 spindle motor on a 3018 is around 60W, or 0.06Kw, or 0.08HP. (Think more of hamster power!) Larger routers may have more powerful motors; 300W is a common option, and many routers have the option to fit much more powerful motors.
Feed Rate
The speed at which the spindle is being moved into or across the stock. The faster you move the bit through the material, the quicker it will cut, but it will also put more stress on the bit and the machine. If you exceed the feed rate that your router is capable of for a cut, then the stepper motor will skip or lose steps, or the spindle motor may stall, ruining your work. Often just referred to as feed.
Max Feed
The maximum achievable feed rate for the router. This is normally the feed rate the router is capable of moving the bit without any cutting load, not the rate at which it will cut! For a Grbl-based machine, this is held in the settings as $110-112 for the X, Y, and Z axes respectively.
Passes
How many times the cut is repeated at a slightly greater depth. If you are using a DOC of 1mm to cut to a depth of 5mm, you will need 5 passes, each one 1mm lower than the last. You may also see a finishing pass mentioned; this is where the final pass is run with a lower DOC to give a better surface finish.
Feed per Revolution
Calculated value dependent on the speed (RPM) and feed. It’s how far the bit moves along the cutting path in a single revolution.
Deflection
Quite simply, as the bit is pushed through the stock, it will bend! Smaller diameter bits will bend more than bigger ones; the greater the stickout and the amount of material it is being asked to remove, the more it will bend. If the bit bends too much, then it will break!
If the bit is bending, the quality of the cut and surface it leaves behind will suffer. This is one of the main reasons that tapered bits exist; it leaves a small cutting area for better detail, but the shaft increases in diameter, reducing deflection.
Runout
If a bit rotated perfectly, then the runout would be zero. But it won’t be quite square in the holder; the spindle of the motor will not be perfectly mounted and straight, so when the bit rotates, there will be a bit of a wobble from side to side; this distance is the runout. Small values are good; if it gets too large, then the cut will suffer.
Stepover
The distance the bit is moved sideways between each cut or line. This can be expressed as either an absolute distance or a percentage of the bit diameter depending on your CAM software. 0 would mean that the bit would just retrace the previous cut; anything over the bit diameter or 100% would mean that ridges would be left between each cut.
The greater the stepover, the more material will be removed on each cut, but at the cost of a lower surface finish.
Stepdown (AKA Depth of Cut)
When cutting, the distance the bit is moved down for each pass. The greater the stepdown, the faster a cut will be made as it will take fewer passes, but it will increase the stresses on the bit and the machine. These are small machines, so use small values to start; 1mm is quite massive on these small machines, but it depends on what you are doing.
Lead In and Out
The way the bit is moved into and out of the cut; the options, if any, you have depend on your CAM software. Some CAM software allows a separate spindle speed and feed to be specified for lead-in and lead-out operations.
Ramp
Where the router gradually applies the stepdown as a ramp rather than plunging and then cutting horizontally.
Plunge
Moving straight down or plunging into the stock. Normally used as a lead-in/out method, though it could apply to drilling. This is the most aggressive lead-in method. NOTE: Do not use with compression or downcut bits! They move the chips down, and so when plunging, leave them with no way to leave the cut and will cause the bit to clog, overheat...!
Spiral and Helical
Other lead-in methods; one will lower the bit in a spiral ramp, while the other will use a helical ramp. These terms can also be used to describe the toolpath used.
Sawdust
What you get when the chips are too fine; basically, they stop being recognizable chips and become sawdust. You can reduce this by slowing the RPM and/or increasing the feed rate so that the bit is cutting larger chips.
Cutting too small chips can greatly increase the heat generated by friction between the bit and the stock, as the chips are supposed to carry a lot of the heat away. This can be especially important when cutting materials such as plastics with a low melting point, where it’s not so much sawdust as the bit looking like the center of a candyfloss machine due to the material melting and sticking.
Chatter
What you get when the chips are too large. Called this as the machine is overstressed trying to push the bit; it starts to make chattering noises and gives an uneven surface finish.
This can be reduced by increasing the RPM, decreasing the feed rate, or possibly decreasing the depth of cut. It is advised to only change one at a time and see what happens.
Surface Speed (AKA Cutting Speed)
This is just the speed that a single flute at the circumference of the bit travels across the surface of the stock. Surface speed is also a measure of the heat generated by friction, which can be especially annoying when cutting materials with a low melting point like plastics.
Chip Load (AKA Feed per Tooth)
This is a measure of the thickness of the chip that each flute will carve out of the stock. Chip load does not depend on the diameter of the bit, but larger bits can normally handle a larger chip load simply due to the fact that the flutes will be larger and the bit more rigid. If the chip load is bigger than the size of the flutes, you will have problems.
Material Removal Rate (MRR)
This is the rate at which material is removed from the stock. Higher MRR values mean you are going to get rid of the waste material faster.
Materials
Wood
A softwood comes (mostly) from evergreen trees (such as Pine, Fir, Spruce….) A hardwood comes from a deciduous tree (mostly). It’s far more complex than that. But just remember that balsa, the softest of all woods, is actually a hardwood! However, the internal structure of the wood is different between soft and hard woods. I hope that helps you as much as it helped me!
Janka Scale
There is a test for the physical hardness of wood called the Janka scale. It measures the hardness by taking a steel ball 11.28mm in diameter and measuring the force needed to push it into the wood to ½ the diameter of the ball. Beware! The force can be measured using different units: pounds-force (lbf), kilograms-force (kgf), or Newtons. Just make sure if comparing woods that all the measurements use the same units!
Ebony takes 14,300N (3,220lbf), rosewood takes