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its type/geometry, which relates to its intended use:
square endmills are used for cutting flat surfaces e.g., pocketing and contouring, and for removing material quickly (roughing).
ballnose endmills are typically used for milling 3D surfaces.
V-bits are used to carve variable-depth grooves based on a 2D feature.
several other types are covered below but may require specific CAM features.
the diameter of its shank (the part that goes in the collet/tool holder)
a large shank is better for reducing deflection.
the choice of shank diameter is constrained by the available collets.
the diameter of its cutting part
the most common sizes used on the Shapeoko are 1/4'' (6.35mm), 1/8'' (3.175mm), 1/16'' (1.5875mm), and 1/32'' (~0.8mm), and their metric cousins (6, 3, 2, and 1mm).
the smallest feature size in a design determines the smallest endmill diameter needed.
smaller endmills are more fragile and more sensitive to runout (more on this below).
the length of its cutting part (Length of cut / LOC), and its overall length (OAL).
a short LOC is better for stiffness, but obviously constrains the max depth of cut.
a long OAL provides better reach, at the expense of rigidity/deflection.
On the smallest diameter endmills, the cutting length is really short otherwise the tool would be extremely fragile and deflect too much
the number of flutes (number of cutting teeth)
see Feeds & speeds for the impact of the number of flutes.
fewer flutes are better for chip evacuation.
more flutes are better for stiffness and finish.
the material it is made of:
carbide is king these days for CNC milling, but it is brittle.
high speed steel (HSS) is cheaper and tougher, but more limited in speeds.
its coating, if any:
Generally, no coating is needed for cutting wood and plastics.
ZrN (Zirconium Nitride) coating is good for non-ferrous metals e.g., aluminium, brass, copper, titanium.
AlTiN (Aluminium Titanium Nitride) coating is good for steel/ferrous-metals.
whether it is center cutting or not:
most are, it means they have the ability to plunge into the material (vertically), like a drill bit does.
non-center cutting tools are more commonly used on manual power tools, think router bits. It is possible to use them on a Shapeoko, but it requires a very careful CAM design.
the helix angle, if it's a spiral endmill:
depending on the helix angle, cutting forces will be oriented differently between the axial and radial directions: it's not really something you should worry about.
the direction of rotation
in practice virtually all endmills are designed for a clockwise tool rotation (as seen from above the cut)
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Typical router collet sizes are 4 mm, 6 mm, 8 mm, 1/8'', 3/16'', 1/4'', 3/8'', so the range of available sizes is limited, especially toward the larger shank diameters. Spindle users have access to specialized collets such as the “ER”-style which has become an industry-standard, that are is available in many more sizes:
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Using a collet adapter/reducer is generally not recommended as it tends to increase runout, but for most jobs, it will still work fine.
Below is a short overview of what runout is, but overall this is not something new users have to worry about.
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The end effect is that the movement of the endmill's tip in the material is the combination of the rotation along its own axis and other unintended deviations. Here's a very (very) exagerated exaggerated view of what happens when cutting a single slot with a lot of runout:
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the answer of course is "it depends" (on the nature of your projects)
the Shapeoko ships with a 1/4'' square endmill (#201 from Carbide 3D store), and the router ships with a 1/4'' collet: this is enough to get started and make a lot of beginner projects actually.
getting a couple of spare 1/4'' square endmills is a good idea: sooner or later, the original #201 will wear out (or chip, or even break in the event of a really big mistake)
the usual next step is to realize that 1/4'' is too large to cut small features: getting a couple of 1/8" square endmills will not go to waste anyway.
this comes with the need to get a 1/8'' collet, or at least a collet adapter.
if you intend to use 3D toolpaths and curvy surfaces, get a ballnose endmill (1/4'' or 1/8'' or smaller depending on the size/precision of your target projects)
get one V-bit: V-carving is quite easy and satisfying, you will probably want to try it, and it's a very common way to engrave text. They come in different shapes (angles), the most common ones are 60 degrees and 90 degrees. Make sure you invest in a good quality V-bit, it makes a big difference (while it is easier to get away with using cheap square endmills)
if you intend to cut mostly plastics, do get an O-flute square endmill.
if you intend to cut mostly aluminiumaluminum, ZrN-coated endmills will help.
a surfacing bit is useful to reduce the time for surfacing your wasteboard, but honestly not required in the starter set, a 1/4'' endmill will do fine.
the other types are very specific, so unless you know you will need them for sure, they can wait.
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