Anatomy of an Endmill
There is a lot of detail involved in endmill anatomy, as seen in the diagram below. Not all are needed in order to properly select a tool, so won’t worry about each minor detail in the diagram.
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Flutes - These are the cutters on the endmill. Typically, 2 and 3 flute endmills are used for aluminum and other non-ferrous metals, while 4 flute endmills are used for steels and other harder materials.
Length of Cut - The length the endmill is able to cut. This is not equivalent to flute length.
Neck - This is the portion of the endmill that can rub on the material as the flutes cut during a deep cutting operation. It includes both the top portions of the flutes not suitable for cutting. A necked endmill has a portion of the length above the flutes at a smaller diameter to give it a deeper reach. Necked endmills aren’t the typical style, but they are an option if required.
Shank - Also known as the shoulder. This is the part of the endmill that is grabbed by the toolholder. It cannot cut and should never rub against material.
Coating & Material
Uncoated
Endmills for aluminum and other soft, nonferrous (no iron) metals are typically uncoated. Some drill bits the team has purchased from McMaster are uncoated. The tool will have no difference in color from shank to cutter.
TiN (Titanium Nitride)
Drill bits and other general purpose tools are usually coated in TiN. They can be used in a variety of metals, but are typically used in ferrous ones. The cutter will be a yellow-gold color.
AlTiN (Aluminum Titanium Nitride)
Endmills meant for steels (both carbon and stainless), other ferrous metals, and high temperature alloys like Inconel are usually coated in AlTiN. The color will be a deep gray/black.
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DO NOT use AlTiN when cutting Aluminum. The Al workpiece will react with the Al in the tool coating, sticking to it, covering up the cutting edge, and make the tool heat up and be very sad. |
Standard Mill Tools
Square / Flat Endmill
Flat endmills have an extremely sharp corner, with a radius of around 0.002”. These corners wear easily, so they should not be used when roughing out a lot of material. They should be used when a better surface finish on the bottom is desired, or a sharp corner is needed.
Although they aren’t ideal for roughing due to the tool wear, these are the endmills that the capstone shop stocks. When working in there it is fine to use these for roughing.
Bullnose / Radiused Endmill
Bullnose endmills are the workhorse of endmills. They can take away material without much tool wear, while also leaving a flat and unscalloped bottom. They have a radius between that of a flat and ball endmill.
Typical radiuses that are used for roughing are 0.015-0.030”, however there are many options depending on the desired outcome.
Ball Endmill
Ball endmills are used where a smooth contour is desired with the cutting direction involving both up and down in the z-axis. It can also be used to give a large radius to an internal corner. The end radius is equal to the radius of the endmill itself.
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Shellmill
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Shellmills are a desirable option for facing. They result in very smooth finishes and take away a lot of material in stepover while not taking away very much in stepdown. They usually have a few cutting inserts that can be flipped or replaced when one gets damaged or worn down. They also have a higher material removal rate (generally) than end mills.
Necked Endmill
Necked endmills allow you to cut further down than the length of the flutes. They are far less rigid than standard end mills, but more rigid than a tool with an equivalent flute length. The shank diameter of a necked end mill is less than the diameter of the cutter to avoid rubbing the part while cutting. They are generally a last resort, but you will likely need to use them.
Specialty Mill Tools
Keyseat Cutters
Keyseat tools are used to undercut some material, where you want to create some sort of internal groove that a typical endmill could not. These are usually purchased on an application specific basis, as the cutter diameter ratio to the neck and the cutter length will likely need to be pretty specific. These are usually used to cut grooves for retaining rings.
Threadmills
Threadmills are a form tool used to mill threads into parts, saving the effort of manually tapping parts.
Toolholding
ER-## Collets
These types of collets are the standard for toolholding in the MIE Capstone Shop. ER collets have slits on both their front and backside, which allows for a wider grip range than their 5C counterparts [see Collet Blocks in Mill Workholding]. The collets come in a few different external sizes shown by the two digit number in their name. ER-20 collets are sufficient for most jobs done in the MIE Capstone shop. If a tool with a shank diameter of >1/2” is needed, a larger collet such as an ER-32 might be used.
These collets are to be installed with a torque wrench. ER-20 holders usually take hex, and ER-32 holders usually take some sort of spanner.
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ER Collet Torque Specs:
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Drill Chucks
Drill chucks for CNC are virtually the same as any other drill chuck. They have three jaws which cinch inward as tightened. They are tightened with a spanner wrench or chuck key by hand. They are meant for drills, so use them as such.
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Never put an endmill in a drill chuck |
Rigidity of Tools
To create nice surface finishes and accurate parts, you need to consider the level of rigidity of your tools.
Things that affect your rigidity:
Length of your tool
The length of the tool relative to the diameter of the tool is generally the ratio you care about the most. Most end mills have a flute length between 3-5 times the diameter, meaning they can only cut that far down. Other options exist to take deeper cuts, such as necked end mills
Type of tool holder
The length and diameter of the tool holder will also play a large role in the rigidity of your tool setup.
Long and skinny = not very rigid
Short and wide = Very rigid
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L/D Ratio is crucial to pay attention to in all aspects of machining. (Tool, Toolholding, Workholding) You don’t always have to calculate it, just feel it out as you go along. |