Milling Cutters and Cutting Fluids

Milling Cutters and Cutting Fluids

Milling cutters are used on the milling machine to remove material from the work piece. Typically the milling cutter is revolving at a calculated speed (RPM) and work is fed to the revolving cutter at a calculated feed rate. Milling cutters exist in a variety of shapes to match the particular requirement of the job.

There are four basic types of materials used to make mill cutters.

High Speed Steel cutting tool (left), also called HSS, is a typical material for an end mill. It is inexpensive and exists in many sizes but has a limited cutting capacity. It will not cut very hard steels because, relatively speaking, it is too soft but is excellent foraluminum , mild steel, and other soft metals.

Cobalt high speed steel cutters (right) are improvement in hardness over the HSS end mill. They are a little more brittle but will stay sharper longer due to the addition of Cobalt as an alloying element. Note that they look very similar in texture to the HSS cutters.

Solid carbide cutters (below) offer an increase in hardness and will allows the machining of many types of materials but are more costly than cobalt HSS cutters. It is very important to have a rigidsetup and the appropriate speeds and feeds in order to maximize the use of the solid carbide cutter. Those cutters are less affected by heat at the cutting point than other types of cutters. They tend to be used for medium to large production runs.

Coated cutters (right) have a solid carbide or HSS body with a coated cutting edge. The purpose of the coating is to make the cutting tool last longer or to be allowed to run the machine at a greater cutting speed.Titanium Nitrite coating is one of the most popular coatings.

End mills have different lengths. The shorter the length the more rigid in setup and the less vibrations on the machine. Short end mills (left) are called stub-length end mills

End mills are also commonly produced with double ends (right). Although double-end end mills are limited in size (rarely exceeding 1" in diameter), they cost less to purchase as compared to two single-end end mills.

Most end mills have spiral flutes with an angle of 30 degrees. That design is the most efficient (all of the above shown end mills have spiral flutes). Straight flutes (rendition below) are used rarely because they are less efficient. You may use them to make two sides of a cut that have to be parallel as in O.D.keyways on a shaft.

Note: Increasing the number of flutes from 2 to 3 or more improves your surface finish at  equivalent feed and speed.

Endmill Geometry

End mills are cutters that have an end-cutting capability. They machine on the end as well as the sides. There are two types of helixes used on end mills: right end and left end. You must be thinking about chip removal to decide which one you want to use. A convention is usually that the cutter is right end if it rotates counterclockwise when viewed from the cutting end.

There are end mills with two flutes (left) and end mills with more than two flutes. (right)
The multi-flute end mill will produce a betterfinish than the two flute end mill at the same RPM and feed but it will have less chip clearance space and therefore will be less convenient in a heavy cut of soft material. Multi-flute end mills are typically more expensive.

There are end mills that are center cutting (like those above) and end mills that are not center cutting (right). In the case of the non-center cutting end mill, you can not plunge into the material and will have to start your cut on the outside of the part. Non-center cutting end mills are more difficult to fabricate and are therefore more expensive, however they do lend themselves to hollow centers that can distribute coolant.

Special Cutters

Roughing end mills ( left) have their flutes serrated to break chips thus reducing horsepower requirement.

Fast-spiral end mills (right) are used when machining relatively soft materials that will get hot easily. The design of this end mill allows for fast removal of the chip and reduces heat transfer to the work piece. Fast-spiral end mills are also called high-spiral end mills.

Ball nose end mills (right) are used when the work piece requires a 90 degree concave fillet or when machining a three dimensionalsurface (example: mold making).

Key slot cutters (right) are used to machine key-slots in shafts. The cutter shown is aWoodruff key cutter.

Corner rounding cutters ( left) are used to make a 90 degree convex radius on a work piece edge. Useful for esthetic removal of a sharp corner.

Cutting Fluids

Using water as a cutting fluid has two problems.

1. Water will rust the machine and possibly the work piece.
2. Plain water does not spread evenly. Chemical waters spread out over the part more uniformly and can therefore remove heat much more efficiently.

Therefore there are special cutting fluids available to solve the problem. The basic four types of cutting fluids for milling operations are all provided in a container similar to those shown here.

1. Chemical fluids (mostly water) - alkaline inorganic and organic compounds, 3 to 10 % concentration Best cooling performance
2. Emulsions (mostly water) - also called soluble oil, emulsifiers keep it mixable, 3 to 10% concentration Good lubrication & heat transfer but not as good as chemical fluids. More inexpensive than chemical fluids.
3. Semi-Chemical fluids, a mix between Chemical fluid and emulsion 5 to 40% mineral oil. Can be used when advantages of both products are desired. More expensive than both other products.
4. Straight Cutting Oils 100% mineral oil with additives - non-emulsifiable, non-diluted, mainly petroleum oil with additives. A good fluid for cutting steel or when tapping diameters over ½”.

Speeds and Feeds

Please note that there are machining conditions where cutting fluid is avoided.

For example, when machining Polypropylene plastic, using a synthetic cutting fluid the plastic will absorb the cutting fluid making it grow in size temporarily.

Another example, when machining cast iron with carbide end mills, it is often recommended to use compressed air rather than cutting fluid to enhance tool life.

When machining a material, it is necessary to use the proper feed and speed for that specific material.

The speed of the cutter or end mill (in RPM) is calculated from the simplified formula:
RPM = (CS x 4 ) / D
Where
CS is the Cutting speed of the particular material (see table) inSurface Feet per Minute
D is the diameter of the milling cutter
Feed is calculated from the following formula:
Feed = RPM x Chip load per tooth x Number of teeth on cutter
Important note: Chip load per tooth and cutting speed vary by manufacturer, material machined, material of the cutter, type of coolant used,... The examples in the chart are for illustration purpose only. Please consult Machinery's Handbook or the manufacturers specifications for further details.

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