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Guide to CNC Milling Tools

guide to cnc milling tools

CNC tools come in nearly as many varieties as products created by the milling process. Understanding the uses of these tools will make it easier to choose the ones you need for your project. Selecting the right tools makes the process of CNC machining more accurate.

What Is CNC Machining?

what is CNC machining

CNC stands for "computer numerical control." The process of CNC machining uses a software program to guide the mills to create the finished product you need. Unlike 3D printing , which adds material as required, CNC machining removes unwanted material from the workpiece instead.

Machinists have several tools to choose from. Each device produces a different result. Sometimes, to get the perfect finish, the producer may use multiple tools.

Types of CNC Milling Tools

Types of CNC milling tools allow for the perfect customization of the final products. Different tools are used to cut into and shape various materials. The final design of the cut also determines which tool to use. Aside from these criteria, professionals select tools to balance the desired finish with speed. One may be a priority over the other, depending on the final use of the finished product.

Here are some of the many tools available for CNC milling .

What Are Chamfer Mills?

Chamfer mills do not have blades along the edges, only at the tip. These are used to cut angled edges. The cutting blades on each chamfer mill are called "flutes." The more flutes on the mill, the more delicate the cut. But fewer flutes on the mill remove more material each turn, for faster production. Balancing speed and finish is essential when selecting a chamfer mill with two, three or four flutes.

What Are End Mills?

End mills come in several types used for different cutting. All end mills cut at 90-degree angles, which distinguishes them from the angle-creating chamfer mills. To cut straight down, you use a center cutting end mill. These mills cut both in the middle and on the sides.

Non-center cutting end mills have a hole in the center and only have cutting edges along the sides of the mill. Roughing end mills are used to make the initial cuts because these have fewer flutes and remove more material at first. To create the finely cut finished product, you'll use finishing end mills, which have more flutes to achieve a closer design to the desired part.

Aside from the number of flutes, the material used for the end mill will also make a difference in the tool used for a project. Cobalt, high-speed steel, and carbide are common materials used to create end mills. Each of these materials has its benefits, but they cannot be used interchangeably. Here's what to know:

  • Cobalt: Cobalt mills are only comprised of eight percent cobalt , with the remainder of the construction made of steel. Thanks to the reinforcement of cobalt, cobalt mills run 10 percent faster compared to high-speed steel mills. This increased speed removes more metal, more quickly, than HSS without sacrificing the finish.
  • Carbide: While carbide is harder and can be turned faster than high-speed steel, it also tends to chip. Don't use carbide mills for rough work. These end mills are best suited for finishing processes.
  • High-Speed Steel: High-speed steel, or HSS, is the standard material used for many mills. It carefully balances longevity and tool cost. Though end mills made of high-speed steel cannot turn as fast as those made from carbide or cobalt, the lower price makes HSS a better option for many. Additionally, HSS has enough flexibility to be used in the cutting of iron and non-iron materials.

End mills have numerous means of cutting into a material. How you will cut determines the tool used:

  • Side Milling: When you want to produce beveled edges with chamfer mills, you perform side milling. This process moves the mill along the corner of the material to cut into it and smooth it to the desired angle.
  • Face Milling: When you simply cut into one surface of a material, it's called "face milling." The surface is the face of the material.
  • Ramping: Ramping cuts directly down into a surface and at an angle, creating a slanted cut down through the material.
  • Plunge: As the name suggests, plunge milling makes the end mill dive straight down into the material being cut. Like ramping, plunge milling requires a center cutting end mill to remove material from the center and edges of the hole produced.
  • Slot Milling: Slot milling produces slots through a material, cutting edges on two sides at once as the end mill makes a groove.

What Are Ball End Mills?

Ball end mills have rounded tips. As the name suggests, these mills produce clean and rounded bases at the bottom of the holes they create. Do not confuse ball end mills with bull nose mills, however. Although both create rounded cuts, bull end mills produce straight bottoms and sides with softened corners. Ball end mills are ideal for carving because they produce spherical bases.

What Are Straight Flute Mills?

Mills usually have twisted flutes. But in straight flute mills, the cutting teeth are vertical. The straight sides are best suited for cutting delicate or easily frayed substances, such as plastics or epoxy composite materials.

What Are Down Spiral End Mills?

Most end mills have up-spiral designs, which pull the cut material up and out of the hole produced. But down-spiral end mills push the cut material downward. These mills do not work well for drilling through an element, but the end mills work well for cutting thin materials while keeping the top clean.

What Are Drill Bits?

CNC drill bits

Drill bits cut holes in a material. Occasionally, a machinist must drill a hole first before adding threads with taps or widening the hole with reamers. Twist drill bits are commonly encountered in general repairs. These bits create small holes.

As with end mills, drill bits can be made from different materials that influence their strength and performance. Coatings on the exterior of the bits may increase longevity. Here are some common materials:

  • Carbide: Like end mills, carbide drill bits increase the bit's longevity. But this delicate metal requires secure and straight positioning to prevent the bit from breaking.
  • High-Speed Steel: The same HSS used for end mills may also be used to create drill bits. These bits have a quality that makes them suitable for general use without breaking the bank.
  • Titanium Nitride Coating: Titanium nitride, or TiN, coating makes the drill bit appear gold on the cutting surface. TiN increases the hardness and longevity of the bit. Additionally, the bit operates more smoothly because the TiN coating slightly lubricates the surface.
  • Titanium Aluminum Nitride Coating: The most common coating for end mills and bits is titanium aluminum nitride, or TiAlN. While TiN makes a bit appear gold, TiAlN gives the exterior of the part a bluish color. The longevity of the element increases while the speeds at which you can use the bit rise by 20 percent when the bit has a TiAlN coating.

What Are Flycutters?

Flycutters are used to create a smooth, flat surface from a large piece of material. Think of table tops and counters. While they get a lot of material off the surface quickly, the design of flycutters can make the engine of the CNC mill work harder. Some people refer to flycutters as "spoilboards."

Choosing CNC Mills for a Project

Selecting the right tools for a project is half the job. Using the wrong tools could require too much time or even ruin the equipment. Several factors play into which device you select for a project. The materials you're cutting into, plus speed, finish, and the direction you need the material to go in, will all help you choose the best bit for the job.

Select the Best Bit for Your Material

Not all end mills and drill bits work well to cut all materials.

For instance, bits designed for plastic use operate at lower temperatures to prevent melting. Bits made for aluminum must pull cuttings out and away from the cutting surface to avoid compressing the shavings and the base together. To avoid irreparable damage, never interchange bits when cutting different materials.

Use the Strongest Bit Available

Using the strongest bit available for the material you're cutting will prevent bit breakage and slippage. Stronger bits last longer while cutting more accurately than weaker bits. However, use caution when selecting solid bits for softer materials, including plastics. The excessive force could damage the material.

Balance Speed With Finish

Finishing faster is not always desirable. The bits that cut more quickly will also make rougher cuts by removing more significant amounts of material at once. If the final product does not need a perfectly smooth surface, using a bit that cuts faster may suffice.

For finer finishing tasks, you'll want an end mill with as many flutes as possible. The more flutes on a bit or end mill, the smoother the finish will be. When using end mills with three or more flutes, the process will take much longer because less material gets cut out with each turn of the mill. Keep this in mind when selecting parts to create smooth finishes or fine details.

To properly hone a piece without sacrificing speed or design, start the project milling with bits that have fewer flutes to create the gross cut. Then finish with end mills that feature more flutes to get a more delicate result. Although this process takes longer, it balances speed with finish quality.

Choose the Appropriate Direction

choosing the right direction

Direction can make or break a product.

CNC millers may choose climb milling, also known as "down" or "conventional" milling. Conventional milling turns the mill end against the direction of the material feed. This method has the advantage of pulling the material up and away from the cutting surface. Unfortunately, it cannot be used with certain metals because pulling the chips away from the surface redistributes the heat produced from the cutting to the metal itself. Some materials experience damage from this excessive heat. Additionally, conventional milling requires more force on the tool, reducing its lifespan. Throughout traditional milling, the chip size gradually increases from the beginning to the end of the process.

Climb milling cuts with the feed. The chips produced get pulled away from the cutting surface, which keeps the surface neat. This cutting direction extends the life of tools by up to 50 percent , which in turn won't require as much force to cut into the material. Despite down milling having the moniker of "conventional," climb milling is becoming more popular due to its advantages. Unfortunately, many shops forbid climb milling because, if improperly done, the machine can pull the material upward, sending shrapnel through the workshop.

Set the Correct Feeds and Speeds

The key to safe and accurate milling starts with setting the correct feeds and speeds. Speed is either measured in revolutions per minute (RPM) or surface feet per minute (SFM). SFM should be a constant, which is based on the material being cut. Multiply the SFM by 3.82. Then, divide this value by the tool's diameter in inches to find the RPM:

RPM = (SFM x 3.82) ÷ tool diameter

If you know the desired RPM, you can find the SFM by multiplying the RPM by the tool diameter in inches and dividing the result by 3.82:

SFM = (RPM x tool diameter) ÷ 3.82

To determine the feed, you'll need to know the speed in RPM. For finding the feed measured in inches per minute (IPM), you need the chip load, also called "inches per tooth" (IPT). The IPM equals the product of the RPM, the IPT, and the number of flutes on the mill:

IPM = RPM x number of flutes x IPT

While you multiply to find the IPM, you'll divide instead to find the chip load (IPT). Divide IPM by the RPM by the number of flutes:

IPT = IPM ÷ RPM ÷ number of flutes

These calculations are critical for achieving the desired finished product with a given material and an end mill or bit. Miscalculating could result in disaster if you put the wrong information into the machine.

Sharp Mills and Resharpening

Sharp mills require less force and produce cleaner cuts. But the problem with many coated and intricately designed mills today is the added difficulty of resharpening them yourself with a standard diamond sharpener. If they are sharpened improperly, the mills may not cut evenly or they could lose their protective coatings.

If you must have your mills sharpened, have a professional do the job. Otherwise, consider purchasing new parts. If you don't want to make repeat purchases of mills, opt for coatings that help the mills last longer. Proper storage also will also increase the longevity of your cutting tools.

Storing Your Mills

Even though mills and bits can cut tough materials, the flutes may still become damaged and dulled with improper storage. This is especially true of carbide, which is deceptively delicate.

Keep your mills in a specially padded storage container so they do not roll around and rub against each other. A carrying case is also essential for toting your tools safely if you travel with them. Resharpening your mills or purchasing new ones can be expensive and disruptive to your workflow. Store your tools correctly to avoid premature wear.

Get the Parts You Want

custom part machining

Don't settle for subpar parts and tools. You need CNC machining professionals who know precisely which tool to use to achieve your desired outcome. We strive to bring you the highest quality possible for the work we do. Contact AMI today for a quote on your part creation from our professionals.