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How To Choose CNC Bits

How To Choose CNC Bits
Issue Time:2017-10-11

Choosing the right bit makes all the difference. It’s the difference between your result being perfect and just being okay. It’s the difference between cutting down on waste and increasing material costs. In the most extreme cases, it’s the difference between operating your CNC machine safely and setting something on fire.

First, what are you cutting?

Do you primarily use your router with wood? Is it hardwood, soft wood, or plywood? Do you use your CNC for composites such as MDF, particle board or laminate? What about phenolic materials? Maybe your shop depends on your CNC for manufacturing large amounts of plastic, acrylic, or aluminum.
You might be cutting solid surfaces, fiberglass, or stainless steel. CNC machines can even etch glass.
Unfortunately, bit selection is not “one size fits all.” There are even different grades of carbide used for different materials. Choosing the wrong bit for the type of material you are processing may deteriorate or even destroy the bit, your material and, in the worst case, even your machine. Remember, the goal is to cut the material, not grind it.
Cutting bits specifically designed for plastic will help avoid melting the material you are processing. Bits for aluminum will clear the chips so they do not fuse to the bit at high operating temperatures. Compression bits are used to avoid chipping or tearing materials such as melamine and plywood.
If you’re new to CNC routing or would simply like to have a multi-purpose bit, consider two-flute spiral bits. They cut a variety of materials including wood, foam, plastics, and aluminum while balancing speed and finish. Another useful bit is the 60 degree V bit which is good to keep on hand for cutting letters, making signs and engraving.

Second, which is more important, speed or finish?

Do you want to cut fast or do you want to cut smooth? Generally, bit selection is based on whether speed or accuracy is the most important consideration at a specific stage of processing. For example, with straight cuts choose an end mill bit for speed. If you are carving, a ball nose bit will give you the best detail.

Speed

Feeds and speed will vary depending on the horsepower of your spindle, whether panels are held by clamps or vacuum, the stability of your CNC machine, and the condition of your collet, among other factors.
Plan to start cutting your material at a lower rate and gradually increase the speed and feed until the quality of your finish becomes unacceptable.
Getting comfortable with trial and error is the mark of a CNC expert.
Chip Load is a calculation which measures the amount of material removed by each cutting edge during a CNC cut. The feed rate is how fast material moves through the machine. Here’s the formula for calculating Chip Load:
Chip Load = Feed Rate (inches per minute)/(RPM x # flutes)
Chip Load is the size of chips created by a bit when making a cut. Chips too large might break a bit while chips too small turn your waste to powder and dull the bit.
The Chip Load number will help you select the right diameter or size of bit. Vortex has a useful chart that will help at this stage:
As an example, let’s say I was cutting at 450” per minute feed rate and had my spindle set to 16,000 RPM. With a three-flute bit, my Chip Load calculation would be as follows:
Chip Load = 450” per minute feed rate = .009” Chip Load
                        16,000 RPM x 3 flutes
If I were cutting hard wood, the Vortex chart shows a .009” Chip Load would call for a ¼” diameter bit. However, if I were using my CNC for aluminum, the chart shows that a ½” bit is recommended for a .009” Chip Load. That’s why it’s important to base your primary bit selection on the material you are cutting with your CNC router machine. Vortex also offers a helpful app for smart phones entitled “Vortex Tool Selection Guide.”
Since Chip Load is based on an average thickness of material for a cutting tool, it’s simply a place to begin. Start with the tool vendor’s recommended settings. Any manufacturer can only guide you in the right direction; it is up to the end user to establish the proper settings for their application. Remember, trial and error is not a bad thing.
Speeds generally refer to the RPM of the spindle and CNC machines offer an extremely wide range of RPM speeds. Generally, higher RPMs result in a higher quality finish. However, it also results in higher friction which increases the wear on your bit. Greater wear means your bit will become dull faster, decreasing the quality of your cut and requiring the expense of sharpening or replacement sooner. Make sure you refer to your spindle manufacturer’s speed parameters before setting speeds. Your goal is to run your CNC machine at the lowest RPM possible while producing the highest quality of cut required for your project.

Finish

If smooth quality cuts are your primary goal for this stage of CNC Routing, know that bits for CNC machines offer multiple cutting edges for additional flexibility. Cutting edges on a bit are referred to as flutes and craftsmen can choose from one, two-, three- and four-edged flutes on their CNC bits.
More flutes or cutting edges on the bit will provide a finer quality of cut.
Two- and three-flute bits are used when finish is more important. And the four cutting edges on four-flute bits will give you the ultimate detail work. Remember that the number of flutes in the bit you are using is a key component of calculating Chip Load.
If your design has intricate components, choose a tapered ball nose bit. Is your machine primarily a sign making CNC? You will likely need to select a “V” bit. Are you manufacturing a smooth flat finish like a table top? Look for a spoil board cutter or a fly cutter bit.

What will be your primary cut direction?

An up-cut bit pulls waste (chips) up and away from the material, but it will splinter plywood and melamine. A down-cut bit presses waste back into the cut, and while it is great for aluminum, a down-cut bit will melt plastics.
Compression bits cut both up and down. Although generally a little more expensive, compression bits are good multi-purpose wood bits and are great for routing plywood. Again, select your bit first based on the material you will be processing.

Other Considerations

Craftsmen should also consider longevity, performance, and finish when investing in the purchase of a new bit. Select a bit that will allow for multiple re-sharpenings. Sharp bits provide clean cuts and your machine will run quieter and use less power, all key factors in operating an affordable CNC shop.
Shorter bits will usually provide cleaner cuts. Long bits tend to increase the level of vibration, which increases the chance of bending the bit (tool deflection). 
A larger shank diameter on the bit will result in cleaner cuts and be more durable. It will also be quieter during operation.
Take care of your bits. Store them properly such as in a tool library and sharpen them when needed. Proper tuning will increase longevity and remember to always warm up your spindle before you begin cutting to insure consistency.
Once you’ve mastered the basics, highly specialized bits are available to bore holes, cut miter folds, or route picture frames. Your CNC Router can fabricate almost anything you can design with the right bit.
It’s likely you chose CNC routing because you have an eye for detail. For you, little things make a big impact. One of the smallest components on your CNC router can have the biggest impact on the finished product. Make the right choice.