CNC Router

About the Tool:

CNC router (or Computer Numerical Control router) is a computer-controlled cutting machine related to the hand held router used for cutting various hard materials, such as woods, composites, aluminum, plastics, cardboard and foams. The CNC uses computer generated tool paths to perform the cutting process. The CNC router typically produces consistent and high-quality work. Using CAD plans or other design software the router can produce one-off parts as effectively as it can repeat identical parts for an assembly.

The CNC works on the Cartesian coordinate system (x, y, z). Different bits and collets are available for various production needs.

The Cutting Bed is where all the action happens. The cutting bed may comes in several types but the function remains the same. The cutting bed is designed to support and secure the piece of material being cut.

The Spindle is the part of the machine doing the cutting. The spindle works by rotating a bit, at varying speeds (depending on material). A typical spindle designed to cut wood, plastics, and other "soft" materials usually have a range of 8,000 to 30,000 revolutions per minute (RPM). Spindles designed to cut metals operate between 2,000 and 10,000 RPMs.

Tooling

  • Straight Flute – great all around bit, decent chip removal

  • Up Spiral – great chip removal, can tear-out the top of thin veneer such as finish grade plywood

  • Down Spiral – poor chip removal, no tear out, slower feed rate

  • Compression – combination of up and down spiral, great all around bit, great for plywood or laminated sheet goods.

Safety:

  • Safety glasses and hearing protection are required

  • Do not leave the machine unattended while it is in operation

  • NEVER reach into the working envelope of the machine while it is running

  • Visually ensure that all tools and bits are completely stopped before removing material or performing clean-up.

  • Only authorized adults can perform maintenance.

Feeds and Speeds

Most spindles (the term for the router attached to your cnc router) will go from about 7,000rpm to 18,000rpm. This speed is termed ‘spindle speed’ and is directly related to the feed rate or surface speed, which most machines are capable of doing up to about 200ipm. The two other variables, step-down and step-over should be kept so that the cross-sectional area engaged with the material is no more than the radius times the diameter of the bit. This is a rule of thumb, but it’s a good starting point for feed and speed calculations.

To recap:

  • Spindle Speed – rotational speed of the cutting tool in revolutions per min

  • Feed Rate – Surface speed at the center of the rotating tool

  • Step down – the distance in the z direction per pass that a cutting tool is plunged into the material

  • Step over – the maximum distance in the x/y direction that a cutting tool will engage with uncut material

Calculating Feeds and Speeds

Below is a formula for calculating feed rate:

ChipLoad x CutterDiameter x NumberOfFlutes x SpindleSpeed = FeedRate

Where chipload is the amount of material cut per tooth (feed per tooth). Feed rate is the surface speed of the cutting tool in inches per min, spindle speed is the rotational speed of the cutting tool in revolutions per min, number of flutes and cutter diameter are determined by your tool. In this case they are ¼ inch and 2 flutes. Depending on the size of your bit, the chipload for plywood is between 0.005 inches 0.01 inches per tooth. For small bits below 1/8 inch start with 0.005 and increase from it there. For bits 1/4 inch and larger you will probably not break anything starting out at 0.01.

The size of the chipload or feed per tooth is a very important factor in machining, larger chips are able to pull away more heat. Smaller chips are easier on your machine and tools but can cause too much heat. You want to make chips that when dropped fall to the floor rather than become dust that stays in the air.

When trying to hone in on your feeds and speeds with a new bit, guess as best you can using the feeds and speeds formula and touch the bit as soon as it stops spinning after making a few cuts (remember: safety first), it should be warm, maybe a little hot to the touch, but it should not burn you. If it is too hot, increase the feed rate or lower the spindle speed. Look at the quality of the edge after the cut has been completed. If it is wavy, that’s tool chatter and you should decrease your feed rate or increase your spindle speed.

Use your ears too, the tool should sound good when cutting … trust your gut.

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