Mar 21, 2014 Calculating Feeds and Speeds. 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.
What’s a Router?Before discussing or using a CNC router it is helpful to know how to use a handheld router. Your typical router has a motor (possibly a variable speed motor), height adjustment (either fixed, or plunge), and a collet, which is a tapered spring that when compressed creates the friction necessary to hold your cutting tool in place.When you use a noncomputerized router you see, hear, and feel how the tool can cut with instant haptic feedback. If you have access to one, go play with it before attempting to create toolpaths on the computer.
Make a quick sketch on a 12”x12” piece of scrap plywood and use a ¼-inch bit to cut it out. If you are working with a fixed base (not a plunge router) be sure to drill a 3/8-inch hole to enable you to safely start the router in the material. Set the bit so that it cuts no more than 1/8 of an inch deep per pass, and be sure to secure the plywood to your table. Be advised that using a router bit/depth not specified could be potentially dangerous. Use a bit smaller than 3/8 inch and use a step of less than the radius of the bit.Begin the cut at the center of your shape and work out in a spiral-like pattern, this will provide support for your router if your shape is larger than its base. As you work your way toward the sketched lines, try making both clockwise and counterclockwise motions and notice that one direction provides far more control and accuracy.
What are Toolpaths?A toolpath is the user-defined coded route which a cutting tool follows to machine a part. They are represented on the screen by lines and curves which represent the path of the bottom center of the cutting tool. Pocket toolpaths etch the surface of the material, while profile toolpaths cut all the way through.
PocketThe process described in the handheld example above is called a “pocket” toolpath. In your first pass you will remove everything inside your lines to a constant depth of 1/8 inch below the surface. If you want to remove more than 1/8 of an inch simply pause after the first pass, lower the bit, and remove a second pass 1/8 of an inch lower and so on. Profile toolpaths being cut.Most CAM software provides what seems to be an insane number of controls and options in the toolpath dialogue. Don’t get overwhelmed and take your time to slowly move through each tab in succession being sure you understand all the options. The most important concepts to take away from the handheld router experiment above are: spindle speed, feed rate, step-down, and step-over. We’ll cover these in more detail below.
Conventional Cutting MotionA standard router bit spins clockwise. If this were to follow the left side of the line it would be a climb cut, if it were to follow the right side of the line it would be a conventional cutting motion. Climb vs Conventional CutsThe main difference between climb and conventional cutting is how the cutter bites into the material. A conventional cut deflects the bit towards the cut and a climb cut pushes the bit away. Climb cutting is often preferred when using a CNC router, as it causes less grain tearing or “tearout.” However, climb cutting can be dangerous on a noncomputer controlled router, as the piece may be hard to control by hand and may “walk away.”In the handheld example above, a clockwise spiral-like pattern from the inside out would also be a conventional cut and will provide more control because we have limited strength when using handheld tools. In fact, many woodworking books refer to this method as the only safe direction to use a router bit.A CNC router however will typically provide a better cut when using a climb cut especially in solid wood because it will eliminate the possibility of tear out along the cutting vector.
Starting out, don’t worry too much about this. Generally you should choose the option which includes both such as ‘mixed’ and the software will choose what to use.
ToolingThere are 4 main types of flute patterns for router bits, plus many types of specialty bits. 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.Each of these flute patterns has their downfalls. If money isn’t an issue and you are mostly cutting finish grade plywood, I really like the Freud 77-202 or 77-204. Straight flutes are also fantastic, inexpensive and underrated. 3D Containment BoundariesI encourage students to spend time with 2D cuts before using full 3D toolpaths. There are a number of reasons for this, but most importantly taking the time to understand 2D machining allows for a much better understanding of the 3D parameters. I’d be happy to cover 3D in more depth at a later date, but for now I want to discuss containment boundaries.Instead of using wireframe geometry or curves and lines as the primary source of input, a 3D toolpath uses surfaces called ‘drive surfaces.’ In order to achieve the desired outcome it is often necessary to use a containment boundary.
This is a curve which defines the limits of motion in x and y on the drive surface. The key to using containment boundaries is that they must be above your geometry. Typically I move the model below to the construction plane before starting and put containment on the CPlane.
Feeds and SpeedsMost 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.