If you’re a CNC Router user (and really, any user of powered cutting tools), you really need to understand the concept of chipload. So what is it, and why is it important?
In a nutshell, the chipload of a router bit (or mill, etc.), is defined as the design width of the little chip of material the bit should be cutting on each rotation.
The chipload is a fundamental number needed to determine the correct spindle feed and speed (F&S) to use for a given tool and material on the big router. Using the correct F&S is hugely important. Too low a chipload, causes bits to overheat and dull quickly, break, and produce fine dust instead of chips, Too fast, and the bit can’t get an appropriate bite of the material, and begins to chatter - making it overheat and dull, or breaking the bit.
Chipload is effected by several factors, including the design of the bit, the number of edges (flutes) of the bit, and type of material being cut. Hardwood produces a lower chipload than plywood, for example, because plywood is much easier to cut. Chipload is also effected by the depth of cut in relation to the diameter of the cutting surface of the tool.
A few important facts:
• Heat produced by the work and friction as the bit moved through the cut, is almost wholly dissapated by the chips produced. If the chips are too small, not enough heat is moved. Too large, and chattering and increased friction cause too much heat to be produced. Heat kills bits, dulls their cutting edges, and causes the steels in them to soften - and break!
• Best finish is always produced at the correct chipload of the bit
• Best speed is always produced at the chipload
• MDF is particularly problematic, since cutting at the wrong F&S increases the health hazard of everyone in the shop, and destroys the filter on our dust collector.
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GWizard produces inappropriately low chiploads for the CNC Router. You’ll need to do the (easy) math yourself.
• Chicks dig chipload studs! Dudes too!
Typical chiploads:
To correctly determine the correct F&S of your bit, use the follolwing simple equations
:
Feedrate (at 1 x cutter diameter depth of cut) = ChipLoad x NumberOfFlutes x SpindleSpeed(RPM)
So, let’s say we’re cutting a piece of Walnut, with a 2-flute spiral 1/4" upcut end mill. For our depth of cut per pass, we select 1/4", and a starting RPM of 18,000.
Feedrate = .009 x 2 x 18000 = 324 IPM.
That’s using the lowest chipload for that size bit, and represents the lowest starting point of our feed rate. If you want to cut deeper that the cutting edge diameter of the bit, use the following factors:
2x the CED = multiply feedrate by .75
3x the CED= multiply the feedrate by .5
Now, an exceptionally hard or tight-grained wood might need a little bit lower chipload, but not a lot. If you run that bit much slower, you’ll start generating a lot of friction, as the bit is rotating too slowly to eject the chips fast enough to take a fresh bite, and you’ll be reducing the ability of the bit to cool itself by tossing out chips. You’ll start to make a lot of dust instead of chips, the bit will heat up over time, get dull, and begin to get soft.
A good comparison, is looking at what happens when you run a board through the jointer too slowly. It makes a lot of dust, and usually burns the wood. You need to keep that board moving, and this is true of your router bits as well.
The same is true if you cut too fast - the bit doesn’t have enough time to cut a chip, so you begin to make dust, the heat doesn’t get dissapated, and you add the additional torque of pushing the bit into the stock faster than you are cutting it. A loud screech, a lot of dust, and possibly a snapped bit are your result.
The above chart is true for most types of bits, but some bits have unusual cutting geometries, like ball nose bit, vBits, compression bits, etc. You need to look up the chipload value for the type of bit you are using.
This seems counter-intuitive to some folks. They rationalize, that a slower running bit is less risky, less likely to break, and the safer choice. This is just flatly wrong, and such people do nothing more than damage our bits, and destroy our dust collector.
Onsrud publishes a really great guide to chiploads at: https://www.onsrud.com/xdoc/feedspeeds