Guide · Tooling
CNC Feeds and Speeds Explained: Chip Load for Beginners
Feeds and speeds feel like a dark art until you see the one idea underneath them: every cutting edge should take a chip of a sensible thickness. That thickness is called chip load, and once you understand it you can work out a starting feed rate for almost any router bit on any machine.
This guide explains the concept rather than handing you a table of magic numbers. Numbers you find online were set for someone else's machine, bit and material. The method below lets you build your own, starting from the tool manufacturer's recommendation and adjusting from there.
What chip load actually is
Picture a two-flute end mill turning once. Each of its two cutting edges passes through the material, and in that time the machine has moved the bit forward a small distance. The distance the bit advances per edge is the chip load, also called feed per tooth. If it is a fraction of a millimetre, each edge peels off a chip roughly that thick.
That chip matters because it carries heat away. A good chip takes most of the cutting heat with it as it leaves. A chip that is too thin can't do that, so the edge rubs rather than cuts, heat goes into the bit, and the edge dulls fast. In wood that shows up as scorch marks; in plastic it shows up as melted, gummy edges; in aluminum it can mean chips welding to the flutes.
Too thick a chip has the opposite problem: more cutting force than the bit, collet and machine can resist. The bit deflects, the cut gets rough, the machine may lose steps, and small bits break.
The one formula you need
Everything follows from this relationship:
Chip load = Feed rate ÷ (RPM × flutes)
Use consistent units. If chip load is in inches, feed comes out in inches per minute. If it is in millimetres, feed comes out in millimetres per minute.
A worked example (illustration only)
Suppose a bit maker's chart suggests a chip load range for a 1/4-inch two-flute bit in a particular hardwood, and you pick a value near the low end of that range: call it 0.004 inches. Your router is set to 16,000 RPM.
That is a starting point for that bit, that material and that chart, nothing more. A different bit, a different wood or a less rigid machine would give a different answer. The point of the example is the arithmetic, not the number.
Why lowering RPM is often the fix
Desktop machines often can't move fast enough, or aren't stiff enough, to run the feed rate a high RPM demands. If the formula says you need a feed your machine can't handle, the instinct is to keep RPM high and slow the feed. That shrinks the chip and moves you toward rubbing and burning. The better move is usually to drop the RPM so a slower feed still produces a healthy chip. Many trim routers and spindles have a usable lower range; check yours.
Where to get a starting chip load
The best source is the manufacturer of the specific bit you are using. Most reputable tool makers publish chip load or feed-and-speed charts by tool diameter, flute count and material. Use those over generic forum numbers whenever you can.
- Start at the low-to-middle of the range on a hobby machine. Charts often assume a rigid machine.
- Diameter matters. Larger bits can take larger chips; tiny bits need very small chips and break easily.
- Material matters. Softwood, hardwood, MDF, plywood, acrylic, HDPE and aluminum all behave differently. MDF and plywood glue lines are hard on edges.
- Your CAM software may include a tool library with feeds pre-filled. Treat those as suggestions and check where they came from.
If you don't have a chart yet, the cleanest fix is to buy bits from a maker that publishes one.
Browse 1/4" two-flute carbide end mills on Amazon
Depth of cut and stepover
Chip load sets how thick each chip is. Depth of cut and stepover set how much of the tool is engaged at once. Both affect the total force on the bit.
| Term | What it is | What happens if it's too high |
|---|---|---|
| Depth of cut (DOC) | How deep each pass goes | Deflection, chatter, lost steps, broken bits |
| Stepover / radial engagement | How much of the bit's width is cutting sideways | Higher load, more heat, poor chip evacuation |
| Slotting | Full-width cut, 100% engagement | The hardest case; chips have nowhere to go |
| Plunge rate | Vertical feed into the material | Burning at the entry point, heavy Z load |
A common conservative starting point on a desktop router is a shallow depth of cut, often expressed as a fraction of the bit diameter, and then increasing it while listening to the machine. Many CAM programs also offer ramping instead of straight plunging, which spreads the entry load and is kinder to both bit and Z axis.
Chip thinning, briefly
When stepover is small (much less than half the bit diameter), the chip each edge actually takes is thinner than the chip load you programmed, because the edge only grazes the material. That is why light finishing passes and adaptive or high-efficiency toolpaths often run at higher feed rates than a slot. The details get mathematical; the practical takeaway is that a light stepover at the same feed can still rub, so feed may need to go up.
Reading the cut: chips, sound and marks
Your senses are the second half of the method. After calculating a starting point, run a short test cut in scrap of the same material and look at what comes off.
| What you see or hear | Likely cause | First thing to try |
|---|---|---|
| Fine powder instead of chips | Chip too thin | Raise feed or lower RPM |
| Scorch marks in wood | Rubbing, dwelling, dull bit | Raise feed, lower RPM, check sharpness |
| Melted, stringy plastic | Heat building up | Bigger chip, single-flute bit, clear chips |
| Chatter or squeal | Too much engagement or not enough rigidity | Reduce DOC or stepover, shorten stickout |
| Machine stalls or loses position | Load beyond what motors can deliver | Reduce DOC and feed together |
| Aluminum stuck in flutes | Heat plus poor chip clearing | Air blast or mist, adjust chip load |
Our guides on fixing CNC router chatter and air blast vs mist for aluminum cover those two problems in more depth.
A simple tuning routine
- Pick the tool and material. Find the maker's chip load range for that diameter and flute count.
- Choose an RPM your machine runs comfortably. Lower is often better on a desktop machine.
- Calculate feed using a chip load near the low end of the range.
- Set a conservative depth of cut and a moderate stepover.
- Cut a short test in scrap, watching chips and listening.
- Adjust one thing at a time. If it rubs, raise feed. If it chatters, cut depth or stickout. Write down what worked.
- Save the result in your CAM tool library so you never have to guess for that combo again.
A small notebook or a spreadsheet of "tool + material + RPM + feed + DOC = result" becomes your most valuable reference within a few months. It is specific to your machine, which no chart can be.
Tools that make this easier
None of these are required, but each removes some guesswork:
- A tachometer to check actual RPM. Dial settings on trim routers are often approximate. Browse non-contact tachometers
- Calipers to measure test cuts and confirm bit diameter.
- A sharp set of reference bits from one maker with published charts, so your notes stay consistent.
- Scrap offcuts of every material you plan to use. Test cuts are cheap; ruined parts are not.
- Hearing protection, because tuning by ear is easier when you are not wincing.
Browse digital calipers on Amazon
FAQ
What is a good chip load for a CNC router?
There isn't one universal value. It depends on bit diameter, flute count, material and machine rigidity. Use the bit manufacturer's chart as a starting range, begin near the low end on a hobby machine, and adjust based on test cuts.
Should I run my router at maximum RPM?
Not by default. High RPM needs a high feed rate to keep the chip healthy. If your machine can't feed that fast, lowering RPM usually gives a better cut and longer bit life than slowing the feed alone.
Why does my CNC burn the wood?
Burning usually means the bit is rubbing instead of cutting: chip load too small, RPM too high for the feed, a dull bit, or the tool dwelling in one spot. Raise the feed or lower RPM, and check that the bit is sharp.
Do feeds and speeds change for plunging?
Yes. Plunge rate is usually set slower than the horizontal feed, and many CAM programs can ramp into the cut instead of plunging straight down, which reduces load on the bit and Z axis.
Can I use someone else's feeds and speeds?
As a rough reference, yes, but only after checking their bit, material and machine match yours. Always confirm with a short test cut in scrap first.
Bottom line
Feeds and speeds stop being mysterious once you think in chip load. Get a chart from the bit maker, calculate a conservative starting feed, test in scrap, and adjust based on chips and sound. Lower the RPM before you starve the chip, and write down every combination that works. After a dozen jobs you will have a feed library built for your exact machine, which is worth more than any generic table.