Laser & CNC guide
CNC Chip Load Guide: How to Set Feed Rate, RPM and Plunge
Feed rate = RPM × flutes × chip load. A worked 1/4 in cutter example, typical starting chip loads by material, and the signs of rubbing or a heavy cut.
Quick answer
Feed rate = RPM × flutes × chip load. A 1/4 in two-flute cutter in softwood or plywood at 18,000 RPM, with a typical starting chip load of 0.007 in/tooth, runs at 18,000 × 2 × 0.007 = 252 in/min (6,400.8 mm/min), with a plunge rate of 126 in/min at 50% of the feed. Surface speed is 1,178 SFM. A feed of 100 in/min on the same setup is a light 0.0028 in/tooth, and 500 in/min is 0.0139 in/tooth, about twice the table value and flagged as heavy.
Formula
feed (in/min) = RPM × flutes × chip load (in/tooth) | chip load = feed ÷ (RPM × flutes) | SFM = π × diameter (in) × RPM ÷ 12
Accuracy
Reference table
Search by name or sort by any column. Each row links to the matching calculator.
6 of 6 rows · sorted by material
| Calculator | ||||||
|---|---|---|---|---|---|---|
| Acrylic (cast) | 0.002 | 0.005 | 0.007 | 0.009 | 180 | Feeds and speeds calculator |
| Aluminium | 0.001 | 0.002 | 0.003 | 0.004 | 72 | Feeds and speeds calculator |
| Hardwood | 0.003 | 0.006 | 0.008 | 0.01 | 216 | Feeds and speeds calculator |
| HDPE / plastic | 0.004 | 0.008 | 0.011 | 0.014 | 288 | Feeds and speeds calculator |
| MDF | 0.005 | 0.01 | 0.013 | 0.015 | 360 | Feeds and speeds calculator |
| Softwood / plywood | 0.004 | 0.007 | 0.01 | 0.012 | 252 | Feeds and speeds calculator |
How it works
What chip load is and how it sets the feed rate
Chip load is how far the cutter advances for each tooth that passes through the material, in inches (or millimetres) per tooth, and it roughly sets the thickness of the chip each tooth takes. Rearranged, feed rate = RPM × flutes × chip load. For a two-flute 1/4 in cutter at 18,000 RPM and 0.007 in/tooth: 18,000 × 2 × 0.007 = 252 in/min, or 6,400.8 mm/min, since 0.007 in is 0.1778 mm per tooth. The same formula works backwards to check a feed you already use: chip load = feed ÷ (RPM × flutes). Changing the RPM, the number of flutes or the feed changes the chip load, so recheck whenever you change any one of them.
Typical starting chip loads by material and cutter size
Larger cutters can take a bigger bite, so typical chip loads rise with diameter. The table on this page lists typical starting values for a rigid machine in four cutter sizes (1/8, 1/4, 3/8 and 1/2 in) and the calculator interpolates between them. For a 1/8 in cutter in hardwood the typical value is 0.003 in/tooth, so at 12,000 RPM with two flutes the feed is 12,000 × 2 × 0.003 = 72 in/min (1,828.8 mm/min), and the surface speed is 392.7 SFM. These are not a cutter maker's numbers. Makers publish their own charts for their own cutters, and those should win. Hobby machines usually cannot hold a rigid machine's feed, so start lower and work up.
RPM, surface speed, plunge rate and the check mode
Surface speed in feet per minute (SFM) = π × diameter in inches × RPM ÷ 12. For 1/4 in at 18,000 RPM that is 1,178.1 SFM (359.1 m/min). Going the other way, a target of 400 SFM on a 1/4 in cutter needs about 6,112 RPM. Many routers and spindles run between about 6,000 and 30,000 RPM, so confirm yours. Plunge rate is the feed multiplied by a percentage: the calculator starts at 50%, giving 126 in/min from 252 in/min. In check mode you enter your planned feed and get the chip load back. Below about 0.001 in/tooth the cutter is likely to rub and heat rather than cut, and above 1.5 times the table value the calculator flags the cut as heavy, with more cutting force and a higher risk of a rough edge or a broken cutter on a light machine.
Work it out
Change the inputs to work out your own numbers. Each step of the working is shown with the answer.
Your measurements
Enter your cutter, material and spindle speed. Everything updates as you type.
What do you want to know?
Works out the feed from a chip load. The chip load comes from the table or from your own number.
Cutter and material
Common cutters
The cutting diameter of the end mill.
Count the cutting edges (flutes) on the end mill.
Sets the typical starting chip load in the table below.
Spindle
Set it on the router dial or spindle controller. A trim router at its top setting is often around 16,000 to 30,000 RPM; check yours.
Leave at 0 if you are not using it. Enter a surface speed from a cutter maker's chart to get the RPM that gives it.
Chip load
Leave at 0 to use the typical starting value from the table (0.007 in/tooth here). Type your own number from the cutter maker's chart or a test cut.
Plunge
50% is a rule of thumb, not a rule. Change it to suit your cutter and machine.
Results
Feed rate
in/min
= 6,400.8 mm/min
18000 RPM × 2 flutes × 0.007 in per tooth (typical starting value).
- Chip load
- 0.007in/tooth
- Typical starting value
- Surface speed
- 1,178.1SFM
- 359.1 m/min
- Plunge rate
- 126 in/min
- 50% of feed, a rule of thumb
Check before you cut
- Chip loads in the table are typical starting values for a rigid machine, not figures from a cutter maker. Go lower on a hobby CNC and check the cutter maker's chart.
Typical starting chip loads (in per tooth)
Typical starting values for a rigid machine, not figures from a cutter maker. Go lower on a hobby CNC and check the chart for your cutter.
| Material | 0.125 in | 0.25 in | 0.375 in | 0.5 in |
|---|---|---|---|---|
| Softwood / plywood | 0.004 | 0.007 | 0.010 | 0.012 |
| Hardwood | 0.003 | 0.006 | 0.008 | 0.010 |
| MDF | 0.005 | 0.010 | 0.013 | 0.015 |
| Acrylic (cast) | 0.002 | 0.005 | 0.007 | 0.009 |
| HDPE / plastic | 0.004 | 0.008 | 0.011 | 0.014 |
| Aluminium | 0.001 | 0.002 | 0.003 | 0.004 |
The columns are 1/8, 1/4, 3/8 and 1/2 in cutters. Sizes in between are interpolated; outside that range the nearest column is used.
How this was calculated
Surface speed, chip load, feed and plunge, with your numbers.
How this was calculated
Surface speed, chip load, feed and plunge, with your numbers.
Step 1: Surface speed
π × diameter (in) × RPM ÷ 12
= π × 0.25 × 18000 ÷ 12
= 1178.1 SFM (359.1 m/min)How fast the cutting edge moves through the material, in feet per minute. It depends only on the cutter diameter and the spindle speed.
Step 2: Chip load from the table
inches cut by each flute on each turn
= Softwood / plywood, 0.25 in cutter: 0.007 in per tooth
= 0.007 in per tooth (0.1778 mm)A typical starting value for a rigid machine, interpolated between the 1/8, 1/4, 3/8 and 1/2 in columns. Go lower on a hobby CNC and check the cutter maker's chart.
Step 3: Feed rate
RPM × flutes × chip load
= 18000 × 2 × 0.007
= 252 in/min (6400.8 mm/min)The speed the machine moves along the cut. Multiply inches per minute by 25.4 for millimetres per minute.
Step 4: Plunge rate
feed rate × plunge percent
= 252 × 0.5
= 126 in/min (3200.4 mm/min)50% of the feed is a rule of thumb for going straight down. Many cutters plunge slower than they cut sideways; use your own percentage if the cutter maker gives one.
Troubleshooting
What usually goes wrong, how to check for it, and how to prevent it.
Burn marks, gummy or melted chips
- Cause
- The chip load is too low so the cutter rubs and heats, the RPM is too high for the feed, or the cutter is dull
- How to check
- Work out feed ÷ (RPM × flutes) and compare it with the typical value for the material and cutter size
- Prevention
- Raise the feed or lower the RPM, keep the cutter sharp and clear the chips out of the cut
A broken cutter, chatter or a rough, torn edge
- Cause
- The chip load is heavy for the cutter and the machine's rigidity, or the cut is too deep for a light machine
- How to check
- The chip load is more than 1.5 times the table value, or the machine visibly flexes or stalls
- Prevention
- Lower the feed or the depth of cut, shorten the cutter's stick-out, and work up from a lower chip load on a hobby machine
The machine cannot reach the calculated feed rate
- Cause
- The machine's maximum feed or acceleration limits the real speed, especially in short moves
- How to check
- Compare the calculated feed with the maximum your machine is rated for
- Prevention
- Lower the RPM along with the feed so the chip load stays near the target, rather than running a fast spindle at a slow feed
Common questions
What is chip load in CNC machining?
It is the distance the cutter advances for each tooth, in inches or millimetres per tooth, which roughly sets the thickness of the chip each tooth cuts. Feed rate = RPM × number of flutes × chip load.
How do I calculate feed rate from RPM and chip load?
Multiply RPM by the number of flutes by the chip load. A two-flute cutter at 18,000 RPM and 0.007 in/tooth runs at 18,000 × 2 × 0.007 = 252 in/min, or 6,400.8 mm/min.
What chip load should I use for a 1/4 inch cutter in plywood?
A typical starting value, as used by this calculator's table, is 0.007 in/tooth for softwood or plywood. It is not a maker's figure, so check the chart for your cutter and go lower on a hobby CNC.
What happens if the chip load is too low or too high?
Too low and the cutter rubs, heats up and dulls instead of cutting a chip, leaving burn marks. Too high and the cutting force rises, which can leave a rough edge or break the cutter on a light machine.