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Radial Chip Thinning Calculation

Guide · Updated October 28, 2026

Chip load charts assume the cutter is engaged about half its diameter or more. When the radial stepover is less than half the cutter diameter, the tooth never reaches its full chip thickness, so the real chip is thinner than the programmed feed per tooth. That thin chip rubs, makes heat and dulls the tool. Chip thinning compensation raises the programmed feed so the actual maximum chip thickness matches the tool maker recommendation.

Step by step

1. Get the recommended chip load: Use the tool maker chip load for the material and diameter.
2. Find the radial engagement: ae is the stepover; compare it with the cutter diameter D.
3. Calculate the factor: If ae < D/2, RCTF = D ÷ (2√(D × ae − ae²)). If ae ≥ D/2, RCTF = 1.
4. Adjust feed per tooth: Programmed IPT = target chip load × RCTF.
5. Calculate feed rate: ipm = rpm × teeth × adjusted IPT.
6. Check the result: Watch chip color and spindle load; back off if the tool or machine cannot take the feed.

The formulas

• Radial chip thinning factor: RCTF = D ÷ (2 × √(D × ae − ae²)), for ae < D/2
• Equivalent: hmax = fz × 2 × √(ae/D − (ae/D)²)
• Engagement angle: φ = arccos(1 − 2 × ae ÷ D)
• Adjusted feed per tooth: fz = hmax target × RCTF

At 50% stepover the factor is exactly 1. The factor rises fast below about 10%.

Factor by stepover

• 50% of D: RCTF 1.00, engagement 90°
• 25%: RCTF 1.15, engagement 60°
• 10%: RCTF 1.67, engagement 36.9°
• 5%: RCTF 2.29, engagement 25.8°
• 2%: RCTF 3.57, engagement 16.3°

These ratios work for any diameter, since only ae ÷ D matters.

Worked example: 1/2 in end mill at 10% stepover

D = 0.500 in, ae = 0.050 in. √(0.5 × 0.05 − 0.05²) = √0.0225 = 0.150. RCTF = 0.500 ÷ 0.300 = 1.67. Target chip load 0.002 in, so program 0.002 × 1.67 = 0.0033 in per tooth.

At 400 SFM in alloy steel, rpm = 3.82 × 400 ÷ 0.5 = 3056. With 4 flutes, feed = 3056 × 4 × 0.0033 = 40.7 ipm instead of 24.4 ipm without the correction.

Worked example: 5% finishing pass

Same tool, ae = 0.025 in. √(0.0125 − 0.000625) = √0.011875 = 0.1090. RCTF = 0.5 ÷ 0.218 = 2.29. A 0.0015 in target chip becomes 0.0034 in programmed per tooth. If the finish gets worse at the higher feed, use more flutes rather than more feed per tooth.

Axial depth and ball end mills

Radial chip thinning pairs with long axial engagement in high-efficiency milling: a light stepover and a deep axial cut spread wear along the flute. Ball end mills also have axial chip thinning when the depth of cut is small; that is handled with the effective diameter rather than the RCTF.

Common problems

Applying the factor to the feed rate twice, using it on slots (no thinning in a full slot), exceeding the maximum feed the corner radius or machine acceleration can handle, and ignoring tool deflection on long tools. Check the finished wall with an indicator or mic.

FAQ

What is chip thinning?

When stepover is under half the cutter, the actual chip is thinner than the feed per tooth.

What is the chip thinning factor at 10% stepover?

About 1.67.

Does chip thinning apply to slotting?

No, a full slot is 100% engagement and the factor is 1.

How do I check a milled wall?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

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General shop information only — follow your drawing, tool maker's data and your quality system.