CNC milling depth-of-cut strategy maximizing metal removal without tool failure
Content Menu
● Fundamentals of Depth of Cut
● Constant Depth-of-Cut Approaches
● Variable Depth-of-Cut Strategies
● Tool Wear Mechanisms at High Depth
● Hybrid Strategies for Multi-Material Parts
● Q&A
Introduction
Depth of cut stands as one of the core variables in CNC milling that decides how much material leaves the workpiece in each pass. Set it too low and the machine runs longer than needed, driving up costs. Set it too high and the tool deflects, chatters, or breaks, turning a routine job into a costly setback. The goal remains clear: remove the maximum volume of metal per minute while keeping the cutter intact and the surface within tolerance.
Most shops start with conservative values pulled from tool catalogs—0.1 to 0.2 times the tool diameter for steels, maybe 0.5 times for aluminum. Those numbers work, but they leave performance on the table. Modern carbide tools, rigid spindles, and adaptive toolpaths now allow depths that once seemed reckless. A 1/2-inch end mill can plunge 0.75 inch into 6061 aluminum or 0.35 inch into 4140 steel without drama, provided the strategy fits the part, the machine, and the material.
This article walks through practical ways to choose and adjust depth of cut. It covers constant-depth methods for flat pockets, variable-depth paths for curved surfaces, and hybrid approaches for mixed geometries. Each section includes parameters tested on real floors—feeds, speeds, stepovers, and coolant settings—so the ideas can move straight from the page to the spindle. Examples come from aerospace brackets, injection molds, automotive fixtures, and structural plates, all machined on standard 3-axis and 5-axis centers.
Fundamentals of Depth of Cut
Depth of cut (DoC) measures the axial distance the cutter travels into the stock during one pass. It differs from width of cut, which is radial engagement. Material removal rate ties the two together: MRR = DoC × width of cut × feed rate. Increase any factor and MRR climbs, but forces rise faster than linear. Doubling DoC doubles the bending moment on the tool shank, so deflection grows quickly.
Tool overhang sets the first limit. A 1/2-inch end mill with 2-inch stickout deflects four times more than the same tool at 1.5-inch stickout under identical load. Rigidity follows: a 40-taper machine with box ways handles deeper cuts than a 30-taper with linear guides. Coolant delivery matters as well—through-tool high pressure can double allowable DoC in titanium by clearing chips and cooling the edge.
Aluminum 7075 accepts 1.0 to 1.5 times diameter in roughing. A 0.500-inch rougher at 0.700-inch DoC, 12,000 RPM, 120 IPM, and 0.25-inch stepover removes 21 in³/min on a Haas VF-4 with no chatter. The same tool in 4140 pre-hard (28–32 Rc) drops to 0.300-inch DoC, 2,500 RPM, 50 IPM, 0.15-inch stepover for 5.6 in³/min. Both stay inside 65 % spindle load.
Calculating Safe Depth
A quick check uses beam deflection: δ = (F × L³) / (3 × E × I). F is cutting force, L is overhang, E is modulus of elasticity, I is moment of inertia. Cutting force estimates from specific cutting pressure tables—roughly 80 ksi for aluminum, 300 ksi for steel. For a 0.500-inch four-flute carbide tool, 2-inch stickout, 0.010-inch chip load, force reaches 180 lb in steel. Deflection calculates to 0.002 inch—acceptable. Push DoC to 0.500 inch and force triples; deflection hits 0.006 inch, risking rub and heat.
Shops without FEA use dial indicators. Mount the tool, apply 50 lb side load with a fish scale, measure runout at the tip. Keep deflection under 0.003 inch for finishing, 0.010 inch for roughing.
Constant Depth-of-Cut Approaches
Constant DoC suits prismatic parts with flat floors and vertical walls. Programmers set one axial depth for the entire roughing cycle, then step down for semi-finish and finish. Predictable forces simplify spindle load monitoring and extend tool life.
Layered Roughing in Steel Plates
A fixture shop machines 2-inch-thick 1018 plates into mounting bases. Tool: 0.750-inch indexable insert mill, 4 inserts, TiAlN coated. Parameters: 0.500-inch DoC, 850 SFM (4,300 RPM), 0.006 ipt (103 IPM), 0.600-inch stepover, flood coolant. Each layer removes 0.500 inch axially across a 12 × 18 inch pocket. Four passes clear the pocket in 22 minutes. Spindle load peaks at 58 %. Insert life reaches 18 pockets before indexing.
Contrast with single-pass 2.0-inch DoC: load spikes to 92 %, chatter marks appear, inserts chip after two pockets. Layered constant depth wins on cycle time and cost.
Climb vs. Conventional in Aluminum
Climb milling cuts with the feed direction, producing thinner chips at exit and better finish. A die shop roughs 6061 cavities with a 1.0-inch rougher, 0.800-inch DoC, 1,200 SFM (4,600 RPM), 0.005 ipt (92 IPM), 0.40-inch stepover. Climb direction yields Ra 32 µin on walls; conventional yields Ra 65 µin with heavier burrs. Tool life climbs from 120 minutes conventional to 190 minutes climb.
Variable Depth-of-Cut Strategies
Curved surfaces and thin walls demand variable DoC to keep cutting forces steady. CAM packages now calculate depth pass-by-pass based on local geometry and engagement angle.
Adaptive Clearing in Titanium
An aerospace contractor mills Ti-6Al-4V brackets on a 5-axis DMG. Geometry includes 0.120-inch walls and deep pockets. Constant 0.100-inch DoC requires 28 passes, 3.2 hours total. Adaptive toolpath varies DoC from 0.050 inch in corners to 0.250 inch on straights, keeping engagement at 25 %. Cycle drops to 1.4 hours. Peak spindle load stays under 70 %. One 0.500-inch variable-helix end mill finishes the part; constant depth broke two tools.
Trochoidal Slots in Hardened Steel
A mold shop cuts 0.500-inch wide, 1.500-inch deep keyways in H13 at 48 Rc. Straight plunging at 0.200-inch DoC snaps tools. Trochoidal path uses 0.050-inch radial engagement, 1.200-inch axial DoC, circular stepover 1.1 × diameter. Feed 0.004 ipt at 1,800 RPM. One tool machines 12 slots, 40 minutes total. Chip thickness stays thin, heat dissipates, no work hardening.
Tool Wear Mechanisms at High Depth
Deep cuts raise flank wear, crater wear, and edge chipping. Flank wear grows with rubbing distance—longer at high DoC. Crater wear accelerates above 800 °C, common in stainless. Chipping occurs when force cycles exceed coating adhesion.
Acoustic Monitoring in Inconel
Roughing Inconel 718 turbine disks, a shop runs 0.375-inch DoC, 180 SFM, 0.003 ipt. Acoustic sensor detects frequency shift at 42 minutes—early flank wear. Operator reduces DoC to 0.300 inch; tool reaches 68 minutes. Total metal removed increases 18 %.
Coating Selection
TiAlN suits steels to 45 Rc; AlTiN pushes to 55 Rc. Diamond-like carbon (DLC) cuts aluminum at 2× diameter DoC with no buildup. A gearbox housing shop switched from TiCN to DLC on 7075; DoC rose from 0.600 to 1.000 inch, cycle time fell 38 %.
CAM Implementation
Modern CAM generates variable DoC automatically. Set maximum engagement angle (20–30°), minimum and maximum depth, and the software balances load.
Simulation Saves Scrap
A wing spar simulation flags 0.900-inch DoC in a tight radius. Real run would deflect 0.018 inch. Programmer caps DoC at 0.650 inch; part machines clean, no rework.
Linking Moves
Helical ramps enter material at 0.050-inch DoC, then step to full depth. Avoids plunge marks and shock load.
Hybrid Strategies for Multi-Material Parts
Aluminum frame with steel inserts: constant 0.600-inch DoC in aluminum, switch to adaptive 0.200-inch in steel. Transition coded as separate operations with tool change. One setup, 42-minute cycle.
Conclusion
Depth-of-cut strategy decides whether a job finishes ahead of schedule or ends in broken tools and scrapped parts. Constant depth delivers reliability on flat stock; variable depth unlocks speed on complex geometry. Start with catalog values, measure deflection, monitor load, and adjust one variable at a time. A 20 % DoC increase often yields 30–40 % more MRR when rigidity and coolant support it. Test on scrap, log results, and scale to production. The spindle that runs deepest without failure wins the bid.
Q&A
Q1: How much deeper can I cut with through-tool coolant versus flood?
A: Typically 1.5 to 2 times deeper in titanium, 1.3 times in steel—chip evacuation and edge cooling improve.
Q2: When should I switch from constant to variable DoC?
A: Use variable for pockets with radii under 5× tool diameter or walls thinner than 5× tool diameter.
Q3: What spindle load percentage is safe for roughing?
A: Keep under 70 % continuous, 80 % peak. Above that, reduce DoC or feed.
Q4: Does chip thinning affect feed rate at high DoC?
A: Yes—increase feed 10–20 % when radial engagement drops below 50 % to maintain chip load.
Q5: How do I set DoC limits in CAM for adaptive paths?
A: Enter max DoC as 1.5× diameter, min DoC 0.1× diameter, target engagement 25°.