5-Axis Machining Aluminum 7075-T6

Why 7075-T6 on 5-axis

7075-T6 is the standard structural aluminum for aerospace. Yield strength 503 MPa, density 2.81 g/cm³ — the best strength-to-weight ratio of any common aluminum alloy. It's everywhere: wing ribs, bulkheads, fuselage frames, actuator housings, avionics enclosures. Nearly all of these parts have complex 3D geometry that requires simultaneous 5-axis machining.

The alloy machines beautifully. It's free-cutting, produces short chips, and doesn't work-harden. The challenge isn't machinability — it's that aerospace parts in 7075 are typically large, thin-walled, and machined from solid billet with 90–95% material removal ratios. A 40 lb billet becomes a 2 lb part. Managing distortion, maintaining thin-wall tolerances, and keeping cycle times competitive are the real problems.

Speeds and feeds

Roughing with indexable cutters: 2,000–4,000 SFM, 0.006–0.012" IPT, 0.500–1.500" axial DOC, 40–60% radial engagement. Face mills and shell mills with polished positive-rake inserts (Kennametal KCPK, Iscar IC28, Sandvik H13A). Uncoated or diamond-coated inserts — TiAlN coating is unnecessary and adds friction on aluminum. Material removal rates of 20–50 in³/min are routine on 50-taper machines with 30+ HP spindles.

Roughing with solid carbide: 1,500–3,000 SFM, 0.004–0.008" IPT, 1.0–2.0x diameter axial DOC, 25–40% radial engagement. 3-flute end mills with polished flutes and variable helix (35/38°). High-efficiency milling (HEM) / trochoidal toolpaths at higher speeds and feeds with reduced radial engagement — the cutter is always engaged but never fully loaded. Helical interpolation for bore roughing.

High-speed finishing: 3,000–6,000 SFM, 0.002–0.004" IPT, 0.010–0.030" axial DOC, 5–15% radial engagement. This requires a high-speed spindle (15,000–30,000 RPM) to maintain SFM with small-diameter cutters. Ball nose end mills for contoured surfaces, bull nose for floors with fillets. Feed rates of 200–500 IPM are common with small tools at high RPM — the machine's acceleration and jerk limits become the constraint, not the cutting parameters.

Drilling: 500–800 SFM, 0.004–0.010" IPR depending on drill diameter. Through-coolant carbide drills. No peck cycle needed for depths under 4D in 7075 — the chips evacuate cleanly. For deep holes (8D+), use gun drills or through-coolant peck cycles.

Thin-wall machining

This is where 5-axis aluminum work gets interesting. Aerospace structural parts routinely have wall thicknesses of 0.040–0.080" with floor thicknesses of 0.060–0.100". Pockets are deep (2–6" deep) with tight radii and stiffening ribs.

Strategy: Machine one side, flip, machine the other side. But the order matters. Rough both sides first, leaving 0.030–0.050" of stock on all finished surfaces. Then finish one side, flip, finish the other. This lets residual stresses redistribute before finish machining locks in the final geometry.

Distortion control: 7075-T6 plate has residual stress from the rolling and heat treatment process. Removing 90% of the material releases these stresses, causing the part to warp. Countermeasures: machine from stress-relieved plate (T7351 temper — pre-stretched), use alternating climb/conventional passes on large faces, and leave a 0.020–0.030" finish allowance that's removed in a separate light pass after the part relaxes.

Thin-wall deflection: A 0.060" wall that's 4" tall will deflect under cutting forces. Reduce radial engagement to 5–10% on finish passes, increase spindle speed to maintain chip load at the reduced engagement, and use very short stick-out end mills. Axial depth of cut on thin walls: maximum 1D of cutter diameter per pass. Vacuum fixturing or wax potting of thin sections prevents vibration.

Achievable tolerances

General profile: ±0.005" is routine on 5-axis machines with proper workholding. This covers 80% of aerospace structural tolerances.

Critical dimensions: ±0.001–0.002" on bore diameters, hole positions, and mating surfaces. Requires probing (Renishaw OMP or OTS) to establish part datums after roughing and before finish passes.

Thin-wall thickness: ±0.005" on walls under 0.080" thick is standard. ±0.003" is achievable with light finish passes and reduced cutting forces. ±0.001" on thin walls requires specialized fixturing and is not routine production work.

Flatness: 0.002" over 12" on machined faces is achievable in temperature-controlled environments. Thermal growth of the part during machining (aluminum expands 13.1 µin/in/°F) means a 10°F temperature rise on a 12" part causes 0.0016" of growth. Flood coolant maintains part temperature.

Surface finish: Ra 32–63 µin is standard. Ra 16–32 µin with high-speed finishing passes. Ra 8–16 µin with diamond-polished tooling. Anodize-ready surfaces (Ra 32 µin or better) are standard expectations for aerospace 7075 parts.

Fixturing for 5-axis

5-axis aluminum parts are typically fixtured on the A/B rotary table using tombstone fixtures, custom soft jaws, or vacuum plates. The key requirement: full access to five sides of the part without interference between the spindle housing, tool, and fixture.

Dovetail fixturing is common for aerospace structural parts. Machine a dovetail feature on the raw stock, clamp the dovetail in a precision vise, and machine the part from five sides. The dovetail is cut off as the last operation. This eliminates clamp marks and provides rigid, repeatable workholding.

Vacuum fixturing works for thin, flat parts (skins, panels, covers) that can't be clamped without distortion. Requires a vacuum plate matched to the part profile — typically machined from MIC-6 cast aluminum plate.

Cost structure

7075-T6 aluminum is inexpensive compared to titanium or Inconel — $3–6/lb for plate, $4–8/lb for bar. But aerospace aluminum parts have high buy-to-fly ratios (10:1 to 20:1), meaning you're paying for 10–20 lbs of material to get 1 lb of finished part. A 15 lb billet at $5/lb = $75 in material for a 1.5 lb part.

Cycle times are the main cost driver. A structural rib with deep pockets and thin walls might take 3–8 hours on a 5-axis machine at $125–175/hr. The same geometry in a simpler alloy would take the same time — it's the geometry, not the material, that drives cost. Programming time for complex 5-axis parts: 4–20 hours at $75–125/hr, amortized over the production quantity.

Where 7075 saves money vs titanium: 5–10x faster cutting speeds, 1/5 the material cost, and 2–3x longer tool life. An aerospace bracket that costs $800 in Ti-6Al-4V might cost $200 in 7075-T6 — if the design loads allow aluminum.