CNC Milling of Aluminum: Grades, Speeds, and Tolerances That Hold

Aluminum's low cutting force and high thermal conductivity let modern machining centers run surface speeds of 1,000-3,000 SFM and chip loads that would destroy a tool in steel. With sharp uncoated or ZrN-coated carbide and proper coolant or air blast, material removal rates on a 40-taper machine routinely exceed 30 cubic inches per minute in 6061. That speed is the whole economic argument for aluminum: the spindle time per part is a fraction of what the same geometry costs in stainless. The trade-off is that aluminum is gummy. Wrought alloys with high silicon or magnesium content load up cutting edges and build up on the flutes if speeds drop or coolant fails, smearing the finish and dragging dimensions. Built-up edge is the number-one finish killer in aluminum milling, and it shows up worst at low RPM with dull tooling. The fix is the opposite of intuition for steel machinists: go faster, keep the edge sharp, and flush the chips aggressively. Polished-flute or high-helix end mills designed specifically for aluminum make a visible difference on wall finish.

6061-T6 is the default for a reason. It machines cleanly, welds and anodizes well, and at roughly 95 on the machinability scale it gives an excellent balance of strength (around 45 ksi tensile, 40 ksi yield) and cost. Choose it for brackets, housings, fixtures, and anything that needs anodizing without surprises. 7075-T73 trades weldability and cost for strength, landing near 73 ksi tensile in the T6 condition; the T73 over-aged temper sacrifices some of that strength to gain stress-corrosion-cracking resistance, which is why airframe structure specifies it. It machines slightly harder than 6061 and can leave a marginally rougher finish, but it holds tight tolerances and is the go-to for structural aerospace parts. 2024 is the fatigue-and-strength alloy of choice for aircraft skins and fittings, but its copper content hurts corrosion resistance and it does not anodize attractively, so it is usually clad or painted. 5052 is the formability and marine-corrosion alloy; it is rarely milled in volume because it is soft and gummy, but it shows up for milled plate parts that also need bending or salt-water service.

On a well-maintained VMC, aluminum holds +/-0.001 in routinely and +/-0.0005 in or tighter on critical features with temperature control and a finishing pass. Bore and flatness work in the tenths range is achievable but starts to demand a climate-controlled shop because aluminum's coefficient of thermal expansion (about 13 microinch/in/F) means a 70 F to 90 F swing moves a 10-inch part by several thousandths. Surface finish of 32 Ra microinch is easy with a finishing pass; 16 Ra and below is reachable with a fly cutter, polished tooling, and the right feed. The real risk on thin-wall and plate parts is distortion from residual stress. Plate stock, especially in 7075 and 2024, carries internal stress from rolling and quenching; when you mill away one face, the part bows. Buyers ordering thin precision parts should expect the shop to rough, stress-relieve or let parts rest, then finish, and should budget for the extra setup. Stress-relieved plate (e.g., 6061-T651) exists specifically to reduce this and is worth specifying for flatness-critical work.

For most aluminum milling, material is cheap relative to machine time, so geometry and setup count dominate the quote. A simple 6061 bracket from billet might run $15-$40 in moderate quantity; a multi-axis aerospace part in 7075 with tight tolerances and several setups can run hundreds. 7075 and 2024 plate cost noticeably more than 6061 per pound, and certified aerospace stock with mill test reports adds both cost and lead time because of traceability paperwork. Lead times for standard aluminum milling run 1-2 weeks at most job shops, often faster than steel because cycle times are short and tooling is rarely the bottleneck. Secondary operations move the needle most: hard anodize (Type III) adds 3-7 days and a real cost, clear or color Type II anodize is cheaper and faster, and chromate conversion (Alodine) per MIL-DTL-5541 is common for aerospace electrical-ground surfaces. Tight-tolerance parts that need stress relief and a rest cycle stretch lead time regardless of how fast the spindle runs.