Aluminum CNC Machining: Alloys, Tolerances & Finishes

Aluminum is the most commonly machined metal, and for good reason. It cuts fast, holds tight tolerances, takes a clean finish, and costs less to run than steel or titanium, which makes it the default choice for prototypes, structural parts, housings, and almost any component that does not need the strength or temperature resistance of a harder alloy. As a method page within CNC machining, this covers the alloys a designer chooses between, the tolerances and finishes they reach, and the few behaviors, edge buildup, galling in tapping, and limited high-temperature use, that shape how aluminum is specified and cut.

The reason aluminum machines so well is thermal. Aluminum conducts heat away from the cut quickly, so the cutting edge stays cooler and the chip carries the heat with it, which lets the machine run high surface speeds and feed rates without overheating the tool. That same conductivity is why aluminum takes a bright, clean machined finish and why it holds dimensional tolerance readily, with 6061-T6 reaching ±0.025mm (±0.001in) in capable hands. The trade-off is that aluminum is soft and gummy, so it can weld itself to a sharp cutting edge, and it needs the right tooling, coating, and coolant to keep that from happening.

The common alloys

The table above lists the three most common machining alloys, and a few more round out the selection. The right choice depends on what the part must do: carry load, take a form, hold a flat reference, or simply ship cheaply and quickly.

6061-T6: the general-purpose alloy

Aluminum 6061-T6 is the general-purpose alloy, balancing strength (tensile 310MPa, or 45ksi), corrosion resistance, machinability, and cost, and it is the default for brackets, housings, plates, and structural parts that are not highly stressed. It machines cleanly to ±0.025mm (±0.001in) in capable hands, takes anodize well, and welds readily with the right filler. For most non-specialized work, 6061-T6 is the alloy a designer should reach for first.

7075-T6: high strength

Aluminum 7075-T6 is far stronger, at 572MPa (83ksi) tensile, and is the choice for aerospace structural fittings and high-stress components, though it costs more and is not suitable for bending. Its strength-to-weight is the reason it appears in airframe and high-performance duties where 6061 would be too weak, and its poorer weldability pushes designs toward bolted or bonded joints.

5052-H32: forming and marine service

Aluminum 5052-H32 is the formability alloy, the best of the three for bending and deep drawing, and it has excellent marine corrosion resistance, which is why it dominates sheet-metal work rather than machining. It is weaker than 6061, so it is chosen when a part must be bent or formed rather than when it must carry maximum load.

Specialty alloys for specific duties

A few other alloys appear in specific duties. Aluminum 2024 is a high-strength aerospace alloy, stronger than 6061 in fatigue, used in aircraft skins and structural parts but with lower corrosion resistance. Aluminum 6061-T651 is the stress-relieved plate version of 6061, chosen for flat, stable plates and fixtures where internal stress would otherwise warp a machined part. Cast aluminum tooling plate, sold under names like MIC-6 or Alca 5, is dimensionally stable and machines flat, which is why it is used for large plates, vacuum chucks, and fixtures. The alloy and temper should be called out on the drawing because properties and machining behavior differ between them.

Why aluminum machines well

Aluminum’s machinability comes from a combination of low hardness, good thermal conductivity, and a tendency to form a clean chip at high speed. Each trait contributes to the fast, clean cut that makes aluminum the most economical metal to machine.

Thermal conductivity and low cutting force

The low hardness means cutting forces are modest, so the machine and workholding see less load. The high thermal conductivity, around 167 W/m·K for 6061, pulls heat out of the cut zone and into the chip and part rather than concentrating it in the tool, which is the opposite of titanium’s behavior. The result is that aluminum runs at surface speeds several times those of steel, with high feed rates, and it finishes to a bright, smooth surface that often needs no secondary operation.

Managing edge buildup

The behavior to manage is edge buildup. Aluminum’s softness and ductility mean that under the wrong conditions, heat and pressure weld a sliver of the workpiece to the cutting edge, which then drags across the surface and ruins the finish. The remedies are well understood: sharp, polished, or uncoated tooling with a high positive rake angle to shear rather than push; high feed per tooth so the chip is thick enough to carry heat away; and generous flood coolant or air blast to clear chips and cool the cut. With those in place, aluminum machines cleanly at high speed and holds tolerance readily.

Design rules for machined aluminum

Aluminum is forgiving, but its few trouble spots, edge buildup, gummy tapping, anodize growth, and thin-wall chatter, are predictable. The rules below group these by the feature or step they affect.

Cutting and tapping behavior

Prevent edge buildup by specifying or expecting sharp, polished tooling and flood coolant, and by designing features that do not force a slow, rubbing cut. Size threads and tapped holes with care, because aluminum is gummy and can tear if chips pack; allow spiral-flute taps for blind holes and appropriate speeds with lubrication.

Fits, finishes, and dimensional growth

Account for anodizing on fits. Anodizing removes about 10 to 15µm (about 400 to 590µin) of base metal per surface and adds a layer, so size mating fits before the anodize, or specify the surface to be left un-anodized on a tight fit. Keep walls thick enough: non-critical walls from 0.5 to 1.0mm (0.020 to 0.040in) are possible but deflect under cutting force, while structural walls should be 1.5mm (0.060in) or more.

Alloy choice and forming

Watch forming and temper, since 6061-T6 is not recommended for complex bending, with springback of 5 to 10 degrees; use 6061-T4 or the 5052 alloy if a part must be machined and bent. Choose the alloy for the load, with 6061 for general work, 7075 for high stress, and 5052 for forming; specifying the right alloy up front avoids a costly change later.

Finishes and surface treatments

Aluminum accepts a wide range of finishes, and the choice depends on function and appearance. Each finish has a dimensional effect, and anodizing in particular both removes and adds material, so critical fits are either finished after anodizing or masked to stay bare.

Anodizing: type II and type III

Anodizing is the most common finish for aluminum. Type II (sulfuric) anodize adds a thin, hard, dyed layer for corrosion resistance and color, which is why it dominates cosmetic and corrosion-protected parts. Type III (hardcoat) anodize builds a thicker, harder layer for wear surfaces, at the cost of more dimensional growth and a darker, less predictable color. Either type removes about 10 to 15µm (about 400 to 590µin) of base metal per surface while adding a layer on top, so tight mating fits must be sized before anodizing or masked.

Conversion coatings and paint

Chromate conversion (chem film) adds corrosion resistance and improves paint adhesion without the dimensional change of anodizing, which makes it the choice where dimensions must stay as-machined. Bead blasting produces a uniform matte finish that hides machining marks, often combined with anodize or clear coat for a consistent appearance. Powder coating and wet paint are used for cosmetic and corrosion protection on larger parts where the coating thickness is not a constraint.

Machining parameters and tooling

Aluminum rewards specific machining practice, and getting it right is the difference between a clean, fast cut and a smeared, built-up mess. Sharp, polished, or uncoated solid-carbide end mills with a high positive rake angle shear the material cleanly, while tooling suited to steel, particularly TiN-coated general-purpose inserts, tends to build up an edge on aluminum and should be avoided for finishing. Surface speeds run high, often several hundred meters per minute on 6061, with feed rates sized to keep the chip thick enough to carry heat away and prevent the tool from rubbing. Flood coolant or through-tool air blast clears chips and cools the cut, and on deep pockets a peck or high-feed strategy keeps chip evacuation under control so chips do not weld to the wall.

Workholding on aluminum is generally straightforward because cutting forces are low, but aluminum’s softness means a vise can mar a cosmetic surface, so soft jaws, tape, or protective pads are used where finish matters. Large aluminum plate can ring and chatter if it is not damped, so rigid clamping and sometimes a damping backing board or a wax-based damping compound help on thin sections. The combination of low cutting force, high speed, and clean chip formation is what makes aluminum the most economical metal to machine per unit of material removed, and it is a large part of why an aluminum part usually costs less to make than the same geometry in steel or titanium.

Cost and lead time

Aluminum is among the quickest materials to take from drawing to finished part, and the reasons stack up. Stock is widely available in standard plate, bar, and extrusion sizes, so material lead times are short and the alloy a designer specifies is usually already on a supplier’s shelf. Cycle times are short because the metal cuts fast and at high feed. Finishes like anodizing and chromate conversion are mature, widely offered, and quick to apply. Together these make aluminum the default for prototyping and low-volume production, where speed and cost matter and the part does not need the strength or temperature resistance of a harder alloy.

The main cost driver on an aluminum part is usually not the aluminum itself but the part’s geometry, tolerance, and finish. Setup time, the number of operations, deep pockets that need many tool passes, tight tolerances that slow the finish cut, and secondary operations like anodizing and assembly all weigh more on the price than the raw material. Designing for fewer setups, sensible tolerances, and standard stock sizes keeps an aluminum job economical, and the metal’s forgiving machinability gives a designer more freedom to do that than a harder alloy would.

Welding and joining

Aluminum welds, but it welds differently from steel and not all alloys weld equally well, so the joining method is worth choosing early. The choice affects wall thicknesses, fit-up tolerances, and finish, so it should be settled during design rather than discovered at assembly.

Which alloys weld, and how

Aluminum 6061 welds readily with the right filler, commonly 4043 or 5356, using TIG or MIG with alternating current to break the tenacious oxide layer; 7075 is generally not recommended for welding because it tends to crack in the heat-affected zone. The oxide on aluminum melts at a far higher temperature than the base metal, which is why welding it needs AC TIG or special techniques to remove the oxide as the weld proceeds, and why a clean, oxide-free joint is essential.

Distortion and alternatives to welding

Welding distorts aluminum more than steel because of its high thermal expansion and conductivity, so welded assemblies need careful fixturing and often a post-weld stress relief or straightening pass. For CNC parts, alternatives to welding are common and sometimes better: threaded fasteners for disassembled joints, adhesive bonding for cosmetic or sealed joints, and press fits for permanent pins and inserts.

Heat treatment and temper

Aluminum alloys are sold in specific tempers, and the temper changes both the properties and the machining behavior, so it should be specified and understood. The T6 temper, common on 6061 and 7075, is a solution-heat-treated and artificially aged condition that delivers the alloy’s full strength; the T4 temper is solution-treated and naturally aged, softer and more formable, which is why it is chosen for parts that must be bent after machining. The O temper is fully annealed, the softest condition, used for forming operations. Stress-relieved tempers like T651 are used for plate that must stay flat and stable through machining, because they relieve the internal stresses that would otherwise warp a part as material is removed. Specifying the right temper on the drawing avoids receiving stock that machines differently or performs differently in service than the design assumes.

Worked examples

Example: a structural bracket in 6061-T6 carries a moderate load and needs a clean cosmetic finish. The part is machined from plate to ±0.025mm (±0.001in) on the bearing bore, then type II anodized for corrosion resistance, with the bore masked so the anodize growth of 10 to 15µm (about 400 to 590µin) per surface does not change the fit.

Example: an aerospace fitting in 7075-T6 takes a high static load that 6061 cannot carry. The fitting is machined from bar stock at 572MPa (83ksi) tensile, with structural walls held at 1.5mm (0.060in) or more to resist chatter, and it is left bare because 7075 does not weld reliably and the joint is designed as a bolted interface.

When not to use aluminum

Aluminum is wrong for duties that exceed its limits. It is too soft for high-temperature service, losing strength well below the temperatures steel or titanium tolerate, so engine and exhaust components near heat call for steel or superalloys. It wears under sliding or abrasive contact faster than hardened steel, so heavily loaded bearing or wear surfaces may need steel or a hard coating. And for the most heavily loaded structural aircraft parts, 7075 is typical where 6061 would be too weak, or titanium and steel where even 7075 is insufficient. For ordinary structural, housing, and prototype work, though, aluminum is the right and economical choice, which is why it dominates CNC machining.

Applications

Aluminum CNC parts appear across nearly every industry. Aerospace structural brackets, fittings, and housings in 7075 and 6061; automotive prototypes, fixtures, and housings; aerospace and automotive components where weight matters; electronic housings and heat sinks that exploit aluminum’s thermal conductivity; optical and instrument mounts that need stability and a clean finish; and a huge range of jigs, fixtures, and plates in tooling. The common thread is a need for light weight, corrosion resistance, a good finish, or fast economical machining, all of which aluminum delivers better than almost any other metal.

Tolerances and surface finish

Aluminum 6061-T6 machines to about ±0.025mm (±0.001in) in capable hands, one of the tightest practical tolerances of the common alloys, precisely because it cuts cleanly and resists deflection. The standard as-machined surface finish is Ra 3.2µm (125µin), and aluminum’s bright appearance makes a fine finish particularly visible, so a finishing pass often brings it to Ra 1.6µm (63µin) for cosmetic or sealing surfaces. Grinding is rarely needed on aluminum because its softness lets a sharp tool reach a fine finish directly, but lapping can produce optical-quality surfaces where required. As with all metals, tighter tolerances and finer finishes cost more through slower feeds and added operations, so they are reserved for the features that need them.