Custom Aluminum CNC Parts

Why is aluminum the default for machined parts?

Aluminum weighs about a third as much as steel, and in air it forms a thin oxide film that protects it from further corrosion by oxygen and water (source 1).

The bigger reason is cutting speed. Tooling makers expect high cutting speeds and long tool life in aluminum, while stainless steel and titanium work harden, run hot and build up on the cutting edge (source 2). Published starting ranges put wrought aluminum, 6061 and 7075 included, at 800-1,500 surface feet per minute (source 3), against 100-350 for stainless steel (source 4). Slower cutting means more machine time, which is why the same part usually quotes lower in aluminum than in stainless or titanium.

Which aluminum alloy should the part be?

6061-T6, unless the part gives a reason to use something else. It is one of the most common aluminum alloys for general-purpose use, with a typical yield strength of 39 ksi. It welds by TIG or MIG, though the metal next to the weld drops to about T4 properties, roughly 40% weaker (source 5).

7075 is the grade for highly stressed parts: 73 ksi yield in the T651 temper (source 6). It is notoriously hard to fusion weld because it hot cracks (source 7). The two alloys are nearly equally stiff, with a modulus of 69 GPa for 6061 and 71.7 GPa for 7075 (sources 5 and 6), so a part that flexes too much in one flexes nearly as much in the other, and the section has to change instead.

When the call is close, ask for both alloys on one quote; the full comparison is on 6061 vs 7075 aluminum. A flat base plate has a third option, cast tooling plate: Clinton Aluminum’s comparison lists MIC-6 flat within 0.005 in on plate 3/4 in and thicker, at a typical yield of 15 ksi (source 8). The tooling plate options are on aluminum tooling plate fixtures, and plates drilled on a tapped and doweled grid for workholding are on aluminum fixture plates. Other alloys and plastics quoted on request. Name the grade on the print.

Is it a machined part, a laser-cut part or a stock item?

A part with pockets, bores, threads, or a face that has to be flat is a machined part: CNC milled from aluminum plate or bar, or CNC turned when it is round. A flat profile cut from sheet, or one bent into a bracket or channel, is a laser-cut part: laser-cut aluminum parts covers that route, and one order can hold both kinds. Brackets can go either way, and custom CNC brackets compares the two. A performance part cut from a solid block, such as a valve cover or a catch can, is covered on billet aluminum parts.

Standard framing extrusion, tube, and plain plate or bar stock are catalog items from their makers and metal distributors. A machined part that bolts to one of them is a custom part, and the connector question below covers what to send for it.

Which finishes are common on machined aluminum?

A finish called out on the print is quoted with the part, and parts come back finished and inspected. The table lists the common callouts.

Anodize is grown out of the aluminum rather than laid on top of it. The oxide forms into the surface as well as out from it, so every anodized surface stands a little proud of where it was machined (source 10). Bores, press fits and threads that must hold size are masked or allowed for on the print, and so is any face that has to carry current, because the anodic layer is an electrical insulator (source 10).

Dyed color varies with the base alloy (source 10), so a mixed 6061 and 7075 assembly in one dyed color may not match. Coating thickness by type, how much a surface grows, masking and color are set out on the anodizing service page.

What should the print say for an aluminum part?

Write the temper with the alloy, 6061-T6 or 7075-T651 rather than just “aluminum”: the same alloy is also sold annealed, as 6061-O, and the strength figures above belong to the temper (source 5). Give the finish as a callout from the table, and mark any surface that has to be masked.

Mark the critical dimensions with their tolerances. A STEP file with no drawing is quoted to a ±0.005 in general tolerance, or the one your notes name, so a tighter bore or fit needs a drawing or a note; what file to send covers that rule in full. No C of C or material certifications ship with parts by default. If your PO needs them, say so before the quote. Inspection reports on critical dimensions aren’t included by default. If you need one, ask when you place the order. If the alloy is still open, say what the part bolts to or carries, so the quote can say whether 6061 is enough. Ask for more than one quantity, such as 1, 25 and 100, and the quote shows each.

Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. Lead time starts at order approval. Put your need date in the part description and the quote answers it. If the date cannot move, rush CNC parts explains when rush fits.

Sources

All accessed 27 September 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Aluminium": en.wikipedia.org/wiki/Aluminium (density about one third that of steel; a thin oxide layer protects the metal from further corrosion by oxygen, water or dilute acid).

2. Sandvik Coromant, "Workpiece materials": sandvik.coromant.com/en-us/knowledge/materials/workpiece-materials (ISO N non-ferrous: high cutting speeds and long tool life; ISO M stainless: heat, notch wear, built-up edge, hardens as it is cut; ISO S including titanium: sticky, built-up edge, hardens as it is cut, heat).

3. Harvey Performance Company, "Attacking Aluminum: a Machining Guide", In The Loupe, 24 April 2018: harveyperformance.com/in-the-loupe/aluminum-machining-guide/ (wrought 2024, 6061 and 7075 at 800-1,500 SFM).

4. Harvey Performance Company, "Slaying Stainless Steel: Machining Guide", In The Loupe, 19 June 2018: harveyperformance.com/in-the-loupe/stainless-steel-machining/ (stainless steels at 100-350 SFM).

5. Wikipedia, "6061 aluminium alloy": en.wikipedia.org/wiki/6061_aluminium_alloy (one of the most common alloys for general-purpose use; T6 typical yield 39 ksi; highly weldable by TIG or MIG, near-weld properties those of T4, about 40% loss; Young’s modulus 69 GPa; O, T6 and T651 tempers).

6. Wikipedia, "7075 aluminium alloy": en.wikipedia.org/wiki/7075_aluminium_alloy (T651 yield 73 ksi; Young’s modulus 71.7 GPa; highly stressed structural applications).

7. M. Sokoluk, C. Cao, S. Pan and X. Li, "Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075", Nature Communications, 2019: pmc.ncbi.nlm.nih.gov/articles/PMC6327098 (fusion welding AA7075 is notoriously difficult; hot cracking during arc welding).

8. Clinton Aluminum, "ATP-5 vs Mic-6: What’s Best For Your Business?": clintonaluminum.com/atp-5-vs-mic-6/ (MIC-6 flat within 0.005 in on 3/4 in and up, 0.015 in on 1/4-5/8 in; typical yield 15 ksi).

9. Wikipedia, "Sandblasting", section "Bead blasting": en.wikipedia.org/wiki/Sandblasting#Bead_blasting (fine glass beads; used to create a uniform surface finish on machined parts).

10. Wikipedia, "Anodizing": en.wikipedia.org/wiki/Anodizing (Type II sulfuric, Type III thicker hard anodize; oxide grown into and out from the surface, increasing the dimensions of each surface; the anodic layer is insulative; common dye colors include black, blue and red, and colors vary with the base alloy).

11. Aluminum Anodizers Council, "Military Specification MIL-A-8625": anodizing.org (six types, including Type II sulfuric and Type III hard anodize; Class 1 non-dyed, Class 2 dyed).

12. Anoplate, "Hardcoat Anodize, MIL-A-8625 Type III": anoplate.com/finishes/hardcoat-anodize/ (undyed hardcoat deep gray-black on many 6xxx alloys, bronze-gray on most 7xxx; black by far the most specified dye, other colors less decorative; wear resistance).

13. Wikipedia, "Powder coating": en.wikipedia.org/wiki/Powder_coating (dry powder applied electrostatically and cured under heat; can produce much thicker coatings than conventional liquid coatings).