An automation integrator in Stuttgart sent us a drawing last year for a robot joint housing: 6061-T6, 2 mm walls, flatness held to 0.05 mm. His previous shop shipped 200 pieces and 41 of them came off the CMM at 0.12 mm. Nothing was wrong with the machine. Nobody had stress relieved the blanks, and nobody had priced that step in either.
Most aluminum cnc machining services quote a part and a tolerance, then leave distortion for you to find. This page is about the part they skipped.
Why Aluminum Keeps Moving After You Cut It
Aluminum expands at roughly 23.1 µm per meter per degree Celsius. A 400 mm plate sitting 8 degrees warmer than the inspection room grows about 0.07 mm across its length. Measure it hot, ship it cold, and your customer measures a different part. It is the most common way aluminum cnc machining services jobs go wrong, and one of the easiest to prevent.
Thermal drift is the easy half, because you can wait it out. Residual stress is the stubborn half. Rolled plate and extruded bar arrive with stresses locked in from the mill. Cut material away unevenly and the balance breaks, so the part bends toward whichever side still carries more metal. It can take days to appear.
That is why two companies selling aluminum cnc machining services can run identical programs on the same alloy and ship different parts. One roughed, let the workpiece rest, then finished. The other cut to size in a single pass. The second shop is not lying when it says the part was good when it left.
What Grade You Pick Changes How Much It Moves
Buyers comparing aluminum cnc machining services usually start with the alloy, so start there properly. 6061-T6 is the default for good reasons: it machines clean, welds, anodizes predictably, and holds dimension when the process is sane. Brackets, housings and fixture plates belong here.
7075-T6 roughly doubles yield strength, and it is stiffer and more abrasive to cut. Structural and aerospace work needs it. You pay twice, in tool wear and in distortion, because higher strength means more stress left in the plate. We watched a 7075 plate move 0.3 mm after a single roughing pass.
2024-T3 sits between the two on strength and is worse on corrosion. 5052 is soft, gummy, and excellent near salt water. Neither belongs in a tight tolerance part by default.
The Three Shapes That Distort Most
Not every part is at risk. Across the aluminum cnc machining services we quote, three shapes cause nearly all the trouble.
Large thin plates. Any face bigger than about 300 mm with a thickness under 6 mm. Material comes out of the middle, the plate cups, and clamping cannot fix it afterwards. Vacuum fixturing plus a rough, rest, finish cycle usually does.
Thin wall rings and housings. Wall thickness under 2.5 mm on a turned part. The wall deflects under chuck pressure and springs back when released, so the diameter measured in the machine is not the diameter in the crate. Soft jaws bored to finish diameter, or an expanding mandrel, is the answer.
Long slender shafts. Past roughly ten to one length to diameter, the workpiece bends away from the tool and cuts a taper. A steady rest and lighter depths of cut cost money. A shop that skips them still ships you something, and that is where most aluminum cnc machining services complaints begin.
What A Shop Should Tell You Before You Accept The Quote
Ask these before the purchase order, not after the first article lands.
Does the quote include a stress relief or ageing step, priced as its own line? If they always machine to size, walk away from a tight flatness callout.
Will they rough, rest, then finish? On a plate or a thin housing this is the biggest single lever, and it adds a day or two rather than a week.
What is the temperature of the inspection room? Anyone selling aluminum cnc machining services who cannot answer is not measuring to 0.01 mm in any meaningful sense.
Is aluminum surface treatment handled in house or brokered out? Type III hard anodizing builds 0.03 to 0.05 mm per surface, which will eat an H7 bore. When two vendors touch the part, somebody has to own that number.
That last question is the one buyers skip, and it decides whether aluminum cnc machining services at this tolerance band are genuinely open to you.
How To Write Distortion Into The Drawing
You do not need a page of notes. Three callouts do most of the work.
Put flatness or parallelism on the face that seats, and leave the cosmetic faces uncontrolled. Datum the surface the part bolts to, not the one that was easiest to machine. Add one note, stress relieve before finish machining.
On turned parts, control the wall with a minimum instead of a symmetric tolerance. A 2.00 plus or minus 0.10 mm wall tells a shop less than a 2.0 mm minimum. That single change is where aluminum cnc machining services quotes start to separate.
Describe what the part does, too. A housing that carries a bearing has a different critical dimension from a housing that only shields a board. Brief a precision cnc machining service on function rather than geometry, and the process plan improves on its own.
Quick Answers On Aluminum Machining Deformation
How tight can flatness be held on a thin aluminum plate?
On a 400 by 300 by 5 mm 6061-T6 plate, 0.05 mm is routine with a rough, rest, finish sequence. Below 0.02 mm gets expensive fast, and a good supplier will say so.
Do you run 7075, 2024 and 5052 as well as 6061?
All four, plus 6082 and 6063. Brass, stainless steel, titanium and engineering plastics run on the same floor, so a mixed assembly does not need three separate aluminum cnc machining services vendors.
What lead time is realistic?
Simple aluminum prototypes run 5 to 8 working days. A thin wall housing with a stress relief cycle and a finish is closer to two to three weeks, because the rest between roughing and finishing is real calendar time.
Do cheap quotes ever work out?
On simple parts with loose tolerances, sometimes. On anything thin, buying aluminum cnc machining services on unit price alone usually means paying twice.
Send the drawing, the alloy grade, and what the part actually does. If your aluminum cnc machining services quote never mentions how the part gets held, the deformation is hiding in that silence. Reach us through our inquiry page, or start from the full list of CNC machined parts we run on this floor.