A medical device customer sent us a 6061-T6 enclosure last spring with a 0.8 mm wall on three sides. The first article came off the machine at 0.78 mm, comfortably inside the callout, and two hours later the CMM operator measured the same part at 0.71 mm with the flatness gone. Nothing was wrong with the program. The part was simply doing what aluminium does when you take eighty percent of the material off one side of it.
Thin walls are the most common reason a part that quotes cheap comes back expensive. Here is what is happening in the cut, which wall thicknesses hold in practice, and what to change on the drawing before it is frozen.
The number that matters is a ratio, not a thickness
Nobody should care that a wall is 1 mm. What matters is how tall and how long that wall is, because stiffness falls with the cube of thickness and only linearly with height. A 1 mm wall standing 12 mm tall is a routine job. The same wall standing 40 mm tall is a piece of sheet metal you are trying to convince to behave like a casting.
Our working rule for general tolerances on aluminium is an unsupported wall height up to about fifteen times the wall thickness. Steel and stainless hold closer to twenty times, because the material is stiffer. Past those ratios we are no longer milling a wall, we are milling a spring, and the deflection shows up as wall thickness taper from the top of the wall to the bottom.
Put a number on it. A 12 mm three-flute carbide end mill in 6061 at 8000 rpm and 0.1 mm per tooth is pulling somewhere in the range of 200 to 400 N of radial load. On a 6 mm wall that is nothing. On a 0.8 mm wall at 30 mm tall, the wall bends away from the cutter by more than the tolerance band, the cutter stops cutting and starts rubbing, and the surface you get is chatter instead of a finish.
Two different problems that get blamed on each other
The first failure is deflection during the cut. The wall pushes away, the tool rubs, and you get taper, chatter marks and a wall that is thick at the base and thin at the top. This one is a machining strategy problem. We rough a thin-wall part leaving 0.6 mm, semi-finish leaving 0.15 mm, then take the last 0.15 mm in passes of 0.05 mm radial depth at high spindle speed. That last pass takes real time and it is the only reason the wall ends up parallel.
The second is stress release, and this is the one that bites buyers. Rolled aluminium plate carries internal stress from the mill. When you hog 80 percent of it away from one side, you unbalance the part, and the remaining material relaxes toward a new shape. It happens after unclamping, sometimes hours after, and it happens again when the part goes through anodising and the seal. Our enclosure measured 0.78 mm at the machine and 0.71 mm two hours later. Same part, same gauge, no cutting in between.
The fix is process, not program. Rough the part, let it relax, then re-clamp and finish. On that enclosure we roughed both sides leaving 0.4 mm, let it sit overnight, then finished on a vacuum chuck. Wall thickness held at 0.81 mm plus or minus 0.02 across a hundred parts.
Fixturing is where thin-wall jobs are won
You cannot clamp a thin wall from the outside and expect it to stay round. Any jaw pressure that holds the part also distorts it, and the part springs back when you release it. For anything under 1.5 mm we move to a vacuum chuck or pot the part in low-melt wax and machine it fully supported. Potting adds about an hour of non-cutting time per batch and it is the difference between a scrap rate of two percent and a scrap rate of thirty. Sacrificial webs do the same job more cheaply when the geometry allows: leave 0.3 mm tabs holding the wall to a thicker frame, machine everything, then cut them by hand.
What thin walls cost, in real numbers
We quoted the same 6061 housing twice. At a 2.5 mm wall, cycle time settled at 14 minutes on a three-axis VMC and the price at 100 pieces was 22 USD each. At a 1.2 mm wall, cycle time went to 26 minutes, plus the wax potting and an extra relax-and-refinish step, and the price landed at 34 USD each. Same envelope, same alloy, same quantity, roughly fifty percent more money, and the entire delta sits in the wall thickness.
Thin-wall work runs on three-axis vertical machines, which as a budget estimate costs around 45 to 60 USD per hour in our region. That is a planning figure, not a firm price. If you need a firm number for a specific drawing, our team confirms within 1 business day.
Design rules that actually reduce the bill
Keep unsupported wall height under fifteen times thickness in aluminium if you want general tolerances without a price premium. If the function forces you past that, expect to pay for slow finishing passes and accept a realistic tolerance of plus or minus 0.05 mm on the wall rather than 0.02.
Break the wall with a rib or a flange whenever the design allows. A 0.5 mm rib every 20 mm adds stiffness for almost no cycle time and converts a spring back into a structure. The same logic applies to symmetry: if you are removing 5 mm from one face, think hard about removing something comparable from the other, because balanced removal is the cheapest stress relief there is.
Call the tolerance on the wall thickness, not only on flatness. Plus or minus 0.05 mm on a 1 mm wall is a five percent band, and that is a very different job from the same tolerance on a 10 mm wall. Buyers who put a generic tolerance block on a thin-wall part are usually promising a number that was never achievable at that price.
Plan your inspection around the part settling. A thin-wall part is not stable the moment it leaves the machine. We measure after a twelve hour relax at 20 degrees C, and on tight work we measure before and after anodising. Incoming inspection ten minutes off the truck will reject parts that are perfectly good.
Before you freeze the drawing
Thin walls are not a reason to redesign the product. They are a reason to have the conversation early, while the drawing is still soft, rather than after the first article moves. The tolerance chart shows what each process realistically holds at different wall thicknesses, and the materials page lists which alloys we keep on the floor and how they behave when you thin them out. Our capabilities page covers the machines and the vacuum and potting setups we run for this kind of work.
Send us the STEP file with the 2D drawing that carries the tolerances, the quantity, and any finish or inspection requirement. If a wall in your part is going to be a problem, we would rather tell you on the drawing than on the first article. Contact our team and we confirm within 1 business day.