Guides / Design

The Wall Is Not Too Thin Until It Starts Moving

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

A designer takes a bracket from 4 mm down to 1.5 mm to save weight, and the quote comes back higher than before. Less material, more money. It is not a mistake.

Stiffness, not thickness

There is no single thickness that makes a wall thin. What matters is whether the wall deflects under the force of the cutter, and that depends on how tall it is relative to how thick it is, plus what is holding it up.

The working rule is the height to thickness ratio. Keep unsupported walls at or below about 4:1 and a shop can machine them normally. Above that, things start to get difficult, and by 10:1 the wall is a spring.

A 2 mm wall that is 8 mm tall is fine. The same 2 mm wall at 40 mm tall is a 20:1 ratio and the machinist is now fighting it.

What goes wrong

Chatter. The wall vibrates against the cutter like a plucked string. It leaves a visible wave pattern, it is loud, and it wrecks both surface finish and dimensional accuracy. Shops respond by dropping feed rates dramatically, sometimes by more than half.

Deflection and springback. The wall bends away from the tool during the cut and springs back after it. The machinist cuts to the right number and the finished wall is thicker than the drawing in the middle. This is why a thin wall cannot hold a tight tolerance whatever you pay.

Heat. Thin sections have nowhere to dump heat. The wall grows during the cut and shrinks afterwards, so the part measures differently warm and cold.

Clamping damage. A vice that would not mark a solid block will crush a thin walled one.

How a shop handles it anyway

• Step down in layers rather than cutting full depth in one pass, so the unmachined material below keeps supporting the wall.
• Climb mill with light radial cuts and adaptive or trochoidal toolpaths that keep the cutting force constant instead of spiking.
• Leave stock and take a spring pass, a final very light cut that removes the springback error.
• Support the wall with a sacrificial rib, wax, a custom soft jaw or a vacuum fixture.

All of these cost time, and custom fixturing costs setup that gets paid once per order. On a run of two hundred it disappears. On five pieces, it is most of the price.

What to change in the design

Add a rib rather than thinning a wall. A ribbed 3 mm wall is stiffer and cheaper than a flat 1.5 mm wall, both in service and on the machine.

Keep the wall short or break it up. A tall wall crossed by a rib becomes two short walls.

Put a radius at the base. A sharp internal corner at the foot of a wall is a stress riser and forces a small cutter. A generous radius lets the shop use a bigger, stiffer tool.

Use the largest internal corner radius you can. Corner radius sets minimum cutter diameter, and a bigger cutter is a stiffer cutter. Going from 1 mm to 3 mm corner radii can transform how a pocket machines.

Avoid deep narrow pockets. A pocket deeper than about three times the cutter diameter needs a long tool, and a long tool is a flexible tool.

Pick a stable material. 6061-T651 is the sensible default. For thin flat parts that must stay true, consider cast tooling plate.

What to say on the RFQ

Tell the shop which wall thicknesses are firm requirements and which are there because the model happened to end up that way. Designers are often surprised how much of a quote comes from a dimension nobody cared about.

WALL 'A' 1.5 mm IS CRITICAL FOR CLEARANCE. ALL OTHER WALLS MAY BE INCREASED TO 3 mm IF IT HELPS MANUFACTURE.

A line like that regularly takes double digits off a price, and it costs you nothing to write.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• Chatter in machining
• Trochoidal milling
• Deflection of structures

Frequently asked questions

Keep reading

• Tolerance Every Feature and You Will Pay for Every Feature
• The Part Was Flat Until the Clamps Came Off