Guides / Alloys

When a Plate Has to Stay Flat, Rolled Aluminum Is the Wrong Start

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

A customer sends a 400 by 300 mm plate with a pocket machined most of the way through. The drawing calls for 6061-T6 and flatness within 0.1 mm. The shop machines it, releases the clamps, and the plate bows by half a millimetre. Nobody did anything wrong. The material was simply the wrong choice.

Where the movement comes from

Rolled aluminum plate is made by squeezing a slab between rollers. That process leaves the metal with internal stress, locked in and balanced against itself. Solution treatment and quenching add more, because the surface cools faster than the core.

While the plate is whole, those stresses cancel out and it lies flat. Machining breaks the symmetry. Every pocket, every skim off one face, removes material that was holding part of the balance, and the plate moves to find a new equilibrium. The more material you take, and the less symmetric the part, the more it moves.

T651 stretching reduces this considerably and should be your default on any rolled plate. It does not eliminate it.

What cast tooling plate does differently

MIC-6 and its equivalents are cast rather than rolled, then machined flat on both faces and stress relieved. There is no rolling operation to lock stress in, so there is very little stress to release when a machinist starts cutting.

The plate arrives flat to a published tolerance, usually around 0.13 mm over a metre, with both faces already machined. Cut a deep pocket in it and it largely stays where it was.

The trade you are making

	MIC-6 cast plate	6061-T651 rolled plate
Yield strength	~110 MPa	~276 MPa
Flatness as supplied	Machined both faces, tight	Mill tolerance, can be wavy
Movement after machining	Very little	Noticeable on large or asymmetric parts
Thread strength	Low, use inserts for repeated assembly	Good
Anodizing	Patchy, not for cosmetic work	Even and repeatable
Weldability	Poor	Good
Cost per kilo	Higher	Lower

MIC-6 buys you stability and costs you strength. That is the whole decision.

When to specify cast plate

• Jig plates, fixture bases and sub plates that locate other things.
• Vacuum tables and optical breadboards where flatness is the function.
• Large thin plates where the height to thickness ratio makes movement likely.
• Any part where you are removing more than half the starting volume and the result has to stay true.

When to stay with 6061

• Anything structural or loaded. MIC-6 simply is not strong enough.
• Anything that gets welded.
• Anything cosmetic that will be anodized to a colour.
• Small parts, where internal stress has little geometry to distort.
• Anything with heavily loaded threads, unless you are fitting inserts.

A third option people forget

If a part needs both flatness and strength, the answer is not always a different alloy. It is often a different process: rough machine the part from 6061-T651, leave it to sit, then come back and take a finishing pass off the critical faces. The roughing releases most of the stress, the rest happens overnight, and the finish pass cuts the part true.

This is the standard answer to stress movement after machining. It costs a second setup and a delay in the schedule. Say on the RFQ that flatness matters more than lead time and a shop can quote it that way. Say nothing and you will get a single operation quote, which is cheaper right up until the parts arrive bent.

Sources

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

• Residual stress
• Aluminium alloy 6061
• Continuous casting

Frequently asked questions

Keep reading

• Giving Up 10 Percent of Strength to Stop a Crack That Starts Alone
• Why Thick Plate Is the Reason 7050 Exists