Workshop practice
The material you order is not the material that arrives
You order 10 mm aluminium and receive 10.10. It is not a supplier error: it is how commercial stock works. Program for 10 and the part pays for it.
You order a 10 mm aluminium plate. It arrives, you measure it, and it is 10.10.
The first time, you think the supplier got it wrong. You call, and they quite rightly explain that it is within tolerance and perfectly normal. The delivery note says 10 mm. The caliper says 10.10.
Both are right. “10 mm” is a commercial designation, not a dimension.
Why there is always extra and never too little
Notice one detail: material almost never arrives below nominal. It arrives on size or above it.
There is industrial logic behind this. Undersized plate guarantees a complaint because some customers cannot make their part. A plate a few tenths thicker bothers almost nobody: the extra is removed or ignored. Rolling and sizing processes are therefore adjusted towards the safe side.
The practical result: expect material to be as thick as or thicker than nominal, never thinner. Expect variation between batches and even within the same plate.
Where it shows up
At the work zero. If you set Z assuming 10 mm thickness and the plate is 10.10, all depths shift by a tenth. In a pocket that is irrelevant. On a supporting dimension that must mate with another part, it is not.
In workholding. A stop or vice designed around nominal size loses some grip or no longer supports where it should.
In flipped parts. This is where it hurts most. Machine one face referenced to the table, then flip and reference the machined face, and the extra thickness enters twice with opposite signs. Recesses on the two sides stop being symmetrical, showing up precisely in the dimension linking them.
In assemblies. Ten spacers cut from the same bar all carry the same extra thickness. Stacked, the deviation multiplies.
In pre-cut material. If you outsource the contour, the thickness arriving at your workshop is decided by their supplier, not you.
The rule
Measure the material before programming. Always. Every plate.
It sounds obvious until you see how many people program to nominal because CAD says 10.
With the measurement, make three simple decisions:
- Set Z zero on the actual top face, not calculated from the table. It is the only reference independent of thickness.
- If thickness is functional, face-mill it. Remove the extra tenths and the rolling irregularity at the same time. Now you know what you have.
- If it is not functional, leave it alone and ensure no program dimension depends on it.
The same happens with round bar and tube
It is not just sheet. Nominal round bar has its tolerance, and tube also has wall-thickness tolerance, usually more generous than its diameter tolerance. If you make a fixture to clamp a tube with a 3 mm wall, do not assume it is 3.00.
A small change in habits
Record the actual thickness in the program or job sheet. Two seconds.
When you repeat the part six months later with a different batch and something does not fit, that number tells you in thirty seconds whether the problem is the material or you. Without it, two hours of chasing a ghost await.
At 7Axis Industrial we manufacture parts, spares and short runs. Measuring the starting stock is the first step in the job, not an optional precaution.