The short answer
On plate RFQs, **flatness** and **parallelism** are not interchangeable GD&T notes. Flatness controls a single surface against itself—how much that face bows or twists, with no datum required. Parallelism controls a surface (or a derived median) relative to a datum plane, so opposite faces stay equally spaced in orientation. Writing “flat and parallel 0.05” without saying **which face is the datum** and **whether you mean both controls** is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the plate outline alone.
This guide is for buyers packaging aluminum plate RFQs for covers, nests and bases. Illustrations use an **Example part** only—not a customer semiconductor nest. Related notes live in [flatness, parallelism and perpendicularity on machined parts](https://3dwk.com/resources/flatness-parallelism-perpendicularity-machined-parts), [face milling vs peripheral milling for flatness](https://3dwk.com/resources/face-milling-vs-peripheral-milling-flatness), and [GD&T callouts that raise CNC quote bands](https://3dwk.com/resources/gdandt-callouts-that-raise-cnc-quote-bands); here we focus on **flatness vs parallelism labeling on plates** so estimators do not guess a double-side skim.
What each control means for quoting
**Flatness** asks whether one face is planar enough for a seal, a stack or a vacuum nest. The shop can often qualify it with a granite check or a mapped scan of that face. **Parallelism** asks whether the opposite face tracks a datum you named—usually the first face after it is skimmed and designated A. That second control is what forces a flip, a matched skim, or a grind sequence. Process pages for semiconductor and automation plate work ([CNC 3-axis Al 6061 semiconductor](https://3dwk.com/en/cnc-3axis/al-6061/semiconductor.html), [CNC 5-axis Al 6061 semiconductor](https://3dwk.com/en/cnc-5axis/al-6061/semiconductor.html), [CNC 3-axis Al 6061 automation](https://3dwk.com/en/cnc-3axis/al-6061/automation.html)) assume datum-face notes when lots mix as-sawed plate with precision skims.
Do not bury “flat // 0.05” inside a notes cloud. Put **FACE A FLATNESS 0.05; FACE B PARALLEL TO A WITHIN 0.08** (values are examples you write, not a shop promise) on the PDF and mirror it in RFQ text. Background: [datum schemes for multi-setup CNC parts](https://3dwk.com/resources/datum-schemes-for-multi-setup-cnc-parts), [plate vs billet stock CNC aluminum](https://3dwk.com/resources/plate-vs-billet-stock-cnc-aluminum).
Stock, residual stress and which face is A
Sawed plate and residual stress move after a heavy skim; calling a tight parallelism without saying how much stock comes off each face leaves the estimator guessing a stress-relief or double-pass plan. Name the datum face (often the larger or sealing face), the opposite face, and whether thickness is a separate size tolerance. Thickness ± a tight band plus silent parallelism is a common conflict: size can be met while faces still wedge. Related: [how stock size changes a CNC quote](https://3dwk.com/resources/how-stock-size-changes-cnc-quote), [CMM sampling plans for CNC lots](https://3dwk.com/resources/cmm-sampling-plans-for-cnc-lots).
Aluminum 6061 plate $/kg planning bands sit in the mid-commodity range under **rates.v1.1** material tables; the flatness/parallelism story is skim strategy and inspection, not kg price.
When you need both controls
Use flatness alone when one sealing or mounting face matters and the back face is non-functional (paint, clearance, or a cover no one measures). Add parallelism when a stack, a rail, or an opposite mounting face must track A. Add perpendicularity of edges only if those edges actually locate something—do not copy an entire GD&T starter set onto a simple plate. The sibling overview [flatness, parallelism and perpendicularity](https://3dwk.com/resources/flatness-parallelism-perpendicularity-machined-parts) covers the three-control family; this page stays on plates and the RFQ habit of blending the first two.
Tolerance and inspection traps
Calling “flat and parallel” with one number and no datum, applying the same 0.02 to a large thin plate without a manufacturing datum, or requiring 100% CMM maps of both faces on a prototype lot, all force **needs_manual**. Default non-critical envelope to ISO 2768-m and put functional flatness/parallelism only on the faces that stack or seal. Related: [surface roughness Ra callouts that raise quotes](https://3dwk.com/resources/surface-roughness-ra-callouts-that-raise-quotes) when a ground Ra is implied by the flatness note but never written.
Illustration 1 shows an **Example part** plate with flatness on datum face A and parallelism on the opposite face versus a single “flat // OK” cloud.
Example part walkthrough
**Example part:** Aluminum 6061-T6 plate ~150×100×8 mm, one sealing face, opposite mounting face, ISO 2768-m general on edges. With STEP + **FACE A (SEAL) FLATNESS 0.05; FACE B PARALLEL TO A 0.08; THICKNESS 8 ±0.10 SEPARATE**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “flat and parallel 0.02 all over” often lands DFM notes or **needs_manual**—without inventing any customer nest PO.
Illustration 2 contrasts a vague RFQ (“flat // as needed”) versus a labeled datum-and-control pack and the clearer estimate path.
What widens bands fastest
One number standing in for both controls, no datum face, thin plate plus a tight parallelism after a heavy one-side skim (stress movement), and inspection of every non-functional back face. Quantity breaks help unit minutes but do not erase datum silence—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost) and [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
What to do next
Mark **FLATNESS** on the functional face, **PARALLELISM TO A** on the opposite face when you truly need it, name thickness separately, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Service pages: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not invoices. If the UI shows **needs_manual**, answer the datum/control questions rather than forcing a fake firm price.
Common questions
Is parallelism tighter than flatness?
Not automatically. Parallelism is a different control (it needs a datum). A plate can be flat on A and still wedge relative to B if parallelism is unnamed—or the reverse if only parallelism is named and A itself bows.
Can I write “flat and parallel 0.05”?
Prefer two callouts: flatness on A, parallelism of B to A, each with its own value. The shorthand hides which face is the datum.
Does plate stock vs billet change the note?
Yes. Plate residual stress after a skim is a common reason parallelism moves. Name stock type; see [plate vs billet stock CNC aluminum](https://3dwk.com/resources/plate-vs-billet-stock-cnc-aluminum).
Do I always need both?
No. If only one face seals or mounts, flatness on that face plus a looser thickness may be enough. Add parallelism when the opposite face has a job.