The short answer
Minimum wall thickness is not one number across CNC and SLM. CNC walls are limited by cutter reach, chatter and clamp distortion; SLM walls are limited by melt-pool stability, orientation and post-machine skim needs. Put the process path and wall intent on the RFQ so [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands stay honest instead of flipping to **needs_manual**.
This guide is for buyers comparing prismatic CNC vs metal AM shells. Illustrations use an **Example part** only—not a customer aerospace skin. Related: [CNC vs 3D printing](https://3dwk.com/resources/cnc-vs-3d-printing-when-to-choose), [Ti-6Al-4V CNC vs SLM](https://3dwk.com/resources/ti6al4v-cnc-vs-slm-cost-lead-time), [cobalt chrome SLM vs CNC](https://3dwk.com/resources/cobalt-chrome-slm-vs-cnc-cost-lead-time).
CNC thin-wall reality for estimates
Open pockets with walls under ~1–1.5 mm in aluminum often need rest machining, soft jaws, or sacrificial ribs. Deeper thin walls raise tool stick-out and chatter—see also deep-reach notes. Soft plastics (PA66 time coeff ×0.8 under **rates.v1.1** estimates) deflect more than 6061; metal-tight walls on nylon invite scrap. 17-4PH (×1.4) and IN718/IN625 (×2.5) punish thin walls with longer cycle times and higher scrap risk; CoCr (×2.2) is similar. Lines whose mid exceeds configured USD thresholds for hot alloys/CoCr auto-route to **needs_manual**.
Bar $/kg planning bands (estimates): PA66 ~5, 17-4PH ~9, IN718 ~55, IN625 ~60. Powder SLM bands (estimates): 17-4PH ~70, IN718 ~120, IN625 ~130, CoCr ~150 $/kg—material choice and wall strategy interact.
Call minimum wall on the PDF, and whether ribs/bosses may stay thicker. Related: [CNC pockets, thin walls and undercuts](https://3dwk.com/resources/cnc-pockets-thin-walls-undercuts), [designing bosses and ribs](https://3dwk.com/resources/designing-bosses-and-ribs-for-cnc-housings).
SLM thin-wall reality for estimates
SLM can print walls CNC cannot mill from solid, but as-built surfaces, residual stress and support scars still need a plan. Functional mates usually need CNC skim—budget that second process. Unsupported horizontal thin floors may warp; orientation notes help. If you need airtight walls, say leak-test method; do not assume as-built density equals a sealing face.
Parallel RFQs (CNC from plate vs SLM + machine datums) beat a single vague “lightest possible” ask. See [IN625 vs IN718 CNC vs SLM](https://3dwk.com/resources/in625-vs-in718-cnc-vs-slm-buyers) and [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
Illustration 1 shows an **Example part** CNC pocket wall vs an SLM lattice-backed shell with machined pads.
What to put on the RFQ
State process preference (CNC only / SLM+CNC / either for trade study), minimum wall for non-mates, thicker keep-outs for clamps or powder removal, and finish (as-built OK vs machine all exterior). Ambiguous “make it as thin as possible” widens bands.
Default non-mates to ISO 2768-m on CNC; on SLM, separate as-built vs post-machined tolerances. Related: [DFM checklist](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote), [what files to send](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
Example part walkthrough
**Example part:** electronics cover ~90×60×20 mm envelope. Path A: CNC 6061 from plate, 1.5 mm min wall, 3 mm floor, open pocket. Path B: SLM 17-4PH shell 1.0 mm nominal wall + CNC skim sealing pad, powder note for internal cavity. PDF picks one path (or asks both). With clear wall callouts, expect usable planning bands on [quote-new.php](https://3dwk.com/quote-new.php). Silent “thin as possible, material TBD” often **needs_manual**. Estimates only—no customer invoices.
Illustration 2 contrasts a vague thin-wall note versus a process+wall RFQ block.
Heat treat and finish interactions
17-4PH H900/H1025 adders (~+$60/lot under **rates.v1.1** estimates when configured) can move thin walls; call heat treat before or after skim. Anodize on thin CNC aluminum walls needs growth allowance. SLM CoCr/IN alloys with thin ribs may need stress relief before machining—say so.
Stock nesting vs AM buy-to-fly
CNC from plate wastes the pocket volume as chips; SLM near-net can reduce buy-to-fly on sparse shells. That does **not** automatically make SLM cheaper—powder $/kg bands (estimates: 17-4PH ~70, IN718 ~120, IN625 ~130, CoCr ~150) plus support removal and CNC skim often dominate. For dense prismatic covers, CNC aluminum still wins on many **rates.v1.1** planning bands. Ask for both paths only when geometry or weight truly forks the process.
When comparing quotes, keep wall thickness, mate skim, and heat-treat notes identical across paths so you are not comparing unequal RFQs. Related: [prototype vs production CNC quotes](https://3dwk.com/resources/prototype-vs-production-cnc-quotes), [how quantity breaks change unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost).
Clamp, powder removal and keep-out zones
CNC needs clamp pads or soft-jaw contact; do not thin every exterior face. SLM needs powder-exit holes and support access—mark keep-outs that must stay thick enough for removal tools. If a cavity is sealed after print with no exit, expect **needs_manual**. Say whether temporary print supports may leave witness marks on non-mates.
Tolerance stacks on thin walls
Do not paste ±0.02 all-over onto 1 mm walls. Prefer ISO 2768-m on non-mates and tighter frames only on sealing pads after skim. Thin walls amplify thermal and residual-stress drift after heat treat—call whether critical dims are post-HT. For PA66 CNC walls, moisture notes matter more than metal GD&T copy-paste; see [PA66 moisture and dimensional stability](https://3dwk.com/resources/pa66-moisture-dimensional-stability-cnc).
What to do next
Pick CNC vs SLM (or both), write min wall + mate skim notes, upload on [quote-new.php](https://3dwk.com/quote-new.php). Services: [CNC machining](https://3dwk.com/cnc-machining-service.php), [metal 3D printing](https://3dwk.com/metal-3d-printing-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not firm quotes.
Common questions
Is 1 mm wall always OK in CNC aluminum?
Not as a blanket rule. Depth, tool reach, and clamp strategy matter. Short shallow walls behave differently from 30 mm deep 1 mm fins.
Does SLM always win on thin walls?
Only when geometry cannot be milled from solid or nesting waste dominates. Prismatic open boxes often stay cheaper in CNC aluminum.
Will tighter wall tolerances tighten the mid?
Opposite if they force hand finishing or 100% CMM. Keep tight controls on mates; leave non-mate walls on general tolerances.
How do powder prices enter the band?
SLM powder $/kg bands (estimates above) raise material share on thick sections; thin shells shift cost toward machine time and post-process—still label as estimates.