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Process Selection

Goal: Understand when to think like a machinist, a sheet metal fabricator, or a molder before arguing about detailed DFM rules.

Executive Summary

DFM starts with process choice, not with isolated geometry checks.

The same feature can be trivial in one process and painful in another. A deep pocket is normal in CNC but irrelevant in sheet metal. A long unsupported wall is a molding and stiffness problem in plastics, but a bend-sequencing and distortion problem in sheet metal. If the process assumption is wrong, the downstream DFM conversation is wrong.

The Three Process Families In Scope

Process Starting material How value is created Best fit Typical cost signature Common DFM failure mode
CNC machining Solid stock, casting, or plate Remove material with controlled tools Precision metal or plastic parts, low to medium volume, complex 3D geometry Lower tooling cost, higher recurring machine-time cost Features that force small tools, extra setups, chatter, or secondary processes
Sheet metal fabrication Constant-thickness sheet Cut, punch, bend, form, and join Enclosures, brackets, covers, frames, ducting Low material-conversion cost when geometry stays 2.5D, cost rises with forming complexity and secondary ops Designers forget the part is still a constant-thickness bent sheet
Injection molding Molten polymer into a mold Pay upfront tooling to make repeated near-net-shape parts Plastic parts at medium to high volume, integrated geometry, cosmetic surfaces Very high tooling cost, low unit cost at scale Designers create geometry that fills, cools, ejects, or tools badly

How To Choose The Right Mental Model

Choose CNC first when:

  • the volume is still uncertain or low
  • the part needs tight tolerances on multiple machined datums
  • material choice is metal or engineering plastic stock
  • geometry is truly three-dimensional and not naturally sheet-based
  • you want design flexibility before committing to expensive tooling

Choose sheet metal first when:

  • the part is basically a constant-thickness folded shell
  • the product is an enclosure, bracket, cover, tray, or frame
  • bends and fastener hardware do most of the structural work
  • cost, lead time, and reparability matter more than sculpted geometry

Choose injection molding first when:

  • the part is plastic and expected to ship at real production volume
  • integrated features like ribs, bosses, snap-fits, and cosmetic surfaces matter
  • per-part cost matters more than tooling flexibility
  • assembly simplification can justify more capable tooling

The Process Logic Behind DFM

Each process has a dominant economic question:

  • CNC asks: how many setups, how much machine time, how awkward is tool access, and what special tooling or finishing will be required?
  • Sheet metal asks: can this stay a simple cut-and-bend part, or are we accidentally creating a forming, tolerance, or assembly problem?
  • Injection molding asks: can this part fill, cool, eject, and survive tooling economics without forcing expensive mold complexity?

Cross-Process Red Flags

These design choices are suspicious in almost every process:

  • abrupt thickness changes
  • geometry that hides the intended datum strategy
  • cosmetic demands without manufacturing-safe geometry
  • parts that are only feasible because of manual rework
  • unnecessary part count or too many unique fasteners
  • tolerances tighter than the function actually needs

Important Contradiction

A single number rarely survives process comparison. What counts as a "deep feature", "thin wall", or "acceptable draft" depends on the process, material, volume, and supplier capability. That is why this wiki separates process understanding from rule authoring.

Where To Go Next

Open Questions

  • Should future process-selection guidance explicitly include production-volume bands?
  • Do we want a separate page comparing prototype route versus production route for the same part?
  • Should supplier capability be modeled as an overlay on top of the process pages?

Sources

  • C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\06_Technical data\06_Manufacturingbooks\85541444-Manufacturing-Engineering-Handbook.pdf
  • C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\06_Technical data\06_Manufacturingbooks\310796791-Definitive-Guide-to-DFM-Success.pdf
  • C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\06_Technical data\06_Manufacturingbooks\307451948-Dfm-Injection-Molding-Analysis-0614.pdf
  • C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\01_Product_Project_Management\TextCAD_Wiki\docs\04_DFM_Research\DFM_Rules_Handbook\packs\B_CNC.md
  • C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\01_Product_Project_Management\TextCAD_Wiki\docs\04_DFM_Research\DFM_Rules_Handbook\packs\C_SHEET.md
  • C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\01_Product_Project_Management\TextCAD_Wiki\docs\04_DFM_Research\DFM_Rules_Handbook\packs\G_BASELINE.md