CNC Machining

Mental model: CNC is flexible because it removes material from stock, but every feature still has to be reached, cut, supported, and inspected by real tools in real setups.

How The Process Actually Works

The part usually starts as billet, plate, bar, or a near-net preform. The shop then decides how to fixture it, which faces establish the datums, how many setups are needed, and which tools can safely reach each feature.

That is the heart of CNC DFM. Most problems are not about whether the machine can move there. They are about whether the tool can reach there without becoming too small, too long, too unstable, too slow, or too expensive.

What The Shop Cares About

• setup count
• tool access
• stiffness of both tool and part
• chip evacuation
• datum repeatability
• whether a nominally simple feature secretly forces EDM, custom tooling, or manual finishing

Five Physics Families

RapidDraft's current CNC rule cleanup works best when the rules are explained in five physics families instead of one long undifferentiated checklist.

Family	What is physically happening	Typical design symptoms
Walls and part stiffness	Thin sections act like springs during machining, so they deflect, vibrate, and move under tool load.	thin webs, thin ribs, tight tolerances on weak walls
Holes, threads, and drilling	Drill stiffness, chip evacuation, coolant access, and thread tool reach worsen with depth and small diameter.	deep small holes, non-standard drills, excessive thread depth, poor hole-to-edge support
Corners, pockets, and cutter geometry	End mills impose real internal radii, and small radii force smaller, longer, slower tools.	sharp internal corners, deep pockets with small radii, inconsistent corner radii
Tool reach and feature slenderness	Tool overhang rises faster than stiffness, so long-reach tools chatter and lose finish or accuracy.	deep narrow cavities, tall slender features, small features needing micro tools
Access, setups, and datums	Each extra setup adds cost and positional uncertainty, and inaccessible faces force awkward fixturing.	hidden faces, one-setup-critical features split across orientations, weak datum strategy

Common DFM Issues

1. Hole design that fights standard tooling

Standard drill sizes are cheaper and simpler than unusual diameters. Deep small holes are risky because drills wander, chip removal worsens, and breakage risk rises. Flat-bottom blind holes also push the design away from standard drilling and toward costlier operations.

This is why CNC review tends to ask:

• Is the diameter standard?
• Is the depth realistic for the chosen diameter?
• Does the functional requirement really need a flat bottom?

2. Internal geometry that forces fragile or expensive tools

Deep radiused corners, sharp internal corners, and narrow pocket regions are classic cost multipliers. They force long slender cutters, smaller cutter diameters, chatter risk, slower feeds, or EDM.

The design question is not "can this be cut somehow?" It is "can this be cut with stable, repeatable, normal tooling?"

3. Thin walls, ribs, and weak local features

When walls or ribs become too thin, the part itself behaves like a spring during machining. That creates vibration, tolerance instability, and scrap risk. Thin features are especially dangerous when tight tolerances are imposed on them anyway.

4. Extra setups hidden inside the geometry

A part may look elegant in CAD but require multiple orientations, custom fixturing, awkward clamping, or special probing in the shop. That turns a seemingly simple part into a schedule and cost problem.

5. Threads, edges, and local breakouts

Threaded holes too close to edges, tight geometry around fasteners, and poor relief choices create local breakout or post-processing problems. These are often caught late because the CAD model looks clean while the cutting sequence does not.

The Most Useful Contradiction To Preserve

The source set does not give one universal hole-depth rule.

• The broad DFM guide is conservative and recommends keeping diameter-to-depth below about 3 for deep drilled holes.
• The existing RapidDraft reference summary from Hubs is more permissive, framing roughly 4x diameter as a recommendation and much larger values as harder-limit territory.

That difference is important. It suggests CNC findings should often be tiered:

• green: well within common tooling comfort
• yellow: possible but efficiency or risk penalty
• red: special process, supplier confirmation, or redesign recommended

Why Tiered Thresholds Matter

This is one of the most useful mental shifts for CNC rule design. Many CNC rules are not true binary capability boundaries. They are comfort bands.

Rule family	Comfortable	Elevated risk	Hard-limit or special-process territory
Hole depth to diameter	common shop tooling range	slower drilling, more breakage and wander risk	supplier confirmation or redesign likely needed
Pocket depth to internal radius	ordinary end-mill reach	long-reach tools and lower removal rate	chatter and access risk become dominant
Wall thickness	stable local stiffness	tolerance and vibration risk rise	high scrap risk or process change
Tool access and setup count	reachable in clean setups	added fixturing or probing burden	unstable or expensive manufacturing route

This is why the best CNC reviewer experience should show both the rule and the band it is using, not just a naked pass/fail.

What Good CNC DFM Looks Like

Good CNC parts usually share a few qualities:

• features are reachable in logical setups
• internal radii reflect real cutters
• holes use standard sizes and realistic depths
• thin walls are avoided unless function truly requires them
• tolerances are reserved for functional surfaces, not sprayed everywhere
• the model does not silently depend on EDM or heroic tooling

How This Connects To RapidDraft

The live rule connection is already present in B_CNC and in the shared DFM Baseline Codex. This page is the explanation layer behind those rules.

The current rollout also adds a CNC family map in the rules handbook: CNC Physics Family Rule Map.

Open Questions

• Should the wiki split CNC milling and CNC turning into separate pages once this section grows?
• Do we want a future page on fixturing and setup strategy, since many CNC cost drivers are actually setup drivers?
• Should we encode conservative versus aggressive supplier thresholds explicitly in the rule system?

Sources

• 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\85541444-Manufacturing-Engineering-Handbook.pdf
• C:\Users\adeel\OneDrive\100_Knowledge\203_TextCAD\01_Product_Project_Management\TextCAD_Wiki\docs\04_DFM_Research\DFM_Rules_Handbook\references\REF-CNC-1.md
• 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\G_BASELINE.md