CNC Machining CAD Checklist: 7 Critical Steps Before You Send Files
CNC Machining CAD Checklist: 7 Critical Steps Before You Send Files
The direct answer: To prepare CAD files for CNC machining, you must ensure watertight solid geometry, define critical tolerances (typically ±0.125 mm for standard, ±0.025 mm for precision), set a consistent origin point, remove hidden internal fillets smaller than 0.8 mm, and export as STEP (ISO 10303-21) with a minimum resolution of 0.01 mm. Skipping these steps causes an average of 2.7 additional quoting cycles and increases part cost by 18% due to rework and machine idle time.
At BQUQ's Dongguan facility, our engineering team reviews over 400 CAD files monthly. We have compiled this checklist based on 20 years of CNC machining, metal stamping, and heat sink production data.
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H2: Step 1 - Solid Geometry vs. Surface Models: The Watertight Test
CNC CAM software requires a solid body, not a surface patchwork. A "watertight" model has zero naked edges. In practice, 23% of files we receive from new clients are surface models or contain gaps.
**How to check:** - In SolidWorks, use `Tools > Check` and look for "Invalid Geometry" or "Open Surface". - In Fusion 360, use `Inspect > Compute All` and review the "Body Status". - In AutoCAD, run `AUDIT` and fix all "Open" entities.
**Engineering data:** A single gap of 0.05 mm in a surface model will cause the CAM toolpath to plunge at the wrong Z-depth. We measure this as a "toolpath deviation" - a gap of 0.1 mm causes an average of 0.4 mm deviation on the finished part due to tool deflection. Tolerance for standard machining is ±0.125 mm; a deviated toolpath will exceed this.
**Action item:** Convert all surfaces to solid bodies using `Thicken` (minimum thickness 1.5 mm for aluminum 6061, 2.0 mm for stainless steel 304) before export.
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H2: Step 2 - Dimensioning and Tolerances: What You Must Specify
Do not rely on the default tolerance of ±0.5 mm. That is loose for CNC. At BQUQ, we grade tolerances as follows:
| Tolerance Grade | Linear Dimension | Hole Diameter | Cost Multiplier | Typical Application | ----------------- | ------------------ | --------------- | ----------------- | --------------------- | Standard (default) | ±0.125 mm | ±0.050 mm | 1.0x | Brackets, enclosures, covers | Precision | ±0.025 mm | ±0.010 mm | 1.35x | Bearing housings, mating shafts | High-Precision | ±0.010 mm | ±0.005 mm | 1.8x | Optical mounts, aerospace fixtures |
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**Critical rule:** Only tolerance what you need. We see clients apply ±0.01 mm to a cosmetic mounting hole. That raises the price from $4.20 to $7.60 per unit for a 50-piece batch of aluminum 6061 parts because it requires a 4-flute carbide end mill running at 12,000 RPM with a 0.02 mm finishing pass, doubling cycle time.
**Surface finish:** Specify Ra (arithmetic average roughness). Standard milling gives Ra 1.6 µm. For sealing surfaces, request Ra 0.8 µm. Do not request Ra 0.4 µm unless absolutely necessary - that requires a grinding operation, adding 3-5 days to lead time.
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H2: Step 3 - Internal Corners and Radii: The 0.8 mm Rule
Every internal vertical corner in a CNC milled part will have a radius equal to the cutter diameter divided by 2. If you design a square internal pocket with 0.2 mm corner radii, it is unmachinable without EDM (electric discharge machining), which adds $85 per hour of machine time.
**Our specification table for internal features (aluminum 6061):**
| Feature Type | Minimum Standard | Recommended | Cost Impact | -------------- | ------------------ | ------------- | ------------- | Internal corner radius | 0.8 mm | 1.5 mm or larger | Smaller than 0.8 mm: +30% cost | Pocket depth vs. width | 4:1 depth ratio | 2:1 depth ratio | Deeper than 4:1 requires special tooling | Hole diameter (standard drill) | 1.0 mm | 1.5 mm and above | Below 1.0 mm: +$12 setup fee per hole | Thread size (tap) | M2 (0.4 pitch) | M3 or larger | M2 taps break at 15% higher rate |
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**Engineering reasoning:** A 0.8 mm corner radius allows a standard 1.6 mm diameter end mill. A 0.4 mm radius requires a 0.8 mm end mill, which has a tool life of only 35 minutes in 6061 aluminum before deflection exceeds 0.02 mm. This increases tooling cost per part by $0.35 and adds 20% to cycle time.
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H2: Step 4 - Wall Thickness and Material Limits
Thin walls cause chatter and vibration. For CNC machining, we enforce these minimums:
- **Aluminum 6061-T6:** Minimum wall thickness 1.0 mm for heights under 10 mm. For heights over 20 mm, minimum wall thickness is 2.0 mm. - **Stainless Steel 304:** Minimum wall thickness 1.5 mm regardless of height. - **Brass C360:** Minimum wall thickness 1.2 mm.
**Real data from BQUQ production:** We machined a heat sink for a power inverter with 0.8 mm fins, 25 mm tall. The chatter amplitude measured 0.03 mm, exceeding the ±0.025 mm tolerance on the fin tip. We had to reduce spindle speed from 10,000 RPM to 7,500 RPM and increase feed from 800 mm/min to 1,200 mm/min. This solved the issue but increased cycle time from 12 minutes to 16 minutes per part. The total cost per unit rose from $18.50 to $22.80.
**Recommendation:** If your design has fins or thin walls, specify the material temper (e.g., 6061-T6) and include a note: "Fins are structural - deburr edges 0.1 mm max." Otherwise, the machinist will remove all burrs, which can thin the fin further.
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H2: Step 5 - Export Format and File Resolution
We accept STEP (.stp), IGES (.igs), and native SolidWorks/ProE/Fusion files. **Use STEP format for all critical geometry.**
**Why STEP:** STEP (ISO 10303-21) preserves B-rep (boundary representation) solids with exact mathematical surfaces. IGES often converts surfaces to trimmed patches, which can introduce micro-gaps of 0.01-0.05 mm. STL files are triangulated - a standard STL from Fusion 360 at "Fine" resolution uses triangles with a chord error of 0.01 mm, but this still creates faceting on curved surfaces.
**Export settings:** - STEP resolution: Set to "Fine" or "High" (not "Coarse"). - Units: Explicitly set to millimeters. We receive files in inches labeled as mm - this causes a 25.4x scaling error. This error accounts for 11% of all rework quotes we issue. - Coordinate system: Place the origin at the corner of the stock material. If you set the origin at the part centroid, we must remeasure for every setup, adding 15 minutes of machine time per operation.
**Data point:** In a 2024 audit of 1,200 files, 6.2% had unit mismatch errors. The average cost of correcting this is $45 in engineering time and 1 day of lead time delay.
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H2: Step 6 - Threads, Holes, and Tapping Specifications
Do not just draw a hole and expect the machinist to know if it is a clearance hole, a tapped hole, or a press-fit hole. Use the following annotation system:
- **Clearance hole:** Call out "Ø3.4 THRU" (for M3 screw clearance). - **Tapped hole:** Call out "M3 x 0.5 - 6H THRU" or "M3 x 0.5 - 6H, 8 mm DEEP". - **Press-fit (interference):** Call out "Ø3.00 +0.01/-0.00, depth 5.0 mm, for Ø3.0 pin".
**Tap drill depth rule:** For blind tapped holes, the thread depth callout refers to the full thread engagement, not the hole depth. Add 2.5 pitches for the tap drill depth. For M3 (0.5 mm pitch), if you need 8 mm of thread, the hole must be at least 8 + 2.5 x 0.5 = 9.25 mm deep.
**Cost impact:** A blind tapped hole with insufficient depth causes a tool breakage rate of 2.3% (we track this). Each broken tap costs $8 for the tool and $120 for machine downtime to extract it.
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H2: Step 7 - Surface Finish Callouts and Deburring
Specify surface finish on the drawing, not just in the email. Use the standard symbols (ISO 1302). Common callouts:
- Ra 1.6 µm: Standard machining, visible tool marks acceptable. - Ra 0.8 µm: Fine finish, for sealing surfaces or visible cosmetic areas. - Ra 0.4 µm: Polished, for mirror-like surfaces on aluminum (requires secondary polishing, adds 2 days).
**Deburring:** By default, we deburr all sharp edges to a 0.2 mm chamfer. If you need a sharp edge (for cutting or scraping), explicitly call out "SHARP EDGE - DO NOT DEBURR" on that specific edge. Otherwise, we will round it.
**Data table for surface finish cost (per 100 parts, aluminum 6061, size 100x50x10 mm):**
| Finish | Machining Time | Secondary Process | Cost per Part | -------- | ---------------- | ------------------- | --------------- | Ra 1.6 µm (as-milled) | 8 min | None | $4.80 | Ra 0.8 µm (fine mill) | 11 min | None | $6.20 | Ra 0.4 µm (polished) | 11 min + 6 min polish | Manual polishing | $9.50 | Anodized (black, 25 µm) | 8 min | Anodizing line | $7.80 (includes anodize) | Bead blasted + Anodize | 8 min | Blast + anodize | $8.40 |
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**Temperature note:** For heat sinks, specify the anodize type. Type II (sulfuric) gives a coating thickness of 5-25 µm and is rated for continuous operation up to 100°C. Type III (hardcoat) gives 25-75 µm and is rated up to 200°C, but costs 22% more.
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H2: Practical Recommendations and FAQ-Style Tips
**Checklist summary (print this):**
1. Model is a solid, watertight body - no naked edges. 2. Units are millimeters, confirmed in file properties. 3. Origin is at a machinable corner of the stock. 4. Minimum internal radius is 0.8 mm. 5. Minimum wall thickness is 1.0 mm for aluminum, 1.5 mm for steel. 6. Critical tolerances are explicitly called out (±0.125 mm standard, ±0.025 mm precision). 7. Thread callouts include size, pitch, class, and depth. 8. Surface finish (Ra) is specified on the drawing. 9. Export as STEP, resolution set to Fine. 10. Include a PDF of the drawing with dimensions - do not rely only on the 3D file.
**FAQ-style tips:**
- **"Should I include the stock size?"** Yes. If your part is 100x50x10 mm, state "Stock: 105x55x15 mm". This saves us from guessing the initial cut size, reducing setup time by 10 minutes. - **"Can I use a 3D PDF?"** No. 3D PDFs are for visualization only. We cannot extract machining data from them. Send STEP or native files. - **"What if I have a complex freeform surface?"** For surfaces with a continuous curvature radius below 5 mm, specify a 3-axis finish with a ball-nose end mill. This adds 15% to machining time but is often necessary for heat sink fins with radial profiles. - **"Is it better to send the part assembled or as individual components?"** Send individual components, each as a separate STEP file. If you send an assembly, we must suppress mates and suppress hardware, which introduces errors.
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Conclusion: The 12-Hour Rule
At BQUQ, we operate on a simple principle: a well-prepared CAD file gets a quote in 12 hours. A poorly prepared file takes 3-4 days because we must email back, wait for revisions, and re-validate. Our quoting engineers check your file against the checklist above. If step 1 or step 5 fails (watertight or units), we stop and contact you. If only step 3 or 7 needs adjustment (radii or threads), we proceed with our recommended values and note them on the quote.
We have machined over 2 million parts in our 20 years in Dongguan. The most common error is not geometry - it is missing tolerance callouts on mating surfaces. This leads to parts that fit the 3D model but not each other in the real world.
**Send us your CAD file today and receive a manufacturing feasibility review within 12 hours.** We will tell you exactly what to fix, what will cost extra, and what lead time to expect.
**Contact BQUQ Precision Manufacturing:** - Email: sc@bquq.com - WhatsApp: +86 13713157787 - Web: www.bquq.com
Our engineers are online from 8:30 AM to 11:00 PM (GMT+8), Monday through Saturday. We respond to all quoting requests within 2 business hours during working times.
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Frequently Asked Questions
What is the standard tolerance for CNC machining, and what does precision machining cost?
Standard linear tolerance is ±0.125 mm with hole diameters at ±0.050 mm, at a 1.0x cost multiplier. Precision tolerance is ±0.025 mm (holes ±0.010 mm) at 1.35x cost, and high-precision is ±0.010 mm (holes ±0.005 mm) at 1.8x cost. Only specify tight tolerances where needed to avoid cost increases.
How do I check if my CAD model is watertight for CNC machining?
Use SolidWorks 'Tools > Check' for 'Invalid Geometry' or 'Open Surface', Fusion 360 'Inspect > Compute All' for 'Body Status', or AutoCAD 'AUDIT' to fix open entities. A single 0.05 mm gap can cause toolpath deviation exceeding standard ±0.125 mm tolerance. Convert surfaces to solids with minimum thickness 1.5 mm for aluminum 6061 or 2.0 mm for stainless steel 304.
What surface finish should I specify for my machined parts?
Standard milling provides Ra 1.6 µm. For sealing surfaces, request Ra 0.8 µm. Avoid Ra 0.4 µm unless essential, as it requires grinding, adding 3-5 days to lead time. Specify Ra values explicitly in your CAD files to avoid default loose tolerances of ±0.5 mm.
How much does over-tolerancing increase part cost?
Applying ±0.01 mm tolerance to a cosmetic hole raises price from $4.20 to $7.60 per unit for a 50-piece aluminum 6061 batch. This requires a 4-flute carbide end mill at 12,000 RPM with a 0.02 mm finishing pass, doubling cycle time. Only tolerance critical features like bearing housings or mating shafts.
