And How to Avoid Them for Better Parts, Faster Deliveries & Lower Rejections
Introduction
You’ve designed hundreds of parts. You know tolerances. You know materials.
But even the most seasoned engineers unknowingly commit design choices that confuse CNC programmers, increase cost, delay production — or worse — cause rejections.
This blog reveals the 10 most common mistakes seen across 5000+ drawings processed on TheSupplier.in, with examples, visuals, and practical advice.
✅ 1. Specifying Tight Tolerances Everywhere
📏 The Mistake: Applying ±0.02 mm to non-critical features like slots, flanges, or chamfers.
💸 Why It Hurts:
Increases machining time & cost
Requires CMM validation unnecessarily
Causes supplier rejection if machine can’t meet it
✅ What to Do Instead:
Tolerate only what’s functionally critical
Use general tolerance notes (e.g., ISO 2768-m)
✅ 2. Sharp Internal Corners That Tools Can’t Reach
📐 The Mistake: Designing 90° internal corners in pockets or cavities.
💡 Reality: Endmills are round — they can’t make perfect internal square corners.
✅ What to Do Instead:
Add fillets ≥ tool radius (e.g., R2, R3)
Ask supplier for their smallest tool size
✅ 3. Unclear Hole Callouts or Missing Thread Details
🧾 The Mistake: Marking holes without specifying:
Ø size
Depth
Whether it’s tapped or clearance
Which side to tap from
🚨 This leads to wrong parts, delays, or cost increase.
✅ What to Do Instead:
Use hole callouts like: “M6 x 1.0 — 15mm depth — tap from top”
Mention fit: clearance/interference/threaded
✅ 4. No Finish Specification
🎨 The Mistake: Leaving surface finish undefined.
This creates problems when:
Parts discolor
Surface is too rough for sealing
Anodizing affects dimensions
✅ What to Do Instead:
Mention clearly: “Finish: Anodized black, 10μm”, or “Machine finish only”
Mark cosmetic areas if needed
✅ 5. Ignoring Bending Direction in Sheet Metal
📄 The Mistake: Not considering grain direction or not providing bend notes.
This can cause:
Cracking
Uneven bend angles
Springback issues
✅ What to Do Instead:
Indicate grain direction
Use minimum bend radius per material
Mention “K-factor” if available
✅ 6. Asking for Unnecessary 5-Axis Machining
⚙️ The Mistake: Designing undercuts or angled features that force 5-axis machining — when simpler solutions exist.
✅ What to Do Instead:
Modify geometry for multi-side 3-axis setups
Ask your supplier for alternate suggestions
✅ 7. Not Including Inspection Access
🔍 The Mistake: Designing closed features that can’t be measured.
If QC can’t probe it, they’ll either:
Reject it
Mark as unverified
Delay shipment
✅ What to Do Instead:
Add access points, flat pads, or reliefs
Design for CMM path or gauge access
✅ 8. Using Rare or Hard-to-Source Materials
🧱 The Mistake: Selecting grades that are:
Not available locally
Need import licenses or MOQ
Difficult to machine
✅ What to Do Instead:
Choose common grades: SS304, EN8, AL6061-T6
Add note: “Use equivalent grade if performance matches”
✅ 9. Not Designing for Clamping & Setup
🗜 The Mistake: No flat clamping surface = unstable machining = vibration = bad part.
✅ What to Do Instead:
Add support tabs, flat edges, or temporary hold areas
Avoid deep cavities unless needed
✅ 10. Missing Drawing Details That Waste Time
⏱ The Mistake: Submitting 1-view PDFs with missing:
Dimensions
Tolerance table
BOM/material notes
Surface callouts
✅ What to Do Instead:
Submit multi-view drawings with 2D + 3D models
Add a note: “Discuss if any info unclear”
📌 Conclusion
Even experienced engineers fall into these traps — especially when speed pressures or repeated design habits creep in.
By avoiding these 10 mistakes, you’ll:
Cut part cost
Speed up deliveries
Increase approval rate on first try
Build long-term trust with your manufacturing partner
Ready to Upload Your Drawing?
Let our sourcing engineers evaluate your design and match you with the right CNC partner.
👉 Upload now at www.TheSupplier.in
