Introduction: Precision Matters

Hi, I’m Barry Zeng, a manufacturing engineer at Shanghai Yunyan Prototype & Mould Manufacture Factory. One of the most common questions I get is: “How accurate is SLA 3D printing compared to CNC machining?” The short answer: CNC is more accurate. But the real question is: “Do you need that extra accuracy?” For many applications — especially when producing CNC machining spare parts for legacy equipment — the lower accuracy of SLA is perfectly acceptable. In this guide, I’ll compare the real‑world dimensional accuracy of SLA vs. CNC machining. You’ll learn about typical tolerances, factors that affect accuracy (thermal expansion, tool wear, resin shrinkage), and when you can safely use SLA instead of CNC. I’ll also share a case study where we replaced a CNC machining spare parts order with SLA, saving 70% cost and 80% lead time — without compromising fit. By the end, you’ll know exactly which technology meets your tolerance requirements.

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Chapter 1: CNC Machining Accuracy – The Gold Standard

CNC machining is the benchmark for dimensional accuracy. Typical achievable tolerances:

• Standard CNC milling: ±0.025–0.05 mm.
• Precision CNC milling (with compensation): ±0.005–0.01 mm.
• CNC turning: ±0.005–0.01 mm on diameter.

For CNC machining spare parts that must mate with existing components (e.g., a bearing housing or a gear), this level of accuracy ensures perfect fit. CNC also offers excellent surface finish (Ra 0.4–1.6 µm) and isotropic material properties. The downsides: high cost, long lead time for complex parts, and material waste.

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Chapter 2: SLA Accuracy – How Good Is It Really?

SLA (stereolithography) 3D printing is often called “high‑precision” additive manufacturing. But how precise is it in real‑world conditions? Typical tolerances:

• Desktop SLA (e.g., Formlabs): ±0.05–0.1 mm or 0.2% of dimension, whichever is larger.
• Industrial SLA (e.g., UnionTech, 3D Systems): ±0.03–0.05 mm or 0.1% of dimension.

For a 50 mm part, SLA can hold ±0.05–0.1 mm. For a 200 mm part, ±0.2–0.4 mm. Factors that affect SLA accuracy:

• Resin shrinkage: Most resins shrink 3–6% during curing. We compensate by scaling the CAD model.
• Post‑curing: Over‑curing causes additional shrinkage and warping.
• Orientation: Holes printed vertically are round; holes printed horizontally become elliptical.
• Temperature: Resin viscosity changes with temperature, affecting layer formation.

For many CNC machining spare parts, SLA accuracy is sufficient — especially for non‑critical dimensions or parts with clearance fits.

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Chapter 3: Real‑World Accuracy Comparison – Testing Data

We tested identical parts (50 mm cube with a 10 mm hole) on industrial SLA and 5‑axis CNC. Results from 10 samples each:

Feature	CNC (mean ± std dev)	SLA (mean ± std dev)	Difference
External dimension (50 mm)	50.002 ± 0.005 mm	50.015 ± 0.025 mm	SLA within 0.03%
Hole diameter (10 mm)	10.000 ± 0.003 mm	9.985 ± 0.020 mm	SLA slightly undersized
Flatness (50 mm face)	0.005 mm	0.035 mm	CNC 7× flatter

CNC is clearly more accurate. But for many CNC machining spare parts — especially non‑rotating, non‑bearing parts — SLA’s ±0.05–0.1 mm tolerance is perfectly adequate. A clearance hole for an M3 screw needs ±0.2 mm. A cosmetic cover needs ±0.5 mm. SLA handles these easily.

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Chapter 4: When SLA Is “Good Enough” for CNC Spare Parts

Many CNC machining spare parts don’t require micron accuracy. Here’s when SLA is sufficient:

• Clearance holes: If the hole passes a screw or bolt, ±0.2 mm is fine. SLA can hold ±0.1 mm.
• Non‑mating surfaces: Covers, guards, handles, and cosmetic parts.
• Low‑load brackets: As long as mounting holes align, a 0.1 mm offset is acceptable.
• Prototypes before committing to CNC: Test fit with SLA, then order CNC machining spare parts for production.
• Parts that will be manually adjusted: If the installer can file or shim, SLA is fine.

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Chapter 5: When You Must Use CNC (SLA Won’t Cut It)

SLA cannot replace CNC machining spare parts in these scenarios:

• Bearing journals: Need ±0.005 mm roundness and surface finish Ra 0.4 µm.
• Sealing surfaces: Flatness <0.01 mm required for gaskets or O‑rings.
• Press‑fit holes: Interference fit of 0.01–0.02 mm. SLA’s ±0.05 mm is too loose.
• Threaded holes: SLA can’t produce accurate threads. You’d need to tap after printing, but the hole position may be off.
• Parts with high load: SLA resin is brittle and may crack under stress.

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Chapter 6: Case Study – Replacing CNC with SLA for 50 Spare Brackets

A factory needed 50 replacement brackets for a conveyor system. The original brackets were CNC‑machined aluminum. Tolerance requirement: mounting holes within ±0.3 mm (non‑critical). CNC quote: $35 each, 10‑day lead time. We proposed SLA (tough resin): $9 each, 3‑day lead time. The client tested one SLA bracket — it fit perfectly. They ordered 50 SLA parts, saving $1,300 and 7 days. For this application, SLA was “good enough.” Not all CNC machining spare parts need CNC accuracy.

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Chapter 7: Factors That Improve SLA Accuracy

To get the best accuracy from SLA for CNC machining spare parts:

• Use industrial SLA machines: Better optics, heated vats, and calibration.
• Calibrate XY scaling: Print a 100 mm test part, measure, apply compensation factor.
• Use 25–50 micron layer height: Thinner layers reduce stair‑stepping.
• Orient holes vertically: Horizontal holes become elliptical.
• Add 0.1–0.2 mm oversize to holes: Compensate for resin shrinkage.
• Post‑cure at low temperature (40–50°C): High temperatures cause warping.

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Chapter 8: Decision Matrix – SLA vs. CNC for Your Spare Part

Requirement	SLA	CNC	Recommendation
Tolerance ±0.05 mm or looser	✅	✅	SLA (cheaper, faster)
Tolerance ±0.01 mm or tighter	❌	✅	CNC
Clearance holes / non‑mating	✅	✅	SLA
	Bearing / sealing surface	❌	✅	CNC
Low load / cosmetic	✅	✅	SLA
High load / metal part	❌	✅	CNC

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Chapter 9: Cost and Lead Time Impact

For CNC machining spare parts, cost and lead time are often as important as accuracy. SLA advantages:

• Cost: SLA is 50–80% cheaper than CNC for small batches (1–50 parts).
• Lead time: SLA parts in 2–5 days vs. 7–14 days for CNC.
• Complexity: SLA handles complex geometries at no extra cost; CNC charges more.

If your spare part doesn’t require tight tolerances, SLA is almost always the better economic choice.

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Chapter 10: Summary – Accuracy Decision Checklist

• ☐ Does the part need ±0.01 mm or tighter? → CNC.
• ☐ Does the part have bearing or sealing surfaces? → CNC.
• ☐ Is the part plastic and non‑structural? → SLA.
• ☐ Are clearance holes the only critical feature? → SLA.
• ☐ Do you need parts in under 5 days? → SLA.
• ☐ Is your budget limited? → SLA.

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Conclusion: Accuracy Is Not One‑Size‑Fits‑All

CNC machining offers superior accuracy, but SLA is often “good enough” for many CNC machining spare parts — especially clearance holes, brackets, and cosmetic covers. By choosing SLA when tolerances allow, you can save 50–80% and get parts in days instead of weeks. We offer both SLA and CNC, and we help clients choose the right process based on their real accuracy needs. Send me your CAD file and tolerance requirements. I’ll recommend SLA or CNC, provide a free DFM report, and quote your project — within 24 hours. Let’s get your spare parts made right.

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👇 Need CNC Machining Spare Parts – But Want to Save Money?

Send me your CAD file and tolerance requirements. I’ll recommend SLA or CNC based on your real accuracy needs — and provide a free DFM report and quote within 24 hours.

Not sure if SLA is accurate enough for your spare part? Just say: “Barry, here’s my part — what tolerance can SLA hold?” I’ll give you a realistic assessment.

🎯 SLA vs. CNC — Real Accuracy for Real Parts 🎯

P.S. Mention “accuracy guide” when you email, and I’ll send you a tolerance comparison chart and a cost savings calculator.

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Barry Zeng
Senior Manufacturing Engineer, Shanghai Yunyan Prototype & Mould Manufacture Factory
(10+ years helping clients choose between SLA and CNC for spare parts — balancing accuracy, cost, and lead time. Let me help you make the right call.)