CNC Machining Wiki
Not another textbook. Practical knowledge from the shop floor — what actually matters when you're selecting materials, specifying tolerances, choosing surface treatments, or evaluating costs. Built on 20+ years of manufacturing experience at Sinbo Precision.
Quick-Find: What do you need?
Which aluminum alloy?
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6061 vs 7075 vs 5052 — strength, machinability, cost
Which steel for my part?
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Carbon, alloy, tool steel — decision guide by application
Which stainless steel?
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304 vs 316 vs 303 vs 17-4PH — corrosion vs machinability
Which surface treatment?
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Anodize, plate, paint, polish — selection by requirement
Does this part need heat treatment?
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When and why — hardness targets, distortion risk, cost
What tolerance is realistic?
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ISO 2768, IT grades, achievable accuracy by process
Thread type or tap drill size?
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Metric, UNC/UNF, NPT, BSP — sizes and callouts
Which fastener and what torque?
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SHCS, Helicoil, dowel pins — grades and torque table
What does this GD&T symbol mean?
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14 symbols, datums, feature control frames explained
T6 vs T651 — which aluminum temper?
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Why T651 matters for CNC machining (distortion)
Key Data Quick-Reference
Material Density
| Material | g/cm³ |
|---|
| Carbon / Alloy Steel | 7.85 |
| Stainless Steel (300 series) | 7.93 |
| Stainless Steel (400 series) | 7.70 |
| Aluminum (all alloys) | 2.70 |
| Titanium (Ti-6Al-4V) | 4.43 |
| Copper | 8.96 |
| Brass (C36000) | 8.50 |
| POM (Delrin) | 1.41 |
| PEEK | 1.30 |
General Tolerances (ISO 2768-m)
| Nominal (mm) | Tol (mm) |
|---|
| 0.5 – 3 | ±0.1 |
| 3 – 6 | ±0.1 |
| 6 – 30 | ±0.2 |
| 30 – 120 | ±0.3 |
| 120 – 400 | ±0.5 |
| 400 – 1000 | ±0.8 |
Surface Roughness Ra / Rz
| Ra (μm) | Rz (μm) | Typical Use |
|---|
| 0.4 | 2.0 | Sealing / sliding faces |
| 0.8 | 4.0 | General mating surfaces |
| 1.6 | 8.0 | Non-critical surfaces |
| 3.2 | 16.0 | Rough machining |
| 6.3 | 25.0 | Saw cut / sand cast |
Hardness Conversion (Approx.)
| HRC | HB | HS |
|---|
| 20 | 225 | 34 |
| 30 | 285 | 43 |
| 40 | 370 | 53 |
| 50 | 480 | 63 |
| 58 | 600 | 75 |
| 62 | 750 | 82 |
Thread Tap Drill Sizes (Metric)
| Thread | Tap Drill | Pitch |
|---|
| M2 | 1.6 mm | 0.40 |
| M3 | 2.5 mm | 0.50 |
| M4 | 3.3 mm | 0.70 |
| M5 | 4.2 mm | 0.80 |
| M6 | 5.0 mm | 1.00 |
| M8 | 6.8 mm | 1.25 |
| M10 | 8.5 mm | 1.50 |
| M12 | 10.2 mm | 1.75 |
Fastener Torque (Nm, dry)
| Size | 8.8 | 10.9 | 12.9 |
|---|
| M3 | 1.1 | 1.5 | 1.8 |
| M5 | 5.0 | 7.0 | 8.3 |
| M6 | 8.5 | 12.0 | 14.0 |
| M8 | 21 | 29 | 34 |
| M10 | 41 | 58 | 68 |
How to use this wiki
Use the Quick-Find above when you have a specific question. Use the topic directory to browse. Every page includes practical decision tables, cost awareness, and common mistakes — not just textbook theory. Written for engineers, procurement, and anyone who works with machined parts daily.