Slide 002: Speed Selection and Turning Theory¶
Slide Visual¶
Slide Overview¶
This slide covers the physics of wood turning—rotational speed selection, the relationship between RPM and rim speed, and the fundamental cutting principles that govern safe and effective turning.
Instruction Notes¶
RPM and Rim Speed¶
The critical safety factor in wood turning is rim speed—the linear velocity at the outermost point of the workpiece. Rim speed (in feet per minute) is calculated as:
Rim Speed = (π × Diameter × RPM) / 12
Safe rim speed for wood turning is generally kept below 6,000 FPM for rough blanks and up to 9,000 FPM for balanced, round stock. This means larger diameters require proportionally lower RPMs.
Speed Selection Guidelines¶
| Workpiece Diameter | Roughing RPM | Finishing RPM |
|---|---|---|
| Under 2" | 2400-3000 | 3000-3600 |
| 2"-4" | 1200-2000 | 2000-2800 |
| 4"-6" | 800-1200 | 1200-1800 |
| 6"-8" | 600-900 | 900-1200 |
| 8"-12" | 400-600 | 600-900 |
| Over 12" | 200-400 | 400-600 |
Rule of thumb: For roughing, divide 6,000 by the workpiece diameter in inches to get a starting RPM. For finishing, use 9,000 divided by diameter.
Cutting Theory Fundamentals¶
Wood turning uses three fundamental cutting actions:
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Cutting (Shearing): The bevel rides the wood surface while the edge slices fibers cleanly. Produces the best surface finish. Used with gouges and skew chisels. Requires bevel-rubbing technique.
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Scraping: The tool is held flat or slightly below center with the edge contacting the wood at a steep angle. Produces a rougher surface with torn grain but is easier to control. Common for beginners and for specific operations like finishing bowl interiors.
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Shear Scraping: A hybrid technique where a scraping tool is rotated 45 degrees to present a shearing cut. Produces a better finish than flat scraping while maintaining scraping's control advantages.
Grain Orientation¶
Understanding grain orientation is essential: - Spindle turning: Grain runs parallel to the lathe axis (between centers). Cutting forces are relatively predictable. - Faceplate turning: Grain runs perpendicular to the lathe axis. The tool encounters alternating end grain and face grain in each revolution, requiring different tool presentation strategies.
Key Talking Points¶
- Always start at a lower RPM than you think necessary, especially with rough or unbalanced stock
- An out-of-balance workpiece at high RPM is a serious safety hazard
- Cutting produces a better finish than scraping—but requires more skill
- Never exceed the lathe's maximum rated RPM
Learning Objectives (Concept Check)¶
- Can students calculate appropriate RPM for a given workpiece diameter?
- Can students distinguish between cutting, scraping, and shear scraping?
- Do students understand why grain orientation matters?
Last Updated: 2026-03-19