Activity 002: Speed Calculation and Selection Exercise

Activity ID: U7M1-ACT-002 Duration: 30 minutes Objective: Calculate appropriate lathe speeds for various workpiece diameters and demonstrate speed adjustment on the lathe.

Overview

Students work through a series of speed calculation problems, then apply their knowledge by setting up workpieces of different diameters and selecting appropriate RPMs. This bridges theoretical knowledge with practical machine operation.

Materials & Equipment Needed

• Wood lathe with variable speed or step-pulley system
• Pre-cut blanks in various sizes: 2"x2"x8", 4"x4"x6", 6" diameter round, 8" diameter round
• Speed calculation worksheet
• Calculator
• Tachometer (handheld or built-in)
• Safety glasses and face shield

Instructions & Procedure

Phase 1: Speed Calculation Problems (10 minutes)

1. For each blank diameter provided, calculate the roughing RPM using the formula: RPM = 6000 / Diameter (inches).
2. Calculate the finishing RPM using: RPM = 9000 / Diameter (inches).
3. Calculate the rim speed in FPM for each scenario using: Rim Speed = (π × D × RPM) / 12.
4. Record all calculations on the worksheet.

Phase 2: Speed Setting Practice (15 minutes)

1. Mount the 2"x2" blank between centers (using Phase 3-4 procedures from Activity 001).
2. Set the lathe to the calculated roughing RPM for 2" diameter stock.
3. Verify actual RPM with the tachometer. Record the reading.
4. Increase speed to the calculated finishing RPM. Verify and record.
5. Repeat with the 4"x4" blank—note how the appropriate speed changes dramatically.
6. Instructor demonstration: mount the 8" round blank at appropriate low speed. Observe the difference in visual appearance and vibration between correct and too-fast speeds.

Phase 3: Speed Decision Scenarios (5 minutes)

1. Discuss as a group: What speed would you choose for a cracked, bark-on 10" log section?
2. What about a perfectly round, pre-balanced 3" maple spindle?
3. A 6" spalted wood bowl blank with soft spots?

Discussion Points

• Why is rim speed the true safety factor rather than RPM alone?
• What are the consequences of turning a large blank too fast?
• How does wood condition (cracks, bark, spalting) affect speed selection?
• When should you deviate from the calculated recommendation?

Expected Outcomes

• Students accurately calculate RPM for given diameters
• Students can set lathe speed and verify with tachometer
• Students understand the inverse relationship between diameter and safe RPM
• Students can make informed speed decisions for irregular stock

Assessment Rubric

Criteria	Excellent (4)	Proficient (3)	Developing (2)	Beginning (1)
Calculations	All correct	1 minor error	2-3 errors	Cannot apply formula
Speed Setting	Sets speed accurately, verifies with tachometer	Minor adjustment needed	Requires guidance to set speed	Cannot operate speed controls
Decision Making	Considers diameter, condition, and balance	Considers diameter only	Applies formula rigidly without context	Cannot determine appropriate speed

Safety Considerations

• Never change belts on a step-pulley system while the lathe is running
• If excessive vibration occurs at any speed, stop immediately and recheck mounting
• The 8" blank demonstration should be instructor-only unless students have demonstrated competency
• Maintain minimum 1/4" tool rest clearance at all times

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Last Updated: 2026-03-19