How the speeds are worked out
The bigger the piece, the faster its rim travels at a given RPM, so safe speed falls as the diameter grows. These are the common rules of thumb: 3,600 ÷ diameter to start an out-of-round or unbalanced piece, 6,000 ÷ diameter for roughing once it runs true, and 9,000 ÷ diameter for finishing cuts, with the diameter in inches.
For small work the rule gives speeds no lathe reaches (18,000 RPM for a 1/2" pen blank), so the calculator stops at 3,000 RPM, as Wood Calc does. Spindle work usually runs at 2,000 to 3,000 RPM.
Lathe speed chart
Speeds for common diameters, from the same rules as the calculator. Start at the low number, and go slower still for a blank that's out of round or unbalanced.
| Diameter | Start | Roughing | Finishing (top) |
|---|---|---|---|
| 1" | 3,000* | 3,000* | 3,000* |
| 2" | 1,800 | 3,000 | 3,000* |
| 3" | 1,200 | 2,000 | 3,000 |
| 4" | 900 | 1,500 | 2,250 |
| 6" | 600 | 1,000 | 1,500 |
| 8" | 450 | 750 | 1,125 |
| 10" | 360 | 600 | 900 |
| 12" | 300 | 500 | 750 |
| 14" | 257 | 429 | 643 |
| 16" | 225 | 375 | 563 |
| 18" | 200 | 333 | 500 |
| 20" | 180 | 300 | 450 |
| 24" | 150 | 250 | 375 |
* The rule gives more than 3,000 RPM here, so the chart stops at 3,000, as the calculator does.
Start slow
Always start at the lowest speed with the tool rest set and stand out of the line of fire. Speed up only once the piece runs smoothly, and slow down if the lathe shakes. A blank with bark, cracks or a lot of weight on one side needs less than the starting speed shown. These numbers are a guide, not a promise of safety; your lathe's manual comes first.
A worked example
An 8" bowl blank starts at 450 RPM while it's out of round, roughs at 750 RPM once it runs true, and takes finishing cuts at about 1,125 RPM. A 12" blank halves that: 300, 500 and 750 RPM.
Turning a segmented bowl? The Segmented Ring Calculator gives the cuts for each ring. Sizing a blank from a log? Try the Bowl Blank Calculator, and for a ball, the Sphere Turning Calculator.