Pen Lathe Speed Calculator

Lathe speed (RPM) is one of the few pen-turning variables that directly affects safety and finish quality. Too slow on a small blank wastes time and produces tear-out. Too fast on a large blank causes vibration, broken blanks, and sometimes broken tools.

The standard relationship between RPM and surface speed:

surface speed (m/min) = π × diameter (mm) × RPM / 1000

For pen turning, the target surface speed varies by material. These are the figures the calculator uses, and they match the sibling Pen Turning Lathe RPM Calculator, which works in surface feet per minute rather than metres:

Typical pen blanks are 15-25 mm diameter, well within the range of a small benchtop lathe (500-3500 RPM). Larger kits and pen ferrules run up to 35 mm, where the speed has to drop noticeably.

Practical chart for pen blanks, which is the calculator’s own output:

• 15 mm hardwood: ~2600 RPM
• 20 mm hardwood: ~1950 RPM
• 25 mm hardwood: ~1550 RPM
• 15 mm acrylic: ~1600 RPM
• 20 mm acrylic: ~1200 RPM
• 25 mm acrylic: ~950 RPM

Acrylic runs slower because a lower RPM reduces friction-melting at the cutting edge. Some turners run acrylic at hardwood speeds with very light cuts and a wax lubricant on the workpiece, which works but takes experience to judge.

Adjust speed for blank balance: a poorly-balanced blank (eccentric grain, crack, undersized blank piece glued to a brass tube) vibrates badly at higher speeds. Run the lathe at 60-70% of recommended speed for the first few cuts to round the blank, then increase to recommended speed for finish cuts.

Sanding speed: many turners drop the lathe to 800-1200 RPM for sanding because higher speeds heat the wood and burn the sandpaper. For acrylic, sanding speeds of 1000-1500 RPM with each progressive grit (240, 320, 400, 600, 800, 1500, 2400, 4000) produce the famous mirror-finish acrylic pen.

The 80-grit-on-acrylic test: if your starting grit is too coarse, the next grit cannot remove the deeper scratches. Most acrylic finishes start at 240 or 320 grit; 600 is a more conservative starting point for already-smooth blanks.

Final polish step: use plastic polish (Novus 2 and 3) or carnauba wax on a clean buffing wheel, off the lathe, to bring the finish to mirror level. Or use friction polish (HUT or Mylands) on the lathe at 800-1200 RPM with a soft cloth, applying very light pressure to build up heat and a wax film.