How to Measure Diameter for Rotary Engraving (Especially on Tapered Cups and Bottles)

I’ve noticed quite a few discussions about how to measure diameter for rotary engraving, and this topic often trips people up when working with tapered or different sized items in the chuck.

When using a chuck-style rotary like the PiBurn Grip 2 or D80 etc, the diameter measurement should always be taken from the exact point where the laser will engrave not from the top, middle, or base of the cup as a whole.

Here’s why:

- The rotary uses the entered diameter to calculate the surface speed and rotational distance.

- If you use a larger diameter (e.g. at the cup’s widest point), the laser will “under-rotate,” stretching your artwork.

- If you use a smaller diameter (e.g. near the base when you’re actually engraving higher up), the rotary will “over-rotate,” compressing the design.

So in short:

Measure the diameter at the exact horizontal line where your engraving will sit (usually with callipers or a flexible tape).

Enter that diameter (or circumference ÷ π) in your rotary settings in LightBurn or your controller.

Extra tip:

If the taper is steep, you need to tilt the rotary slightly so the engraving area runs level with the laser’s focus line that keeps the engraving sharp and evenly sized across the design.

If you measure the largest diameter but engrave lower down on a smaller section, here’s what happens:

- The rotary will rotate too slowly because it’s using a circumference larger than the actual one at the engraving point.

- Since the laser output (X-axis movement) doesn’t match the true surface rotation, your artwork will appear stretched horizontally around the cup.

- Text or logos may also lose alignment, especially on wider designs circles can look oval, and fonts may look wider or “fatter” than intended.

In short:

Larger diameter entered = under-rotation = stretched engraving.

Smaller diameter entered = over-rotation = compressed engraving.

For small logos on tapers if you don’t want to mess with exact formulas, you can just take the average of the two:

(Eg. 90 + 60) / 2 = 75 mm

That’s nearly the same as 74mm using a close enough for most engravings except full wraps.

There is a fancy formula that makes it slightly more precise when the taper is steeper.

On these glasses for example I just used the calipers to measure approx the centre to get an average for where the engraving will be placed.