Achieving a seamless wrap around a tumbler doesn’t just depend on alignment—it starts with precision calibration of your rotary’s Steps Per Rotation (SPR). If your SPR is off, your design won’t meet perfectly at the seam, even if everything else is right. (If you haven’t yet, check out our blog on Perfecting a Flawless Seam for tips on alignment and seam testing.)
What Is SPR and Why Does It Matter?
Steps Per Rotation (SPR) tells your rotary how many motor steps it takes to complete one full rotation of the tumbler. If this number isn’t dialed in correctly, your laser will either under-rotate or over-rotate the cup.
You’ll see the effects most clearly in wrap designs that are supposed to meet seamlessly at the back. A misaligned seam is a dead giveaway that something’s off—and often, it’s your SPR.
How to Test and Dial In Your SPR
One of the easiest ways to fine-tune your Steps Per Rotation is to run a test using blue painter’s tape and a simple rectangle.
🔧 Here’s how:
- Wrap the circumference of your tumbler with blue painter’s tape (as smooth as possible).
- In LightBurn, draw a long, narrow rectangle that spans the entire circumference of your tumbler (this should match your actual design area).
- Engrave the rectangle—just enough to mark the tape.
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Examine the seam:
- If the ends overlap, your SPR is too high.
- If they don’t meet, your SPR is too low.
- If they align cleanly, you’ve nailed it.
This might take a few passes to get just right—but once it’s dialed in, it becomes a set-it-and-forget-it value. You won’t need to redo this for every job. Most tumblers (and even brands) will work off the same SPR value as long as you’re using the same rotary and setup.
Pro Tip: Save Your Settings
Once you’ve found the right SPR, write it down, save it in LightBurn, or jot it on your whiteboard. This one number can save you a ton of frustration in the long run.
Bonus: Combine with Alignment Tips
Finding the correct SPR is only part of the equation—perfect alignment matters just as much. Check out Perfecting a Flawless Seam for the techniques I use with blue painter’s tape and reference marks to ensure your wrap joins exactly where it should.