First 1/16” being cut off during tumbler engraving

I’m working on engraving metal cups but the first 1/16 of the first letter gets cut off and is completely straight I make a square and it’s on the money. Using a 100watt rud controller. The rest of the engraving is fine, did the square trick that measures out fine…

That is backlash in the rollers or belts on the rotary. The rotary turns to go to the bottom of the engraving, then reverses direction to engrave from bottom to top. If the belts or set screws are loose, or gears have play in them, that direction change will take a moment to start turning the cup.

If you can’t fix it mechanically, you can add a single line below the design by about 1/16", set that on its own layer, with the power set to 0.1% or as low as it goes. The laser will move there, but not fire, then move backwards from that spot, taking up the slack.