Still stumped about my rotary

I have a 60W Chinese laser etcher with a Ruida controller. I am running Lightburn. I am trying to etch glass cylinders using a rotary attachment. It is a cylinder style rotary. I have calculated my steps per rotation by creating a line box in Lightburn, and setting rotary numbers accordingly: SPR - 1180.5. Roller diameter - 24mm. object diameter - 38mm. Circumference - 119.381. Running my laser at speed - 100 and power - 20%.

To test the calibration, I created a box and sized it to fit the cylinder - 119.381mm high x 80mm wide. The box etched perfectly with the horizontal lines matching exactly.

Then I created a graphic in photoshop and brought it into Lightburn. I set the size at the exact same size, in fact setting it precisely over the box I created earlier.

The graphic etched with a 6mm gap! But the box still etched perfectly.

How can two different objects that are exactly the same size under exactly the same rotary settings etch at different sizes?? The only thing I can think of, the only variable that I cannot account for, is that etching the box takes seconds and only four movements while the graphic takes 1200 movements and about 20 minutes. Does this mean my rotary is inaccurate?

I might look into the object being engraved slipping during these movements. This can happen with many direction changes, with smooth hard objects, and lightweight objects, as they need some holding-power help to not slip on the rotary. Large rubber bands or weights or both help to reduce slippage of the object in the rotary. Could this be what you are seeing?

It certainly is a possibility. But if that were the case, wouldn’t there be some distortion of the image? There isn’t any that I can see. Whatever slippage there is must be extremely consistent to create a 6mm gap without distorting the image.

I am thinking of upgrading to a chuck rotary which will hold the piece firmly.

Image? Is that Raster? What settings and please show result. Does this look correct in the ‘Preview’?

It’s a jpg. But it looks perfect in the preview window. Like I said, I arranged the box and the graphic to be exactly superimposed on each other. In the preview window, they match perfectly but the final etch the graphic is 6mm too short. I believe that there is indeed some sort of slippage happening with the graphic. It must be very consistent so as not to distort the image, just adds up to 6mm over the course of the etch.

Is there any way to upload images or video here?

Images, yes. Click here.

And you are presented with this.

For video, post to a shared hosting service such as YouTube, DropBox, GDrive or the like and make it public. Provide link here and we can take a look.

I could be wrong but look for the relation for DPI and images brought into LightScape in this forum. Unless of course you’re only experiencing this with the cylinder and have been successful in the past with flat layouts.

Maybe you could glean something from there.

One for LightBurn and Inkscape

new guy here. I noticed a mismatch in my lines per inch when i combined some svg and images, the images did not have enough detail (dpi) to match the lines per inch in the svg. I have to go back into gimp and upscale the image. I think i ended up getting a better image and letting lightburn use the jarvis dithering on it.

I assembled everything here. If you have time, take a look. If not…

HAPPY NEW YEAR!!

In cut settings I have line interval set at .085 which corresponds to 300 dpi, same dpi as the image in question.

Please show just the side that does not match up. This is the interesting bit, and you don’t show much of that.

Looks like your glass is sliding and creeping.
I suggest you cut two 3mm bb circles and hot glue them to the ends. Then cut a triangle and clamp it to one end, to keep the piece centered in the roller assembly.
That’s how I do all my glass on my rotary.

Like this…

Very clever! So you set your rotary object diameter to the disks instead of the glass itself?

I believe you are right about what is happening. I have ordered a chuck rotary which should eliminate the problem entirely.

I believe I may have solved the problem. I re-positioned the rollers so they were further apart. The 38mm tube sits deeper between the rollers resulting in a more firm grip. My last test was perfect. No gap at all.