Matte flask not engraving properly on back side

I have engraved on the top side of a matte black flask with no issue. However, when I try to engrave the back side, it sparks within the center and leaves marks outside of the engraved area (in the center). I tried covering the flask with tape, but that didn’t help neither. The outer edges of the flask engrave perfectly, the center part of the flask leaves a touch of matte in the engraved area. My focus is set to the lowest (center) part of the flask so you would think the center would engrave best and the outer edges not, however, this is not the issue.

How can I stop the sparking towards the center of the flask when engraving? Has anyone had this issue before? As I’ve said ,the front of the flask engraves fine, it’s the back side that I’m having trouble with.

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I am guessing that the flasks you are engraving are curved and the back is concave. Are you using the set focus tool or calculating the height? Can you share a quick video of the sparks you are seeing?

Yes, my flasks are curved. I’m calculating the height to the lowest part of the flask. Yet the outer (higher) edges engrave perfectly and the center (lower section) leaves a bit of matte finish within the engrave. Not sure about sending a video.

I am using the MacBook pro settings also.

Sounds like your focus is incorrect if the edges are perfect, that is where the Glowforge is focusing. How are you calculating the height? Are you using the set focus tool?

I am unable to send a video. But here is a picture to show where the sparks land. The bottom two scrolls have been wiped off with acetone already. Rhe top one is the most recent. You can see how the center part doesn’t engrave all that well also.

I am calculating the height by measuring the lowest part of the flask (as well as the wood pieces underneath to bring it to proper height). My flask at lowest point (in center) as well as the risers measure 1.8, my tray is 1.6, therefore, my manual focus is set at .2

Use the set focus and make sure it lands at the lowest point of the flask and see how it changes the center part. Most trays are not 1.6" as the feet sit in indentations.

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OK. Just tried the set focus on the lowest point of the flask. It is leaving burn marks just the same if not worse. Maybe I’ll just tell this person that it cannot be engraved on the back and be done with it. :frowning:

It has crossed my mind that there’s an issue with the quality of the flask, but as I said before, the top side engraves just fine. :woman_shrugging:

When I look at the smaller set of scrolls to the top, it looks as though the sparking was less. I wonder if the sparks are falling from the outside of the flasks towards the center - as it looks as though the wider scrolls have more of an issue. Maybe I’ll try a narrower engrave with more success.

This may indicate why there is no issue on the top side of the flask when engraving as the sparks would not be redirected towards the middle :woman_shrugging:

Yup, I think the width of the engrave is definitely the issue. If I shrink the width of the engrave and use ‘set focus’, there is no sparking issue. I think the outer edges on wide engraves spark back to the center causing the finish to burn spots in the center.