You needed to do two steps. First, Stroke-To-Path (Ctrl Alt C). Second, Union (Ctrl +).
a) two lines with thin (red, green stroke) stroke, not overlapping.
b) two lines with thick stroke, now overlapping
c) After the selected lines have Stroked-To-Path and Union applied, the shape is now Red-filled and No Stroke. This will work for engraving.
Very good illustration of the challenges to understanding vector production in a design program and the way a Glowforge can process them. I second @mpipesâs vector dissection.
Iâm sure you know this, but I run into this problem all the time. You really can only change one variable at a time if you are doing a test. I always want to try and tweak two or three settings at the same time to save time. It ends up confusing me. From what I read in your post, you changed the file to have filled, closed vectors AND you increased speed. The speed really didnât make the engrave happen, changing the file did. So your caption on the top left photo left out a critical aspect of the design to give the most salient answer. The design file is always helpful to have and itâs good that you posted it.
As to the appropriate place to post design questions, it looks like we might want to start a new topic on this. As it is, since this is not Proofgrade or Selective Service Board, itâs beyond the manual, regardless of the fact that it really is a design problem. The manual does discuss this issue, so technically it isnât beyond the manual. But evidently using non-Proofgrade warranted the change.
We are still learning the ropes here. At least no matter where you post, you can get some forum help. I most likely would post in Beyond the Manual if I have an issue, just because that might allow the issue to get a longer discussion. Iâm in a different situation having logged so many hours on a Glowforge and on the forum, and being loathe to ask anyone for help!
That is a good point. Even if it is a design problem, the Glowforge spent time trying to figure it out and then elected not to do anything, rather than a popup that says, âHey, we noticed that you are trying to convert a single stroke to an engrave and that just wonât work.â I get these little reminders all the time in Slic3r when I am doing supports.
Stay safe Jules. My dad used to say âI need you in my businessââŚwe need you in ours, here, too!
Thanks, and Iâll certainly try! 
(The tornado producing band of the storm has moved on now, so itâs down to just rain. Lots and lots of rain.) 
oo oo are you seeing any âSharknadoesâ ?Stay safe !Pleeeeeeeaaaassssse !
Chuckle! Naw weâre good - I just always think of that stupid movie whenever we get bad weather that causes flooding around here. 
I wonder about that. When I was trying to engrave open paths, the software did its best to close them (resulting in weird filled places where there were none, which is another issue). So there may be some strokes that will convert to an engrave that has black points in it, and others that will convert to an engrave that doesnât. Still has to do all the thinking. (And maybe you want to engrave an area with no points zappedâŚ)
Iâve noted that if it isnât a completely closed path but still has a fill, the Glowforge will process it. Sometimes it doesnât turn out the way you expect. If it is just a single stroke with no shape and no fill, thatâs when it seems not to do anything.