That’s a great project and the fact that you got to do it with your daughter is amazing. 

Yeah I can see using pipe for tree insert, much quicker than rolling one lol. I can definitely help ya out with cone layout as well when you’re ready. I have a decent amount of fabrication experience, feel free to reach out with any questions bud. 

Hey Jorge, I’ve used slip rollers a decent amount throughout the years. 18 gauge will roll easily in a slip roller, make sure you adjust the spacing on the roller on both sides. If your roller isn’t evenly spaced you parts will come out in a cone/spiral shape instead of round. Now if you need to roll a cone you do the opposite, tight spacing on one side and a larger spacing on the other. The shape may affect your roll so pay attention to how you feed the parts. After you cut the star out to roll it, it will roll the arms of the star slightly different if your feeding them in differently. You will need to put the stars in the same way every time or they won’t match. With 18 gauge you could also use a smaller piece of pipe or a tank and bend it by hand cleanly. If you have a fire pit or something that has a design already cut out of it you may have sections that aren’t rolling the same and you may have to work it by hand a little. Hope this helps, looking forward to the end results 

I know you mentioned the cracks are starting at the beginning of the weld, Aren is on point with his answer on filler metal. I did want to just remind you to taper off your amperage/heat at the end of your welds as well. If you quickly let off the pedal that too will affect the heat cycle and can cause cracking. Like Aren said adding a little dab or two as you taper off will help ya out.