TwiztedEng said:
Well, you are right in that the tabs might make things just a little easier to weld together, but then you would have the thickness of the material changing the height of things all over the place, and it just wouldn't look right - not to mention that the pieces wouldn't fit together properly after a few were welded. Without the tabs, everything should work together just fine. "Butt welding sheetmetal" isn't impossible, and if you were really into fabrication you would have seen that I said 14-guage sheetmetal and would know that it is plenty thick enough to weld. Most of the edges will be at fairly good angles also, so there will actually be only a few places where a flat butt weld is required. After welding I plan to grind everything smooth so that it all blends together. Yes it will be difficult, but so is papering, fiberglassing, sanding, bondo-ing, sanding, etc...
And maybe it just won't work at all, but I'm gonna give it a shot and see
OH, and I forgot to mention that what I have is a CNC plasma table, so I can't just throw it into a nice 5-axis mill. I WISH I had something like that, but I have a very small business and things like that are far out of reach for me
-Darren
P.S. - Before you accuse someone of not knowing what they are talking about, please review what they said more carefully. I DO know what I am doing, and I was pretty taken aback by your response here. Sorry if I was harsh back to you.
I'm not accusing you of not knowing what you're talking about, I'm stating it as a fact. You don't know. I don't know. Neither one of us has made a sheetmetal helmet, and it's the very reason we're here. What I also stated, very clearly, and after reading everything you wrote, is that I think it's unwise to leave the tabs off. I'm not trying to start a flame war, but let's be realistic about why we're talking about this in this venue.
Having the tabs on will create a sealed edge on all seams, and leave the front with a presentable edge. A quick touch with a grinder on the seams will smooth it out. Trying to weld each seam, which probably totals several hundred inches or more, is going to take dozens of hours. You didn't mention if you have access and experience with TIG welding or not. If you use MIG, welding the sheetmetal probably won't be easy, and MIG tends to eat at the edges easily. Stick or gas are probably just out of the quesiton, but could be mildly possible. And after all that work, you've still only completed the folded model. So all that pretty welding is going to be bondo'd over, despite it having taken you many times longer than it would have with paper. You will have the same issues with metal thickness regardless of which method you use, since the Pepakura software is not accounting for it.
Spot welding through the tabs is probably an even better option, if you have access to a spot welder with enough reach to get in there good.
14ga steel weighs about 3.5lb/sq-ft. A 12" sphere has a surface area of about 6 1/3 sq-ft, meaning it would weigh about 22lb. The helmet obviously isn't a sphere, but is probably somewhere in that neighborhood in terms of surface area. It could be less, because of the mask cutout, or more, because of all the varied faces. That's probably way too heavy to actually (i.e. comfortably) wear, especially when you add bondo, paint, lights, padding, etc. I would personally stay under 20ga (1/2 that weight), and maybe even under 24ga (1/3 that weight).
gingersnapples said:
wait, whats the difference between a plasma table and a 5-axis mill they both are CNC machines and do generally the same thing right?
A plasma CNC table cuts out sheetmetal the same way that you would cut out paper with an Xacto knife. They are 3-axis machines, that work in 2D. The machine can move the plasma cutter tool around the surface of the sheetmetal in two dimensions, and it uses the 3rd axis to pick the cutter up and keep a constant distance from the material.
3-axis mills are basically the same thing, only they use a rotating cutting bit, like a RotoZIP or Dremel tool. The machine can move the bit in all 3 axes (left/right, forward/back, up/down). These can cut out any 3D object, within the dimensional limits of the machine, as long as it doesn't have any undercuts (areas that are hidden when viewed from above; it could cut out a dome, but not a sphere, for example, since it can only move the bit straight down and can't get underneath the widest point to cut the skinny bottom).
5-axis mills can not only move the cutter in the 3 primary axes, but can also tilt the cutter in two more sets of directions (usually tilt forward/back, and tilt left/right). These machines can mill extremely complex parts, that include undercuts/holes/depressions in all faces. These types of machines are pretty amazing, can basically cut any 3D part you can imagine, and are usually most limited by the methods of holding/clamping the material (e.g. if you want to cut a sphere from a cube of material, where do you hold it?).
-------Edit-------
I know SCA uses helmets made of 14 or 16ga steel, so the weight is bearable. But I also looked up the weight of a military keval helmet, and they are about 4lb (when not wearing NVGs). A motorcycle helmet weighs 2-5lb (
http://www.webbikeworld.com/motorcycle-helmets/motorcycle-helmet-weights.htm ). After having worn SCA combat helms, I can tell you that you won't really want to walk around in one for any length of time. After having worn military kevlar helmets, they are much more comfortable in weight, although they can be a bit heavy when you first are getting used to wearing it for extended periods.
