As others have said, engraving on a curved plate is a level of complexity over on the flat. There are a number of reasons, but, before that, I would suggest that you look at Fusion360 (it is free and way powerful) and try:
- model the curved surface as a segment of a cylinder (or sphere, or whatever the surface is). For a segment of a cylinder, I would model a cylinder and intersect it with an extrusion of the outside profile of the tag. No worry about the thickness or edges being the perfect angle, as you only care about the surface.
- build the text on a work plane over the surface to engrave. This would be the same plane that was used to make the profile, but a different sketch.
- in the CAM workspace, use project to get the text to the surface. If the curvature isn’t too great (the part isn’t too far from flat), this will look fine. If the part is too far from flat for this to look ok, then it will be difficult to do the engraving anyway, since the tool will not be close to perpendicular to the surface. In the CAM setup, pick an origin that will be usable when setting up on the machine. For the tags you linked to, I would use one of the hole centers at the top surface, for reasons below.
- generate toolpaths.
- Post-process for the machine. Fusion360 has a post processor for the Nomad that works well. The .nc file from fusion goes directly to the machine.
To do this, you will need the curvatures of the plate involved in both directions (is it a segment of a cylinder? A sphere? Different cylinders in two directions?) This can be measured fairly easily, but must be done precisely. The tool I use is called a lens clock, but it can be done many other ways. Search SPHEROMETER for more info. If the surface is NOT a segment of cylinder of sphere, it gets real hard to measure, and you either need to get the info from the manufacturer (good luck…) or use a CMM (coordinate measuring machine… Not a hobbiest tool, but, if you are associated with a university with an engineering department, or in industry where precision measurement is done, there is like one available)
To set up, I would use a waste piece and cut the profile of the part bottom surface (same as the top for this?) into the waste piece. The surface of this will support the part, and, if cut below the surface of the waste piece, the edges will locate the part. At a minimum, I would put a conical dimple in this fixture for each of the holes in the part. Better would be to have the fixture surface up a little (easier to mount and remove the parts) and use holes for alignment pins.
Holding the part down can be done any number of ways, including fixture wax, super glue, clamps, etc. For small numbers of pieces (a dozen or fewer per day), I would use wax. For production numbers (as fast as the machine can crank them out, maybe several hundred per day for simple pieces), you will need a well designed fixture, which is beyond what I am prepared to go into here. For one good method, see the penny carving thread a few weeks ago.
WHY curved surfaces are harder
- modeling the surface becomes critical. Being slightly off on curvature makes the result look bad, as cut depth will vary over the surface.
- fixturing in more involves, as you need the position and alignment very close to avoid unmatched cut depth, and you need good support to prevent the part from flexing during the cut.
- the tool will not be perpindicular to the surface. For a ball-end tool, this isn’t a big deal (though the appearance may vary a bit depending on the angle of attack), but for a vee cutter, this will give different leg lengths depending on the direction of cut relative to the curvature. For near-flat parts, this doesn’t show. For larger curves, it is a real distraction. By the time you get to this point, the method describe above will begin showing distorted characters, anyway.
Engraving curved parts is one of the few places that manual methods and some of the traditional machine tools still hold fort in some cases. I would LOVE to be able to afford a Deckel universal type engraving pantograph with a rocking table. (This is basically a copy mill with the ability to precisely scale. Then again, I would also like to be able to fly by flapping my arms. They are equally likely at this time, though, in my daily life, flying would be more useful…
Ok, enough. If you’re still reading, have fun.