A dubious idea I have about cutting curved dovetails so a box would look like an impossible dovetail box, but would in fact have the top open on an arc rather than a diagonal slide like most impossible dovetails.
The axis of rotation is the left corner, where there's a green line.
@smellsofbikes So I'm now working on a completely parametric design based entirely on robust constraints. I would expect a design rotating on that corner edge to be a right pain to actually assemble, so what I'm modeling puts a pin in the corner defining the axis of rotation. A reamed pin hole will provide a precise reference for mounting on a rotary table, and then when it's finished will align the top and the base to fit together.
For manufacture, my idea is to make drawings that show a distance across spheres for measuring the dovetails, since you can't measure a curved dovetail with gauge pins...
However, I've run into what I think is a bug in the development version of FreeCAD I'm running with the TNP mitigations in place, so I might have a short detour for a bug report. We'll see... ☺
@smellsofbikes Second time in a few days that git pull and rebuild fixed a bug before I could even report it. I haven't done the lid yet, nor any drawings for measuring across spheres, but the lid will be easier because subtracting a shapebinder of the base from the lid will make the corresponding dovetails.
@smellsofbikes Oh now I understand the problem! I didn't see it until I got it modeled this far. Now I realize we're talking four custom dovetail cutters for perfection. (You have a d-bit grinder? 😀) Realistically the straighter of the two dovetails will be good enough to look right if cut with a normal dovetail cutter, but the extremely angled one won't.
Now I understand why it was hard to constrain robustly without mathing it out! Back to the drawing board for me!
@smellsofbikes Perhaps the hardest part is that the dovetail cutter itself would need a curved profile in order to project a straight line on the box surface. I think that the steeper the dovetail angle, the more obvious this effect would be. I can't tell from your screen shot; were you accounting for this in your design?
@smellsofbikes OK, I think that sketching on the face and revolving around the axis of rotation, then cutting off the extra, gives me a theoretically-correct object. However, to machine it might actually take eight different dovetail cutters; one for each face on each part, because the base would need convex cutters and the lid would need concave cutters.
I'm unlikely to machine this myself, but https://gitlab.com/mcdanlj/RotationalImpossibleDovetail has both the original wrong version and the updated files. I haven't yet set up the spreadsheet with the driving parameters to actually be a configuration table, but it's set up to trivially convert to a configuration table.
If you try to machine any variant of this, I do not envy you the headache but will enjoy pictures... 😀
@f4grx It's not obvious to me why the cover is harder than the base of the box. Given all 8 custom dovetail cutters necessary to produce truly straight lines on the surfaces, it seems like both are the same work?
With my idea of the lid rotating on a locating pin in the corner, I'd think that machining the lid first, then the box base, and starting from the inmost surface out, would let you take light passes until each surface in turn engages between the base and the lid, until all four surfaces match?
@smellsofbikes Addendum: Somewhat ironically, this was done in very latest #FreeCAD (as of a couple hours ago) with the TNP mitigations enabled (FC_USE_TNP_FIX defined), but this particular model is, I think, fully robust against topological ambiguity even without the mitigation, since I tried to define everything against fixed datums, nothing relative to geometrical elements.
@smellsofbikes I'm trying to validate my construction by creating an assembly, making the lid partially transparent, and rotating it around the pin, but I still can't figure out how to get the lid to rotate in the assembly.
@smellsofbikes Ok, when I'm back at the computer I'll model the lid dovetail separately instead of subtracting the base from the lid, and add a clearance parameter.
For fudging the visible edge, I'd consider getting a 40° internal angle, 20° per side cutter, to reduce the visible curve at the edge.
Another idea would be SLM or SLS now that it's relatively cheap. But... not in 316 stainless!
@smellsofbikes They have made excellent progress on the new integrated Assembly Workbench. It's fairly streamlined, and they have kept it to a small number of flexible constraints ("joints"). It's not finished yet, but they paid attention to UX while building it. They are clearly intending it to be generally usable in a few months, since it was one of the defining features for "1.0" and they are currently hoping to get there by August, last I read. 🤞
Unfortunately, the author of Assembly4 took umbrage at some of what Ondsel wrote in their rationale for building a new workbench with a C++ solver, and he became so abusive that he was permanently banned from the FreeCAD forum. 😭
@smellsofbikes The eye probably wants the clearance to be normal to the surfaces, but actually what we want is horizontal and vertical clearance, so despite the illusion, the vertical and horizontal clearances here are the same 0.1mm, which is a parameter so people can adjust for their printers.
I have pushed the new files. I suspect that stable FreeCAD can read the Parts file. A current weekly build should be able to read the Assembly file that puts them together properly. The lid should be able to rotate in the assembly because I use a Revolute joint, but I haven't figured out how to do that.
I'm a novice on Lattice2, so I'd appreciate testing of all the functionality of the workbench both on recent development builds and on stable FreeCAD. I want to make sure that the fix is complete and correct on both stable and development versions before suggesting that it is ready to merge.
If you use git to install workbenches, you can do something like this if you want to test and help:
cd .local/share/FreeCAD/Mod/lattice2<br></br>git remote add johnsonm git@github.com:johnsonm/Lattice2.git<br></br>git fetch johnsonm<br></br>git checkout johnsonm/mkj-attachment-support<br></br>
Feel free to comment on the pull request itself or here, I'll see it either way.
DeepSOIC gave me helpful feedback, and ultimately merged my PR and fixed a bug I introduced in the process. Lattice2 should work again on development builds of FreeCAD. 🎉