Adjustments for 19mm ply

The default plywood thickness in New Zealand is 19mm, I understand that I will likely have to adjust the CNC templates to account for this.

Any tips on where specifically I should pay attention to? My assumption is any perpendicular connection between two pieces, but would love to have that confirmed.

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Hi @lachlan It’s a bit of a tricky one, although Skylark standard (repeated) blocks should make this easier than it’s been in the past. I don’t think you can uniformly apply a blanket 1mm offset/tolerance to all the cutting files in the same way we usually do for the 0.25mm machining tolerance (example here). This is because the material thickness is changed, but not the length of tabs or slots. However, in saying that, the small differences of <1mm may not matter much when you factor in the manufacturer’s variation in plywood batch thicknesses. So there’s maybe two ways you could go about it:

  1. The surest way would be to manually tweak the cutting files (and maybe even 3d models) for 19mm depth on all slots and tabs for every Skylark block, but leave the non-material-based dimensions (ie. length of slots) the same.

  2. The other option is to test a uniform 0.5mm offset on holes/slots and 0.5mm inset on outer profiles, on top of the already applied 0.25mm cutting file machining tolerances, and cut and assemble a few full-size test blocks. If these work, and parts fit together well, this may be a shortcut for converting future version releases that doesn’t involve so much manual adjustment.

19 mm is a hair less than 3/4 inch, widely available in the US. @Clayton are the block models not parameterized for a material thickness?

Hi @Steve The block models are broken down into ‘parts-of-parts’ CAD components, as you might have seen in the v0.2 development branch Rhino or Sketchup models. We haven’t gone fully parametric with things like material thickness as this logic is not well supported in either software, and it hasn’t been a priority to have different material thickness versions of the Skylark block library to date. I think the 1mm difference is small enough that it could be mitigated by tolerances on the cutting files, but open to other parametric possibilities if you know of a good tool for achieving this across the entire block library?

I’m not sure such a tool exists since all the tab lengths and slots are a function of material thickness. You would have had to parameterize the models from the get-go. There are import/export parameter tools for Fusion and Inventor but they are janky. I’ve tried to use those tools in the past but the 2D sketches on faces of materials, critical for generating DXF files, often do not update with parameter changes.
An alternative is using a programmatic 3D CAD package, such as OpenSCAD, where global variables can be set and revised without much change to the workflow. The 3D objects are created with Boolean operations from primitive shapes, meaning that the entire library would need to be revised. However, version control and updating assemblies to the latest versions would be easier than the brute force approach from conventional CAD packages.

I have been dealing with designing a forking proprietary building system that is adaptable for both sizes 18mm EU&USA and 19mm AU&NZ native production.

We have made basically the whole library double by parametric design in Fusion 360.

For simpler parts (similar to the Skylark parts) it is no problem and parts can be adjusted by changing the value in the mother formula table.

So I can from experience say that parametric design in Fusion 360 is surely possible for Skylark and all can be changed on by just changing the thickness value.

Still we decided for our own system for future projects to just give CNC cutters in AU&NZ the order to program with a router bit 7mm but cut with 8mm. Thus creating enough tolerance to make the 19mm fit.

But also ordering import there instead of native production can solve it as well.

Our experience is that in general all fits well when it comes directly from the machine. But some weeks later, or when transported through humidity to location. All get very hard work to make it fit.

So we now cut everything up to 0.5mm bigger in any case. But rely on glue and staples to secure structural integrity.

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