Designing CNC-cut plywood joinery
September 16, 2016 4:41 AM   Subscribe

I have basically unlimited access to machines with CNC routing capabilities, but I'm not any good at design and layout. So I need some guidance.

I work for a leading provider of digital finishing machines, which in addition to cutting also have routing/milling capabilities. I can use the machines for basically any hobby project I care to do. I have no skill in using "proper" design software. What I've usually done is used Inkscape to lay out my design and then export it to a PDF file, which the machine will happily cut or mill for me.

Now I want to do CNC cut plywood joinery as per this page. I then need to align the joints on different faces on my design, and add a fraction of a mm here and there because of tolerances. I'm finding this really slow going in Inkscape.

The thing is, I've got a fairly good idea of how I would do this in Sketchup. I would dimension the part, and place the the joinery tenons and mortises etc with the help of Sketchup's tape measure tool and dividing aids, and then use the push/pull tool to add and subtract 0.1 mm here and there. But Sketchup won't give me a file that's much use for milling flat sheets.

So do you know a workflow in Inkscape that could help me? Or should I try some other kind of software?
posted by Harald74 to Computers & Internet (9 answers total) 6 users marked this as a favorite
I used to do this kind of think in rhino (with rhinocam) and it's not bad but it's tedious. In the end I mostly wrote my own code to do it (that is, I wrote python code that writes gcode). I found it easier than drawing stuff, but then, programming is what I do for a living, so...
posted by RustyBrooks at 5:22 AM on September 16, 2016

You can do this in SketchUp by exporting a 2D DWG that the CNC router can follow.

Disclaimer: I'm a SketchUp employee.
posted by nickggully at 5:49 AM on September 16, 2016 [5 favorites]

I had a cabinet shop make some joints like the ones you link to. I used SketchUp, and then exported to DXF which seemed to be what the cabinet shop's CNC machine needed.

What kind of file would be ideal for your CNC machines?
posted by gregr at 6:37 AM on September 16, 2016 [1 favorite]

What you're doing is going from a drawing to a tool path, a 3D representation of the cuts, including the profile of the cutting tool . So you're looking for tool-pathing software or CAM. Inkscape's a decent CAD drawing stand-in if you're careful with units (DXF export can be a bit hit or miss until you work out your workflow) and if you use Geometric Bounding Box (Edit → Preferences → Tools / Bounding Box to use) so lines follow the CAD convention of having zero width.

Y'know, if you asked some folks at work for recommendations and blogged about how you went from simple PDF drawings to furniture, they'd be very happy with the free press …
posted by scruss at 7:45 AM on September 16, 2016 [1 favorite]

Response by poster: What kind of file would be ideal for your CNC machines?

It happily eats PDFs, so that's what I've been using. DXF can also be used (along with .acm, .ard, .mfg, .ai, .cf2, .ds2).

Inkscape's a decent CAD drawing stand-in if you're careful with units

Yes, and it does work, only that working with repeated patterns like the joinery I linked to is very fiddly. In Sketchup I think I could do it quite quickly with alignment and push-pull as I outlined, but I lack the knowledge to do it efficiently in Inkscape (or other applications).

A full Sketchup license is unfortunately out of the question for me at this time, nickgully, but thank you for the tip.
posted by Harald74 at 8:54 AM on September 16, 2016

Fusion 360 is free for hobbyists and startups. I've found it just as easy to use as Sketchup, but with more advanced features, including CAM tools for generating toolpaths.
posted by bradf at 11:26 AM on September 16, 2016

Response by poster: I think I've progressed a bit further now, thanks to you guys both here and in MeMail:

Some realisations:
Guides in Inkscape are next-to-useless, I've started to draw helper objects to align to instead. If I start with a known point (0, 0 say), it's easy to manually input offset and such to precisely position elements.

To not have everything off by some small amount due to line widths, set Geometric Bounding Box (Edit → Preferences → Tools / Bounding Box).

Getting a large number of tenons along an edge is easiest by drawing the rectangle that defines one tenon and one "hole", and then grouping them and using cloning to get the rest in place. Then they need to be "un-cloned" and converted to paths. And all the tenon rectangles removed. Using Path → Difference will cut out all "holes" from the edge of the part. All the Boolean operations between paths are useful, btw.

I've milled out two samples already. They were a tiny bit loose. I'm going to find out if the milling bit was ever-so-slightly undersize (maybe the nearest inch size to 5 mm or something). The fit should be such that you need to slightly force the pieces together.
posted by Harald74 at 12:36 PM on September 18, 2016

Response by poster: Note: Telling the SW that a 6 mm bit is 5.7 mm gives a nice tight fit in MDF.
posted by Harald74 at 5:19 AM on September 29, 2016

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