Guy Dickinson

More ShopBot Plywood CNC Joinery for Dilettantes

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Dec 042017
 

“When all you have is a hammer, everything starts to look like a nail,” or so the saying goes. Well, when all you have is a cartoonishly large CNC router, everything starts to look like you should cut it on there, even if a better tool would be, say, a table saw. Fortunately, the ShopBot is pretty much the ultimate multitasker, and this weekend I set about expanding my very slight knowledge of plywood joinery by making a few rabbet joints. I’ve more or less skated by with just box or finger joints for all my projects to date, and while those are extremely useful and strong, they do require filleting the corners of each joint to make them fit correctly, which isn’t always ideal or pretty.

What on earth is a rabbet joint?, I hear you ask. Is that some sort of drug paraphernalia for small fuzzy mammals? Alas, no (although that does sound like an entertaining ShopBot project in its own right.) A rabbet–or a rebate if you’re from Britain–is a groove cut into the edge of a board. Rabbet joints use them to strengthen and straighten what would otherwise be a butt joint, which is simply where the two parts meet without any modification. Rabbet joints are much stronger because they allow any load to be distributed over a greater surface area, and give glue more crevices to get into. I’m particularly interested in a double rabbet because less of the ugly outer edges of the plywood are visible.

Traditionally, rabbets are cut with a table saw and a dado set, and truth be told, that’s a much faster, simpler, and elegant way to go about this. But I set myself a challenge: I wanted to figure out how to do this on a ShopBot, and I also wanted to take Fusion 360’s CAM package for a spin, and this seemed like a simple way to get started.

I began with the simplest starting point: just one part made out of a bit of scrap:

One of the things that I keep tripping over in Fusion is that because you absolutely have to model everything in three dimensions before you cut it, assumptions that you made early on can come back to haunt you. For example, it’s easy to assume that your stock is exactly, say, half an inch thick, but when you measure it with the calipers, it’s 0.469″ or somesuch. That 1/32″ isn’t usually too much of a problem, but if you want everything to fit precisely without any gaps or bits sticking out that you have to account for it. I find it immensely helpful to define a variable that holds the measured thickness of the stock material, and then define everything else in terms of it.

This is especially helpful if you’re designing something speculatively, before you’ve purchased the wood, and then you just need to go back and make this change in one place rather than re-drawing the entire thing from scratch. So in my case, I started with the nominal plywood thickness of 0.5″, defined the rabbet depth to be 1/3 of that, and the rabbet width to be 2/3  on one side and 1/3 on the other.

This first attempt went pretty well. This was my first meaningful attempt at using Fusion 360 as a full-fledged CAM suite in addition to its design capabilities. Things I noticed right off the bat about the resulting toolpaths were:

  • The “adaptive clearance” mode for pocketing out large areas results in a very good surface finish at the expense of a bit more waste stock. Fusion is used most frequently for machining metal, so it makes some sense that the programming is especially well-suited for doing very fine finishing passes.
  • If you want to sacrifice some of the spoilboard to ensure that cuts go completely through the material, the adjustment to make is the “bottom” plane of the cutting operation, not by monkeying with the dimensions of the board or the zero plane, both of which I tried with bad results.
  • If using triangular hold-down tabs, which tend to cut off more easily, be sure to make them long and thick enough to hold up during the final finishing pass if you have one programmed.
  • For pocketing/clearance operations where perfect fit is needed, be sure to uncheck “stock to leave” which is, annoyingly, configured to some random value by default.

My new skill in hand, I set about making something more useful: a simple five-sided box, open at the top.

Once I’d sketched out the parts in Fusion, I set up the CNC operations:

This time, I actually remembered to simulate the whole thing to make sure I’d put in tabs and make sure I had set everything else up properly

 

I was pretty pleased with the surface finish, especially against the sides of the material. I cleaned everything up by hand with a bit of 220-grit sandpaper. Wood, as they say, is a forgiving medium, and sandpaper is the apology.

The moment of truth is the dry-fit to confirm that the design was correct, which it fortunately was.

After glue-up, a quick douse down with a coat of lacquer and it’s…almost presentable.

You can see here what I meant about not seeing too much of the edges of the plywood. Of course, they’re visible on the top–there’s not much to be done about that. But on the sides of the box, only at most 1/3 of the ply side is visible. Even more elegant rabbet joints mitre the edges to hide everything completely. I’d like to try that as a version 2.0.

You can find me on Twitter at @gdickinson.

ShopBot Furniture Making by a Woodwork Dilletante

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Feb 272017
 

Recently, I moved to a new apartment, and this presented me with a problem. You see, some years ago, my wife and I bought a nightstand when we had space for just one of them. Now, we have space for two, so we needed a second one. Of course, that nightstand has been discontinued for years and years. We could have purchased two new nightstands, but that seemed like a waste. Fortunately, we have a ShopBot and a supply of plywood – so I took it as a challenge to make a nightstand that was as close to the original as possible.

A nightstand that is now discontinued.

The original nightstand, made by a retailer popular with recent college graduates, that isn’t Ikea.

There were some design requirements from the outset:

  • The finished piece has to look very close to the original.
  • The finished piece must not have rough edges or visible joinery, just as the original doesn’t. My own tolerance for plywood furniture where the edges of the wood are visible is quite high, but my wife’s isn’t.
  • The finished piece should hold up as a daily-use piece; it should be solidly built.

Continue reading »

Feb 092015
 

LED Cube

It’s something of a rite of passage for hackers and electronics tinkerers to put together an LED Cube. It’s a great way to build something from a relatively minimal amount of components while building knowledge of diodes and microcontrollers, as well as a test of soldering skill. It’s also something that I’d never done myself, so I set out a few weeks ago to put one together.

I found plenty of illustrated guides online; Instructables has at least two: http://www.instructables.com/id/LED-Cube-4x4x4/ and http://www.instructables.com/id/The-4x4x4-LED-cube-Arduino/ are quite good. Putting the cube itself together was a great challenge and took me a day or two. However, once that was all done I found that the software was a bit of an afterthought (and in some cases left almost entirely as an exercise to the constructor.) So this weekend I spent some time putting together a C library for Arduino (or Arduino-like) microcontrollers for controlling a 4x4x4 LED cube (although it could easily be adapted for different sizes.

You can find the code on GitHub.

Dec 172014
 

Untitled

By day, I tell computers what to do. I like to think that I’m okay at doing that. On the weekend, I sometimes make radio things or build off-the-wall electronics projects at Resistor. I’m not quite as okay at that. It’s quite rare, though, that I make something tangible which has no physical function other than its own form. Long ago, I watched my grandfather build things out of wood: tables, benches, once even a dollhouse. Recently, my friends welcomed their son into the world, and as the holidays are approaching I thought a perfect gift would be a set of wooden blocks made by hand. I thought it would be a perfect opportunity to dabble in woodworking and create something completely non-electronic, non-code for a change. Continue reading »

Mar 032014
 

Are you interested in programming and software but have never known where to begin? Some of our most popular classes have been our

Python!

introductory programming ones, and I’ll be teaching one this Saturday. The idea is to give students a gentle  introduction to software concepts using Python, a very widely-used but accessible language, and practical examples.

I don’t assume any prior programming experience: I’ll teach you everything you need to know to get started. Come and join us!

Learn to Program!

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Sep 062012
 

Computer Cat 01

We’re often asked if we can host an introductory programming class or two; in fact, our resident Classmaster informs me that a full 19.76% of former students report they’d be “very interested” in programming courses. To that 19.76% of a representative sample size, boy have I got an offer for you!

I’ll be teaching Introduction to Programming and Python on October 6th, right here at NYCR. What will you learn, as a vaunted and privileged member of the 19.76%? The goal is to bring you through the basics of programming using some practical and simple examples, to a point where you can begin to learn independently and take home some working code that you can customize and hack on.

What do you need to bring with you? In the hardware department, you need a laptop with some variant of UNIX, like OS X or Linux. If you’re a Windows user, a virtual machine running Linux will be fine. In the brainware department, I won’t assume any prior programming experience, but I will assume a moderate amount of familiarity with computers in general. You should know your way around your own system and be curious about how things work. This is designed to be a gentle introduction, not a crash-course for the impatient, but I’ll be happy to change the pace of the course as necessary.

Interested? Head over to the class page and sign up!

Questions? Feel free to get in touch with me at guy@gdickinson.co.uk

Photo via Flickr user steve caddy, because cats.