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The CNC table - First thoughts and musings...

This has been a long time coming. Over the last couple of years, it turns out that I have seen an increase in the time I have available to devote to projects that involve building things. I’m not entirely sure how this came to be, but I’m sure not going to raise a serious objection. In any case, the more I delve into the possibilities of what can roll, walk, crawl, or be carried out of a well-run workshop, the more I see the value in being able to rapidly manufacture parts. This could cover anything from one-off printed circuit boards to making six identical sets of legs for a hexapod robot. In almost all cases, the most efficient and effective way to make this happen is with some sort of rapid prototyping. If this is the first time you’ve encountered that phrase, it basically means going from a drawing (generally on a computer) to a part that you can put your hands on in a relatively short amount of time. I am actually probably using that term slightly incorrectly, but I don’t really care. The point is, I want to draw something, then shortly thereafter attach it to something in the physical world.

Question one: Addition or Subtraction?

This is where I may have slightly misused the term “Rapid Prototyping”. The way I see it, there are two ways to get from a CAD drawing to a physical part: CNC (Computer Numerical Control) routing, in which a computer drives a router or a mill head over a piece of material, and basically cuts the part out. This is what is called a “Subtractive” method, because you start with more material than you need, and cut away until you are left with the part you want. Contrast this with “Additive” methods, such as 3D printing, which is in some ways similar: the same computer moves a head around, but this time, instead of a cutter, the head actually deposits material onto a build platform, and the object is built up in layers.

Both types of rapid prototyping have their pros and cons: The subtractive approach absolutely, positively requires waste. No matter how you slice it (so to speak 😎 ), you are turning some material into dust or chips. The additive approach, on the other hand, does not necessarily require waste, although in a large number of circumstances, something called “Supporting Material” is used. This is material that is deposited during the building process in order to support, for example, an overhanging section. At the completion of the build, this material is removed. Additive methods are somewhat limited in what materials can be used. For the most part, the solutions that are available for home shops use ABS plastic filament. This is fine as long as you’re okay with a part that is made from ABS. Subtractive solutions, on the other hand, can carve in materials like wood, plastic, and in many cases easily cut materials like brass and aluminum. The current state of the art in home 3D printers also suffers greatly from a very small print area. 4″ x 4″ x 6″ (10cm x 10cm x15cm) is about the limit on a stock printer. CNC router tables, on the other hand, can be made to much larger sizes. 13″ x13″ x 4″ (32cm x 32cm x 10cm) is considered VERY small, and tables that can accommodate a full sheet of MDF or plywood are not unheard of. Also, CNC Router tables are essentially a mature technology. There are plenty of them out there in the world, in all kinds of shapes and sizes, and in forms all the way from a turn-key unit complete with custom software installation (WAAAYYY out of my consideration, for tens of thousands of reasons!), to kits to bound books of plans. 3D printers, while showing great promise, are still at the level of maturity that PCs were when most of what they did involved knowing what do do when met with “C:\”. At the moment, there are but two major choices for a home shop. The Makerbot Cupcake looks like a great project with a great future. It’s also available in kit form, which makes it a lot more convenient than the RepRap, which currently exists only as a set of plans. Both of these projects are open source, which is a huge plus, and sometime after they have been developed a little more, I am interested in building one or both of them. But I digress.

For my decision, the final consideration is the resolution attainable by both of these methods. Unfortunately, the 3D printers just aren’t there. At the moment, a print that is spherical, for example, will have significant ridges. In all fairness, it seems unlikely that a homebuilt CNC Router Table would be able to produce a sphere that didn’t require at least a little sanding, either, but when you look at specs, the router tables have their decimal points one or two places further down than the 3D printers.

So, after considering the trade-offs and the state of the technologies, I have decided to go the CNC Router Table route.

Question two: Okay, so it’s a table. Now What?

If you’ve done even 5 minutes of Google research on CNC, you know about CNC Zone. This is the place where everyone talks about their builds, shares advice and experience, and does all that great on-line community stuff. Naturally, this is where I started. What I was hoping to find was an “Industry standard” CNC table kit. It didn’t have to be the end-all, be-all of tables, I was just hoping for something that I could enjoy building, get some good use out of, and learn the process. Absent that, I was hoping to build a basic CNC vocabulary, so that, if I didn’t know what the answers were, I at least would have a handle on the questions.

I didn’t quite find what I was hoping for. There were some absolutely amazing stories of engineer types who cobbled something together in their garage from parts they found laying around the workshop. That’s not going to be my machine.

I did manage to learn what the basic building blocks of a CNC Router Table are:

  • The table itself:
  • This is essentially a work surface that has some form of gantry, and some method of either moving a cutter head over the workpiece, or alternately, moving the workpiece beneath the cutter head. This is where discussions of things like lead screws or rack-and-pinion systems happen. When discussing this part of the build by itself, there are no motors attached, nor is there any interface to the computer that drives the whole thing.

  • The motors:
  • Naturally, if you’re going to be asking your computer to drive a carriage with a router strapped into it back and forth across a work surface, you’re going to need to discuss motors. Of course, there are a huge number of options and considerations to be looked at. The main divisions seems to be between using servo motors and stepper motors. Each have their own sets of pluses and minuses, but it seems that for most entry-level CNC machines stepper motors are most often used.

  • The motor controller:
  • Closely related to the motors, this is the actual interface that sits between the controlling computer and the motors. There are a number of these available that are aimed at the hobby/DIY crowd. They can also be had as completed units, or in kit form. Most controllers that I have found are based around a parallel port interface, rather than the now more common USB interface. USB controllers are out there, but they are quite a bit more expensive, to the point where it is significantly less expensive to install a parallel port card, assuming you are using a desktop computer. If you are trying to use a laptop (which is generally ill-advised in the DIY CNC community, due to the use of power-saving features which tend to muck with the timing of pulses), the price difference might nearly equal the cost of a used desktop , or even a minimal new desktop build!

  • The computer and software
  • This seems like it should be the easy part, but it also seems to be where a lot of the stumbling occurs. Things will be a lot less complicated if you’re mainly interested in 2D routing. There are several stages a design must go through to get from a concept to G Code, the commands used to specify a tool path, and tell the router where to cut. This seems to often involve a number of programs. This is also, surprisingly, an area where Linux seems to be lagging behind somewhat. One would think that by operating close to the hardware, an operating system like Linux would have an inherent advantage, and that the open source community would embrace the opportunity. Well, maybe not yet, but maybe soon. There are some very promising projects on the horizon (mainly Dan Heeks’ HeeksCAD and HeeksCNC), but it seems that for the moment, the “Standard” software suite runs on MS Windows 8-( .

    So, now that I know what I need to be looking for, I went out looking for it. I was a little disappointed that there didn’t appear to be a complete kit solution that was more or less the gold standard for home CNC. I was just about to start getting frustrated when I stumbled across the Solsylva, a plans-based project that offers a number of options in terms of table size and configuration (dual lead screw or rack-and-pinion). The Solsylva is also designed to work with hardware store/home center materials, such as 2×4 construction lumber and regular old threaded rod. The design also allows for future upgrades, such as replacing the hardware store threaded rod with acme screws, which should increase performance significantly. So it appears that this would be a very good way to get into a DIY CNC Router Table with low relative cost, and afford the opportunity to upgrade later, after I’ve determined what my needs have become. The final point in favor of the Solsylva was that the plans for all three standard configurations are available in one volume for just under fifty dollars as of the time of this writing. So I went ahead and ordered the plans, and we’ll see how this develops. Right now,my thought is to go ahead and start building the smallest table, and if I like the way it’s coming together, I’ll order one of the recommended motor/controller kits. The plans should be here early next week, so I’ll be sure to report on what I find.

    1 comment to The CNC table – First thoughts and musings…

    • Robert Teeter

      A couple of things on this. You now can build a CNC machine based upon the OpenBuilds.com site as they have multiple projects there on doing just that thing.
      They also have projects on building 3d printers and laser cutters also. I don’t own one yet or own the company but I am assembling all the parts to do so.

      Bob Teeter

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