Building Set Limitations Make for Z-Height Follies

I’m working on a small CNC mill that uses a robotics building set as a starting point. I don’t know what to expect from the process. Maybe the connections will be too wobbly for the machine to be anything but a curiosity. Maybe I’ll be able to do pen plotting and balsa carving but nothing tougher than that. My goal is to have it carve PCBs, but what ultimately is important is that I have a tool whose awesomeness justifies the expense I’ve put into the project.

So far the process has been fun and interesting. But recently the Z-axis build has been especially so. It raises a really interesting question: where does the balance between unknown finished design and known material parameters fall?

Designing the Z

I’m a beginner at this, but one can intuit the basic measurements needed. The total travel requirement is the length of the tool, the thickness of the workpiece and/or spoilboard, plus the clearance needed for mounting clamps. Most cutting tools I anticipate using are two inches or less, including the collet. Bearings also factor in. The length of the rail minus the spread of the two blocks determines how far the Z travels

But minimum does not mean I want the minimum. I’d also like a little room so I can rapidly remove the workpiece. There definitely are advantages and disadvantages to a taller Z. For one thing, stability is a factor. The shorter the reach, the more rigid the construction. But a shorter Z also limits what you can do with it, toolwise. For instance, a 4th axis to turn the toolhead to hit the workpiece from an angle. Maybe I’d need a drag knife with a micro stepper turning the blade. One thing I’m definitely not consider is to mill thick materials. I seriously don’t even want to flirt with carving much into the Z. Even an inch would be more than I’m interested in for this project.

I’m choosing a fairly standard gantry setup where the Z is (currently) actuated by a NEMA-17 stepper that turns a lead screw. The product line includes two different nuts and they’re designed so well that gravity alone makes them turn — hold onto the nut and the lead screw spins out of it. Right now the 12″ lead screws in place make the gantry seem very tall to me. The top of the Z motor is around 17″ off the table. How much lift the toolhead will have when done is hard to guess when I don’t have a spoilboard in place and I don’t have a spindle.

One of the biggest problems I’ve encountered thus far with my “building set” approach is that I find myself limited by the strictures that make the set a set and not a random assortment of pieces. There has to be a system of some sort, and right now the system is challenging me.

Hack It Judiciously

Building a project out of a building set forces you to build it according to the system of that set — either that, or at some point you have to make a conscious decision to abandon the system. If the beams are only so big, you can’t magically make them bigger. If a beam is 100mm long, you don’t get out the hacksaw and make it shorter.

::record scratch noise::

Hell yes you can. Here on Hackaday we follow the ethos that if I bought it, I may, can, and should hack the hell out of it if that’s what I want to do. That said, if it costs money, and I can’t afford to buy another, I definitely will make double sure I’m doing the right thing.

Actobotics, the product I’m using features metal channels that are peppered with two different hole patterns, allowing a wide variety of parts to connect. The scheme has enough iterations that you can theoretically make a light-duty CNC mill, is my thinking. The advantage of a building set over custom machined parts is that I can change my project around at will. If I don’t like 9″ beams in a project, I can remove them and add 6″ beams. A set makes iterations a dream. Depending on the set you can also be assured that add-ons like motor mounts and bearings will be compatible.

But sets also restrict you. When I built my tall-ass Z and decided it was too big, my anticipated changes were limited by what the product provided me. Should I hack down some parts to make it the perfect height? Maybe, but not yet.

I have to acknowledge, and reject, the psychological component to this — that the parts have an intrinsic value such that the purity of the set is somehow more important than the projects I do with it. The most logical decision is to use the chopping bandsaw at the hackerspace to cut down the lead screw to whatever size I want. Either that or go out and source my own damned part without relying on what is available for a set.

However, I’m not doing any of that yet for the best possible reason: I still don’t know to what height I should build my Z. Do you have some advice for my current build? Have you been in the same ‘hack it or preserve it?’ conundrum? Let us know in the comments below.

Filed under: cnc hacks, Hackaday Columns, Interest

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