An axis is a direction or a line of travel, so the head on a 3-axis CNC machine can travel in three linear directions: forward and back (X), left and right (Y), plus up and down (Z). In woodshops, these machines are usually employed to work on flat stock (that is, panel processing). Even though CNC machining is becoming highly complex in many other areas of manufacturing, 3-axis configurations are still very common in woodshops. That’s because cabinets are generally built with flat parts.
Those more complex (and usually more expensive) 4- and 5-axis machines add one or two circular, as opposed to linear, functions. They rotate in A (which revolves around the X-axis) and/or B (which circles around Y). There is technically a sixth movement, C, which rotates around the Z-axis, but this offers so few advantages that it is rarely offered or used in woodshop applications. Plus, parts need to be secured for machining in a way that would often physically interfere with the sixth axis.
So, in 4- and 5-axis configurations, the head itself can rotate in addition to traveling across the platform and moving up and down. This allows a tool (a router bit, laser or 3-D printer) to reach a part from virtually any direction. The result is that more complicated, sculpted pieces can be milled without having to stop and reposition the work in a new orientation, if such repositioning is even possible.
Some basic CNC platforms are described as 2.5-axis. In these, travel along all three linear axes still exists, but only two can be operational simultaneously. So, for example, a router bit might travel to the entry point, stop, plunge to the correct depth, stop and then travel only along the X- and Y-axes while milling a part to size. A true 3-axis machine can vary the depth of cut as it travels, while a 2.5-axis head would, in this case, maintain the same depth of cut until it stops moving.
The nomenclature here (X, Y and so on) is used as it exists in a standard vertical machine. If the milling center is rotated to the horizontal position, the axes’ designations change. For example, the vertical Z-axis will be what was originally the X-axis.
Confused yet? Hang on, there’s more.
A 3-axis CNC can be configured to emulate many of the properties of a 5-axis machine. For example, the head on some models can be tilted to a predetermined angle (that is, indexed) before it travels or the table can be adjusted or the part can be supported in a custom holder that holds it at a specified angle.
A woodshop can also install an aggregate head, which is an affordable way to add a fourth axis to a 3-axis setup. This has the effect of turning the spindle’s business end up to 90 degrees. In that case, a head traveling along the X-axis (which would usually just mill vertically or directly below it) can now work horizontally (sideways, if you will). Many aggregate heads also tilt to variable stops between 0 and 90 degrees, adding even more complex abilities.
Is three axes enough?
If you’re building boxes and drawers, doors and fronts, filler strips and appliance panels, then 3-axis milling will probably work. If you’re machining architectural details that incorporate carving or work in the round, then you might want to look at 4- or 5-axis units.
For woodshops looking at buying a first 3-axis router or upgrading an existing one, the price can vary a lot depending on needs. Beyond the bed size, some of the major concerns are travel speed, horsepower, work-holding options and tooling capacities and changers. The ability to add an aggregate head might be important, too, especially for sign shops and furniture makers.
In addition to looking at stepper motors and travel, it’s a good idea to ask the salesperson about the relationship between the new machine’s spindle speed (how fast it rotates) and its horsepower rating. Are there different power ratings for different speed ranges? Think about the materials you mill and the profiles you cut. Shallow grooves in softwood window parts are going to require a whole lot less work than through-cuts in thick cabinet sides. Are you buying too much power or too little?
The CNC setup for a shop that plans on buying in its RTA boxes and just doing a little custom work is going to be a lot different than the arrangement for an architectural millwork fabricator, or a freeform furniture or stair builder. Those shops might need to look beyond three axes.
Keep in mind too that 3-axis machines have come a long way in a short time and so have their tooling, spindles and especially their software. When buying new, you’ll need to be very specific about explaining to your salesperson what tasks the machine will be asked to perform. If you’re looking at used equipment, begin with the software and its ability to be upgraded, even before you discuss spindles and tool changers. Rapid advances in the worlds of imaging and printing, data storage and transmission all affect the way that your computer talks to your router. Who would have thought 10 years ago that we could someday take a picture with a phone, send it to a CNC and engrave a panel for an appliance or a set of doors?
One of the more salient observations made by CNC salespeople is that many shops buy a 5-axis machine and then operate it as though it only had three. They don’t push the envelope, look for design challenges and discover what the machine can do.
The same can be said, in a way, for 3-axis CNC. A lot of shops use their machine to process panels and nothing more. That’s fine if the jobs are all boxes and doors. But if your shop has a chance to bid on more interesting fare and hesitates because you lack the confidence, the knowledge or experience to do so, then you might not need a new CNC.
Perhaps what you really need is an imaginative designer.
This article originally appeared in the September 2016 issue.