Know your options when CNC shopping

In the first of a two-part series, here’s what you need to know about spindles, axes and tables
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As a CNC router instructor, I once had a student who dipped into his retirement savings to buy a mid-range machine. A year later, it still hadn’t cut a single part. He was in over his head and it took several weeks to get him up and running.

Another student was excited by the prospect of purchasing an industrial class CNC router from an overseas source. He crowed about the low price, but it was difficult for me to assess the machine because the literature was in a language that I don’t speak.

These folks didn’t ask the right questions before buying, and first among those is – what will it be used for?

Scale is everything. Casework factories and high-volume custom shops are at one end of the spectrum. Then there are small shops, educators, and hobbyists. Many shops primarily reduce and mill sheet stock, where others work solid wood or advanced materials. Those large shops would be wise to partner with a well-established industry leader that not only provides the router but also the training, service, support and customization required, such as material handling, robotics and lean manufacturing cells. Here, shop management must be free to focus on many complex aspects of their business and not feel the need to become CNC experts.

For small shop owners, high volume isn’t usually the top priority. They may need to use the router for a wider variety of tasks – sheet goods one day, hardwoods the next. These shops will find that their needs can be best met by a large group of mid-size machinery manufacturers.

Educators and hobbyists most often find that small and less costly machines are the most flexible, and also the most upgradable, as skills or needs evolve.

Once you’ve decided on a market segment, it’s time to explore what it offers.

Spindles

Some desktop machines use a portable router but most CNCs employ a spindle. Routers are designed for hand-held use and modest, intermittent duty cycles. A router in a CNC might have an automated on/off switch but the spindle speed will be manual, and not programmable. You’ll need to stop the process and change it manually. Routers are usually very loud and difficult to be around for long periods.

Spindle speed is programmable, and most spindles are well suited to the extended duty cycles seen in high volume production. Many are available with enough horsepower to drive large bits in heavy materials.

Spindles can be programmed for automatic tool changing (ATC) capabilities, if the machine offers that option. That’s important – who can afford the time to be changing tools back and forth during a significant production run? ATC also avoids the opportunity for error when one is manually changing tools and trying to set the tool height with precise repeatability.

Higher end spindles can accommodate aggregate heads. These are devices that allow a blade or bit to approach the work from an angle other than vertical. An aggregate head can often do things that let a shop avoid the expense of upgrading to true 4- or 5-axis machines.

Another common spindle option is a drill block that is populated with multiple drill heads. It’s ideal for drilling shelf holes for cabinets, as it is capable of drilling several co-linear holes in sequence. And, of course, it eliminates the need for tool changes during drilling operations.

Axes

Is the proposed CNC a 3-, 4- or 5-axis setup?

A 3-axis system is the most common and these are appropriate for a wide range of product fabrication from cabinets to signs and furniture. Three axes are all that’s needed for jobs that can be cut using a vertical spindle. By flipping stock over or using creative fixturing, a 3-axis machine can often cut two-sided jobs or mill parts with three or more faces.

A 4-axis machine adds the ability to rotate and position stock about a rotary axis, for cutting parts such as spindles or furniture legs. While it might look like a lathe, there’s a core difference. On a lathe the workpiece is spun rapidly against a fixed tool. A 4-axis CNC positions the stock around the rotary axis while the tool is manipulated in three axes.

A 5-axis table provides the support and clearance for a spindle that is capable of moving much like a human wrist. The spindle is moved in the usual three axes and is also able to rotate on its own in two orthogonal rotational directions. (Orthogonal describes a geometry of two objects that are at right angles, or perpendicular to each other.) That means the tool is capable of approaching the part from any direction, so it is limited only by the mechanical constraints of the spindle mounting mechanism.

On a 2-axis machine the cutter moves side to side (X) and back and forth (Y), so it has two linear axes. On a 3-axis CNC, the cutter moves side to side (X), back and forth (Y), and up and down (Z), so it has three linear axes. With a 4-axis set-up, the cutter moves side to side (X), back and forth (Y), up and down (Z), and can rotate in one plane. And on a 5-axis machine the cutter moves side to side (X), back and forth (Y), up and down (Z) and can rotate in two planes.

Tables

The maximum size of a part that can be cut is determined by the size and configuration of the table, which is also called the cutting bed. A 4’ x 8’ bed can usually handle a full sheet of plywood or MDF; 5’ x 10’ beds are favored for handling 10’ long sheet goods such as solid surface countertop materials, or for machining 5’ x 5’ sheets of multi-ply. For sheet goods, a 4” vertical travel (Z) may be all that is required as that’s usually enough to allow the tool to plunge. But most solid wood applications will benefit considerably from more vertical travel. Five-axis machines can offer several feet of vertical travel (what’s your ceiling height?).

Small-bed CNC routers usually require that panel stock is reduced (cut into smaller pieces) prior to being secured for machining. Due to a lack of space, tight budgets, and the nature of the markets they address, these machines don’t generally come with automatic tool changers, although that’s beginning to change.

In smaller and less expensive machines, work is usually secured with mechanical fasteners or clamps. In mid-size and larger machines, vacuum clamping enables more rapid and efficient loading and unloading of part.

Nested based systems, where the whole table is a vacuum clamp (usually with sections that can be turned on and off) are optimal for most sheet goods machining, while pod and rail (also known as point to point) systems are most often used for hardwood or more complex shapes. Pods are little cubes that slide along beams and use very strong and finely focused vacuums to hold parts in place.

When considering the purchase of any vacuum-based system, it is also important to consider the vacuum source. Industrial vacuum pumps are designed for continuous duty. While common shop vac units can develop enough vacuum for some small applications for short periods of time, there are plenty of stories of those products overheating and shutting down in the middle of a job.

This is part one of a two-part series. Part two, scheduled for the March 2021 issue, will cover motor, cutting speed, drive system, and controller options.

This article was originally published in the February 2021 issue.

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