An Up-Close Look at Signmaking Routers

“This year,” I said, “let’s do it differently.” I was talking to Susan Conner, our Senior Editor, and said I’m tired of running the same laundry list of router/engraver manufacturers’ products every year. Not that this is bad, because it does provide a reference — but it really doesn’t tell you what you may want or need to know about buying and operating computer-driven, computer numeric control (CNC) routers. If you’re fabricating either electric or intricate wood-type signs, I believe you need a signmaking router. But there’s more to it. Meaning, once you buy one, you’ll want to gain proper expertise. Also, you’ll need related shop tools if you don’t have them now. For example, to make routed wooden signs, you’ll want several boxes of power hand tools, a table or radial saw (I prefer a table saw), and a vertically mounted (wall) circular saw. Were it my shop, I’d add a planer/joiner to the mix, and a miter, scroll and band saw. You may also want a drill press and several types of bench-mounted sanders, plus a vacuum-type exhaust system for the more aggressive devices. Also plan to buy an assortment of clamps and general hand tools. This isn’t a complete list, but, to fabricate metal or electric signs, add sheetmetal-bending devices (such as an apron brake), pipe benders, a TIG and acetylene welder, a metal cut-off saw, a disc grinder, a nibbler and a box of vice-grip clamps. What else? Oh yes, you need space. Space for the router and its accoutrements, space to store materials as well as load them onto the router, and space for any additional tools or machines. I’m presuming you already have an area in which to assemble the sign. Remember, too, that routers are noisy and, even with vacuum clearing systems, they can create dust or other airborne particles that you won’t enjoy breathing. And, until you become proficient, be emotionally prepared for the learning curve because routers aren’t as easy to operate as, say, digital printers and sign cutters. Still want a router? Say yes. A computer-driven CNC router is a first step into automation for most signshops. And it will expand your product line, cut production costs, and add to your shop’s profits. Best of all, its abilities will absolutely fascinate you. Automation The early machining systems — termed numerical control (NC) — were directed by word and number codes that translated into electrical signals, and these initiated such motor-drive actions as positioning the gantry or triggering the spindle. Blueprint changes meant numerical code changes, and thus, time-consuming reprogramming and the production of a new tape or punch card. Modern, computer-driven, CNC machining equipment allows immediate changes. On a modern router, the computer software’s binary codes, translated into CNC symbols and assembled into a transmittable package, or “block,” are the instruction codes. These blocks provide commands, positions and functions to the machine and govern the machine’s movement across three axes, x, y and z. Understandably, the codes must contain significant continuous-feed electronic data, including tool paths, tool data, feed rates, gantry and spindle movements, tool-change operations and machining methods and patterns. Some machines provide closed-loop feedback to ensure all processes are on track. The machine-control unit Whatever the originating system, CNC routers must be programmed for each different cut they make, be it a Chevrolet truck part or a sign. With signs, generally, the total project is designed with computer software that, when directed, communicates with the router in binary language. Think of this software as a translator, a Rosetta Stone of sorts. It’s basic to a router signmaking software package, and it converts the binary codes to CNC language. This is simpler than what you’d see on the big machines at GM’s concept engineering shop, but it works on the same principles. But — because it’s a sign — things aren’t that easy. Not surprisingly, sign-design variables tangle things up. Routing the average sign is a lot more complicated than, say, cutting a six-bolt truck flywheel. Because of this — and to keep your life simple — sign software engineers create interactive signmaking router-driver programs. These programs provide you with either (clickable) instruction blocks or interactive on-or-off type questions that inquire, for example, if you’d prefer a recessed or prismatic cut. The instructional codes from your completed design, and your clicked-on answers, now flow into the machine control unit (MCU). The MCU, in part, includes a post processor that works in conjunction with the computer program. In most systems, this processor is where the machine accepts the software’s binary output and converts it to CNC, the MCU’s language. The MCU sends CNC instructions in real time. In the process, it continues to react to the computer’s coded input, translating it and determining the necessary action, and then sees that the machine acts properly. You could describe the MCU as a First Sergeant implementing his company commander’s orders, because it, like an Army first-shirt, brings real work onto the scene. If the MCU has a closed-loop positioning system, it supervises the machine’s work by calculating and confirming positions, that is, by checking the spindle’s relative coordinates from the absolute ones forwarded by the software. Drive motors Four types of drive motors move and position a CNC machine’s components. They are: stepper motors, direct-current (DC) servos, alternating-current (AC) servos and hydraulic servos. A stepper motor moves in increments, that is, it moves a predetermined number of degrees every time it receives an electric pulse. Servo motors are the free spirits of the electric motor world. Unlike stepper motors, and just like your electric fan, they turn when you apply current and roll to a standstill when the current stops. A servo motor’s speed is controlled by the electrical input. A servo is an excellent choice when the device needs acceleration and deceleration control, or fast motor speed and quick reversing. If both are the same size, an AC servo produces more power than a DC, and it appears to be the motor of choice for routers. Hydraulic motors are more for boring into engine blocks, and you’ll seldom, if ever, see them in routers. Rotary to linear motion Eggbeaters convert rotary motion to linear; so do automobile differentials, bicycle-chain sprockets and a router’s ball-lead screws, commonly called “ball screws.” All routers and engravers use a type of mechanical/electrical motion-control (positioning) system that locates the spindle (with its cutting tool) at a desired point. The type of motion-control system may vary because of either the designing engineer’s preferences, cost or the type of work required. You may find one or more of the following motion-control systems on routers or engravers: ballscrew, lead screw, rack and pinion, belt or chain. Ballscrews are similar to the rack-and-pinion steering system on your car, except they are different. In its simplest form, a rack-and-pinion steering system comprises two parts. The “rack” is a linearly threaded (on its topside), flat, steel bar that connects, through constant-velocity (CV) joints and tie rods, to a car’s front wheels. The “pinion” is the threaded (or geared) end of the car’s steering shaft. Its threads match those of the rack, and these two work in unison. The bearing-mounted pinion turns when the steering wheel turns. It, in turn, moves the rack, which turns the car’s wheels left or right. The similarity to a ballscrew is that both move an object by using a nut (carrier) along a threaded shaft system. A router’s ballscrews are motor-driven, threaded shafts permanently mounted into pillow-block bearings with, typically, the drive motor mounted on one end. Like ball screws, lead screws are threaded rods fitted with a nut carrier, sans the sophisticated, recirculating bearing system. This motor, instructed by the MCU, turns the shaft (threads), and this action sends the carriage, forward or back, along the bar’s axis. The carriage carries and supports moving, working parts, such as the gantry or the spindle. Interestingly, a ballscrew’s carrier system includes recirculating circuits of steel balls that roll between the carrier’s nut and the ballscrew shaft. In theory, a ballscrew has long life and high efficiency. It can convert 90% of a motor’s torque to thrust. Engineers use diameter and length figures to determine the maximum rpm of the ballscrew because, as the mass of a ballscrew increases with its length, so does its tendency to whip and chatter at speed. Some engineers set a maximum length at 13 ft. Motion control For the record, there are 1,000 books — and 10,000 theories — on the subject of motion control in machines, which, unlike that required for nine-year-old kids, is the simple practice of writing coded actions and designing a machine that responds to those codes. Signmakers generally require their machines to act on three axes, but high-grade, machine-shop machines may have six or eight axes. Imagine the software. Everything — software, motors, ball screws and closed-loop positioning systems — are part of the motion-control system. And yes, there are more — digital systems, for example — but, by necessity of page count and time, I’m being cursory here. In practice, you’ll usually find three motion-control systems on a router: the table (x axis), the gantry (y axis), and the up-and-down spindle (z axis) systems. If you’re buying a ballscrew-equipped router, know that they are the key components for accuracy. When buying, remember that larger-diameter ballscrews are more accurate and less inclined to whip under stress, but may lose some accuracy due to heat expansion along the threaded shaft. A cheaper version is the Acme Screw system. It uses a plastic or soft-metal nut that slides along a threaded shaft. These systems are poorly suited for routers, but look for them on machining equipment that must hold a vertical load in a power-off position. Other drives You’ll also see belt drives in use. Before you grimace, remember that Harley-Davidson uses belt drives on its Fat Boy and Ultraglide motorcycles. I’ve talked to riders who have these systems, and they tell me the belts outlast chains. Also, I’ve read performance reviews indicating they perform better than standard chain-drive systems. Are belts better than ballscrews? I wouldn’t think they’d be as accurate, but the belt systems have certain advantages, such as fewer moving parts. Also, they’re recommended for high-speed applications. Digital-drive systems are the newest buzz for the machining industry. Digital systems instruct the machine in digital, not CNC, codes, and, although (initially anyway) digital cranks up the costs, it gives quicker response times and more accuracy when measuring an actual cutting path against a programmed path. And, to the disdain of some old-time engineers and machinists, digital cuts out the CNC codes. Closed-loop positioning Closed-loop positioning combines segments of the machine and software. In most cases, you’ll find both velocity and position-control loops. A tachometer reads the velocity feedback, and an encoder reads the positions. In actual operation, the MCU surveys for and receives electronic pulses (called “polls”) dispatched from the actual work units — the spindle and the gantry — and its feedback system checks these against the software’s original coordinates. If the coordinates don’t match, the system either corrects or shuts down, depending upon the software engineer’s incorporated preferences. Some final basics Like Humpty Dumpty, no person or gee-whiz technology can help a wrecked frame. Routers, and their gantries, are subjected to various forces, and unless the basic framework is incredibly strong, the machine will eventually weaken. Modular, framed machines are becoming more popular because, the engineers decree, modular designs decrease harmonic vibrations and increase the table’s overall rigidity. They also have fewer pieces. You’ll see various ideas in play — trapezoidal columns, for example — and I wouldn’t begin to say which is best. Just remember that the table must take forces — “bounce” — from every direction but down. Think of it like buying a billiard table. There, you’ll want a solid table that won’t move while in use, nor twist or settle over time. The gantry should be strong but not necessarily heavy. Its efficiencies are affected by the span (width), travel distances, the weight and torque of the spindle motor. All these affect the gantry where, over time, design weaknesses affect the machine’s tolerances and increase its aptitude for skewing. Several designs exist to minimize these problems, including dual-drive systems. Look carefully at lubrication systems because routers operate at very high speeds and contain many precise fitting parts. Like cars, dry systems seize quickly, and repair costs are high. Reservoir lubrication systems are cool if they cover all points. Finally, don’t forget safety. In his Computer Numerical Control book, writer William Luggen says, “The first and most important thing to learn about numerical control (NC) and CNC machines is safety.” He cautions that automated machines are unforgiving and dangerous. He says you should stow long hair and wear safety glasses, but remove rings, wristwatches, bracelets, loose clothing and, heaven forbid, neckties. Luggen emphasizes that the machine operator is responsible for the well being of the machine and the safety of all nearby persons, including him or herself. If you’re into computers and machining, you may want to look at www.metalworking.com (click on “Shareware”). This site provides a freeware version (think Linux) of a CNC control program written by Kevin Carroll. Written in BASIC, it drives a three-axis stepper motor control using the printer port of an IBM PC.