First, let’s get the facts straight and realize that just about any CNC engravers on the market with linear bearings can more than likely engrave and mill stainless steel. How fast and with what accuracy said table will be able to do this is a question of construction/design of the table and controller. Second, the rigidness of a table is based on the gantry construction and how those linear bearings are mounted to it, not the solidness of the table stand or the diameter of the Z-axis leadscrews. Anyone who tells you that has a salesmen’s view, not an engraver’s view, of table construction.

If all we needed was a rock-hard table stand, then we would all mount the CNC engravers to a Wells Fargo Bank Safe. Look more at details like X-Y leadscrew pitch and the motion system, and how that affects accuracy, speed and rigidness. What I will attempt to do is explain a little on the basics of the controller signal and table construction, then point out just what to look for in an engraving system that will help mark and mill stainless and industrial metals.

Next, we’ll move on to the controller, because without a strong dead-on tap signal to a micro-stepper or servo-controlled table, we will not be able to translate the signal image to the material in a precise and controlled manner. In all engraving systems, there is a software program and the hardware to support it. A controller receives the data from the program and then sends it to the CNC engraving table and puts your programmed images onto the material; without this signal being hyper-accurate, a precision product cannot be produced. All the advanced engraving systems presently utilize PCs using a Windows XP or 2000 OS (Operating System) to run the software programs, so the main controllers all receive their engraving signal from a Windows-based PC.

The controller has had several names: MPU (master/main processing unit), SCU (system control unit), controller, etc. It is simply that set of electronics that links the commands from your engraving jobs to the table, so you can make money.

First, there is a cable that goes from the PC that inputs into the controller. This is done via a strap or a cable. The cable usually screws on the MPU box, and the strap usually slides on. In more advanced controllers, this does not connect at the PC with an ISA or PCI Card that is mounted to your PC Chassis, but to the Ethernet port of the PC. The first main component in the controller is the power supply. It supplies different levels of DC power to the rest of the components in the control unit. This is done in a circuit of the power supply that converts AC (Alternating Current) from an electrical outlet in the wall into DC (Direct Current) that runs the control unit.

The power supply also steps the juice down into various levels. One level runs the logic circuit that thinks for the controller, (usually at 5 volts +/- DC.) Another voltage level runs the driver circuit that powers the table, and so on. Some controllers also filter the AC so it can run the spindle motor drive. From the motherboard/interface on our modern controller, we come to the driver circuit.

Some units have one card, with three different areas of the card for each motor it drives. Some control boxes have three different cards for a driver circuit. These are the best. They are easier to work on and cheaper to fix—the reason why driver circuits and motors are divided into threes is that there are three different axes on a standard engraving table. The X-axis usually runs left to right; the Y-axis runs back and forth, and the Z-axis lifts the spindle up and down, to and from the engraving material. Many of the new MPU controllers have a backup driver card on board as a fourth axis that can be assigned to the other axes of the table in case of problems.

The driver boards are rated by the power of the motors that they drive. The closer the output of the driver cards to the amp rating of the motor, the quieter the table, and the longer the board and the motor will last. The X- and Y-axis motors are usually rated the same, and so are the driver boards that run them. The Z-axis may be smaller than the other two. And, of course, the driver board will be different than the other two. The Z-axis may be air-actuated if it is hooked to an older table, as in an engraving system that has been upgraded with a retrofit kit. This Z-card for air is a neutral keying component and produces either an on or off signal.

The controller has two basic user interfaces; one is the front controls, the other is a pendant. The pendant is just the front panel controls on a wire to give flexibility. Some of the interface controls have a membrane cover. The toggle switches with interface controllers are much stronger and easier to fix if they do break. It is a pain to spend $300-plus to replace a front panel for a $10 switch. New pendants are actually programmable, like ATM touch screens, and can be programmed and updated like PDAs or pocket PCs; upgrades for these are usually free and allow your system to grow with you.

THE TABLE

Now we can move to the table. There are many different types of tables out there. Back in the days of the pantograph—a manual engraving monster—that was the driving force behind getting the images onto the material in the beginning. Then there were the first computerized units like the Dahlgren System One. That ran from a TRS-80 Radio Shack computer. New Hermes may have started manual engraving; but Will Dahlgren and Jay Hoffpauir invented and developed the first computerized engravers. These marvels of strong U.S. competition are what we make our livings with; knowledge about their operation should help you keep some of that money they earn in your pocket.

The tables range in average sizes from 5” x 8” to 25” x 50”, and 52” x 52” engraving/light-routers. Whatever table size you have, it probably runs pretty much the same.

Looking at the table systematically, the first part of the table we see is the cable input from the controller. This area is associated with an electronic board, usually called a break-out board. The break-out board is used for channeling the signals from the controller via the table cable. These signals usually use keying called polarity keying, which means the voltage moves from positive to negative voltage, and back again. The table signals are, of course, going to the different axes, where they result in the movement of the carriages.

All standard tables have an X-axis, a Y-axis, and the Z-movement axis. The X-axis usually runs left to right and controls positioning within the standard margins. The Y-axis usually runs front to back and controls the baseline on a standard plate. The Z-movement axis lifts the spindle to and from the material. The properties of the X and Y axes are usually the same, with the exception of the rails. These rails have carriages that travel across them. They use bearings like Thompson bearings, which give them the ability to glide across the table.

Some older systems use V-wheel technology, which is like a roller-skate wheel that the carriage rides on, but as I mentioned, this is pretty much outdated stuff. The systems that we are talking about that are rated for high precision and speed are the ones I believe will do great on stainless steel; they are the ones that use linear bearings. They have no give and almost no wear life.

These two axes (X & Y) have a train of parts; let’s follow the track from the motors forward. The DC-stepping and/or micro-stepping motors are the main types of motors used. Some tables use other types, but these are the most common. These motors are rated by amps. Many times they are either 2.9 or 4.6 amps. It is important that you know what the rating is for your motors—that way, if you have a problem, you can order the correct part, as they must match their driver card in the controller. This is important to keep you from burning up a motor slowly.

When we say stepping, what we mean is that the motor will move a small distance clockwise or counter-clockwise, depending on the positive or negative voltage command it receives. This area of the motor properties is measured in steps per inch. Old units received a signal to run at about 600 to 800 steps per inch. Now some of the top-notch units run from 3,200 spi to 6,400 spi. As you can see, the higher the steps per inch, the higher the accuracy can be for the table.

After the motors, we come to the coupling units. This component joins the motor shaft with the end of the lead screw. A solid coupler is more advanced than a helical coupler. A solid coupler tells me that the design of the bridge is milled to the point that the motor shaft and the lead screw are directly aligned to give greater transfer of signal to the cutter. A helical coupled unit is not engineered to be that accurate, and thus the coupler has some flex in it and will not transfer the precise rotation, and at some point it could break in the middle of a job.

Moving forward, the lead screw moves through the lead screw nut in the axis carriage. As the lead screw turns through the lead screw (LDS) nut; the axis carriage moves by the power of its motor. The lead screw nut is attached to your carriage in the case of the X-axis; the X-axis carriage is made up of the spindle motor, the spindle and the Z-movement axis mechanism.

The spindle motor can be AC or DC; these are designed to spin at a range of speeds and are controlled from the controller box or a motor speed attachment. Next we come to the spindles; they can range from 11/64 to 1/4 to 1/8 or even 1/2 diameters, which indicate what diameter cutter shaft the spindle can hold. There are also colleted spindles, which hold the cutter at the top and at the bottom of the cutter shaft—these are best for cutting/milling stainless steel.

Next we come to the Z-movement axis, which is mounted onto the X-axis carriage. This is one of the main areas that separate the quality of a table from other models and manufactures. The Z-axis has a smaller motor as a rule, and a smaller lead screw and lead screw nut, plus its own terminal block of wires for signal input.

That signal goes through to the Z-movement axis that moves a ball-slide. This ball-slide must be of heavy duty design; it is what the spindle is screwed to. Its operation is just up and down—but up and down is a major part of quality. It can have no sloppy movement at all. The more heavy duty this mechanism the better, and the longer the quality of engraving will be maintained, especially when engraving deep and/or harder materials. In this area of the table is also the pressure mechanism; this determines how much pressure and/or firmness there will be at the point of engraving. It is also a major control on quality.

Getting back to the X-Y axis, the lead screw-LDS nut interface is another major hot spot on the table. The grooves of the lead screw and the grooves of the LDS nut must have a perfect marriage. If they are too tight, not only is the quality bad, but you can put too much torque on the motors and their driver circuit, thus causing problems and premature aging. If they are too loose, you will get misshaped characters.

There are two types of lead screw LDS nut interface: self adjusting and standard LDS nuts that can be adjusted. The self-adjusting nut is set with a load at the factory. They have a tension spring, where the adjusting standard LDS nuts have small screw(s) in the nut, to tighten the grooves together. To check the LDS nut, just grab the carriage and try to wiggle it in the direction it travels—if you have no wiggle and the lead screw turns smoothly with normal lubrication, then you are okay. If not, call your tech.

Note: if your manufacturer has changed LDS pitch and diameter a lot over the years, by the time you need a new one, it may be hard to match. Knowing this can help you plan for the future. Furthermore, there are two-part LDS self-adjusting nuts and three-part self-adjusting nuts. The two-part nut is better because it needs to be changed far less frequently, since it is more precisely married to the lead screw itself. Three-part nuts are just old technology.

Even though the Z-axis has a shorter distance to travel, the same rules apply. This is where most of the big innovations have happened in the newer table designs. A unit with an interactive proximity sensor usually can go into milling mode by locking the Z-axis into a solid state. But I have found that for marking stainless steel, a little give so I can float is good for bit life and chip load.

An interactive proximity sensor not only feeds signal back to the controller on every up and down motion to where the material is but will do it within a letter. This allows you to engrave at angles and on uneven materials. Whether you are buying a table or a whole system, solid construction is what is important, not just bells and whistles.

The last thing we are going to cover is lubrication. Make sure it’s what the factory recommends, as a rule. But I recommend that you never use silicon or greases. You have a unit that produces residue, and almost all the LDS nuts are plastic—now you have sand grinding against plastic—not good! Also, silicon will get sticky after a time. Anyone that tells you to use silicon on an engraving table has never had to overhaul one or is just too stubborn to see reality.

THE FUTURE

Many times in the last five or six years I have heard that the laser is the only future for our industry. There is no mistaking that the laser has changed our industry. But the next era in engraving is here, and it involves CNC desktop engravers and light-duty routers—in other words, engravers that take engraving bits and not laser beams. To those that believe the laser is the only future, go to an IMTS Show or a sign or machinery shop, or even a proactive trophy shop. The engraving cutter or bit is still king.

We all know that lasers will perform with features that a standard engraver will not and vice versa, but there are many features that they both can perform, one better than the other, of course. This area is where the new CNC technology catches up to, and many times surpasses, the laser. The new engraving tables have added a much-higher level of precision; this precision has led to massive increased production speeds. These new units use micro-steppers at 6400 SPI, giving them a precision of .000161, repeatable.

This allows us 8” to 10” a second of vector engraving speed, and in the future even higher. As spindle motor RPMs raise to 100K, we can see engraving tables outproducing standard class 1 lasers. But getting back to our point about stainless steel, let’s summarize:

To do stainless steel, I suggest a linear bearing table with self-adjusting two-part leadscrew nuts, with 18K-plus spindle speed measured not at the motor, but at the bit. You also need a good corkscrew cutter about .020th, and a suction table if you are going to do cut outs; if not, it is optional. And you need a unit that steps at 3200-6400 SPI, good cutting solution, and a program that will automatically step down to your final depth, so we do not go through a lot of cutters to complete a production run.

Moving the bit through the material faster with smaller step downs is about equal to hogging the part out in one to two passes time-wise, but saves on time spent sharpening bits. Additionally, if an engraver has an interactive proximity sensor, that does not mean it will not mill correctly; that feature lays in the software’s ability to surface set and the Z-axis’s ability to lock itself down to varying degrees of pressure until it is locked down completely. A good unit can do both angled/uneven engraving and rigid milling.

Remember, for the most part, quick, high-production units are more accurate than slow ones. That accuracy is what leads to the speed, and with spindles out there that are up to100K RPM, you can do stainless at 8 IPS. I have seen them; I have run them. I hope this basic information on controllers and tables will give you enough knowledge to take my introduction and summary and get a system that will not only cut and mill stainless steel, but one you can use for your many other marking evolutions. Good luck.