GUIDES

Build a Hobby CNC for PCB Milling and General Engraving

CNC machines are the ultimate hobby accessory. The building of a CNC machine is a hobby in itself, but it is the application of CNC toward other hobbies that makes it perhaps the most productive of all possible hobby activities. With todays technology and Ebay, anyone can afford and accomplish computer control of machinery toward hobby applications. This guide is targeted toward developing a small scale light duty high precision low cost CNC machine.

The first most important tool required is Ebay to shop for all the requisite components and materials. Without Ebay, this discussion would be moot, as there would be no good safe place to obtain products, and few could afford them if they could find them. As it is, there is no other source, in general or in particular, where one might find so much such specialized equipment so cheap. Ebay brings the whole entire world of CNC right to you. In the dark ages, before Ebay, hobby CNC wasn't even an industry. Now, the hardest aspect is just choosing from the multitude of readily available products. Which brings us to the purpose of this article.

By limiting this discussion to a narrow spectrum of CNC mechanics as applies to small scale hobby level engraving, we shall avoid much of the complicated science of CNC that would apply in a broader based discussion of larger ranges of CNC applications. Here we shall focus on simple 2D 3 axis flat surface marking and shallow milling applications such as general engraving and printed circuit board prototype manufacturing. Anyone who's ever made a printed circuit board by hand will definitely want a CNC alternative. Modest hobby CNC equipment can cut extremely fine detail, and then drill about 2 holes per second, accurate to within .001", easily, repeatably, and inexpensively. It's just a matter of getting the right equipment, tools and software together in one system.

First we need an XY axis. This forms the work table, or working surface, to which the work piece is attached. The Y axis moves back and forth, while the x axis moves left and right and rides on the y axis, so the table can move in any direction and follow any desired 2D tool path or shape being engraved. The sliding mechanism is called a linear bearing which provides for near friction free straight line motion. We use two, one stacked on the other at right angles.

Small linear bearings with 6 inches or less of travel are much cheaper than say 12 inch or larger devices. So it is a good idea to just build a small machine with about 6 inches of travel before investing in larger projects. You can always expand it later. Motors, belts, electronics and all can be reused on subsequent projects. A small system is just easier to build and handle, requires less material and investment, and makes it easier to achieve high quality results.

For detailed engraving, it is impractical to expect to cover an area much larger than 6 x 6 because hobby cnc travel speed is going to be low enough to where the time it takes to engrave a given area is a major consideration. For instance, running at a rate of 6 inches per minute can take an hour or more to mill out a complex circuit board just 3 x 3 inches. And depending on the overall tightness and quality of the entire machine, 6 inches per minute may be an optimistic expectation. When micro tiny end mills and engraving bits are being used, the practical speed limit drops to 2 to 3 inches per minute.

The reduced weight of a small XY table also minimizes the inertial effects and mechanical momentum associated with moving and accelerating a heavy weight as opposed to a lighter one. In other words, a smaller table can move faster, more accurately, with smaller motors than a larger one can. And we need that speed so it won't take all day to make a detailed engraving.

We will use typical small cheap bipolar stepping motor with simple timing belts to drive each axis.

A stepper motor is a special motor that only turns an accurate fraction of a turn at a time. Most have 200 steps per revoltion. Any typical cheap bipolar stepper motor on ebay will be just fine. Our XY table and tooling is so small it won't require very much motor power. Just get two identical stepper motors for the X and Y axii.

Belt drives are much easier to obtain, mount, tune, and upgrade than screw drives. Belt drives are both versatile and scalable. Screw drives are not. As shown below, we use 2 belt stages to gear the motor way down. Depending on the ratio of sprocket sizes we can make the table move say .0005" for each motor step. Any motor sprocket combination that moves about .0005" per step should be a good starting point. Try to design the motor mount and drive shafting to accomodate various sprocket sizes and you'll be able to tune the ratio if necessary.

The Z axis is positioned over the XY axis on a rigid frame. We use dual linear bearings on the Z axis to better support the weight of the spindle. The spindle is usually a motor used to turn a cutting tool but it can be a vibratory engraver, a wood burner, or almost any type of marking or cutting tool. By far, the best hobby cnc spindle motor is a Porter Cable laminate trim router. They are amazingly accurate and vibration free. A router will work but the additional weight is less desirable.

Most engraving bits are 1/8" dia. Most router chucks are 1/4". Precision tool adapters that hold the bit in with a set screw are easiest to change bits during a project without disturbing the machine. With any spindle motor, you must check and ensure that the total tool tip runout is minimized. Anything over .0005" runout will cause excessive burring and bit failure.

Tool Tip Runout Must Be < .0005"

The Z axis range of travel required is very small so we only need a small linear bearing set 1 or 2 inches long. Such short linear bearings are usually very inexpensive, so use 2 or 3 or even 4. It's a good idea to build the Z axis more heavy duty. It will work better and it can be used as is on other bigger better projects later on.

The base and Z axis support frame can be made of wood to start out, but wood shrinks and swells with the weather and therefore it cannot hold the XY axis flatness relative to the Z axis to any high degree of accuracy. Engraving and PCB milling typically requires the work surface to be very flat because you're only cutting .003" or so deep. With a wood frame the machine will cut different depths in different places on different days, so it is not suitable for serious repeatable accuracy. If a metal frame is too difficult to establish at first, by all means, just use wood to get started then switch to a metal frame after everything else is in order.

A small light duty machine like this will only require about 2 amps maximum power. More amps might work better in some cases, but amperage is like machine size, it is best to limit it as much as possible (at first). At 3 to 5 amps, small motors can get very very hot and deterioration will occur at an accelerated rate.

The stepper motor control board of choice is a xylotex brand 3 axis microstepping motor controller. It has adjustable amperage up to 5 amps and 1/8 microstep capability. 1/8 microsteps are probably about as accurate as a hobby machine can take advantage of.

Standard 5 amp max, 200 step per revolution stepper motor running at 2 amps, with timing belt drive geared down to 1/1600" per microstep resolution moves plenty fast and achieves plenty of accuracy for any PCB milling or engraving that can be achieved under any hobby level designation. Don't overspend on an oversized machine, with over elaborate ambitions. Especially on your first project. Even if your ultimate ambition is a 10 foot square plasma torch cutter, just build a small machine first, and learn all about how to use the electronics and software on a small scale. Then you'll be in a much better position to make the more costly decisions (or not).

Mach2 is a very comprehensive and inexpensive CNC control software package good for hobby CNC or professional purposes. It translates gcode motion control instructions into motor control signals, which causes the motors to move accordingly. It can import DXF vector clipart from graphics programs like Corel Draw, which is all you really need for 2D engraving and milling. You don't need to know gcodes or have fancy expensive CAM software such as might be required for 3D milling of complex parts.

To hold the work piece on the table, a t-slot clamp plate can be attached to the table, or can even be used as the table. For PCB milling, you can just clamp down a small aluminum fixture with #4-40 tapped holes at convenient spacings, and flathead screws through countersunk holes to hold the PCB material down. With which you can flip a PCB board over to make double sided boards using the screws and fixture to maintain alignment. When drilling, use a thin composite spacer under the PCB so the drill won't penetrate into the aluminum fixture or work surface.