CNC Basics
A CNC is simply a machine which controlled by a computer to perform a repetitive or accurate task otherwise not possible or cost effective by hand. The basics are true for all machines, whether it's a CNC router, a laser cutter, a 3D printer, a rotary tube cutter, or a robotic arm: a series of commands are sent to a controller which drives a motor which moves the machine.
The following page specifically focuses on flatbed cutting machines: such as CNC routers, plasma tables, and laser cutters.
CNC Routers, etc.
The three axis flatbed CNC (I'll just refer to it as a CNC beyond this point) is essentially a table with a cutter positioned over it. The cutter is able to move left and right and back and forth across the table. This will be the 'X' axis and the 'Y' axis and will depend on the manufacturer on which is which. The cutter will move up and down, the 'Z' axis. There will be variations to this, such as an addition cutter head or a cutter head which rotates away from a vertical position, and these additional items will receive axis identifiers. But the basic three axes are two in plane with the table and one perpendicular to it.
Each axis will be driven by a special motor. This motor is designed for accurate control of the rotational position and speed. This will require a special motor controlling system, and there is more information about that further on. The computer in the equation is used to transmit the motion commands to the controlling system and to provide a human interface with the machine. Finally, there is usually a second computer in the mix used by the engineer/designer and it the source of the CAD work and motion command file.
The scenario is usually like this: The designer wants a shape cut, or 1000 of the same shape cut, and a cutting file is created. This file is sent to the computer connected to the CNC. The CNC operator loads the file into the CNC control software and initiates the file. Each line of this file is something like 'move cutter to here', 'move cutter down', 'move cutter to here and here and here', 'move cutter up'. These commands are interpreted by the controlling system to 'spin this motor at this rate, in this direction, and this many times'. The CNC then operates in a controlled fashion.
It is not necessary to understand the details involved with the operation of a CNC machine. The following information will touch on these details, but only to the point where it benefits someone interested in selecting a new machine. The goal of this writing is to provide insight into specific differences between similar machines as an aid for comparison.
Materials to be Cut
A typical flatbed CNC will be used as a router to cut plywood, foam, or plastic sheet or as a plasma/laser cutter for steel or aluminum plate. On occasion, you will find one that uses a special cutter or marker for fabric, paper, or thin plastic sheet. Each will have its own specific requirements. So not all CNC's do all things well. For instance, it is possible to put a plasma cutter on a router style CNC, and many people do just that. But a CNC set up specifically for a plasma cutter will have a water table below to catch the sparks and slag, a spoil support to protect the table, and a table designed for the unique weight and size of metal plate (which is not always 4' x 8' and will likely weigh several hundred pounds). Therefore, it is obvious that the machine to select will be designed for the material that will be cut.
CNC Router Table Design
In most cases the cutter will be mounted on a head which is suspended above the table on a gantry. The head is able to move left and right along the gantry and is able to move the cutter up and down. At this point, flatbed CNC's fall into two categories for motion in the remaining axis: moving gantry style and moving table style.
A moving table style will have a fixed gantry and the table will move back and forth. The primary benefit of this is that the gantry can be built very stout (and likely weigh more than the moving table). A stout gantry is desirable when multiple cutting heads are mounted or a large Z-axis clearance is necessary which translates to higher twisting moments on the gantry.
A moving gantry style will have a fixed table with a gantry that rides down along the table on rails. This design uses less floor space since clearance is not required for a moving table. Additionally, it simplifies the table, which is largest component of the CNC, making the whole thing less expensive.
Why the difference and which is better? It boils down to which is heavier and harder to move, the gantry loaded with spindles and tools, or the table loaded with material. The heavier or more complex one gets fixed. Practically speaking, a moving table style is really only found in very heavy high production factory work or very light 3D printers and laser etchers. Moving gantry is common everywhere else.
Linear Motion Track, Screws, and Motors
Once you have a table and something that moves around to cut the material, you need something that actually does the moving. A motor with a rotary motion must be converted to a linear motion and this is done with one of two systems: by screw drive or by gear and track. Each manufacturer will tout that whatever system they have installed is best, but they both have their pros and cons.
A screw drive (ball screw, Acme treads, etc.) makes for what appears to be an easy installation. The motor at the end of a fixed screw rotates the screw to move a nut. The nut is attached to the thing that moves. Each rotation of the screw moves the item only a fraction of an inch so a gear reduction isn't needed. Easy. However, the nut on the screw has some looseness to it so when you stop and try to go the other direction, there is a period of which the nut doesn't move until it contacts the other side of the screw threads (this is called backlash). So the nut needs an anti-backlash device which complicates things. The screw also needs bearings and devices to allow it to rotate but hold it in place and this complicates things. And one final limitation is the length of the screw. If you plan on a 16' long table, odds are it won't be driven by a screw.
A gear and track drive also appears to be an easy installation. A toothed track runs the entire length of the path to travel and a motor with a toothed gear fits into that track. The motor is attached to the thing that moves. Also easy. However, the force from the motor which pushes the item down the track also tries to push the gear out of the track. Each rotation of the motor moves the item several inches, depending of the diameter of the gear, and this necessitates a gear reduction or a large motor with a micro-stepping drive to provide the torque and resolution required.
Servo Motors vs. Stepper Motors.
A few decades ago, stepper motors and drives were only commonly available in smaller sizes. Servo motors (which are just DC motors with a location encoder) have been available in whatever size, as long as you had the money to spend. Now steppers are available in some pretty large sizes. How to choose? If you apply enough force to an energized stepper motor, you can get it to jump over to the next step (this is called loosing a step). A servo motor won't do this. However, this can be good and bad. You would not want this to happen under normal conditions, but if something goes wrong and the machine gets hung up, it's preferable to loose steps over stripping off a gear, etc. There are more pros and cons to this argument, but bottom line: both types will work just fine. And all in, the stepper is less complex and less expensive so you will see this one as the standard.
Routers vs. Spindles
A router is a single speed (or slightly variable speed) hand held tool designed to do intermittent work with bearings designed to resist forces imparted by the operators hands.
A spindle is a three phase motor with much larger and stronger bearings, and when coupled with a variable frequency drive, is capable of reverse rotation and a speed range of between 100 to 18,000 rpm.
In a CNC environment, a router is a disposable item and is barley adequate. If you plan on actually cutting materials, just skip the router step and get a spindle. You will end up there anyway.
Computers, Controllers, Drivers, and Design Software
The interface and software is one of the most important parts of the whole system. However, which is best and what works for an individual's situation can be totally different from person to person. On one extreme, one can use an old PC, write the programs in a text editor, and run the CNC with an open-source free program. On the other extreme, a completely automated design, nesting, CAM, and driver set can cost as much as the machine and require a yearly maintenance fee (these are typically found in specific systems, like high-production cabinet making or HVAC ductwork fabricators).
Because everyone's needs are different, I can only recommend that you do as much research on the software as you would with the rest of the machine. Start with something simple! It is easier to learn a simple program and then upgrade later than it is to be overwhelmed by a complex CAD/CAM package.
CAM Software Suggestion
May I suggest that if you are operating a panel router, don't every buy something called bla-bla-CAM MILL. To explain, let me define the difference between a MILL and a ROUTER:
A MILL usually takes a block of material and mills it into some three dimensional single piece, like a mounting bracket, a cylinder head, or a bust of George Washington. The material is typically locked onto the table with a fixture and rarely cut loose. Holes are 'pocketed', which means that all the material in the hole is turned into shavings. Typically, the cuts don't go all the way through the piece and so any cutting order is acceptable.
A ROUTER usually takes a large sheet of material and cuts it into a bunch of loose pieces. Center holes are simply cut away into single smaller pieces, not milled completely down into cuttings. Almost everything is cut all the way through. Because of this, it is very important to define what needs to be cut first, which direction it needs to be cut, and where the starting point is.
There are some very popular MILL software packages which are marketing towards the people using CNC routers. You can somewhat make them work, but everything ends up being an inefficient work-around. I really can't stress this enough: ONLY GET SOFTWARE WHICH IS PRIMARILY DESIGNED FOR A ROUTER! Here are a few functions you should keep an eye out for:
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The ability to (1) pick the next piece you want to cut, (2) pick the starting point on that piece, and (3) pick the direction to cut.
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The ability to automatically calculate holding tabs (I never use them, but it's a good indication that the software is intended for a router).
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The ability to drill a batch of holes (i.e. provide a G81 drill cycle for a series of points).
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Simplicity; the more options you have, the more things you need to turn off or work-around to get the software to work correctly. A lot of time is wasted trying to get a full featured package set up and working properly.
I originally used a very simply CAM program from the early 1990's written in DOS. My parts and cutting paths were written in a separate CAD program and imported as a DXF file. Because of its simplicity, the time between opening the DXF file to closing with a G-code file took me about three to five minutes. Over the years I've used many different CAM packages, but for simple panel work (also called '2-1/2D' as a term similar to '3D') nothing remained in the market that was simple. Features are too easy to add, so every CAM package got too complex... so I wrote my own.
Elsewhere on this site, worktablecnc.us, there is free CAM software that I wrote in Python. It is all I use for 2-1/2D stuff. It is really very basic, but easy to learn.