Since I work with ZTW, I'm sure this reply will be suspect of a bias point of view. But oh well...
Quality - It is the most important factor (next to customer service) in everything we do. Everything is tested repeatedly and most electronics devices are tested prior to shipping. But I'll let other customers give you their opinion of it. You may find some posts on this at the ZTW forums (forums.zentoolworks.com) as well.
Complete Kits - We had this going for about a month last year and it was very successful. We make it available periodically but we can't keep up with the restocking right now. In order to keep prices at the lowest, we take the "longer routes" in shipping, assembly, etc which saves us money and in turn saves you money. But we will work with you to design a complete kit - no problem!
Kits, Parts, and Pieces - Desktop CNC systems are very configurable; they are not so proprietary that you have to use the sellers equipment. I'll describe the major parts of the CNC setup:
- CNC Frame - as you might guess, this is the actual skeleton of the system. While there are different frame designs, they are generally the same mechanically. You will want to know the limitations of the frame: max weight can it support; PSI tolerance, etc. This is the primary part provided in a base CNC kit, along with the motors, guide rods, lead screws, and anti-backlash nut for each axis it supports.
- Guide Rods / Lead Screws - The lead screw is what the motors will turn which results in moving the assembly that is attached to it (cutter or the work table). The chrome/steel guide rods are smooth and is what holds the assembly/gantry on it. It keeps axis movements absolutely straight as the lead screw is flimsy and is only used for movement.
- Axis Motors - The motors are responsible for turning the "lead screw" for each axis. The most popular motor to use on CNC systems is the Stepper motor. This type of motor rotates in 'steps' and allows for mini/micro steps in between that allow for very fine placement. You will want to be aware of the Max. and operating voltage and current settings for the motor of choice. You need to make sure your power sources can supply these requirements without going over.
- Anti-Backlash Nut - When you put a nut onto a machine screw, you can wiggle the nut just a little bit. An Anti-Backlash nut (AB or ABN) tries to minimize this movement. Because in the world of CNC, this could be a nightmare. Axis motors move the spindle cutter back and forth, up and down, line by line. If there is no AB nut, when an axis changes direction, the gap (probably around 0.01mm to 0.1mm) will not be calculated in its "known position". If you're dong big carvings, it might not be noticeable; many CNC systems work without an ABN because of the type of carvings they do. But, if you are doing something detailed and fine and require return accuracy (ie: returning the cutter bit to the exact same spot every time), then you require an AB nut. By default - you should have one. We try to keep a return accuracy at 0.01mm which works well, even for making printed circuit boards which is what I do a lot of. You can get even stronger AB nuts for even more accuracy, but it will result in much higher costs.
- Spindle/Cutter - The drill that does the carving and cutting. CNC frames usually provide you a flat vertical "plate" on the Z axis (up/down) to fasten a drill to. You can use pretty much any drill you want as long as you can fasten it to the plate and I've seen some creative fasteners before; from glue to duck-tape... We provide a very nice 8000rpm DC spindle that works really well for general CNC. I use it myself to create those PCBs I mentioned. But cutters have limitations too and you'll want to be aware of them. The things you want to pay attention to are: Shaft sizes it supports, Maximum RPM (how fast), Torque force (how powerful), and the most over-looked value - the runout. Runout is the amount of jiggle the bit does when it spins. If you put in a 0.5mm bit, you expect a 0.5mm carve to be made. But if the drill has a high runout, the carving may be 1mm wide or more. The runout is usually a product of the drill chuck but there could be other contributors as well. As everything else, more accuracy equates to more expensive. Our spindle kit was chosen to provide a general starting point.
- Controller (Motor Driver) - The hardware that converts computer commands to actual motor signals. These are usually available as a "card" as not many are offered with an enclosure. Usually, this is because of what it would cost to manufacture the enclosures. So, we leave the manufacturing of an enclosure to the customer as one of their first CNC 'jobs' to do. It's a great start-up project! Controllers have many functions and features. The things you want to be aware of is: The max operating voltage of the card and the motor supply, number of axis ports (3 or more), available terminals for Emergency Stop switch and Limit/Home switches, adjusters for the amount of current, number of micro steps it supports, and a few other common properties (decay, etc).
- EStop and Limit/Home Switches - The motor driver card controls all CNC movement. Having a button you can hit that shuts everything off right away is necessary in case of an Emergency (hence EStop). The Limit/Home switches are fastened at the ends of each axis. When the cutter gets within a certain range or touches the switch, it sends a signal to the driver card telling it that this is the limit of movement in the operating direction. The name "Home" is given to these switches because they are used in "homing" your machine. This is the process of zeroing your machine position in the software you are using to load and execute jobs. Not everyone uses or has an EStop switch or limit switches. You can definitely operate a CNC without them as long as you know how to avoid hitting the frame when setting the x/y/z starting point (zeroing your virtual position; not the same as machine position).
- Parallel Connections - Controller cards are normally provided with a Parallel connector. It could be a male or female port so you need to be aware of this and get the right cable. You computer has to have a parallel port too. They used to be on every PC sold as the default "printer port" (LPT1), but has been replaced by the USB connections so some computers do not have them. If you do not have one, you need to get a PCI parallel adapter (Ebay) and install it . There are some cards out there which claim to work with USB or Parallel to USB adapters. We all highly advise against the usage of these. Most CNC sellers stick with parallel connectors for good reasons. Parallels were primarily used by printers. So they were built to support different kinds of queued/streaming data which is exactly what CNC systems are all about. USB is not built with the same features. It does not queue and in most cases its transfer rate is too slow. It can cause issues that are simply untraceable and make you think it's the card. The number of issues with USB (known and unknown alike) has prevented it from becoming a suitable replacement to the parallel port for CNC data communication.
- CNC Software - The computer software used to communicate with the driver card. The software should support the same number of axis motors you have on your system and provide you with a 3D preview of the cutting path (called "Tool Path"). Most software is proprietary to the driver card, but, there are a few Windows and Linux versions out there that work very well and supported by both manufacture and CNC community. For Windows there is Mach3 (ArtSoft USA - Home of Mach3 and LazyCam). It is a shareware application that limits you to 500 command lines. This can be thought of as a limit of 500 cuts per job. A licensed version will give you a limit of something like 1,000,000 I think. Linux (Ubuntu version) has a free application called EMC2 (LinuxCNC.org - Home). This is a very detailed application that allows you finer control over everything, but is not as easy to figure out as Mach3.
I hope I didn't fumble in there somewhere. But you get the point/basics and that is what is important. You will have lots of fun working with own CNC system. The learning curve is not that hard really and there is a ton of help in the form of videos or forum searches. The "hump" you will need to get over is the initial setup and configuration of everything. Once you get past it you will be ready and comfortable starting your first project!
Anyway, keep asking questions and get the most out of them before making a final decision. You want to be sure that you know what you are getting and whether or not its worth the price and time you will need to spend getting it setup.
Good luck and hopefully we'll see you cutting and carving soon! Don't forget to post and show-of your first few jobs. We love seeing the different works people make with their new CNC unit. It's great!