Well, you asked for it...here's a start on the FAQ.
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0. What is a lead screw.
A lead screw is the screw that makes stuff move on a router. The screw is installed, usually under the table, and a nut is attached to the table itself. By holding the screw in place, but rotating it, the nut is made to move back and forth. Since the table is attached to the nut, it drags the table along with it.
1. Difference between ball screw and acme screw.
There are many kinds of screws that are available. Three primary types are used in CNC machines.
ALLTHREAD:
This is the standard "threaded rod" that you can get at the hardware store. It uses a 60 degree triangular thread, and is the least desireable type of screw thread for several reasons. It is not manufactured to high tolerances, and consequently a lot of inaccuracy can be introduced into your machine. And, the thread is not designed for transmitting power, so significant transmission losses exist in a system using this type of threaded rod.
It has several advantages, as well. First of all, it is cheap. It is also locally available, almost everywhere in the world. And, it is available in many different varieties, with fine threads, regular threads, reverse threads, etc, in many different materials. Taps (thread cutters) are commonly available at local hardware stores, too.
Many successful machines use this type of threaded rod, despite its disadvantages.
ACME THREAD:
This is the common type of screw used for power transmission. Its primary advantages are its thread profile, which is designed specifically to transmit power, which is what we do in a CNC router, and its availability. It is usually manufactured to higher tolerances, which means that your machine will be more accurate. While it's not as easily available as hardware store allthread, it's easily ordered from any number of online parts houses, and is fairly low in cost. At the time I write this, a 10 foot piece costs around $6US.
This is the standard "entry level" choice for most serious CNC home builders.
BALL SCREW:
Ball screws are a specialized type of screw. The threads form a channel in which ball bearings ride...the screw and the mating nut actually work as a helical ball bearing. Needless to say, this has the lowest level of transmission losses -- so low that the nut can actually "fall" down the threads unless a braking system is employed.
Benefits include extremely low friction, extremely high tolerances, and extremely low backlash.
Disadvantages include limited availability, and extremely high price.
2. What is Pitch.
Pitch, simply defined, is how many threads there are per inch of screw. Practically, this means that an 18 pitch screw (18 threads per inch) will have to rotate 18 complete turns, in order for the nut to move a distance of 1 inch.
3. What is backlash.
Backlash is the "looseness" or "wiggle" present in a machine. Go to your garage, and put together a screw and a nut. Now "wiggle" the nut. You'll notice that there is some looseness. This is backlash.
The practical consequence of backlash is that, when changing directions, your drive screw can actually turn the other direction 1/2 a turn (more or less, depending on your tolerances) before the table actually moves. Needless to say, this can mess up your parts.
There is backlash present in all machines. Even the so-called zero backlash nuts still have backlash...it's just so tiny that it's negligible.
Handling backlash can be done many ways, including mechanical compensation, preloading, split nuts, and even software compensation. The most commonly used designs involve a two-part nut that can be compressed or expanded in some way, to press against both sides of the screw thread. These solutions work quite well.
4. What is rack and pinion.
Rack and pinion is another type of linear motion system. It involves a motor driven gear riding on a "flat" rod with gear teeth, that runs the length of the machine. The gear teeth guarantee that there is no slippage, even under load.
5. How do I mount a bearing.
There are many ways to mount a bearing, depending on its use. Some bearings need to simply be held in place, others need to take loads from a certain direction, and are called thrust bearings. There are many threads on this forum that show homebuilt bearing mounts, most of which work very well.
6. What is the difference between stepper and servo motors?
Stepper motors are a special type of motor, with more than 2 wires. Each set of wires is connected to a discrete coil, and manipulating the power to these coils allows the motor to move in discrete steps.
The common type of these motors, used in CNC machines, have 200 steps per revolution. That means that each time the motor takes a step, it moves 1.8 degrees around.
The advantage to using a stepper is simple. They are already designed for computer control, and since their rotation is defined in tiny steps, you always know where your cutter head is located.
Due to the large number of steppers used in computer products randing from tape and floppy drives, to the biggest LASER printers, stepper motors are fairly cheap. Their control systems are also reasonably priced.
The disadvantage to using stepper motors is that their control is somewhat complicated. They require a special board to run them, and 1 board per motor! This controller costs additional money...recently driver boards suitable for homebuilt routers have come to the market, at an attractive price point.
Another disadvantage to steppers is that there is no feedback that the cutter head has actually moved. This is called an open loop system, and is more of a theoretical problem than a real one, but it does deserve to be mentioned.
Now, the servo motor. Usually built into the motor is an optical encoder, that lets the motor know whether or not the motion actually happened. In this way, feedback is generated, and the position of your cutter head is virtually assured. This feedback is sent back to the motor, and is called a closed loop system.
However, servo motors are very expensive compared to stepper motors, and also require a different expensive board to run them. Coupled with their greater speed, this makes servos the better solution. With their greater speed, however, comes lower torque, which means that they have to be geared down for actual use.
The practical difference between steppers and servos, for a small router, is extremely small. Most home builders use stepper motors and generate excellent results, at minimal cost.
7. etc............ad infinitum  |