Flying Gantry Machine Overview: A flying-gantry or moving-gantry cnc machine is designed in to save space, but at the same time has the option of being made into an open-table bed. This is useful for applications like cutting straight through the table; it is slightly more complex than one traversing axis (example Axis Y will traverse with Axis X and Z). This design arrangement must bear the load of all three axis,' and is not considered to be as strong as a fixed gantry layout.
Fixed Gantry Machine Overview: With a fixed-gantry CNC Router machine style, one axis or slide will be fixed to the gantry (example Axis X) with Axis Z traveling or 
attached to it; Axis Y is separate and fixed to the base. A fixed gantry can be made stronger because two of the three axis' can be attached to a solid, non-moving structure, but at the same time this arrangement will result in a larger machine footprint. A fixed-gantry design is almost exclusively used on full-size, vertical milling machines. A fixed gantry cannot be made in open table design.
Open Table: An open-table CNC Router Machine design is designed for cutting straight through - it is basically a flying gantry with the bed removed, and the machine is usually propped up on four legs or is mounted to side rails. This style of machine is useful for applications like plasma, laser, or if you need to cut a part that cannot be moved, like an inlay on a floor or wall. It can also be mounted to a table, if required; this style is prone to flex in the sideways axis, as the side rail bearings cannot be supported along the entire length.
Closed Table: A closed table CNC Router is a machine that will have the cutting surface designed in a way that incorporates the side rails onto the base. With this style of design, the machine can be made more rigid, but will have limitations on the part size to be cut, as the bearing rails block off space on the sides.
Selecting a Linear Bearing Style
CNC Linear Bearing:
Linear Bearings on a CNC Router, CNC Lathe or any machine, are bearings that can travel in a straight line, with any type of machine cutting the linear bearing must be as rigid as possible and able to glide as free as possible, with a machine like a cnc router the clearance between rail and runner block should be very small or zero, large cnc milling machine will actually be preloaded, there are many types of commercial and DIY linear bearings, the quality of the linear bearings will directly reflect the quality of the cutting and hardness of material that the machine can cut.
Supported Linear Bearing:
A supported rail system is the most rigid system; in a supported rail system, the rail will be supported or bolted down the entire length of the rail, the runner blocks will house hardened ball bearings, and the rail will be a hardened shaft or rail, supported linear bearings will cost more than all other types of linear bearings, and will the best results.
Unsupported CNC Linear Bearing:
In an unsupported rail system the rail will be mounted only at the ends, this style of linear bearing will have some flex and will not be as rigid as supporting the entire length of the rail, but will cost less than a supported rail system, and can be good for light cutting, if made in shorter lengths, an unsupported linear bearing can be purchased or made at a low cost with bronze bearings and precision ground shafting.
DIY CNC Linear Bearing:
Because of the high cost of purchasing a linear bearing from a manufacturer, some builders make their own. A DIY linear bearing can be made for both supported or un-supported versions. A linear bearing made yourself can have great results, and can be made with standard tools. Our version is a fully supported design.
| NEMA Motor Frame Mounting Dimensions inch |
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Dimension | NEMA 17 | NEMA 23 | NEMA 34 | NEMA 42 |
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| Motor Shaft Diameter | 0.1968 | 0.250 | 0.375 | 0.625 |
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| Motor Shaft Length | 0.945 | 0.81 | 1.25 | 1.380 |
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| Pilot Diameter | 0.866 | 1.500 | 2.875 | 2.186 |
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| | Pilot Length | 0.080 | 0.062 | 0.062 | 0.062 |
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| | Mounting Bolt Circle | 1.725 | 2.625 | 3.875 | 4.950 |
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| | Bolt Hole Size | 4-40 | 0.195 | 0.218 | 0.281 |
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| Bolt Hole Distance square | 1.220 | 1.856 | 2.740 | 3.500 |
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CNC Router Z-Axis Slide Design
Machine Fixed Z-Axis Motor Design:
The Z axis can be arranged to only move the slide and cutting tool; in this arrangement there will be less weight to move, as the motor is fixed to a opposite axis.
Machine Moving Z-Axis Motor Design:
At Rockcliff we have always designed the Z as moving motor and make it the standard, similar to most commercial applications; it is usually as strong and will give more clearance in moving motor design the rail and motor move while the runner block is fixed to the opposing axis.
Linear Drive Machine Mechanics A linear drive is the mechanics required to transfer from rotation of the motor to straight line machine motion, as used in all the slides of a CNC router, there are many choices, screw, rack and pinion, timing belt.
Machine Lead Screw: 
A lead screw is basically a long screw made with a continuous thread. In a CNC machine this lead screw is attached to a motor, either directly by a coupling or by timing belt with reduction. And the drive nut, which is attached to the table or slide as the fixed motor spins the screw, will drive the table to to specified position, it is important to have minimum clearance between screw and nut, excessive clearance will result in chatter and inaccurate cutting. Resolution / Accuracy: Whether you select servo or stepper motors, they will both have a number or count associated to one revolution of the motor shaft - this number or count is called resolution. For example, a stepper motor with 1.8 degrees, with no micro stepping will have to step or index 200 times per turn. The resolution can be increased, by using a Micro stepping controller, this will allow one single step to be increased by 8 or 1600 times per turn. However, a servo motor resolution is determined by the encoder. An alternative way to increase resolution is to select a finer pitch lead screw; however, with the increased finer pitch, a slower maximum speed will incur.
Machine Lead Screw Diameters: The lead screw diameter is measured from the tip of the threads - Example: 3/8-16 this thread diameter would equal 3/8".
Machine Lead Screw Pitch: Pitch is used to determine the thread spacing of the lead screw and matching nut. Pitch is the distance the nut travels along the screw when the drive nut is turned. One turn example: 1/4-20 is a 1/4" in diameter and has a pitch of .05", if the nut turns 20 times it will move one inch 20 x 0.05" =1.0"
Machine Lead Screw and Nut Options: In low cost lead screw applications, a drive nut will often be made from plastic or bronze; making the drive nut from these materials will give a low friction solution, at the same time no lubrication is needed.
UN Thread: A UN thread (Unified Thread) is the standard thread shape that is used on general nuts and bolts that you find on most items attached with nuts and bolt.
Acme Screws: Acme screw threads can be made much more coarse than UN threads. They are square-cut threads and can hold much more load than a v style or UN thread; they are common components that are generally found in industrial machines, they are readily available.
Ball Screws: A ball screw uses ball bearings in a recalculating housing. Ball screws use hardened ball bearings and the screw is hardened steel; it either come in a lower cost, rolled screw or a precision ground screw. Most high- end milling machines will use ball screws - they have high efficiency and can be very accurate. They will, however, be the most costly to purchase, and, as there is no ball retainers in the drive nuts, they will have to be handled and assembled with care.
Multiple Start Threads:
Most general threads will be a single start, multiple start threads are used to increase the linear travel distance, per one turn,
and at the same time, using multiple starts will also increase the thread contact area, you can easily identify the amount of starts
by looking at the end of the screw, a single start will have one entry point, a two start, will have two, and so on, manufacturers of screws can use many starts, but in general 2 to 5 starts are more common in CNC machines, for example a 1/2-10 single start
will move a slide 0.100" per one turn, a 1/2-10 two start will move a slide 0.200" per 1 turn, you also must match the nut to the
screw, for example a 1/2-10 two start screw must be matched to a 1/2-10 two start nut.
All-thread:
All-thread screws or threaded rod will come fully threaded and are threaded with UN threads. This is the standard thread found on most machine screws and fasteners. Allthread is very common and can be purchased at most hardware stores and can be used as a very low cost, lead screw. The only issues with all-thread is inconsistant thread pitch, and it is normally only available in fine threads.
Anti-backlash Nut:
An anti-backlash nut will have zero clearance between the screw and nut, it is manufactured to allow preload to be applied to the screw. An anti-backlash nut is very useful in a CNC machine, as it allows very precise transmission from a rotating screw to linear motion. When cutting, with any machine, holding the work as rigid as possible will make for smoother, chatter-free cutting. Anti-backlash nuts can be purchased in UN, Acme and ball screws.
Supported Lead Screws:
In longer length slides screw, end supporting is important. A supported screw will have a bearing mounted to the lead screw on the opposite end of the motor, this bearing support will help eliminate a condition called lead screw "whip"; at higher, lead-screw speeds an un-supported screw will become unbalanced. To mount a bearing correctly, the threads will have to be machined off to a clean diameter, in order to fit the inside of the bearing; and the bearing will have to be mounted to the machine.
Rack and Pinion:
In longer length machines, a rack and pinion can work well. One drawback with a R/P is that carefull alignment is more time consuming, and some reduction from the motor to pinion is usually needed for closer machine cutting resolution. A second major drawback is backlash - a properly designed anti-backlash pinion, works well; I see a lot of incorrect setups, where the pinion is preloaded into the rack.
Timing Belt:
In longer length machines, timing belt drives are great. The main advantage is their easy alignment. As with R/P some reduction from the motor to the drive pulley is usually needed for closer machine cutting resolution. A second major advantage is anti-backlash - there is no backlash in a timing belt set-up.
Linear Motor:
Found in most high-end CNC machine centers, one minor drawback, they are extremely expensive, prohibiting most hobby use.
Sprocket and Chain:
Normally not recomended for CNC linear drives, due to a cogging effect, caused when the solid chain links enters and exits the sprocket. this cogging effect, normally will occilate a slide giving undesirable cutting effects A chain drive also needs some adjustment slack to function properly, which will cause backlash.
see movie
Wire / Cable Drive
A wire drive can be used succesfully, if engineered properly, some pen plotters and printers used wire, good for lite load applications