Terminology
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If your new to the CNC world here are some names and terms you are going to come across, click (More) for detailed explanations:

 

Anti-Backlash

It's a term that refers to mechanical components that have been designed to have very little or no unwanted movement or looseness.

 

Axis

There are several of these and it usually refers to one of two things, the first is the direction of movement of the cutting tool and the second is a grouping of components named for their direction of movement.

 

Axis - Movement

These are:

  • X Axis    -        Left and Right travel or East-West, if you where standing in front of a machine the cutting component would move from side to side.
  • Y Axis    -        Forward and backward travel or North-South, If you where standing in front of a machine the cutting component would move towards or away from you.
  • Z Axis    -        Up and Down travel, not surprisingly if you where standing in front of a machine the cutting component would move up and down.
  • A Axis    -        Often refers to an add-on component allowing your machine to work as a lathe (rotating the material being cut along the X Axis). It can also refer to the replication of X Y and Z Axis. Another use is the rotational movement of the Z Axis, usually allows the cutting component to rotate to get to the sides of the material being cut.
  • B Axis    -        Pretty much as per A Axis

 

Axis - Component Grouping

 

 

Backlash

Unwanted movement of cutting head due to movement or looseness of mechanical parts.

 

Ball Screw

A precision threaded rod used as a drive mechanism for the Axis’s of a CNC machine. Its primary function is to convert rotary motion to linear motion. The ball screw rotates clockwise or counter clockwise and transfers its energy to a fixed nut (see Ball Nut) that is in turn forced to move in one direction or another. The actual form of contact between the screw and the nut is via a line of ball bearings, this means the groves in the screw are U shaped and not V shaped as you would find in normal threaded rod or generic bolts. As ball bearings are used the thread is also designed to allow for oil as well.

 

 

Ball screws are usually only supported at each end by a bearing, some additional support is offered by the nut (but this is more of a deficit to the axis being moved).

 

 

Ball screws come in many varieties and there is a wide selection of manufacturers. There is also a grate deal of variation from type to type. When talking about specifications you will usually hear the following terms:

  • Major Diameter

  • Minor Diameter

  • Pitch

  • Starts

  • Thread direction

 

Major diameter

Major diameter refers to the width of the shaft from thread peak to thread peak. This is diameter usually used to describe the screw size, eg RM1605, screw is 16mm in diameter. This is also the diameter you will need allow clearance for.

 

Minor diameter

Minor diameter is measured from the lowest part of the thread trough to adjacent thread trough. This is the smaller diameter and is used when calculating the actual strength of the screw. See Deflection (This is something I’m not overly knowledgeable about but will endeavour to get more information about and post here).

 

Pitch

Pitch is the centre distance from one thread to another. Pitch is also the major determining factor in the movement speed of the nut on the screw. A wide pitch will make the nut move faster as there are less turns per cm or inch. A tighter thread pitch will give you a slower moving nut as the screw will have to turn more times to travel the same distance as the wide pitch. The main advantage of the tighter pitch is resolution which translates to more intricate cuts. The wider pitch would be found commonly in larger machines working on industrial scales. A wider pitch will also require more energy to move as a single turn moves a grater distance than a tight pitched screw.

 

Starts

Starts or start is the actual number of individual threads on a screw. There are three types of starts; Single Start, Double Start and Four Start. We’re all familiar with single start as it is how all hardware bolts and screws are made. There is a single thread wound around the screw shaft from start to finish. Well “Double” is a second thread wound around the same screw shaft that never crosses the first. And “Four Start” is 4 threads wound around the same shaft. Why? If you have a look at the wide pitch shaft above you can see there is space to fit a second thread but why would you do it. Well the wide pitch thread allows less ball bearings to be in contact with the shaft and nut at any one time. Put this together with the higher mechanical forces (torque) of moving a load quicker and you have fewer bearings dealing with higher stresses. The logical solution, add a second thread and you get double the bearings (in two separate circuits) in the same amount of space. Four start is exactly the same, however the pitch is much grater allowing for some pretty impressive speeds. The four rows of bearings allow for a good distribution of loads and stresses.

 

Ball screw pro’s and cons

 

          Pro’s

                   Efficient energy transfer

                   Accuracy

                   Durability

          Cons

                   Cost

                   Maintenance 

 

Ball Nut

The fixed component of a ball screw assembly through which energy is transferred to facilitating the movement of an axis. The ball nut is usually fixed firmly to a particular axis and as the ball screw rotates within it, it will cause the nut (and axis) to move in the direction of rotation. (See Ball Screw)

 

 

CNC - Computer Numerically Controlled

Cutting, milling, surfacing machines controlled by computer.

 

Collet

Collets fit in a router/spindles head to change the shaft size of a bit it will accept. Typical routers accept 6mm or 6.35mm shafted router bits but can often be fitted with an 8mm collets which in turn allows for bigger bits. Larger routes may have 12.7mm or even 25.4mm collet. Collets range in shaft bore size from 1mm through to 25.4 and usually rise in 0.5mm increments. Collets also come in different size formats and standards, see ER for more information.

 

 

 

 

NEMA – National Electrical Manufactures Association        www.nema.org

NEMA is an American trade association for the electrical manufacturing industry. It has created a number of standards for the manufacturing of products used in the generation, transmission and distribution, control, and end-use of electricity. Many of the CNC components manufactured today are designed to meet many these standards. In particular servo and stepper motors usually conform to these standards. The standards usually cover:

Voltage

Physical dimensions

Mounting specifications, and

Other Codes and Standards  

 

 

NEMA 17

Is a standard for manufacturing small stepper and servo motors and relates to the way the motor is mounted. These standards are:

Face plate              42.67mm x 42.67mm (Max)

Screw hole pos       31.0mm x 31.0mm

Flange diameter     21.95mm

Screw diameter      4.5mm

 

 

NEMA 23

Is a standard for manufacturing medium stepper and servo motors and relates to the way the motor is mounted. These standards are:

Face plate              56.9mm x 56.9mm (Max)

Screw hole pos       47.14mm x 47.14mm

Flange diameter     38.1mm

Screw diameter      5.21mm

 

  

NEMA 34

Is a standard for manufacturing Large stepper and servo motors and relates to the way the motor is mounted. These standards are:

Face plate              85.852mm x 85.852mm (Max)

Screw hole pos       73.025mm x 73.025mm

Flange diameter     46.025mm

Screw diameter      5.537mm

 

 

Servo Motor

This is an AC or DC electric motor used to drive the moving parts of a machine. This type of motor consists of the motor itself, and encoder/decoder, a reader, and, some form of break/hold mechanism. The encoder/decoder and reader are used to position the motor (by counting) or to learn it’s current position (by sampling the decoder). The breaking/hold system (usually on large machines) is used to slow and hold the moving components in position. Servo motors using AC currents are often powerful and fast. Common resolution is 1024 positions (or steps) per complete revolution (9).

 

Pro’s

Power

Speed

Resolution

 

Con’s

Expensive

High voltag

 

Stepper Motor

This is a DC electric motor used to drive the moving parts of a machine. This type of motor is wound in such a way that there are a large number of “hold” positions per revolution. Unlike a normal electric motor, when a current is applied it does not rotate; rather the magnetic field generated is used to hold motor firmly in position. The motor requires an electrical pulse to initiate rotation; any single pulse will cause the motor to move to the next hold position. Because of this “hold” system the stepper motor is usually always on even though it might not be moving (the hold current can be up to 80% of the motors power ratting). Stepper motors provide no feed back to the controller while moving so it is not possible to “Read” their current position. Common resolution is 200 positions (steps) per complete revolution (9).

 Pro’s

Low Cost

Low to medium voltage

Resolution 

Con’s

Accuracy

Power

 

Still a work in progress. If there are any errors or something that needs changing please contact me at

 

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This site was last updated 08/10/14