CNC Dictionary: 2.5 Axis to Brushless DC Motor
2.5 Axis or 2.5D CNC
2.5D refers to a machining scenario in which there are no overhanging elements to the part. In other words, everything that must be reached can be reached by plunging straight down from above.
3 Axis or 3D CNC
Refers to a machine or CNC scenario that has 3 axes of motion available, usually X, Y, and Z.
3DM (see also “Rhino”)
3DM is the file extension for a Rhino 3D CAD file.
4 Axis CNC
Refers to a scenario or machine in which 4-axes of motion are available. Usually these are X, Y, Z, and a 4th rotational axis. The 4th axis is typically the ability to rotate along an axis normally parallel to the X-axis, and is usually referred to as the “A” axis.
This capability is extremely useful for a lot of situations. For example, it allows machining features onto a curved surface, such as a shaft or cylinder, as the shaft or cylinder is rotated in the 4th axis. It can also allow access to the underside of a part or direct access to the sides without having to manually flip the part to another orientation.
5 Axis CNC adds a second rotational axis to the X, Y, Z, and A axes for even greater flexibility.
Programming a 4 axis system can be more complex, and a 5 axis system is extremely complex to program.
See also “Axes”, “4th Axis” and “5 Axis CNC”.
The term “4th Axis” often refers to the hardware necessary to add a 4th axis to a 3 axis CNC machine. This hardware typically consists of a motorized rotary table mounted on the machines table and aligned so its axis is congruent with the x-axis of the machine.
5 Axis CNC
5-Axis CNC is generally a mill or router that has an X, Y, and Z-axis working perpendicular to one another together with two rotating axes. Imagine a 4th axis rotating table that is used to rotate a 5th axis that can also rotate at right angles to the 4th axis.
This configuration provides the maximum machining flexibility in a single setup of any of the CNC configurations. Certain types of applications almost require 5-axis machining to be successful. For example, cylinder head porting requires the milling cutter to reach inside a convoluted intake or exhaust port in a cylinder head, a task that would be just about impossible to do in more conventional configurations.
5-axis machines are generally quite expensive and difficult to program, however. See also “Axes” for a diagram of the 5 different axes.
5-axes are required for a general purpose CNC tool grinder/sharpener, and you can see why if you examine the geometry involved:
5-axes needed for Tool Grinding…
The “A” axis is used to access the cutter around its circumference. The “B” axis is needed for cutters such as ball mills that are not just cylindrical the whole length of the tool.
5-Axis machines may be characterized as “continuous” (also called “simultaneous”) or “positional” (sometimes called “indexing”). A 5-axis continuous machine is capable of cutting while all 5 axes move. This allows continuous machining over complex shapes, and is more powerful than 5-axis positional. A positional machine can only cut in 3 axes at a time, but it can position before cutting using all 5 axes. The positional approach is much simpler to implement, both for software and hardware, and so may be found in older or lower end machines. 5-axis positional or indexed is also sometimes called 3+2 axis.
In theory, you can cut any part with 5-axis positional that you could cut with 5-axis continuous. A machine designed for continuous should be faster. In addition, 5-axis positional may introduce dwells (See also dwell) that result in visible marks in the surface finish.
The A-Axis is the rotational axis about the X-Axis. See “Axes” for a diagram.
ABEC is the abbreviation for Annular Bearing Engineering Council. It is a term commonly used in conjunction with rating bearing tolerances or precision. A number follows the ABEC designation, and the higher the number, the more precise the bearing. An ABEC 7 bearing is an excellent machine tool grade bearing.
The ABEC system relates to other bearing precision rating systems as follows:
|ABEC SYSTEM||ISO SYSTEM||DIN SYSTEM|
|ABEC 3||Class 6||P6|
|ABEC 5||Class 5||P5|
|ABEC 7||Class 4||P4|
|ABEC 9||Class 2||P2|
Absolute coordinates are expressed relative to a fixed position whereas Relative Coordinates (see also) are relative to the current position of the cutting tool. The G90 command places the part program into absolute coordinate mode. G91 cancels absolute coordinate mode and places the part program into relative coordinate mode.
An absolute move is made using absolute coordinates, so it commands motion to an exact location specified relative to the origin of the coordinate system. A relative move is made using relative coordinates and so is specified as a move from the current position
Accuracy is the difference between the expected value and the actual value of some measurement.
When combined with PTFE (Teflon), acetal makes an excellent low friction material with which to coat machine ways. This is the material used on the Tormach CNC mill, for example. Rulon and Turcite are all trade names for the PTFE + Acetal combination. Delrin is the trade name for pure Acetal. See Also Delrin.
ACME is the most common type of leadscrew found in machine tool applications. The ACME thread is a particular type of thread. Compared to a ballscrew (see also Ballscrew), ACME leadscrews have very high friction and backlash, both of which are undesirable attributes for high performance CNC applications.
On the other hand, ACME leadscrews can be made to very high precision, and they are often cheaper than ballscrews. They are almost universally the choice for manual machines.
Active High/Low Logic
The terms “Active High” and “Active Low” refer to how electrical signals are interpreted. In Active High logic, the presence of a higher voltage (within a specified range) indicates a signal is present. Active High logic is also referred to as “Open Collector” logic. While it may seem odd to use the absence of voltage to indicate the presence of a signal, there are advantages to this approach. In general, it gives the circuit a much broader tolerance for the voltages used.
In Active Low logic, a ground or low voltage (within a specified range) indicates a signal is present.
Angular Contact Bearing
Angular Contact Bearings are ball bearings designed to deal with heavier axial loads. A typical ball bearing is designed to deal with pure radial loads and are not well supported the closer the load comes to the axis of the bearing. They are commonly used to support leadscrews and in building precision spindles for machine tools.
There is a complex system of standardized nomenclature used to identify the characteristics of an AC Bearing from the part number. For example, AC Bearings have a contact angle that determines how far off axis they are supported for loads, they have a precision (See also ABEC), the have preload specifications (See also Preload), they have dimensional characteristics in terms of their bore, outer diameter, etc., and they have ratings that relate to their maximum operating speeds in RPM.
The best place to learn more about angular contact bearings is from the manufacturer’s bearing catalogs:
Annealing is a heat treating process that is designed to remove the hardness of a material so that it can be machined.
An arbor can be either a workholding device, or a toolholder. As a workholder, it generally refers to a device that can expand it’s OD to clamp the ID of a workpiece. As a toolholder, it is similar to a spindle and is used in conjunction with grinding and buffing. A very similar term is “Mandrel”.
ASE is the file extension for 3DS Max ASCII export files from Autodesk. In other words, a CAD file.
ATC: Automatic Tool Changer
See Also Toolchanger.
Automatic Grinding Center
A CNC machining center that does grinding. Most automatic grinding centers are used to grind and sharpen tools and inserts.
See also Machining Center.
Following are diagrams depicting the basic axes for mills and lathes:
On a 4-axis or 5-axis mill there are additional rotational axes. Here is an example of a 5-axis mill with trunion table:
5-Axis Mill With Trunion Table
A 4-Axis Mill simply omits the B-Axis.
Back Driving is the tendency a ballscrew (or any leadscrew really) can have to spin when a force is applied to the nut along an axis. Because they are such inherently low friction devices, it becomes possible for the cutting force of the machine to back drive the ballscrew and move the table unless a force is provided to prevent that.
This is one reason why manual operation of ballscrew equipped machines may not be possible unless the machine is carefully designed to prevent back driving either by careful selection of the ballscrew pitch or fitting of an adjustable friction device to hold the axis in place by preventing the ballscrew from turning.
In CNC applications, the stepper or servo drive locks the ballscrew in place so there is no problem with back driving. In some cases, it may be possible for a heavy vertical load to backdrive downward due to its weight after the machine is turned off.
The formula to calculate back driving torque is:
Td = Drive Torque (pound-inches)
P = Load (lbs.)
L = Screw Lead (inches/turn)
e = Ball Bearing Screw Efficiency (90%)
Back EMF or Back Electromotive Force
When a step motor or servo motor is decelerated, it generates “Back EMF”. In essence, it will try to pump electricity back into the power supply. This can be destructive to the components in the worst case, as that electricity must find somewhere to go. In a system where multiple motors share the same power supply, it may be that another motor can use the electricity. Alternately, a capacitor in the power supply may be able to absorb and buffer the Back EMF. Some power supplies employ specific circuitry to deal with the Back EMF.
Backlash is any kind of unexpected play in an axis due to clearance or looseness of mechanical parts. When the axis is commanded to move, the drive motor may turn briefly before movement begins. That delay is the backlash.
Backlash has a variety of causes. The most common is play between the leadscrew threads and those of the nut. ACME screws can have considerable backlash of this kind, while ballscrews may have almost none. Another source is any tendency for the screw to move axial in the bearings that hold it, or any other such play in the system. Precision angular contact bearings with preload are often used to combat this tendency. Gears, belts, and chains can all introduce backlash into a mechanical system. Even loose fasteners or flex in the mounting plates or chassis can be a source of backlash.
In some cases, backlash can be compensated for in the driving software using a feature called “Backlash Compensation” (see Backlash Compensation).
See also the CNC Cookbook articles on Backlash:
as well as the Stepper/Servo/Backlash Simulator
Backlash compensation is a feature of the software used to control the drive motors of a CNC system. By telling the software how much backlash exists in the system, it will try to compensate for that backlash. It does this in a manner similar to the way manual machinists do. Namely, the software expects that when an axis reverses direction, it will not move until the backlash has been taken up.
Backlash compensation is a useful feature, but it is no panacea or excuse not to try to eliminate all backlash from a CNC system. Sometimes, an axis needs to smoothly change direction without waiting for the backlash to catch up. The classic example is when machining a circle, where the direction changes multiple times. Backlash compensation will not eliminate errors in machining the circle.
A device that may be added to a leadscrew to reduce or eliminate backlash. Backlash eliminators were sold or provided as a feature of machines that did not have ballscrews in order to control backlash and enable climb milling. A backlash eliminator could be as simple as a double nut setup with a precision spacer between the nuts to eliminate the backlash on an ACME leadscrew. The term is less prevalent in more recent times as ballscrews have become common on machine tools.
See Also ACME, Backlash, and Climb Milling.
Backplotting is a feature of CAM programs whereby the path followed by the tool can be backplotted onto the drawing. A backplot typically shows all the passes at once. Here is a typical backplot from OneCNC Turn for lathes:
A backplot: green lines are cuts, dotted red are retracts between passes…
Balanced Turning (See “Pinch Turning”)
Ball mills are end mills that have a half spherical bottom. They’re favored for 3D profiling because they don’t have a flat bottom. They have some unique cutting problems at the bottom because the relative speed varies all along the ball. The cutter is moving fast out at the diameter of the ball than it is at the center when viewed along the axis.
The ballnut is the nut component that is paired with a ballscrew. They’re filled with ball bearings, and have connecting passages that recirculate the ball bearings back to the top of the nut after the roll over the threads of the ballscrew all the way to the bottom.
Ballnuts may be preloaded to reduce backlash either by using oversized ball bearings in them to take up the slack, or by using them in pairs with tension or preload between the two nuts.
They are are typically mounted to the machine tool using either a flange or a thread on the nut. They must be lubricated and protected from chips and other debris.
Ballscrews are highly efficient low friction and low backlash leadscrew devices that use ball bearings rolling in a channel cut into the screw. The low friction and backlash attributes are extremely valuable for precision CNC applications where they are used to drive the axes of the machine.
Ballscrews come in rolled and ground types, with the latter being more accurate, having lower backlash, but also being quite a bit more expensive than rolled ballscrews.
Ballscrew Mapping is a software feature of the CNC control software (such as Mach 3) wherein a map of the errors in the ballscrew is entered and the software attempts to automatically compensate for these errors. The errors are expressed in terms of how far off from the desired position the screw is at various distances along the screw.
Ballscrew mapping can be a very effective way to increase accuracy in the machine, but it requires extremely accurate methods to measure the true position of the axis along the length of the screw. This is typically done using gage blocks and a dial indicator or even better by using linear scales and a DRO. The most precise method of calibrating a ballscrew map is laser interferometry.
Bar Feeders and Pullers are devices used to automate a continuous feed of bar stock to a lathe so that the operator doesn’t have to keep loading new stock as parts are made.
Bar Puller (see Bar Feeder)
The B-Axis is the rotational axis about the Y-Axis. See “Axes” for a diagram.
A bed mill, sometimes called a column mill, is the most common way to construct a vertical CNC mill. In this design, the spindle is affixed to a vertical column and slides on ways up and down the column.
Bellows are used to protect the ways and leadscrews on a machine from chips and coolant.
Belt drives may be used in machine tools to connect a drive motor to an axis leadscrew, or to connect the motor to the spindle. They can even do double duty as a replacement for the leadscrew on a plasma or router table when the belt runs the entire length of the axis and the axis is attached to the belt at one point.
Common types of belts include Timing Belts (See also Timing Belts) and V-Belts. Timing belts will generally present lower backlash and be a more efficient, so V-Belts are primarily used for motor to spindle cases. Even there, a timing belt may be preferable.
Bipolar Stepper or Driver
Stepper motor controllers can be either bipolar or unipolar. The term bipolar means current can be supplied to the windings of the stepper motor in either direction. The alternative configuration is unipolar (see also Unipolar), which involves using a center tap and only half the windings at a time. Bipolars have the advantage of producing 40% more torque for a given number of windings, all other things being equal. The controller for a bipolar motor is slightly more complex than unipolar, and the bipolar may run hotter, but in general, bipolars deliver higher performance.
Step motors are typically 4, 6, or 8 wire devices. Bipolars have no center taps, so if there are only 4 connections, the motor is a bipolar only design.
A block is the smallest operational unit of a G-Code part program. Think of it as a single statement of code, or a single operation that the machine could perform. In printed form, a block is a line of text.
A block consists of:
- An optional delete character: “/”
- An optional line number
- Any number of segments, where a segment is a word or comment
- An end of line character
The execution of segments within a block will be the same regardless of their order. This can be a bit confusing to read, so it is often good practice to order them in a way that makes it clear what should happen.
Blocks ignore whitespaces, which can also make for some confusing formatting. Blocks are also case insensitive.
There are some further limitations on the number and type of words that can appear in a block depending on their interactions with one another.
If the block delete control is on, lines of code which start with a slash (“/”) are not executed.
In a drawing, a bounding box is the smallest rectangular space aligned with the axes of the coordinate system that would completely contain the object and with no extra room to spare. It is a useful way to gain a crude understanding of the overall dimensions of a thing, perhaps so that an appropriate piece of stock could be chosen from which to make it.
A box tool is an integrated follow rest and cutting tool combination designed for use where it may not be possible to employ a tailstock to help support the workpiece:
Sliding ways are used by many machine tools so that their axes may slide against one another. There are four prominent designs:
– Dovetail Ways: Very common on mills, dovetail ways look like the dovetails used in wood joinery.
– Box Ways: Box Ways are rectangular cross section, as opposed to the angular shape of dovetail ways. Box ways are very strong, but they suffer from two shortcomings. First, they involve a lot of surface contact area, so managing friction is key. Second, in order to slide at all, some clearance is required, which results in some slop in their travel. They are the strongest and most rigid design, but they are difficult and expensive to manufacture. Because of the friction issues, box ways are limited to about 600 in/minute travel speeds.
– Prismatic Ways: These are prism shaped ways common to lathes.
– Linear Ways: Linear ways use linear slides as rails and so are much different than the other three styles. The roll on ball bearings as opposed to having more metal-to-metal contact in the other designs. Linear ways will have the least rigidity, but for the price, they are ideal for precision and high speeds.
Box Ways vs Dovetail Ways…
Breakout boards are generically boards designed to take signals that come in on a particular cable or protocol and “break them out” to individual connections for point to point wiring purposes.
In CNC terms, the most common breakout board is a parallel breakout board, which is used to take a parallel printer connection from a PC and break it out to individual I/O signals that may then be wired to relays, Gecko drivers, and the like.
Usually a breakout board will incorporate opto isolators (See also Opto Isolator) so that anything that happens on the point to point wiring is isolated and cannot get back to the PC. So for example, if a high powered line was inadvertently connected to the breakout board, the opto isolator would protect the PC from damage, potentially at the cost of burning out the opto, which is a cheap part to replace. Opto isolators also eliminate the need for common grounds and create greater noise immunity.
A Parallel Breakout Board With No Opto Isolation…
Bridge rectifiers are “full wave rectifiers”, meaning they convert the negative going portion of the AC signal to fill in the gaps with a positive way between the normal positives in the AC. They do this using diodes in a “bridge” configuration that effectively reverse the polarity on the AC signal for the negative going portion so that it becomes positive.
They’re typically a component of the DC or Linear Power Supply (see also Linear Power Supply) used to power DC stepper and servo motors. Bridge Rectifiers are robust, relatively inexpensive solid state devices with 4 terminals. Two terminals are for the AC, and 2 are for the DC that comes out of the device. A capacitor (See Capacitor) is used to smooth the ripple left over in the process.
Bridgeport Taper (see “R8 Taper”)
Brushless DC Motor
Brushless DC motors use a permanent magnet rotor and coils in the stator. The stator coil currents are sequenced by an external motor controller. This eliminates noise caused by the arcing of the brushes and greatly improves motor life, and therefore reliability of the motor.
|2.5D to Brushless DC Motor||Last||Next|
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