Numerical Control Kernel
The NC kernel is the unit responsible for the geometrical data processing and motion planning in the control unit. It accepts inputs like the required contour defined by the part program in the form of linear, circular, helical, polynomial or spline blocks and has access to machine data such as the maximal axis velocity, acceleration and jerk. The output of the NC kernel are a time optimal trajectories serving as reference setpoints for the servo controllers. An abstract overview of the basic functional blocks incorporated in the NC kernel is shown in Figure 2.3.
Figure 2.3 - Components of the NC kernel (without position control)
Compressor: It is the first process in the NC kernel. It has the task of approximating the various part program blocks using smooth spline elements within a predefined tolerance. The compressor step substantially reduces the number of blocks used to describe the contour and thus reduces the stored program volume as well as the computing time required by the NC kernel. The compressor step results in a smooth and continuous spline representation of the given contour.
Contour Rounding: It has the objective of removing discontinuous transitions between neighboring contour blocks. It transforms the discontinuous blocks into tangential (C1) or curvature (C2) continuous blocks (C1 continuity implies equal 1st derivative, whereas C2 continuity implies equal 2nd derivative at the joining points). The contour rounding function modifies the programmed contour locally by introducing extra “contour rounding blocks” to overcome the discontinuity of the original blocks. Figure 2.4 shows an illustrating example of two linear blocks with the additional rounding block. Whether or not to add the rounding as well as the type and size of that rounding are normally selected by the user in the form of part program commands or machine data.
Figure 2.4 – Contour rounding example
Look-Ahead Function: It is the part responsible for calculating the axial restrictions in the NC kernel. It identifies the “critical points” in the contour, where the programmed path federate or the velocity, acceleration and jerk limits of the axes may not be maintained. The “critical points” which can influence the axial restrictions include, for example, the singular or semi-singular points and areas with extreme curvatures. The output of this function are the path velocity, acceleration and jerk limitation curves defined for each block in the programed contour. The limitation curves represent a road map for designing the path velocity, acceleration and jerk profiles, which will be discussed in the trajectory planning step later on.
Motion Control Function: Its basic task is the planning of path trajectories. It transposes a motion state vector, which is composed of the scalar path variables (jerk, acceleration, velocity and position) from an existing initial state to a desired final state with minimal time and under axial restrictions. The inputs to the motion control function are the contour description given by the compressor, the contour rounding function and the axial restrictions defined by the look-ahead function, whereas the output are time optimal path jerk, acceleration, velocity and position trajectories.
Interpolation: The interpolation function has the task of sampling the trajectories and projecting the generated path trajectories into the Basic Coordinate System (BCS). The sampling is done according to a predefined interpolation type and machine sampling rate called the Interpolator (IPO) sampling cycle. The type of interpolation can be linear, quadratic or cubic interpolation. The projection process splits or distributes the path trajectories into Cartesian trajectories such that their combined motion reconstitutes the initially defined contour.
Kinematic Transformation: This function has the task of transforming or projecting the time optimal setpoints generated by the interpolator from the BCS to the Machine Coordinate System (MCS). The existence of the transformation step and the kind of transformation to be used depends on the machine in use. For example, Cartesian machines do not need a kinematic transformation step since the MCS is the same as the BCS.
Fine Interpolation: As a final stage in the NC kernel comes the fine interpolation function. It is responsible for mapping the MCS setpoints from the IPO sampling cycle to servo control sampling cycle which is also known as the Fine Interpolator (FIPO) sampling cycle. The type of interpolation can be linear, quadratic or cubic interpolation which is defined via machine data.
References: Gen [1-3].
Assessing question:
What the difference between CAD and CAM systems?
What is NC Kernel?
What the function of compressor?
How can be defined the Contour Rounding?
What is the feature of the Kinematic Transformation?
Lecture 3. Specifications and design features of the class of NC and CNC systems, specifications and design features of the other numerical systems.
Machined parts can be classified as rotational or nonrotational (Figure 3.1). A rotational workparth as a cylindrical or disk-like shape. The characteristic operation that produces this geometry is one in which a cutting tool removes material from a rotating workpart. Examples include turning and boring. A nonrotational (also called prismatic) workpart is block-like or plate-like, as in Figure 3.1(b). This geometry is achieved by linear motions of the workpart, combined with either rotating or linear tool motions. Operations in this category include milling, shaping, planing, and sawing.
Figure 3.1 – Machined parts are classified as (a) rotational, or (b) nonrotational, shown here by block and flat parts.
A part shape is created as generating and forming. In generating, the geometry of the workpart is determined by the feed trajectory of the cutting tool. Examples of generating the work shape in machining include straight turning, taper turning, contour turning, peripheral milling, and profile milling, all illustrated in Figure 3.2.
Figure 3.2 – Generating shape in machining: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, and (e) profile milling.
In forming, the shape of the part is created by the geometry of the cutting tool. Form turning, drilling, and broaching are examples of this case. In these operations, illustrated in Figure 3.3, the shape of the cutting tool is imparted to the work in order to create part geometry.
Forming and generating are sometimes combined in one operation, as illustrated in Figure 3.4 for thread cutting on a lathe and slotting on a milling machine. In thread cutting, the pointed shape of the cutting tool determines the form of the threads, but the large feed rate generates the threads. In slotting (also called slotmilling), the width of the cutter determines the width of the slot, but the feed motion creates the slot.
Machining is classified as a secondary process. In general, secondary processes follow basic processes, whose purpose is to establish the initial shape of a workpiece. Examples of basic processes include casting, forging, and bar rolling (to produce rod and bar stock). The shapes produced by these processes usually require refinement by secondary processes. Machining operations serve to transform the starting shapes into the final geometries specified by the part designer. For example, bar stock is the initial shape, but the final geometry after a series of machining operations is a shaft.
Figure 3.3 – Forming to create shape in machining: (a) form turning, (b) drilling, and (c) broaching.
Figure 3.4 – Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling.
References: Gen [1-3].
Assessing questions:
What the difference between rotational or nonrotational machined part?
How can be a part shape created by generating?
What is the features of creating a part shape by forming?
Common things in straight turning and taper turning.
What is broaching?
Lecture 4. Coordinates systems and points. CNC machine motions.
World Axis Standards
There are nine standard axes universally used in CNC machining. Three are the familiar primary linear (straight-line) movements X, Y, and Z. Three primary rotary axes (A, B, and C) are used to identify arc or circular movements such as a programmable turntable, lathe spindle, or an articulating, wrist action milling head (rotary motion). Finally, we have three secondary, straight-line motions called the auxiliary linear axes (U, V, and W).
Figure 4.1 - The three primary linear axes of X, Y, and Z.
Three Primary Planes
Combining any two primary axis lines defines a flat plane. There are three planes: X-Y, X-Z, and Y-Z (Fig. 4.2). For example, when viewing a part placed on a vertical milling machine, the table represents the X-Y plane, while a lathe object is viewed in the X-Z plane - usually from overhead.
When the machine control is capable of cutting curves in more than one of these three discrete (unique) planes, the programmer must add a code word to define in which plane the motion is to occur.
Figure 4.2 – The three primary planes X-Y, X-Z, and Y-Z.
Axis ID on a CNC Machine
When facing a new machine for the first time, the world orientation of its axis set (relationship to the floor and to the operator) can often be identified this way, in this order (Fig. 4.3).
Figure 4.3 - The primary axes as they apply to three familiar machines
Z – The axis parallel to the main spindle;
X – Usually the longest axis, usually parallel to the floor;
Y – The axis perpendicular to both X and Z.
Use the right-hand rule for a mill in a shop (Fig. 4.4). Depending on the world perspective of the axis set, you may find your hand in any position, but it will be found to fit the rule.
Figure 4.4 – The right-hand rule helps identify the machine axes.
The most common example of skewed world orientation, shown in Figs. 4.5 is a slant bed lathe, where the X-axis has been tilted relative to the floor. This modification makes chip and coolant ejection more efficient and improves operator access for setting up tools.
Figure 4.5 – A slant bed lathe features a tilted X axis relative to the floor to improve chip ejection and operator access to tools
The Primary Rotary Axes A, B, and C
Some CNC machines feature programmable axes that rotate or articulate. According to the EIA267-B standard, there are three primary rotary axes:
A, B, and C.
Each is identified by the central primary linear axis around which it pivots.
A axis rotates around a line parallel to X
B axis rotates around a line parallel to Y
C axis rotates around a line parallel to Z
Mill Rotary Axes
When angled or warped surfaces are required, but they cannot be cut with standard milling equipment, we turn to rotary axis machines. There are two ways a CNC mill might employ a programmable rotary axis: by rotating the part or by rotating the cutter head, or a combination of the two.
Articulating Mill Heads. These machines are equipped with a spindle head that can rotate in one or two planes during a cut (A or A 1 B). The articulation is similar to the two Bridgeport type mill head rotations, but here it has empowered to feed in an arc (Fig. 4.6).
Plus or minus rotary motion – Rule of Thumb
To define which direction a rotary motion is to occur, clockwise or counterclockwise, we use a plus or minus sign on the coordinate.
Rule of thumb – rotary axis sign value. To identify whether the rotary axis direction is positive or negative (clockwise or counterclockwise), use the rule of thumb. First, identify the positive direction for the central axis around which the rotation occurs (1X, 1Y, or 1Z). Then, pointing the thumb of your right hand along that positive direction, your fingers will curl in the positive rotary axis direction. Negative rotary motion would be against your fingers.
Figure 4.6 – A five-axis mill features two articulating motions – A and B.