Motions of cnc machine
There are some basic terms, which need to be clarified. If it is necessary to create an arc in two or more axes by machining with specific feed rate is called the circular interpolation. When entering data surpass controller limits or axis the default (fallback number) can appear. To machine in one or multiple axis in a straight line with specific feed rate is called the linear interpolation. To do effective position travelling it is used rapid travel (rapid), where a machine produces the fastest axial speed. Before rapid positioning safe position for a tool, which is toward or backed away from a work, is called, retract height (R). Machining a circle by using two axis by one time is controlled by 2-D control function.
During linear interpolation in the 3rd axis the circular interpolation in two axis can be done by using 2 ½ -D control. The last function is 3-D control. The main idea of this function is that at a specific feed rate to produce circular interpolation.
Lathes and mills have 4 ways in tool motion:
1) A machine can produce the fastest speed by using rapid travel. It is effective way to move or reposition a tool to cut or to change it. The range of speed varies from a slow 25 meter per minute (MPM) on training lathes to the fastest 250 MPM on factory machines. During rapid travel tool enters in one-speed, which means velocity does not specified.
Rapid Height or Distance Off. When you plan a program, it needs to be defined the closest rapid approach movement to the specimen. From the specimen distance to the tool feed rate needs to be decreased. Clamps, vise, chuck jaws, and other holding tooling considers the safe distance. As an advise, stop cutting tool rapid travel when it achieves 12 mm from the specimen surface. For industrial case it is not normal but safe.
Retract height on milling machines (R) is a safe travel height above the specimen/tool. Here tool need to be pull back numerous times then move over the specimen surface as in drilling a series of holes. In terms of lathe, it also called the departure point in certain commands. The tool must retract back to a safe starting place before taking another pass. Retract moves away from the specimen are usually made at rapid travel.
Linear and Nonlinear Rapid. The path a CNC machine during rapid travel has two versions according to the microprocessor’s speed in the controller (Fig. 2.1). To know the path of the machine travel is crucial. The cutter can run into the part or the holding tools, if the odd path goes unaccounted for in a program.
Figure 2.1 – The difference of older controls corresponding to true straight-line path from point to point in rapid travel mode.
In the illustration, two controls reposition the tool from A to B at rapid speed. Note the difference between the slower CPU dash line and the newer solid line. The slower CPU cannot output motor control data fast enough to portion the two drive motors to move in a straight line from A to B in rapid travel.
When given a rapid command, the older control rotates both axis drive motors at their full speed until one of the axes reaches its destination, producing a 450 angle for a distance. Then the other axis continues on at full speed to its destination.
The faster CPU control portions out the motor control speeds such that both X and Y arrive at the destination at the exact same time—producing a straight line from A to B. That true linear movement requires more computer speed.
Linear Interpolation—Straight-Line Motion at Feed Rate G01
Linear motion occurs at the specified feed rate in the program. Like twiddling the dials on an Etch-A-Sketch® or both X and Y handles at differing rates on a mill, you could almost produce the diagonal line shown in Fig. 2.2. The result would not be very smooth or accurate. Additionally, achieving the desired feed rate would be an impossible challenge.
As in Fig. 17-42, when the CNC machine is commanded to move in linear interpolation, one, two, or three axes must move at coordinated speeds to arrive at the destination, in a straight line, at the tool feed rate specified in the program.
From the drawing you can see that when more than one axis motor is driving the tool, each must be turning at some subrate below that which is specified. Their combined effect produces tool movement at the specified rate. This is known as interpolation (interpolation means to find a value between two others). For example, to linear interpolate a straight line at 200 going from A to B, the Y axis must rotate at 36.3 percent of the X axis speed (the tangent ratio of the line’s slope).
Figure 2.2 – The combined action of differential X and Y feed rates creates the linear path at the programmed feed rate.
If the programmed feed rate is 400 mm per minute (mm/M); for example, the X axis must drive at 375.87 mm/M while the Y axis turns at 136.81 mm/M. The CPU controls each axis drive to achieve the result.
Feed Rate Override Similar to rapid movement, feed rate movements can be overridden by the operator to fine-tune the cutting action. But there’s a difference between rapid and feed override control. The usual adjustment range for feed rate is from 0 percent up to 150 percent of the programmed rate. Unless the programmed rate is near or at the machine’s fastest rate already, the feed can be increased above the programmed rate.
Circular Interpolation: G02 Clockwise, G03 Counterclockwise
Circular interpolation is the next higher CNC motion. It produces a full circle or a part of one (arc), at the programmed feed rate. Circular motion adds an additional challenge for the computer beyond linear interpolation. When producing a true circle, the control must continuously change the ratios between the drive motors for each increment of arc.
Each axis is interpolating below the target feed rate, but their combined action creates the correct tool velocity. Keep in mind that in addition to the axis motor control calculations, the CPU is also comparing resultant feedback to keep everything on track (Fig. 2.3).
Figure 2.3 – By constantly varying the X and Y rates, a true circle can be produced at the programmed feed rate.
Default Rates During circular interpolation, due to the high number of calculations per second, some older (slower CPUs) must limit feed rates to a maximum default rate.
These controls cannot produce circles at the same speed as straight lines. For example, an older control can perform linear interpolation at 200 IPM but must slow to 140 IPM for arcs. Even though the program is at a higher rate, the control will fall back or default during the circular movement. Newer 32-bit (or higher) CPUs have no such limits until extreme feed rates are encountered.
The term default will be used elsewhere in CNC. It means a fallback or preset bias built into the control. A default also occurs when a choice must be made by the control when other information is not available.
Axis Combinations for Milling Machines
CNC lathes can perform the tool motions of linear and circular interpolation as already described in the primary plane of X-Z only. This discussion is about mills only. Three-axis milling machines are further categorized by their ability to perform various circular moves:
Two-dimensional motion
Three-dimensional motion
Two-and-one-half-dimensional motion
The difference is significant.
Two-Dimensional Motion. The lower level 2-D control is restricted to moving two axes while producing a circle. That means arcs in the primary planes of X-Y, X-Z, or Y-Z only—one plane at a time.
The program must include the command G17 to indicate circular interpolation in the X-Y plane, G18 to produce a circle in the X-Z plane, and G19 for Y-Z circular interpolation.
Three-Dimensional Motion. A 3-D mill requires a fast processor or very slow feed rates. It must interpolate and drive three axes simultaneously to produce arcs. Perfect circular tool motion involving X, Y, and Z axes at the programmed feed rate is a great challenge for the processor. 3-D controls are not common in industry.
Two-and-One-Half-Dimensional Controls. Most controls today fall between the preceding two, producing circular motion in one of the primary planes X-Y, X-Z, or Y-Z, while moving the third axis in a straight line at the same time. That motion, called a 2-1/2-D move (two axis circular while one moves linear), produces a spiral similar to a thread – always parallel to a primary axis. Machines with 2-1/2-D control are able to machine the spiral ramp pass.
CAM-Generated Shapes
Because most machine controls are limited to linear and circular motions, CAM software cannot simply generate an irregular surface. Instead, the CAM software breaks the surface into tiny arcs or straight lines, then links them together to approximate the contour.
The length of these approximation lines is defined by a set tolerance away from the perfect shape, and how steep the curvature is. Advanced software such as Mastercam, our example CAM program, can reprocess the program once it has been generated to create more efficient arcs that fit the shape better, but there is still a lot of data compared to a true 3-D process.
However long the data might be, the CAM process for complex shapes is the method of choice today. Down to the microscopic level the difference between true 3-D generated shapes and CAM-approximated is indistinguishable. Additionally, using CAM, the CNC machine is far less costly and complex.
Polar Coordinates
The two-dimensional polar universe is a series of concentric circles with one radial line extending out, used for the angular reference. Distance out from the origin determines the radius ordinate, while angular displacement from the polar reference line (PRL) completes the pair of coordinates. Polar coordinates are expressed as R, A (radius, angle).
Absolute Polar Coordinates
If the central reference is the PRZ point for the part geometry and the angular reference line is horizontal (zero degrees), then the coordinate is an absolute value polar coordinate.
For example, in Fig. 2.4, point A is absolutely identified in the flat plane as:
R7.16 A52.00
That coordinate identifies point A as unique. Many (but not all) CNC controls can act on it as reference or geometry, as easily as a Cartesian value.
Figure 2.4 Some drawing dimensions are polar rather than rectangular.
Incremental Polar Coordinates
Sometimes the point cannot be absolutely identified from the PRZ but it can be usefully located using another known point as a reference. If the radial and angular dimension is local, then an incremental polar coordinate can do the job.
In Fig. 2.4, point B is identified using the center of the radius as its origin and the radial line to point A as its angular reference.
Figure 2.4 – Polar motion clockwise is negative, counterclockwise is positive.
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