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About CNC Controllers

CNC controllers are devices that control machines and processes. They range in capability from simple point-to-point linear control to highly complex algorithms that involve multiple axes of control. CNC controllers can be used to control various types of machine shop equipment. These include horizontal mills, vertical mills, lathes and turning centers, grinders, electro discharge machines (EDM), welding machines, and inspection machines. The number of axes controlled by CNC controllers can range anywhere from one to five, with some CNC controllers configured to control greater than six axes. Mounting types for CNC controllers include board, stand alone, desktop, pendant, pedestal, and rack mount. Some units may have integral displays, touch screen displays, and keypads for control and programming.

Industrial communications options for CNC controllers include ARCNet, CANBus, ControlNET, Data Highway Plus, DeviceNet, Ethernet 10/100 Base-T, parallel, PROFIBUS, SERCOS, Universal Serial Bus (USB), serial (RS232, RS422, RS485), and web-enabled. Communications language choices include bitmap, conversational, DXF file, G/M codes, Hewlett Packard graphics language, and ladder logic. A bit map (often spelled "bitmap") defines a display space and the color for each pixel or "bit" in the display space. Conversational language is a higher level, easy to learn programming tool. It performs the same functions as the standard G-code commands. Drawing eXchange Format (DXF) file that was created as a standard to freely exchange 2 and 3 dimensional drawings between different CAD programs. It basically represents a shape as a wire frame mesh of x, y, z coordinates. G-code is the programming language for the Computer Numerically Controlled (CNC) machine tools that can be downloaded to the controller to operate the machine. M-code is the standard machine tool codes that are normally used to switch on the spindle, coolant or auxiliary devices. Hewlett Packard Graphical Language (HPGL) was originally created to send 2 dimensional drawing information to pen plotters, but has since become a good standard for the exchange of 2 dimensional drawing information between CAD programs. Ladder logic is a programming language used to program programmable logic controllers (PLC). This graphical language closely resembles electrical relay logic diagrams.

CNC controllers have several choices for operation. These include polar coordinate command, cutter compensation, linear and circular interpolation, stored pitch error, helical interpolation, canned cycles, rigid tapping, and auto-scaling. Polar coordinate command is a numerical control system in which all the coordinates are referred to a certain pole. The position is defined by the polar radius and polar angle. Cutter compensation is the distance you want the CNC control to offset for the tool radius away from the programmed path. Linear and circular interpolation is the programmed path of the machine, which appears to be straight or curved, but is actually a series of very small steps along that path. Machine precision can be remarkably improved through such features as stored pitch error compensation, which corrects for lead screw pitch error and other mechanical positioning errors. Helical interpolation is a technique used to make large diameter holes in workpieces. It allows for high metal removal rates with a minimum of tool wear. There are machine routines like drilling, deep drilling, reaming, tapping, boring, etc. that involve a series of machine operations but are specified by a single G-code with appropriate parameters. Rigid tapping is a CNC tapping feature where the tap is fed into the work piece at the precise rate needed for a perfect tapped hole. It also needs to retract at the same precise rate otherwise it will shave the hole and create an out of spec tapped hole. Auto scaling translates the parameters of the CNC program to fit the work piece.

Features common to CNC controllers include alarm and event monitoring, behind tape reader, diskette floppy storage, tape storage, zip disk storage, multi-program storage, self diagnostics, simultaneous control, tape reader, and teach mode.

Encoder interface closes the loop on control

The EnDat interface can transmit position values from incremental and absolute encoders as well as transmitting or updating information stored in the encoder, or saving new information.
Note: A free brochure or catalogue is available from http://www.heidenhain.co.uk on the products in this news release.
Digital drive systems and feedback loops with position encoders for measured value acquisition require fast data transfer with high transmission reliability from the encoders. Further data, such as drive-specific parameters, compensation tables, etc must also be made available. For high system reliability, the encoders must be integrated in routines for error detection and have diagnostic capabilities.
The EnDat interface from Heidenhain is a digital, bidirectional interface for encoders.
It is capable both of transmitting position values from incremental and absolute encoders as well as transmitting or updating information stored in the encoder, or saving new information.
Thanks to the serial transmission method only four signal lines are required.
The data are transmitted in synchronism with the clock signal from the subsequent electronics.
The type of transmission (position values, parameters, diagnostics etc) is selected by mode commands that the subsequent electronics send to the encoder.
The EnDat interface provides everything needed to reduce system cost per axis up to 50% - and at the same time improve the technical standard.
The most significant benefits are: cost optimisation, improved quality and higher availability.

Measurement technology is key to automation

Advances in machine accuracy, on-machine touch probing technology and noncontact tool setting provide powerful tools for automating and speeding mould machining, says Barry Rogers.
Note: A free brochure or catalogue is available from http://www.renishaw.com on the products in this news release.
Drives to faster, leaner, more flexible manufacturing are shifting industry focus away from traditional post-process quality control. The most expensive, non-value-added process in most shops is part inspection. Inspecting good parts - parts that meet all print specifications - is a waste of time, money and manpower.
Rather than back-end detection, attention is shifting to front-end prevention.
The aim is to make 100% good parts, right the first time, to ever-tighter tolerances in the lowest possible total processing time.
Under that mantra, a variety of practices and technologies are being applied to machine tools to achieve greater process control.
Automated process checks can keep process and parts in control, while minimising downtime for operator intervention.
These process control improvements can be particularly vital for mouldmaking.
The one-off nature of most mould/die work and the high accumulated value that can go into a complex mould demand right-the-first time processing.
At the same time, shorter lead times and global competition force the need for faster mould processing.
By minimising need for operator intervention, these process controls give mouldmakers an 'eye on the job' during long machining runs and lightly staffed second and third shifts.
Front-end prevention takes three forms: identifying and maintaining machine capability; in-process probing; and automated tool monitoring.
A technology leader in all three areas, Renishaw offers single-source expertise and assistance in creating an integrated programme of mouldmaking process control.
To move from defect prevention, you must be able to document your process capability and the accuracy of your machine tools.
To do this, inspect them to a nationally recognised and accepted standard, such as ISO230 or ASME B5.54.
Both call for a ballbar and laser interferometer to be used with a recommended procedure for checking machine tool accuracy.
The purpose of these standards is not to specify an accuracy the machine must meet, but to find out what accuracy level it can meet - its process capability.
The part print dictates the accuracy your machine must have to make good parts - where to set the accuracy bar.
Testing tells you how high your machine can jump.
As long as your machine can top the bar, you have process capability.
Test and calibration technology are now available - and affordable - to enable shops to ensure the accuracy and health of their machine tools.
Plants and large shops increasingly maintain their own laser interferometers and electronic levels, while rental equipment and diagnostics services are commercially available to small shops from various sources and competitively priced.
Renishaw's QC10 ballbar system is readily affordable by virtually any shop and provides a fast, 15-minute check-up for prevention and diagnosis in maintaining machine accuracy.
The ballbar test allows precise assessment of machine geometry, circularity and stick/slip error, servo gain mismatch, vibration, backlash, repeatability and scale mismatch.
Renishaw's Ballbar5 software provides diagnosis of specific errors in accordance with ISO230-4 and ASME B5.54 and B5.57 standards, then provides a plain-English list of error sources rank-ordered according to their overall effect on machine accuracy.
This allows maintenance people to target those factors which most need attention.
Periodic ballbar testing enables trend tracking of machine performance.
Preventive maintenance can be scheduled before a machine drifts out of process capability.
The industry trend is to calibrate the machine on need, not time.
There is no reason for maintenance to pull a perfectly good machine out of production for calibration.
Let the ballbar and the accuracy of your parts determine when something has gone awry.
Meantime, run production.
Today's standard machine tools can deliver accuracy and repeatability approaching levels formerly available only on CMMs.
This enables the machine tool itself to be used for probing checks of workpieces during critical stages of the machining process.
Once a machine tool's performance as a measuring instrument has been established, the touch probe becomes the operator's CNC gauge.
Probing routines can be programmed as part of the machining process and automatically run at various points to check feature dimensions and locations and apply necessary compensations.
This saves operators from using dial indicators and shim stock, or eliminates errors in manually entering fixture, part and tool offsets into the control.
Probing on the machine makes it part of the process - a powerful process improvement tool for making parts right the first time in the shortest throughput time.
Used to locate the part automatically and establish a work co-ordinate system, probing cuts setup time, increases spindle availability, lowers fixture costs, and eliminates nonproductive machining passes.
On complex parts, 45 minutes of fixture alignment can be replaced by 45 seconds of touch probing - performed automatically by the CNC.
When starting with a casting or forging, probing can determine workpiece shape to avoid wasted time in air-cutting and help determine best tool approach angle.
In-process control uses touch probing to monitor size and position of machine features during the cutting process, as well as verify precise dimensional relationships between various features at each step to avoid problems.
A touch probe can be programmed to check actual machined results at various stages against the program and automatically apply cutter compensation - particularly after rough machining or semi-finish machining.
Reference probing - comparing part features to a dimensional master or reference surface of know location or dimension - enables the CNC to determine positioning discrepancies and generate an offset to make up the difference.
By probing the artefact before a critical machining pass, the CNC can check its own positioning against the master's known dimensions and program an offset.
If the dimensional master is mounted on the machine and exposed to the same environmental conditions, reference probing can used to monitor and compensate for thermal growth.
What results is a closed-loop process requiring no operator intervention.
Every machine has its own set of numerous small errors in its motions and structure.
As a result, there is always a slight discrepancy between a CNC's programmed position and the true position of the tool tip, even after laser compensation has brought the two into closer agreement.
Programmable artefact probing provides a way to further compensate for remaining machine errors.
It gives process control feedback to enable positioning accuracy that can approach the machine's repeatability specification.
Such closed-loop process control can allow a machining centre to achieve accuracies comparable to boring mills and other high-precision machines.
Many probing operations are accomplished through the use of memory- resident macro programs.
Work co-ordinate updates, tool geometry changes, part measurement etc, are automatically determined by the CNC after the successful completion of a probing cycle.
This eliminates costly errors resulting from miskeyed information or incorrect calculations.
Used to inspect parts after machining, probing can reduce the length and complexity of off-line inspection, and it some cases eliminate it altogether.
Inspecting on the machine is particularly beneficial with large, expensive workpieces, such as mould or dies, which can be especially difficult and time-consuming to move.
Here, too, reference probing against a traceable artefact can be used to compare final dimensions to the known dimensions for a metrology master.
When making this comparison, the CNC can determine if the specific machining tolerances were actually achieved.
Based on these results, an intelligent decision can be made on corrective actions, while the workpiece is still on the machine tool.
Laser tool setters provide a fast, automated means to verify tool dimensions, especially critical in checking for wear during the long machining runs in mouldmaking.
A cost-effective solution to high-speed, high-precision tool setting and broken tool detection, laser tool setters rapidly measure tool length and diameter on-the-fly, while the tool is indexing through the laser beam and rotating at normal speeds.
Laser checking at working spindle speeds identifies errors caused by clamping inconsistencies and radial run-out of the spindle, tool and toolholders - not feasible with static tool setting systems.
Renishaw's NC family tool setters can perform broken tool detection at maximum traverse to further minimise out-of-cut time.
As the tool moves through the laser beam, system electronics detect when the beam is broke and issues and output signal to the controller.
The NC systems can accurately measure tools as small as 0.2mm diameter anywhere in the beam.
The system triggers when the laser beam is broken beyond a 50% threshold by the tool being checked.
The noncontact tool setting system uses a visible-red diode laser proven reliable in machining conditions.
Advanced electronics and simplified design makes noncontact tool setting an affordable alternative to contact systems.
No moving parts make NC systems virtually maintenance free.
The design avoids the brackets and actuators with contact-based systems.
Housed in a rugged stainless steel unit, the NC laser tool setters feature Renishaw's MicroHoleTM protection system.
This uses a continuous stream of compressed air to keep out contaminants and provide uninterrupted protection from chips, graphite and coolant ingress, even during measuring routines.
Three different Renishaw NC systems enable installation on nearly any size and configuration of machine tool without impinging on the work envelope.
These proven, affordable control technologies can allow greater automation of mould machining with greater process control.
They can make it possible for mouldmakers to produce moulds faster, with greater geometric and dimensional accuracy, and less operator intervention, rework or manual finishing. Request a free brochure from http://www.renishaw.com

High Speed Milling Machines

High speed machining is characterized by low cutting forces and high metal removal. High Speed Milling is a technique used in the CNC Machining Industry that combines high spindle speeds with increased feed rates. This results in a high chip-forming rate and lower milling forces, producing an improved surface quality finish and closer tolerances. In high speed milling, the electronics can make all the difference. The right CNC coupled with other elements of the control system can let a slower machine mill a given form faster than a machine with a higher top feed rates.
1. High Speed Uses
High-speed CNC milling is used, for example, to machine the titanium rotors of the first high-pressure compressor stages of the EJ200 engine. High speed CNC milling allows cost-effective milling of the different airfoil geometry from the solids. By subsequent finishing operations the planned surface finish is achieved. The CNC milling which caters to high speed must be structured with an axis movement system that is suitable for CNC machining.
2. Axis Movement
The high-speed CNC milling machines required for the process must be fitted with an axis movement system suitable for machining blisks, which should be at least 5 axes simultaneously, depending on the milling task involved and an efficiently high-speed control system.
3. 3D Surfaces
High Speed CNC milling machines working on 3D surfaces in any materials produce a finer surface finish and higher accuracy in less time that the traditional milling machine. Acceleration is the most critical factor that affects the high speed machining. Since one or more axis are always increasing or decreasing velocity in a 3-D cut, ultimate feed rate is directly related to acceleration
4. What Can A High Speed Control Possibly Do?
A CNC milling machine which possesses a higher structural stiffness has a greater potential acceleration rate. Box shaped high speed CNC milling machine, like Bridge and Gantry is the mostly widely used types of High speed CNC milling tools. The overhead type Gantry exudes the highest stiffness, acceleration and accuracy among other high speed CNC milling tools. Due to its scalability, this machine type is available in sizes to match the work piece, from small to large.
In usual terms, it simply gives you the ability to finish one task faster and move along to the next sooner, making work output higher. In drilling and tapping, this can result in faster hole-to-hole times, quicker spindle reversals for tapping, and substantial cycle-time reductions. The most dramatic benefits, though, come in 3D designs machining. Few, drilling and tapping jobs require a million lines of machine codes. In molds, dies, patterns, and prototypes, complex surfaces comprising a million or more line segments are not at all uncommon. Saving just a fraction of a second per move can result in substantial cycle-time improvements.
5. Downsides - When Is Fast Too Fast?
But despite all these benefits, in high milling, the tool path segments can be so short that a machining center moving at a high feed rate can't accelerate or decelerate fast enough to make direction changes accurately. Corners may be rounded off and the work piece surface may be gouged. Look-ahead is one answer. Look-ahead capability can let the CNC read ahead a certain number of blocks in the program, to anticipate sudden direction changes and slow the feed rate accordingly.
6. Additional Benefits:
- Improved accuracy - Better fit - Superior finish - Better life - Produce more work in less time - Improving the accuracy and finish - Reducing polishing and fitting time - Tools simply last longer because their chip load is more consistent.

Part Program Selection on a Fanuc Power Mate with a Selector Switch

Author: ControlOn

Synopsis:
Fanuc Power Mate (Models D, F) has an option called “Workpiece Number Search”. Using this option, the PMC can initiate the execution of a specified part program. This is especially useful in cases where the Power Mate is used to control special purpose machines that produce a known set of components. Such applications are found in batch production factories.

How it Works:
To synchronize the PMC and CNC functions, there is a communication area in the Fanuc Power Mate where the PMC & CNC exchange signals. These are a set of bytes and there are two areas namely:

a) The “G” area - signals from the PMC to the CNC

b) The “F” area - signals from the CNC to the PMC

By selectively loading G009 with a value in the range 1-255, the CNC will execute a program between O001 to O255.

To read the complete article please visit http://www.controlon.com/resources/default.asp

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