4 LABORATORY - AXIS AND MOTION CONTROL
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14. CNC MACHINES
• Computer Numerical Control machines use a computer to guide a process that might otherwise be done manually.
14.1 MACHINE AXES
14.2 NUMERICAL CONTROL (NC)
• The use of numerical data to drive a machine for processes such as,
- milling
- turning
- drilling
- grinding
- shot peening
- tube bending
- flame cutting
- automated knitting machines
- automatic riveting
- etc.
• Basic components of NC systems,
- program
- controller unit
- machine tool
• Most suited to,
- parts are processed frequently in small lot sizes
- complex part geometry
- close tolerances on workpart
- many operations on part in processing
- large amounts of metal to be removed
- engineering design will possibly change
- parts that are too expensive for mistakes
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• The methods for developing NC programs include,
- manual part programming
- computer-assisted part programming
- computer generated programs
• The manual and computer aided methods use various NC programming languages,
- APT (Automatically Programmed Tools)
- AUTOSPOT (Automatic System for Positioning Tools)
- SPLIT (Sundstrand Processing Language Internally Translated)
- COMPACT II
- ADAPT (ADaptation of APT)
- EXAPT (Extended Subset of APT)
- UNIAPT
• These languages are used by a parts programmer to define the motion of the cutting tool.
• The languages may be preprocessed, and then used for a number of various control types,
such as,
- punched paper tape
- Computer Numerical Control (CNC)
- Direct Numerical Control (DNC)
• The automatic methods work with geometry created in a CAD program.
14.2.1 NC Tapes
• NC Programs are preprocessed on computers, and punched onto paper or mylar tapes.
• Simple NC machines can use a tape reader to direct the machine.
• Problems,
- required storage, transportation, and manual loading of NC tapes
- has to reread the tape for each new part
- tapes tend to wear, and become dirty, thus causing misreadings
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- the readers are slow, and can cause ‘dwell marks’ on complex pieces
- the mechanical parts in the readers reduced reliability
- testing had to be done on the NC machine
- no program editing abilities (increased lead time)
• The end of tapes was the result of two competing developments
- DNC used remote computers to replace tape readers, these were displaced in most cases
by CNC
- CNC allowed the use of a local computer to overcome problems with tapes, and the
problems with distant computers. While CNC was used to enhance tapes for a
while, they eventually allowed the use of other storage media, and currently program transfer media are not required.
14.2.2 Computer Numerical Control (CNC)
• A computer controller is used to drive an NC machine directly.
• Characteristics are,
- controls a single machine
- located very close to machine tool
- allows storage/retrieval/entry of NC programs without preprocessing of NC code
• Advantages of CNC,
- program is only entered into memory once, so it is more reliable
- the programs can be tested and altered at the machine
- increased flexibility and control options on the local computer
- easy to integrate into FMS systems
• The Background,
- the problems with NC tapes were approached using DNC networks
- the communication problems with DNC systems became obvious, and local computers
were added to act as tape readers which would read tapes once, and play them back
to the NC machine indefinitely
- CNC controllers began using other storage media like magnetic tapes, and floppy disks
- CNC now offers features like,
- local programming,
- communication over interfaces,
- hard disk storage,
- program simulation
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- etc.
• ASIDE: Direct Numerical Control is similar to CNC, except a remote computer is used to
control a number of machines. This gives the advantage of more computer power. This approach
is no longer popular, as the dropping cost of computers offsets any advantages.
• Some companies use proprietary NC Languages, such as the example of DYNA Mill NC
code shown later
• These machines are often programmed by downloading NC code from a computer, or manually programming the controller computer.
• Future trends involve,
- adaptive feed rates to increase speeds as the metal removal rate varies
- tool wear detection
-
14.2.3 Direct/Distributed Numerical Control (DNC)
• Uses a few methods,
- the oldest methods used modems, and a mainframe which emulated a tape reader, to control the NC machine (no storage)
- a more recent advance used a local computer which acts as a storage buffer. Programs are
downloaded from the main DNC computer, and then the local controller feeds
instructions to the hardwired NC machine, as if they have been read from tape.
- the newer methods use a central computer which communicates with local CNC computers (also called Direct Numerical Control)
• DNC controllers came before CNC machines, but as computer technology improved it
became practical to place a computer beside the NC machine, and DNC changed in form.
• Characteristics of modern DNC systems are,
- uses a server (with large storage capacity) to store a large number of part programs
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- the server will download part programs on demand to local machines
- may have abilities to,
- display and edit part programs
- transmit operator instructions and other data needed at the machines
- collect and process machine status information for management purposes
• Advantages are,
- eliminates the need for NC tapes (the advantages are obvious)
- design changes are immediate
- NC programs may be edited quickly
- can be used to support an FMS system
- increase efficiency of individual machine tools
- more shop up-time than with stand alone machines
- simplifies implementation of group technology, computer aided process planning, and
other CIM concepts
- reduces peripheral costs with NC tapes
• A Brief History,
• Mid 60’s
- concept proved by Cincinnati Milacron and G.E.
- telephone links used to send instructions from large computers to hard wired NC
machines. Basically replaced a tape reader.
• 1970
- several commercial DNC systems announced.
• Mid 70’s
- Aerospace companies used DNC because of the large number of distributed
machines in their facilities.
• Initial resistance to DNC technology was (previously) based on,
- high cost of computer hardware
- the number of machines which could be controlled by one computer was limited
- computer software was limited for maintenance, scheduling, control, and data collection
- a backup computer was usually required
- was hard to justify on the basis of downloading parts programs
• when downloading programs there are two popular opinions,
- a program should only be downloaded in part, this accommodates easy engineering
changes in a real-time environment.
- many programs should be downloaded to the local controller to provide protection
against system failure, and eliminating the cost of real-time response in the DNC
central computer.
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14.3 EXAMPLES OF EQUIPMENT
• The number of NC machines available commercially will be well into the thousands.
14.3.1 EMCO PC Turn 50
• This is a small desktop lathe capable of turning parts in metal.
• The basic physical specifications are,
Cutting Volume
radial travel 48mm rad.
axial travel 228mm
Max. Holding Volume
radial 30-65mm
axial 300mm
12mm by 12mm
Max. Tool Size
max 80mm dia.
Chuck
130-3000 rpm
Spindle
0.001mm
Resolution
0-750 mm/min
Feed
<=600N below 500mm/min
Feed Force
100/110/230VAC, 0-6KVA
Power
840 by 695 by 345 mm
• The basic sequence of operations for this machine are,
1. Unpack components.
2. Connect devices to power, air supply, and attach interface cables.
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3. Install RS-485 card in PC.
4. Install software.
5. Test basic system (Done initial setup here).
6. Start and initialize lathe and PC with software.
7. Setup tools for new job. Find zero positions/offsets, and enter values for turret.
8. Load NC code.
9. Simulate program.
10. Load stock and close automatic chuck.
11. Close door.
12. Run program on Lathe.
13. Open door and open chuck.
14. If cutting a similar part go to step 8, if doing a new setup go to step 7.
14.3.2 Light Machines Corp. proLIGHT Mill
• This is a small desktop lathe capable of turning parts in metal.
• The basic physical specifications are,
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Cutting Volume
Max. Holding Volume
Max. Tool Size
Spindle
Resolution
Feed
Feed Force
Power
Dimensions
Weight
Controller
Control Interface
Programming
Spindle
200-5000 rpm
50ipm x,y and 40ipm z
IBM compatible computer
IBM compatible computer
G-Codes and Dos software
1 H.P.
• The basic sequence of operations for this machine are,
1. Unpack components.
2. Connect devices to power, air supply, and attach interface cables.
3. Install software.
4. Test basic system (Done initial setup here).
5. Start and initialize mill and PC with software.
6. Setup tool for new job. Find zero position/offset.
7. Load NC code.
8. Simulate program.
9. Run program on Mill.
10. If cutting a similar part go to step 7, if doing a new setup go to step 6.
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14.4 PRACTICE PROBLEMS
1.
14.5 TUTORIAL - EMCO MAIER PCTURN 50 LATHE (OLD)
• The lathe is shipped with software that is meant to emulate shop floor interfaces. We don’t have
the standard keyboard, so we need to use special key stroke sequences on the PC keyboard.
• Procedure:
1. Connect the air supply to the lathe and make sure that the regulator on the lathe is
between 25 and 75 psi - 50 psi is good. Ensure that the lath is connected to the PC
with the DNC cable. The computer card must also have a terminator on the second
connector - this is an empty connector. Turn on the lathe, and the PC.
2. Once the PC is booted, run the emco control software. The screen may come up with
warnings. If these warnings don’t disappear when you hit ‘ESC’ call the instructor.
3. First we must zero the lathe. To do this first hit ‘F1’ and then ‘F7?-ZRN’. A small label
‘ZRN’ should appear near the bottom of the screen. Press ‘4’ on the number pad of
the keyboard - the lathe should move in the ‘x’ direction. Next, press ‘8’ on the
keyboard, the lathe should move in the ‘z’ direction. After all motion has stopped
the lathe is calibrated, and it will be put in jog mode.
4. You can move the lathe with the keys on the number pad as well as perform other function.
4 - move carriage left
6 - move carriage right
2 - cross slide out
8 - move cross slide in
+/- - increase/decrease feed
5. You can now put the mill in MDI mode by pressing ‘F1’ then ‘F6?-MDI’. Push the
door open and hold it for a second, it will then stay open. Clear the error on the
screen with ‘ESC’ and press
work piece and then close the door.
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6. Put the computer in program mode ----------------
14.6 TUTORIAL - PC TURN 50 LATHE DOCUMENTATION: (By Jonathan
DeBoer)
• SETUP:
The lathe is controlled by a computer through an RS485 port. RS485 is a serial data bus
that can be chained from one device to another and must be terminated.
The controlling computer must be running Windows 3.1 or 3.11 and must have the RS485
card installed. Windows 95 will not get along with the interface card, and the software refuses to use an RS232 port with an RS485 adapter. The machine should
have as few peripherals as possible; if one device happens to use any of the IRQs/
DMAs/IO ports as the RS485 card, there will be problems. So remove sound
cards, extra interface cards, etc. The RS485 card has two DB9F connectors on the
back, plug the cable from the lathe in one and a terminator in the other.
Install WinNC (the control software) under Windows 3.1. There are two disks; the
installer and a machine data disk.
The lathe needs to be plugged in to the computer, to a power outlet (of course), and to an
air supply at 50-75 psi (less than 50 and there isn't enough pressure to open the
door). A pressure gauge is on the left side of the machine, all plugs/etc are on the
right.
• POWER ON/OFF:
To Turn On:
Turn on the computer and machine. To turn on the machine, turn the key on the
right side. On the computer, launch Windows if neccisary. Once windows is
running, launch WinNC. Make sure NumLock is on before launching
WinNC. WinNC will then establish communication with the machine.
To Turn Off:
To just shut off the lathe but not the computer, just turn the key on the lathe. An
error will come up in WinNC indicating it lost RS485 communication. Not to
worry; when the lathe is turned back on later, hit ESC and the error will go away.
To turn off both, exit WinNC by hitting Alt-F4 and then exit Windows. Then
Simply switch off both the machine and the computer.
• OPERATION:
Some notes:
The EMCO software is distinguished by having the most counter-intuitive, unnatural, information-withholding, and ornery interface known to man. Most technical references available are in German.
The software periodically pops up error messages for minor and major errors.
