The Next Generation of Mechanical CAD/CAM Software
Source: Amir Lapid
Throughout my years in the field of CAD/CAM (Computer Aided Design and Manufacturing), and specificainit where metal machining applications are involved, there was always the question of the direction that Mechanical CAD might take. What does the future hold for CAD/CAM? What more can be achieved with the help of this facility? Can further savings and efficiencies be applied to metal machining? And most important of all - have we reached our developmental limits with respect to the tools of this technological area?
We will attempt to confront these questions within the scope of this article, and we intend to raise further thoughts and ideas on "the next generation" of CAD/CAM.
The history of mechanical CAD/CAM is extremely short compared to the ancient history of metal working, which was seen as a prime necessity for the very existence of primitive man. In prehistoric times, man had to use stones, or a variety of hammering devices to process the iron which was available to him, for the manufacture of weapons and utensils for his mere survival.
The need for high precision metal working arose at the beginning of the last century, during the development of the automotive and aviation industries. The essential need to integrate moving parts forced manufacturers to achieve high degrees of precision and accuracy if any design was to be made into a tangible, working product.
Initially, designs were made on paper, with pencils and some mathematical equations for the calculation of strength, but as machines became more complicated, resistant and sophisticated, and above all safer, and in view of developments and the expanded options provided by computer programs, advanced computation software was developed to cope with design tasks. This move carried with it a significant upgrading of the required precision requirements, which, for mechanical machining, have now reached micro-millimeter and even nanometer dimensions - we have frequently encountered the rejection of parts by QC inspectors due to a deviation of a single micron from a specified tolerance - and manufacturing must start all over again.
The history of mechanical CAD/CAM is extremely short compared to the ancient history of metal working, which was seen as a prime necessity for the very existence of primitive man....
The need for high precision metal working arose at the beginning of the last century, during the development of the automotive and aviation industries. The essential need to integrate moving parts forced manufacturers to achieve high degrees of precision and accuracy if any design was to be made into a tangible, working product.
Cimatron's comprehensive CAD/CAM software for the tooling industry, provides several methodologies, allowing utilization of the machine's HSC capabilities:Currently, as design programs become more advanced, the need arises for "stronger" manufacturing programs, with higher technological capabilities. In order to enable the manufacture of a part which has been designed by an ambitious engineer, who has seen his idea with his own eyes as a finished product on his computer screen - most design engineers are unable to comprehend the significance of implementing their design through a metal machining process. Manufacture by metal machining is in many cases, a highly complex procedure which cannot always be achieved.
The utopian dream of every owner of a metal machining plant is that only a few minutes elapse between receiving approval to manufacture and deliver a finished product, packed and ready for shipping, after having passed through the lengthy process of design, manufacture, chemical finishing and final inspection, in order to proceed to collecting payment from the customer. Some companies try to bridge the existing gap with CAD/CAM software programs, which are able to substantially accelerate design time (the time that elapses from approval of a part intended for manufacture until the start of the machining process), and to shorten processing time on the machine.
High Speed Cutting
The most advanced technology currently available for CNC (Computer Numerical Control) machines is HSC (High Speed Cutting) - i.e. high speed metal machining, characterized by the machine's ability to operate at high speeds, while maintaining a uniform speed between work piece and tool - no stoppages or slowdowns, proper cooling of the tool while dispersing shavings, and a "smart" computer capable of reading and processing tremendous amounts of G-Code1 lines at high speed while "looking" at the on-coming program in order to maintain a smooth and continuous motion - without stopping and at the correct cutting speed, as defined by the tool manufacturer, in order to obtain the best possible surface quality.
1. Intelligent utilization of an "optimizer", which applies a uniform load to the cutting tool at all stages and for as long as it engages the material. This is done by ensuring that within each machining stage, the program remains aware of the amount of material remaining on the work piece2 after the last pass, and the volume of raw material on the tool during contact between it and the work piece. The operator defines the preferred machining data (as defined by the manufacturer) which serves as a relative reference marker between shavings volume and feed rate3. Accordingly, Cimatron E's software calculates the new machining speed for every given moment, according to the amount of raw material being removed by the tool, e.g., if the tool is overloaded (100% of tool diameter), the program will slow the feed rate down; if the load percentage is only partial, the feed rate will be increased. This option allows the machine operator to maximize the capabilities of both tool and machine, thus enhancing and significantly shortening processing times.
2. Another option - without changing the machine's feed rate, is when Cimatron E identifies excessive shavings load on the tool, and it modifies the cutting tool's motion to trochoidal4 mode - i.e., linear forward feed with a spiral motion of the cutting tool on the XY plane - allowing the operator to maintain maximum feed rate without risking tool breakage that may be caused by overloading. The possibility to work at high speeds with low machining loads on the tool extends its life, raises machine efficiency and shortens machining times.
3. An additional and possibly the most important option, is the elimination of sharp directional change movements. For example: any transition from motion along the X axis to motion along the Y axis entails a slowdown of the machine on the X axis, a momentary halt at the rotation point and a reacceleration to feed rate velocity on the Y axis. It emerges that these velocity changes with associated decelerations and accelerations are highly significant in respect to increased processing times for the parts being machined. Here, Cimatron E cancels out any sharp motions and rounds off all corners, obviously taking the angle of change into account, which allows the machine to operate at a uniform tool-work piece speed, thus reducing machining times and achieving better surface quality than with regular machining.
5-Axis Machining
An additional technology which has recently found its way into smaller machining plants (equipped with 3-6 CNC machines), is computerized machining with 5-axis milling machines. This phenomenon is the result of the lower prices of these machines, relative to the recent past and powerful CAD/CAM capabilities, enabling high-speed, accurate programming for the 5-axis environment.
This technology allows near-final machining within a single clamping5 cycle, and when the part is removed from the machine, only one side remains for further machining.
The above capability provides for shorter machining time, enhanced precision and better ratios between the various machining cycles located on spatial machining planes.
Cimatron E provides on-going work capabilities on five axes simultaneously, while taking account of the location of the work piece on the table (it is possible to position the work piece not at the center of the rotation axis, and the software will automatically calculate its precise position, with no need to define "home"6 for each machining angle), and taking into consideration the length of the cutting tool relative to the collet7 and the spindle8. Cimatron E applications are responsible for the inclination of the machine head (the spindle) or the table, so that a safety range is maintained between them and the work piece.
Work with a 5-axis machine allows surfaces to be processed at a high level of accuracy, with enhanced surface quality and shorter processing times, compared with 3-axis machines. The ability to continuously change the preferred angle of the tool relative to the work piece provides two machining options:
1. Milling a surface with a flat tool and larger lateral increments than when working with a ball nose tool. Here, Cimatron E ensures maximum perpendicularity between the base of the cutting tool and the work piece. This mode is especially effective for convex surfaces.
2. Processing a surface with a ball nose tool entails small lateral increments. The significance of which is that many milling strokes are required to achieve the required surface quality. Cimatron E controls the work of the cutting tool at the point of contact, in order to prevent working with the base of the tool. The software ensures that the tool will not be perpendicular to the work surface, because cutting speed at the tool's center is zero and work with the base of a ball nose tool results in poor surface quality. It is thus highly important that the point of contact be located at the side of the ball nose tool. This point of contact capability is available only with 5-axis machines working simultaneously.
Processing Remaining Material
The progress of 3-axis milling technologies is directly affected by the rapid development of CAD/CAM software programs. In the past, when machining a three dimensional surface, operators were required to use a manual calculator in order to solve the appropriate equation, write it in G-Code, and calculate the coordinates for the machine's action. Even though the program was very short, a skilled software programmer and a great deal of time were needed in order to arrive at the correct equation. Any change in the surface necessitated a new and tedious mathematical calculation.
Today, when surfaces are provided as mathematical models directly from the designer, the CNC programmer needs no advanced mathematical skills. He simply allows Cimatron E to run all the necessary calculation work. Machine downtime is thus minimized, and the machine no longer stands idle, waiting for the programmer to finish writing and running the code. Consequently, it is possible to immediately introduce modifications to the product. Although the program is a long one, this fact is insignificant, as the machine controller is capable of receiving and downloaded a program of almost unlimited length.
The "smart" CAD/CAM software also leaves its mark on the processing area of work on a three dimensional object where the correct sequencing of operating tools is of considerable importance. It would not be wise to work with the same tool for both coarse (preliminary) milling and final (fine) milling.
Cimatron E provides an option for executing the work piece in various stages, with different cutting tools, taking into consideration the material which has been left behind by one tool for the next. For example: the first tool (20 mm crusher) is used to initially remove material - coarse machining with large increments of 0.2 mm off the final dimension. This tool is incapable of reaching all the corners of the part, and it leaves rough steps - which is why we will then use a second tool (10 mm crusher) and continue the removal of material in small increments with the same offset of 0.2 mm off the final dimension.
In view of the fact that Cimatron E is "aware" at each stage of the process of the amount of material which was removed and the amount of material yet to be removed, it will generate work strokes for the second tool, but only in those areas where raw material has been left for removal, thus preventing unnecessary free strokes by the tool. As a consequence of such preliminary "smart" machining, a uniform offset of around 0.2 mm off the final dimension is left for the finishing tool, so that the forces applied to it are uniform, and tool wear is minimal and its lifetime is extended.
When machining parts with three dimensional surfaces, the software enables the utilization of several cutting tools with identical technology - usually by ball nose cutting tools, or flat tools with corner radii - such that the operator selects the optimal sequence of tools preferred by him (from larger to smaller). Cimatron E optimally calculates tool strokes, while giving maximum consideration to the amount of raw material remaining from the previous tool, and the ability of the current tool to remove such remaining material.
This powerful capability provided by Cimatron E, provides the machine operator with maximum flexibility in manufacturing a part, even when changes occur during programming. For example: a modification to the original design, no suitable cutting tool available at the plant, the need to manufacture similar parts but with different dimensions, etc.
Summary
This article has reviewed the latest developments in the fields of mechanical CAD/CAM. The question still to be answered is: have we reached the limits of development within this area? I am unable to answer this question, as the changes occurring in machining tools and the machining field, are revolutionary and on-going.
Allow me to present a view of "next generation" mechanical CAD/CAM as I foresee it:
Following the planning and design of a product, a (solid) mathematical model is submitted to the manufacturing department. The "next generation" CAD/CAM software then slices the model into horizontal sections, each one millimeter thick. From these layered sections, a "super printer" constructs the part from the inner layer outwards, layer upon layer (similar to the Stereo Lithography method, the 3D model is manufactured from any polymeric material).
Within the scope of this technology it is possible to manufacture any part, in any shape, even if metal machining is incapable of manufacturing such a part using currently available technologies.
Such "printing" has several significant characteristics: the transition from model to start of production could be achieved within minutes, parts could be made of any material, including types of various hard metals and/or such as could not be manufactured by metal machining, production speed would equal the speed of plastic extrusion into a mold (seconds per lot), multiple parts could be manufactured within a single lot, and superior surface quality of N1-N39 could be achieved. The characteristics of the material would not be altered, and we could possibly even improve some of them (hardness, strength, fiber orientation, etc.), preferred external coating, and above all - the accuracy of all parts would be as required and identical for all.
So, is the "next generation" CAD/CAM a figment of the imagination? In view of presently on-going developments, it seems that the vision is already becoming a reality.
Glossary of Terms
- G-Code - Metal machining programming language. The file is created by CAD/CAM software and differs from one machine to the next.
- Work piece - the part being processed by the machine.
- Feed rate - the feed velocity of the machine in relation to the work piece, in mm per minute (mm/min).
- Trochoidal motion - forward feed with a spiral motion, a kind of movement consisting of successive circles.
- Clamping - holding down the part at the required angle, allowing maximum machining with no interference to the cutting tool.
- Home - coordinates' point of origin as calculated by the program, axes' temporary point of origin, where X=0, Y=0, Z=0.
- Collet - a cutting tool holding device. Cutting tools are secured by several types of devices: bolt or clamping nut, shrinkable clamping (heating, insertion then cooling), or a collet which is an inseparable part of the cutting tool.
- Spindle - the collet housing, driven by a motor which is connected directly or through a transmission consisting of gears and belts.
- N3 - method for marking surface quality. The allowed depth of surface roughening as defined on a scale of N1 (better than 0.025 micron) to N12 (the lowest - 50 micron).