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Turning to CNC modeling.

If you need thousands of parts, a machine that can make part in three minutes instead of five is an attractive investment. you can make a part in one setup instead of three, cost per part will decrease and accuracy will improve. That's why machines that can do these things multiaxis lathes and mill/turn machines--are rapidly gaining popularity.

These improvements in CNC turning equipment were motivated by the need to increase productivity. Although most of their increased value results from making parts faster, some increased capability results from changes in machine configuration.

Anyone watching two tools cutting at the same time on a dual-turret lathe can appreciate the obvious time savings. But the more you watch these machines, the more apparent it becomes that these tool paths are intricate and complex. The thought is "How do you program this machine so that everything happens at the right time?" Add a C-axis milling tool to eliminate setups, and you have a formidable programming task.

Benefits of advanced turning

Four-axis lathes increase productivity by allowing two cutting tools to work at the same time. Both can cut an OD, or one an OD while the other cuts an ID. Twice as much material is removed in the same amount of time. Reductions in cycle time per part are between 30% and 40%.

Four-axis lathes give you the ability to do "pinch" turning: two tools working on opposite sides of the part at the same time, offsetting tool pressure to eliminate deflection in long, thin cylindrical parts. With live tooling in one or more index positions, lathes with C-axis capability can use a milling tool to cut slots, drill cross holes, or cut profiles wrapped around an OD.

The need for new programming

These new machines are part of a major trend in machining improvement. However, programming techniques have not changed appreciably since the first CNC machines were introduced. Programming has been done either with languages, manually, or with modified CAD systems. Computers have been used to speed up the same programming process that originated with those first CNC machines.

Because programming has not kept pace with machine technology, programming multiaxis lathes can be very difficult. This complexity has kept a wide range of potential users from investing in this technology or from realizing the full benefit of such an investment.

The programmer must know where the tools and toolholders are to avoid collisions and know that cutting is occurring at the correct time and place. This requires extensive and sometimes complex calculations and error-free code preparation. Worst of all, programmers must keep all this information organized in their heads because programming systems have no way to visualize the entire process. Most have not even attempted to directly address these machines.

The problem lies in the procedure that language-based or CAD-oriented systems use to produce code. The first step is to create part geometry. This is then chained into a profile, and a tool assigned to it. Offsets are calculated to produce a cutter-location (CL) file. This file is then back-plotted to show where the tool would be cutting. Unfortunately, this process does not give a precise representation of what would happen on the machine. Also, if any changes in tool path are needed, the complete routine must be repeated.

To get a more accurate back-plotted simulation of tool path, the user must first post-process the CL file to get the finished code, and then back-plot from that code. This adds yet another step to the language-code generation process. Add to this the need to create a tool path for two tools, calculations for them to operate at the same time, and merging these two codes. Programming systems were not created with this situation in mind.

For the C-axis machine, tool paths for turning and milling tools need to be created in separate modules--one for lathes, one for mills. These must then be combined, usually in a text editor. It is not possible to see a visualization of the complete tool path with both tools operating.

Process modeling

Fortunately, these time and quality problems can be corrected by interacting directly with tool path using a capability known as CNC process modeling. At Point Control, we have developed a modeling environment for the CNC process. SmartCAM maintains within its database a model of the entire tool-path sequence, including a graphic 3D image of part geometry, the tool path to cut it, and a list of all events or elements tied to that image. Changes on the screen in part geometry are immediately reflected in the tool-path database, or the user can work directly with the database to access elements or events, and these changes are immediately reflected in the part's graphic image.

Because the database is "dynamic," the tool path is automatically modified. The system "knows" now to translate the graphic image into code. No longer a "CNC programmer," the user becomes a "CNC process engineer" because it is no longer necessary to work at the program-code level.

Multiaxis modeling

Modeling is particularly suited for multiaxis lathes. Most turning programming operates in a 2D work space. With a new software product called "SmartCAM Advanced Turning," that becomes a 3D environment. It is important to know exact positioning as tools move around the part, are changed, or timing sequences adjusted. It is equally important to be able to make continual adjustments to the tool path and see the results.

This new system allows the user to view part, tools, toolholders, and turrets from any angle. With a "Show Path" command, positions and timing can be visually verified, or a Wait" command used to delay one tool to sequence it with another. A "Linked" command synchronizes tools by selecting profiles, or you can select a range of elements to synchronize. The software automatically sorts by primary and secondary turrets, displaying merged tool motion with accurate proportional feed rates and identifying potential collisions.

When using a milling tool for C-axis work, Advanced Turning recognizes its function when the tool is selected and automatically anticipates the procedure associated with it. There's no need to switch to a milling module and merge code later. An automatic function wraps 2D profiles around Cylinder OD.

Process optimization

One of the most powerful capabilities of true CNC process modeling is being able to experiment with various types of cutting procedures to reduce cycle time, improve cutting efficiency, consider alternative work-holding options, and adapt to a changing machining situation.

This is easily accomplished with the "Undo" command. It operates expressly on tool path, not geometry, and lets the user try out different tools, sequences of simultaneous actions, or alternative roughing routines, yet return immediately to the previous state. No longer is a machine used less than optimally because code is too difficult to change.

Evaluating tips

As CAM software continues to evolve, individual features tend to sound similar. All systems should be able to produce standard code, one way or another. The difference will be in how well each system assists the programmer.

The evaluation questions to ask are: Can you make changes with little effort? Do you have to edit code directly? Do you have to start at the beginning each time? Are complex parts as easy to make as simple ones? Can you feel confident that your code is right, or must you test it on the shop floor? The issue is not just programming, the issue is production.

For more information on CAM software from Point Control Co, circle 298.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:computer numerical control
Author:Blakely, Jerry
Publication:Tooling & Production
Date:Jul 1, 1992
Previous Article:Palletizing for greater efficiency.
Next Article:Software cures tolerance stack up.

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