by Harsh Bibra and Todd Drane
How controls contribute to better precision machining performance
When you’re machining tight tolerances in the range of 0.013 to 0.003 mm (+/- 0.0005 to 0.0001 in.) controls play a critical role in ensuring those parameters are maintained.
Shop Metalworking Technology (SMT) magazine spoke with Fagor Automation’s Harsh Bibra and Todd Drane to learn more about how controls contribute to better machine tool performance in precision machining applications.
What new functions do controls offer ?
Todd Drane:I call it “speed meets technology.” With newer and faster processing capabilities, the ability to micro-manage the machining process in real time becomes easier and more viable. In the past, a servo command was presented to the drive, it would begin its execution, the CNC would read the position as provided by the encoder and then update the servo with a new command. Today we anticipate the position and modify it hundreds of times per second via servo control techniques that modify the programmed feeds and speeds based upon the reported encoder position, and the machine dynamics such as cutter load, ambient temperature and programmed contour are taken into consideration. Different manufacturers call this different things; we call it High Speed Surface Accuracy (HSSA). HSSA anticipates conflicting areas of the executing program and automatically smooths the transitions while following the machining tool path. At the same time, algorithms smooth the profile while cross confirmation is done with the axes for jerk control and multiple acceleration/deceleration curves when the movement of a rotary axes requires a better response than the linear axes while RTCP is active. This is one example of a new CNC feature capable of improving precision and part finish.
Another important feature is in part gauging and collision detection, where a CNC based on the program and machining process can dynamically predict collision between par/fixture and tool.
What key functions on a control are a must for a precision machining application?
TD and Harsh Bibra: Good feedback and the ability to truly compensate for machine error. Traditional linear comp or cross-comp tables are not always adequate and must be complemented with more advanced tools that adapt to the different designs of the machine and specific kinematics. Volumetric Compensation is a feature that has the ability to compensate in 21 different degrees of freedom. It has proven to show real gains of up to 80 per cent in accuracy and repeatability.
The encoder feedback you use is important. Absolute protocols are the standard for precision machine tool applications. As well, to assist in thermal performance, a Thermal Determined Mounting System (TDMS) should be utilized to help eliminate feedback error due to thermal expansion/contraction. This has proven to be a useful feature on machines exceeding three meter travels.
Many machine tool builders offer controls built into their machines. How does a manufacturer determine whether the control that comes with the machine tool is the best one?
TD and HB: We believe it’s important that a customer understand the versatility of a CNC. Many manufacturers will purchase a machine tool based upon a particular project/job they have in mind for the machine. However, in due time, they have new more challenging jobs for the machine, only to find out they did not have the features or capability from the CNC perspective. Machine tools are being built to do more and last longer today, thus so should the CNC capability. We recommend a customer look at the machines capability for doing a singular job, and also consider foreseen or unforeseen jobs they may have in the future for it. This entails looking closely at the features the CNC has as standard and its expandability/flexibility to adapt to your growing needs.
Simplicity is another aspect customers should never overlook. CNC advanced technology is only useful if your staff understands how to utilize it.
Can controls be easily retrofitted on machine tools? What cost and time is involved in retrofitting a machine with new controls?
TD: Yes, and moreso today thanks to online diagnostics and easy PLC access as well as an organized easy-to-understand machine parameter layout. Cost and time are of course machine dependent. Small machines that require nothing more than a pre-engineered turnkey package can be retrofitted in a day or two from start to finish. Costs in many cases are as little as 10 per cent of the machine cost, thus making retrofitting not just an easy solution, but a smart one.
If your machine tool is mechanically in good shape, retrofitting with the latest CNC and servo motor technology becomes the obvious choice.
What are the key considerations a manufacturer should look at to determine the best control for his machine tool?
TD: CNCs are available that offer flexibility, allow for additional axes (i.e. rotary table), and have the on-board ability to create custom cycles and screens right at the keyboard. You want seamless integration of the CNC into a PC network and data acquisition software for production management. You want to foresee any needs you will have, hence, you need to ensure the control has ample memory with multiple methods of communicating to the CNC. Also, look closely at the G-code set. Do you have mirror imaging, auto pattern repeat, high level language programming? These are all value-added features that need to be considered.
How does a control on a machine tool contribute to cost savings and production improvements in a manufacturing facility?
TD: Reduction of set-up times. A good CNC will have easy-to-utilize touch tool methods utilizing a graphical conversational type screen that allows the operator to set tool and work offsets in just a few minutes. The goal of having the spindle on is substantially improved for this reason alone and will result in increased efficiency. Today’s CNC controls have the ability to do multiple operations simultaneously. A good example of this is a feature on a twin turret machine that automatically controls the flow of the G-code program from each channel through spindle sharing. The CNC monitors each individual operation and shares the spindle between the two turrets.
A more obvious advantage is the faster and faster cycle times industry is able to accomplish today. CNCs have improved processor speed and power. This just doesn’t assist in improved cutting speeds, but also allows multi-tasking of the processor to monitor and improve the entire cutting process.
The ability to create custom OEM or user on-board editors and screens is often overlooked. This feature means you can adapt the CNC to match the application from the operator/programmers viewpoint. All data entry screens are pulled up in the order you desire. The graphics and variables you want your operator/programmer to see and modify are put in front of him in the format that best increases efficiency.
What changes can manufacturers expect to see in machine tool controls over the next five years?
TD: You are going see a more interactive CNC with new advanced technology. Tools that allow application customization of the CNC will be easier and more accessible to the user. A continued refinement of the algorithms that allow for faster cycle times and better part-finish will continue to advance. We will see CNCs with a predictive behaviour, further reducing the dependence on human skills.
Harsh Bibra is general manager for North America and Todd Drane is marketing manager, Fagor Automation Corp., Elk Grove Village, IL.