A closer look at look-ahead

Modern Machine Shop, March, 1996 by Todd Schuett

Is it possible that your existing fast CNC mills can mill your same 3D contours in 1/10th the time? Not just 10 percent faster, but ten times faster? Is it possible that the part could be even more accurate at the same. time? The answer to these questions is a definitive yes from people who daily use controls featuring look-ahead. Some are more conservative, citing three to five times the improvements, but in competitive industries like molds, dies, patterns and prototypes, the ability to mill accurate part surfaces in a small fraction of the historic time is still exciting.

If you mill 3D surfaces from CAD/CAM data, look-ahead may be the single greatest productivity feature your CNC control can (or may not) offer. Look-ahead for 3D milling may be referred to as "automatic feed rate override," "geometricintelligence" or other things, but the bottom line is that look-ahead can improve your overall milling productivity dramatically.

"You can't talk about high speed without look-ahead," says Dave Long, of Pro-Mold, Schaumburg, Illinois. "We bought our latest CNC machining center with feed rates to 600 inches per minute (ipm) based on the promise of high speed. We haven't realized the benefit from that machine, though, because it doesn't have look-ahead. The addition of new high speed controls with look-ahead on four of our older machines has actually relegated our newest machine to mundane, slower work. The oldest machines all outperform the newest, primarily because of look-ahead." The importance of look-ahead is emphasized by the fact that Pro-Mold now mills 3D surfaces on a former tracing mill with a maximum 100 ipm feed rate. It can mill the same part as the newer and "faster" machining center in a fraction of the time, because of the look-ahead. A 100 ipm machine with look-ahead is actually faster than a 600 ipm machine without look-ahead. Mr. Long's claims of milling that is ten times faster are echoed by Merl Widup of Ehlert Tool, New Berlin, Wisconsin and David Simon of Simco Industries, Roseville, Maryland, and others.

Maximum Feed Rates

Imagine a machine capable of 1200 ipm feed rates milling a part for which total X/Y surface measures only 1/2 inch square. The part isn't large enough to ever accelerate to the machine's maximum of 1200 ipm. Still, if your cutter and material allow cutting at 1200 ipm, why not cut as fast as possible? With look-ahead, the machine's CNC controller is responsible for maintaining the feed rate as close to the maximum programmed rate as possible without violation of the part's geometric integrity. The point is that the responsibility for establishing the balance of optimal feed rate for productivity with the required accuracy is no longer in the hands of the programmer or the operator, but rather rests on the computerized control. Imagine an operator capable of adjusting the feed rate override selector more than 2,000 times per second to go fast where possible and to slow down just enough where needed. With a really fast control featuring look-ahead, that is the net effect: the fastest part possible, combined with optimal accuracy.

The Way Things Were

To really understand the challenge, a brief history on look-ahead in CNC can be helpful. As you read, you will see that look-ahead has had different connotations in the CNC industry. Then we'll look at what actually happens with look-ahead while we're milling 3D contours.

When NC and CNC were developed, the idea was to pre-plan each move that an operator made, and cause them to be executed in rapid succession, without the thought, delays, and likelihood of error that accompanies manual operations. I can remember the first time I saw an NC machine drilling and milling a part as I watched in amazement. The same operation by hand would have taken easily 20 times as long, and it wouldn't have been nearly as accurate. It would likely have had errors as well. The finish and detail locations were perfect. Feed rates for each move were identical. Tools lasted longer because of the consistency of their workload. NC brought about a new consistency to machined parts.

As NC developed, a buffer was added to allow the control to read a block of data before it was ready to be executed, thus speeding up operation. This buffer gave a sort of "look-ahead" to anticipate the next move and minimize dwell time before the execution of each move.

Things developed to where NC and CNC controls weren't just "point-to-point," which meant it was limited to linear moves. Rather, they came to have circular contouring, helical contouring, and so on. Dwell at any surface transition became less and less acceptable, so "no-deceleration" codes were added to suppress the normal delay at the end of each surface. No-deceleration modes didn't really look ahead, but allowed for the flow of data without dwells by eliminating the need to really get into an accurate position at the end of one move before continuing on to the next. This created a fluid motion of the machine axes, but introduced inaccuracies.