Speed vs. Accuracy: You Can Have Both
For the past few years, most shops that perform high-speed CNC machining have been in hot pursuit of faster cycle times. The idea, of course, is to speed up part production. But as you may have noticed, speed sometimes “kills” when it comes to sacrificing finish quality. Do you have to choose one over the other? There are a few factors you should consider in addition to cycle time – and they may have a greater impact on your profitability.
Fixed Overhead Costs – Use ‘Em, Don’t Lose ‘Em
Anytime you have an operator on the clock, this is fixed overhead. That person will draw the same payroll regardless of what they’re doing or producing on any given day. So, for example, if an operator is monitoring and tweaking what should be an automated process, you’ve lost the opportunity to have them performing more productive functions. Once you pay that wage for the day, it’s gone, and you can’t recoup that. I call this “the biggest problem you have in your business.” There’s a big bottleneck, and it’s the operator at the machine. It is therefore critical to employ every technology advantage that enables your machines to run absolutely unattended and complete parts in one set-up on the machine tool.
The Race for Cycle Time – Running in the Wrong Direction?
In the pursuit of profitability, shops and tool builders have made great progress in squeezing down cycle times. We’ve now reached the point where there’s not much more that can be taken out of cycle times. Let’s take the example of die mold machining. Today we have one-second tool changes, and we’ve progressed from 100 to 1,800 inches per minute – the technology is available to do that. But that’s only part of the equation. Typically, with high-speed CNC machining we’re using very small end mills, we make a lot of passes at a very high feed rate, and on a die you have to deal with large guide pin holes. And everyone assumes you have to have an extremely high RPM to machine at high speed, but this is not the case. With today’s technology you want to stay under 20,000 RPM, because of factors like torque curves. All complicating factors, which pose the question: is cycle time the holy grail? I propose that maximizing throughput should be the goal, and this means achieving top speeds without sacrificing quality.
Pay Now or Pay Later
At Okuma, we have a 15K spindle that can do one part to completion. We can put the holes in because we have enough torque in the spindle to do that, and we can high-speed mill. But there are other considerations inherent to any machine tool when it comes to high speed machining: the ballscrews and the servos can only react so fast. This leads to surface finish problems that have to be hand-polished out. So in this scenario, the gains that are made with the speed of the machining are lost when you have to take time for hand finishing later.
Super-NURBS Hits the Sweet Spot
Okuma has a technology called Super-NURBS that delivers both high-quality and high-speed machining simultaneously. With Super-NURBS, you can set yourself up for productivity from the start. You can set up a tolerance band around the finished dimensions that you want to machine, and you can open this tolerance band up to go extremely fast, or you can narrow it down to concentrate on finish. This gives the operator control to find the desired sweet spot between high speed and high accuracy, and you can do this without rewriting programs repeatedly to dial this in. The machine automatically adjusts the feed rate to meet that tolerance.
The key here is – the machine is now doing the work and taking it out of the operator’s hands. This means that a less experienced operator can now perform a simultaneous high-speed/high-quality process. This also means you can have greater control over throughput without being as operator-dependent. You can even use Super-NURBS as one of the tools for running unattended. With Super-NURBS, cycle time is reduced, finish quality is complete on the machine, and overall shop throughput is increased. And throughput is where the money is made, not individual part cycle time. Now you don’t have to choose between speed and accuracy, you can have both!
When it comes to investment, adding Super-NURBS is a lot less expensive than highly-skilled workers. And, much easier to acquire and retain. But those highly skilled workers you do have – you can make the most of them elsewhere in your shop, and you’ll increase throughput even more.
Winning Your Race
Technology like Super-NURBS has been widely used in Europe for about 20 years now. Here in the U.S., I’d say that three years ago maybe 10% of shops understood how to gain these kinds of throughput advantages. Today, probably 50% do. The interesting thing is…the shops that “get it” are very, very successful. So, you can still gain a competitive edge if you focus on throughput, and optimize both speed and quality.
Jim Endsley is Machining Center Product Specialist, Okuma America Corporation.
[For a more detailed review of this technology, check out our Super-NURBS white paper.]