Why Do I Have to Know a Range of SFM’s and Chiploads?

Saw a great question over on CNCZone this morning. A beginner was trying to understand why so many cutter manufacturers publish ranges of surface speed (SFM) and chipload (inches per tooth) values for their cutters. I suspect his broader question was how to know what number in the range to use when calculating the feeds and speeds of his specific machining operation. He also couldn’t understand why the calculator software he was using also had to know the range. After all, it’s a computer, doesn’t it just know the right answer?

Here was my response:

The “right” SFM or chipload depends. The ranges of surface speed are quoted because there is a range of cutting conditions. The more your calculator can take those conditions into account, the less you need to look at it as a range.

Take a look at Niagara’s charts, for example:

http://www.niagaracutter.com/techinf…mat/index.html

Head into 6061 aluminum. Lots of numbers, but they’re organized based on what you can tell it about cutting conditions. The principle variables are:

- Are you peripheral machining or cutting a full slot?

- What is your axial or radial depth of cut?

- What specific endmill is in use in terms of finisher/rougher and coating?

Many companies have not bothered to produce the level of detail Niagara does. They just give a range and leave it up to the machinist to decide. However, even in the case where a fair amount of detail is available, even more elaborate models are possible that take more variables. At some point, it becomes impossible to keep up with it using paper, pencil, tables, and the school-taught formulas for rpm and feedrate. To get the optimal starting point, you need a calculator that figures all that out for you.  In fact, this is exactly the reason I created the G-Wizard Machinist’s Calculator.

For example, you may want to smoothly interpolate all those depths of cut rather than going in steps as Niagara does. You may want to figure both axial and radial allowances for every cut. You may want to take into account geometric effects like chip thinning and ball nosed compensation. And on and on.

Figuring all that stuff out is what got me started with G-Wizard. There’s math for all of it, and it makes a difference to use it. If you’re prepared to use a calculator like that, you don’t have to worry about a range of SFM for most jobs.

The exception would be the case where you’re producing very large quantities of some part and it’s worth it to eek out every last iota of performance. For that case, you’re going to start with canned parameters like what a calculator provides, and then your going to hand tune that stuff to get as aggressive as you can. In fact, you’ll probably get too aggressive and then back off slightly.  BTW, the right software can help there too using a technology called “Knowledge Based Machining.”  This is a feature usually reserved for high-end CAM programs like Esprit, but many shops also use paper and pencil, spreadsheets, or even custom programs.  The idea is to build a database of what worked and what didn’t.  You can go faster than the manufacturer’s recommendations in almost every case.  The trick is knowing how much faster and exactly which case you’re talking about.  So you perform experiments until you’ve gone too far, and then you back off the feeds and speeds a little bit.  Shops that do this gain a real productivity advantage over those that haven’t.   G-Wizard includes a feature called the “Cut KB” or Cut Knowledge Base that is a tool for collecting exactly this kind of information.

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As an aside, a great deal of the information and tools machinists use are based on approximations and rules of thumb. These approximations simplify what we have to know about what is actually an extremely complex science. I’ve been taking Sandvik’s graduate course in metal cutting technology and theory (highly recommended, BTW), and there’s a lot going on! But, the approximations also hold us back from the potential that’s possible.

The approximations start from the idea that old school-taught spindle rpm and feedrate formulas tell the whole story. Of course they don’t. Consider the number of approximations that make up the average pocketing operation in 6061 aluminum:

- We may be assuming the simple feedrate and rpm formulas are enough. But varying the axial or radial depth of cut matters a lot, as do geometric variables like chip thinning and whether we’re climb or conventional milling.

- We assume constant cutter engagement, unless our CAM software supports high speed machining toolpaths like Surfcam and similar strategies. Just look at what a huge difference that makes. Without it, all the speeds and feeds have to assume worst case corner engagement is the norm throughout the cut.

- We assume all carbide cutters with a particular coating are the same.

- We assume all VMC’s, coolants, and all setups are the same in terms of rigidity.

The list goes on. The challenge in really optimal machining performance is to get our arms around all these variables and account for them. The more variables your software and experience can take into account, the closer you’ll get to the ideal. In this world of thin manufacturing and machining profit margins, its worth it to try to take more variables into account than the other guy can. Why use a fancy feeds and speeds calculator like G-Wizard? Why use CAM software with the best possible toolpaths? Same reasons–because in the end they give you control over more variables to get a more optimal result.

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