I see a lot of interesting discussions about feeds and speeds out on the Internet. There are people that think you can get good feeds and speeds just by reading manufacturer’s charts, by ear, from their CAM program, and through a whole host of other shortcuts. Some say they’ve had enough experience as machinists that they just know the best feeds and speeds. Or, they may say their shop only does a few materials and they’ve learned them by trial and error. I’ve heard in the same breath that tooling manufacturers don’t test near enough variables to have complete data but that an individual machinist can accumulate enough knowledge through trial and error to come out with near optimal feeds and speeds for all the situations that they encounter.
These kinds of things make no sense to me. You can’t do this by ear, though many claim they can. If you could, you’d be able to buy audio tapes to get the “ear training” needed to judge your feeds and speeds. Big manufacturers and tooling companies would enroll machinists in ear training courses. After all, having the right feeds and speeds makes a huge difference in costs, whether by savings in tool life or savings in overall machining time (or finishing time if you can get a better surface finish right off the machine).
I have yet to see a CAM program that has very good speeds and feeds built in with the possible exception of SolidCam’s iMachining. Look at all the noise they’re making about that capability. It still doesn’t cover most of the examples of difficult feeds and speeds issues I mention below. Other CAM programs are light years behind iMachining.
The world’s foremost experts on this stuff are the tooling companies and the big manufacturers. They won’t send you to a course to train your ears but they do have binder after binder full of math (way beyond the simple chip loads, surface speeds, rpms, and feedrates formulas), tables, and other data. Here are just a few examples of things I’ve come across:
– The number one request for a new tool type in our G-Wizard Feeds and Speeds Calculator is the button cutter (and close relative bullnose endmill). A button cutter (toroidal indexable tooling) is a wonderful tool. Yet, to get proper feedrates, it needs to account for chip thinning in two different dimensions. Both the cut width and cut depth can lead to chip thinning in one dimension or the other. That’s not simple math you do in your head. But if you ignore it in a material like stainless, you can get to rubbing real fast, work harden the material, and lose tool life. You can be off a little and all will seem well, but you’re leaving money on the table every inch you feed the tool. Money either from reduced tool life or from not pushing the tool as hard as it could otherwise go. BTW, I should have bullnoses and button cutters available in G-Wizard shortly, so this math is top of mind right now.
– Any endmill less than about 3/4″ in diameter is subject to tool deflection. With enough horsepower and a big enough machine, even bigger cutters than that deflect. Tool deflection affects chatter, tool life (it’s equivalent to runout if you think about it), and the accuracy of your work (I deflected 0.001″ but I can hold a tenth all day long?). The math associated with it is too complex for any simple chart or rule of thumb.
– Micromills all have negative rake geometry because the effective radius of their cutting edge is large relative to the chip loads you have to work with. It takes a completely different physics model to get the right feeds and speeds for micromilling.
– Most machinists always climb mill, but that’s wrong. Dapra and many others have discovered cases where you get much better results conventional milling. I describe some of them here:
I could go on like this for literally hundreds of cases (many are covered in our Feeds and Speeds Tutorial, one reason it has so many chapters). The secret is in how many variables you can really master, and it takes more than you can keep in your head or you can estimate from your gut. People will spend $10,000 to $15,000 a seat and more with maintenance and upgrades for CAM, yet they balk at a few dollars for better feeds and speeds?
For those who don’t believe mastering more variables matters, look at the difference between HSM and “Regular” toolpaths. It’s night and day in terms of productivity and MRR. All of that is because the CAM program has mastered just one more variable that CAM programs without HSM cannot–tool engagement angle. Now imagine mastering 5, 10, 15 or more variables your competition isn’t even considering.
They say the spindle is the heart and soul of any good machine. If that’s true, and I believe it is, then surely speeds and feeds are the heart and soul of the programming. To be really good at this takes a lot of math (or a good calculator that builds in a lot of math) and some very systematic data collection. You need the feeds and speeds formulas for a starting point. You need the data collection to fine tune for the specifics of your machines, tooling, and shop best practices. Then you need the calculator again to be able to take the specific data you collect and apply it to new situations. The ideal tool is a combination of calculator and database, which is exactly what our G-Wizard Calculator is.
In the old days of manual machining, it used to be possible to get a handle on feeds and speeds pretty easily, but feeds and speeds for CNC are much harder than for manual machines (click that link to learn why).
Bigger companies are often already up this learning curve. Many smaller shops are not yet, but the more competitive things get, the more it will become a requirement. Right now, it’s just profit left on the table. When competing shops bidding for the same jobs have it nailed and you don’t, it’s going to start to hurt a lot worse because it will mean the difference between getting the job or not and making money on it or not.
You never can afford to stop learning in this business or take anything for granted. Or to look at it another way, do you have the right digital tooling? For CNC, digital tooling is at least as important as physical tooling.
This should all be pretty obvious to Pros because it impacts their profitability. But it’s also important to hobbyists. You typically only build each thing once. You don’t have time for trial and error. You don’t like to needlessly break tooling. In the end, if you’re doing CNC for any reason, it matters.
How will you use digital tooling to gain the advantage?
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Bob is responsible for the development and implementation of the popular G-Wizard CNC Software. Bob is also the founder of CNCCookbook, the largest CNC-related blog on the Internet.