While most machining is 2 1/2 D, 3D Surfacing is still very popular. What’s the difference?
With 2 1/2D, it is rare that X and Y change at the same time as Z on the surface of an object. It can happen during cutting, usually due to ramping or helixing down into a cut. But otherwise, we largely move in either the XY plane or Z, and not at the same time. With 3D, all three axes are frequently in motion at the same time.
Another difference is we typically are using flat nosed or bull nosed (a flat end mill with a radius much less than the cutter radius) end mills for 2 1/2D. For 3D, we typically use ball nosed end mills because they can cut a surface at any angle.
When it comes to feeds and speeds, there are also many complications in 3D versus 2 1/2 D. The toolpaths can be more complex, and we have to think a lot more in terms of multiple passes and multiple toolpath types to get a smooth 3D surface.
This video of a Datron M8Cube machining a mold for a quadcopter is a pretty typical 3D machining job:
[youtube width=”800″ height=”540″]http://www.youtube.com/watch?v=8Lh600hVyt8[/youtube]
3D Machining a Quadcopter mold.
In it, you can clearly see the different nature of the roughing versus the finishing pass. The roughing pass is basically doing 2 1/2D machining, hence the “layer cake” appearance. This is not an uncommon approach, although there are many other approaches possible. The finish pass relies on a ballnosed cutter and small stepover (distance between successive passes or contour lines on the part) to create a smooth flowing 3D surface.
The latest editions of G-Wizard Calculator have added the ability to calculate Feeds and Speeds for 3D Surfaces via the CADCAM Wizards. Before we go much further, let’s answer the question, “What are CADCAM Wizards and how are the different from the regular Feeds and Speeds Calculator?”
There’s a lot more to it than this simple explanation, but let’s keep it short and you can click the link if you want more:
Think of CADCAM Wizard as being an extra employee, who is a pretty good machinist, who you delegate the operation of Feeds/Speeds to. You tell the CADCAM Wizard some basic high level stuff. In effect you say, “Hey Joe, I need a machining recipe for a pocket. The pocket is 2″ by 3″ in 4130 Steel. Figure out the best parameters for Cut Depth and Cut Width for a roughing pass, and also give me parameters for a finish pass that’s 0.015″. I need Feeds and Speeds for all that.” It is important to note that Feeds/Speeds and CADCAM Wizards use exactly the same code to calculate the feeds and speeds, so given all the same inputs, they will return the same answer.
I hope that the idea of having an associate get a very high level direction so they can do the work of producing a detailed cutting recipe makes sense, because that’s exactly what CADCAM Wizards do. Here is the new 3D Surface Wizard:
Some things to note:
- It only took a few simple questions to get a result. The Wizard needed to know Machine, Material, Operation (in this case 3D Surface), Depth, Max Tool Diameter, and Tolerance. The rest it will figure out, though you have options you can override.
- The Feeds area gives you a Roughing Pass, a SemiFinish Pass, and a Finish Pass. You get a complete recipe including what tool to use, Feeds, Speeds, and an estimate of things like how long it will take and material removal rates.
- We can see from the DFM area at the bottom that the Wizard considered 651 scenarios before it found a satisfactory answer. That answer is based on maximizing the material removal rates on the first 2 passes and on achieving the finish dictated by the Tolerance in the finish pass.
As you can see, the Wizard does a tremendous amount of work for you if you give it just a little information. When was the last time you tried over 600 scenarios to find the best combination of Cut Width and Cut Depth for you job? A lot goes on under the covers in the form of Machine Learning Algorithms to make the Wizard’s choice of which 600 scenarios to test very effective. In fact, it is exponentially better than randomly choosing the scenarios.
If you haven’t tried G-Wizard Calculator’s CADCAM Wizards feature, you should check them out. We have a free 30-day trial that makes it easy:
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Why the low feedrate on the finish pass? I tend to run 2-4 times the feedrate on a finish pass, then I do on my roughing passes. And I often feel thats being conservative with the small stepovers.
Mike, if all you’re concerned about is Tool Life, you certainly could go a lot faster. After all, a finish pass is removing very little material by definition, so it isn’t really stressing the cutter much at all.
But if you want the best surface finish, you should be reducing the feedrate relative to the rpms–it should be less than that of the the roughing pass. You want to take the smallest possible chipload before the onset of rubbing.
Another thing to consider–with a ballnose and a shallow depth of cut, you’re using a slower moving part of the ball. If you think about it, the exact tip is almost not moving at all. That’s another reason to take it easier on feedrates, all other things being equal.
But, there are a lot of ways to approach this. In many applications, there’s going to be some finishing done post-CNC. For example, in a lot of the guitar-contouring applications I’ve spoken with, they know they’re going to be sanding the wood. They only bother to get the scallops to 0.001 because it’s very easy to clean things up at that stage with a little sanding. So for them, they’ll be running much higher feedrates too.
I absolutely agree that keeping the feed low relative to the rpm gives smaller scallops and smoother finish. My queries would be:
Firstly, if the feed is per rev, does it actually make a difference that the cutting edge at the centre of the tool is moving slower than at the max radius? I figure there is only ever 1 flute that reaches the centre of any tool, so if the feed per rev stays less than the effective cutting radius then the tool still makes a clean cut. Likewise the horizontal stepover also has to be less than the effective cutting radius. Feeds and speeds calculator will show what stepover gives a specified scallop height, so I aim for a feed per rev to match this. Did I miss anything there?
Secondly, this is a finishing cut on a 3D form, so unless I am cutting a fairly flat face, the contact point of the tool will be further from the centre, possibly even at the full tool radius, regardless of how deep the actual z step is. How does GWizard cope with this variance, and when would I have to manually correct – for example if Gwizard assumes a flat part, do I increase or decrease the feeds/speeds if I’m cutting a steep area of the part? How do I set up when my CAM software can generate variable Z steps (full 3D paths)?
Dan, G-Wizard pretty much has to make worst case assumptions here. Without knowing the exact geometry, it really has no other option. The thing is, most CAM software does exactly the same thing. It would have to dynamically vary the feeds and speeds for each segment cut on a 3D surface. I’m not familiar with any that are smart enough to do that, though I don’t doubt that one exists some whee. Now if you know for a fact that your surface is simple enough that the geometry won’t vary across a vary wide range, you could do the math on it to determine some faster feeds and speeds. For example, if you’re profiling what is basically a 2 1/2D pocket that has a lot of draft of a fixed angle on the walls, this would be a viable solution. What I’d do is either do some math, or more easily, use you CAD software to determine the effective radius of the ball where it touches a wall at that angle. It’s an arc and tangent problem, in other words. Given that radius, just use the 2 1/2D rpm from a cutter with that effective diameter.
It will take a bit of fiddling, but will yield a much faster cycle time.
For complex surfaces, this sort of thing is just too crazy to do manually. What’s needed is a marriage of CAM and Feeds and Speeds engine smart enough to dynamically optimize every section of the surface. I will eagerly await contact from CAM vendors for help from G-Wizard to do this!
8D Cheers!
BW
Yeah, figured… anyhow, Gwizard geometry tab shows me the effective radius, so I’ll work from there.
Thanks.
Oops, you got an extra negative in there.
“With 2 1/2D, it is rare that X and Y do not change at the same time as Z on the surface of an object. “
Thanks!