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Choosing the Best Non-Cutting Move Smoothing Options for Your Machine

Siemens Legend Siemens Legend
Siemens Legend

 The option to smooth the corners of non-cutting move (NCM) motions was extended to planar operations such as Floor & Wall, Planar Mill, Cavity Mill, and etc. in NX12 for manufacturing. This can be seen in the NCM dialog of these planar operations and others under the Smoothing tab as the Smooth Corners check box.

 

NCM_Smooth_On.JPG

 

Smoothing the NCM motions of a tool path is useful for reducing wear on a machine tool. The hard corners of the transfers, engages, and retracts are turned into arc or spline motions based on the NCM types that were chosen. Some people will argue that these additional arc and spline motions will cause wear on the machine tool motors driving the axes. However, our conversations with a machine tool manufacturer revealed that while this may be true, there is a reduction in wear on the machine tool's ball screws. Since the cost of replacing a ball screw is typically much greater in both parts and labor time than replacing a worn out axis motor, it becomes economical to use the Smooth Corners functionality. This is especially true for businesses doing high production runs, or with machine tools that work around the clock.

 

However, not all machine tools are created equal. Smoothing the transfer motions, which usually get output as a rapid move (G00), can have less than favorable consequences in the real motion of the machine for certain controllers. Trying to move through these transfer arcs while rapid positioning can actually cause the machine to slow down and have choppy motions.

 

Here is an example. The tool path shown in the picture below is a simple Floor & Wall operation with the engage and retract motions set to Linear and the smoothing set to the parameters shown in the NCM dialog picture above.

 

FW_smooth_all.JPG

 

In this case, all the hard corners of every NCM have been fitted with a 15mm smoothing radius as specified. The tool path looks nice, but the particular machine this was made for didn't handle the smooth G00 motions very well. Here is a video of that tool path in action. No material is being cut because this study is only focusing on the NCM motions.

 

(view in My Videos)

 

The interesting thing is that the engage and retract motions, which are output as linear motions (G01) at the same feed rate as the cutting motions, are handled very well by the machine and actually seem faster than the rapid arc motions. Some NX CAM software users would then advocate changing the rapid outputs to a feed mode, which can certainly be done the Feeds and Speeds dialog of an operation, but there is a newer way that may be more favorable to some.

 

The project to extend the NCM smoothing capabilities to other operation types came with an added parameter named Smooth Transfer Move Corners. This can be seen in the picture of the NCM dialog previously shown. This check box gives the ability for the user to decide whether or not to smooth the transfer motions while smoothing the engage and retract motions. The tool path in the picture below shows the output when Smooth Corners is turned on and Smooth Transfer Move Corners is turned off.

 

NCM_Smooth_On_partial.JPG

 

The linear engage and retract motions are smoothed to the specified radius while the transfer motions retain the hard corners. Here is that tool path in action.

 

(view in My Videos)

 

The motions are much better now. This partial option reduces wear on the machine tool in some NCM motions while retaining the maximum feed rate of G00 when transferring from one cut to the next.

 

As an added bonus, there will likely be a reduction in cycle time when using NCM smoothing. Take the example of the part file shown in the pictures. Three separate tool paths had their total times recorded after running on the machine tool. The first Floor & Wall path had no NCM smoothing at all and took 23 seconds. The second path was the one that had all of the NCM motions smoothed and took 27 seconds. This is due to the slow down in the rapid arc motions. The third path was the partially smoothed Floor & Wall tool path with transfers unsmoothed. This path took 21 seconds. These differences are small but so are the tool paths. Imagine scaling these differences to a much bigger tool path. Also, there has been feedback from customers using this on bigger paths with a huge reduction in cycle time in a production machining environment.

 

So, with options to smooth NCM motions for multiple machine tool capabilities, and potential savings in machine maintenance and cycle time, who can afford not to be smooth?

Comments
Pioneer
Pioneer

The following effects the net result of smoothing moves.

1.  If the Postprocessor "squares" all Rapid moves, it must be adapted or you must use High feed moves instead.  

2.  Machine may have a delay for each and every Rapid move.   I have seen this as high as 75ms per Rapid move.  Using a High feed instead of Rapid may work much better/faster.

3. Not enough points.   More points can make the machine run smoother and faster.  

 

When you know how to make this match the machine, it can reduce overall air time and surprise the person running the machine.  

Siemens Legend Siemens Legend
Siemens Legend

VH - Agree with your comment about the number of points. to control it use the Tolerance field seen in the first picture Justin posted. in most cases "From Cutting" is good. "Minimize points" is there to replicate the default settings we had in the past (might be necessary for old controllers, but not for modern ones). and ultimately if you like you can specify your own Tolerance (rarely needed).

 

FYI,  

Eddy

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