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CNC WEST February/March 2018 www.CNC-West.com 47 accurate then it can move around faster. These set- tings are tuned from the machine tool company and are usually controllable at the control and within the G-Code. These settings have a significant difference on both part quality and cycle time. See pic of the Mazak Smoothing Machine configurations. CAD Part Design: On the CAD side of a part some- times splines are required to get the desired shape of a line. Often a very similar shape can be achieved with tangent arcs. If your machine processes a limited amount of code then starting with a CAD design that uses tangent arcs whenever possible will be ideal. CAM Tolerance: The machining tolerance is the sum of the tolerances used for the toolpath generation and the CAD Geometry triangulation. CNC machining motion is controlled by G1 straight lines and G2, G3 arc moves. With a CAD model designed with lines and arcs the CAM system will create G1 moves for the straight lines and G2 and G3 moves for the arcs. For CAD models with splines and surfaces the CAM system will approximate the splines and surfaces by linearizing them; creating many short line segments to approximate the desired surface. How accurately the toolpath matches the desired shape largely depends on the number of line segments used. The smaller the tolerance setting the smaller the line segments will be and the closer the toolpath will match the spline or surface. Machinists often have the urge to make a very small value for the CAM tolerance. The tradeoffs to having a small CAM tolerance are longer toolpath calculation time, larger G-Code program file size, and short line segments. Depending on your CAM software, com- puter, machine memory, and your machine’s ability to process code and look ahead these tradeoffs will carry a different weight with you. CAM Smoothing: CAM Smoothing has different names in different CAM software. Smoothing smooths the toolpath by removing excessive points and fit- ting arcs where possible within the given tolerance. Smoothing replaces line segments with tangent arcs where possible within the given tolerance. Reducing the code size with smoothing can have a dramatic effect on part quality and cycle time on machines with limited blocks per second processing and look ahead. Smoothing can also dramatically reduce program file size which is very useful for older machines with limited memory. Toolpaths that lay primarily in major planes (XY, XZ, YZ) such as 2D tool paths and 3D tool paths like parallel will find significant point reduc- tion. Toolpaths that don’t, such as 3D Scallop and ramp produce much less point reduction. Fun stats: I designed a few parts using splines where linking tangent 3 point arcs would have also worked. I found similar results with each part. We set all our toolpath tolerance values to .0005” and tracked some stats on one part using our Haas VF2SS. With High Speed Machining turned off the part took 12 minutes to rough and finish machine and had a very poor fin- ish. We turned on High Speed Machining on and the cycle time went down to 8 minutes and the part quality was better but not great. With High Speed Machin- ing still on we turned on smoothing for each toolpath with a .0005” value and the cycle time went down to 6 minutes and the part finish was very nice. It should be noted that I ran out of time to test a part that was purely designed with tangent arcs. But, designing parts with arcs often has a dramatic effect on the machined part quality on machines that have limited processing and lookahead. My takeaway: Like most things in our trade the devil really is in the details. I’ve seen very professional quality parts produced on hobby level machines while I’ve also seen very poor-quality parts produced on machines that even the snobbiest of machine snobs would love to have. I wish I could wrap up this article with a silver bullet magic formula but I can’t. It really comes down to the basics of learning the small details of how your tools work and how they work together. In this case there are details in your CAD, CAM and machine tool control that all affect the outcome. The better you understand how they work together the better your parts will likely be.