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Any reason to bother with NURBS/spline paths on 840D with compression?

erikh

Plastic
Joined
Mar 20, 2017
I have a 2014 DMU65 with Siemens 840D SL control. The path compression package came included with the controller, and it definitely works quite well. CYCLE832 + COMPCURV is very nice.

Although I have NX 11, I use Grasshopper in Rhino to make 5-axis toolpaths for aesthetic effects with ball nose mills on generative geometry. There are a handful of cases where incident curves can be represented exactly with NURBS, but cubic spline ("akima" splines) and bspline is always possible. Right now, I'm sampling NURBS surfaces and outputting linear path segments - just plain G1 commands with unit vector (A3, B3, C3) tool orientation.

Fitting tangent continuous cubic segments or bsplines and outputting these as CSPLINE or BSPLINE in X, Y, Z, XH, YH, ZH, PL format has proven tougher than I anticipated. Before dumping more hours into this, I'm curious to know if anyone has found situations where witness marks can only be avoided by using spline paths. Is 840D compression so good that I can totally lean on it and stick with linear paths? So far, I'm happy. I'm worried that I'll find out that I do need splines at some point and end up blowing deadlines in order to implement it under duress.
 
I'm not using NURBS output on my machines at the moment but in theory spline output is supposed to be smoother/faster at high feed rates. However, even the Siemens NX docs are somewhat vague about it. From what I've read it seems there isn't many programmers using it which somewhat leads me to think it might be a niche solution.

I would think if your geometry is decent quality you shouldn't see much of a difference between linear and spline output if you use small intol/outtol values. The only time I have dwell in a path when there are very small model anomalies which make for sharp edges; typically a minute inside corner which causes a zero interpolation intersection. Turning on smoothing and using a small intol/outtol usually takes care of it. Usually I receive very nice models from my customers but once in a while there is some poor geomenty we are forced to deal with. (hello China!) I see superb finishes coming from our NX and Mastercam programs when using tight tolerances and obviously tight tols are a must. A friend who uses CamTool is getting excellent results as well. All of us are using only linear and arc code.

Sorry I can't be of more help but at this point I would do some testing to do a direct A-B comparison. Perhaps try samples with different settings too.
 








 
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