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DJ, we've done conversions to DXF for so many years I can't count them all (OK, about 20yrs ±) ... I have yet to see a spline come through as anything other than a number of line segments. (Unless of course export all splines as polylines is checked) Even then, they are segments once you explode the polyline.
Depending where the file goes next there is some "massaging" that can be done though. On or about the ACAD 2012 release they allowed for the option to edit the precision of the spline. For this to work you of course require ACAD and you want to export your model geometry as splines so the "Export all splines as splines" must be selected.
From here, once you open the file within ACAD you can do a "pedit" to change the splines to a polyline and it will ask you for the precision you would like. Initial precision is ~.050" per segment which can be edited to be ~.001" per segment at the maximum precision allowed which is 99 (precision). This of course can be a bit taxing depending what you're running for a system but I routinely set my precision to 99 for Wire EDM work and have not found any gotcha's as it were.
Good luck with it ...
I got some help from Doug Enns at MLC Cad systems.
It took a while, but I learned that in order to save your sketch
as splines, you must first extrude the sketch into a feature (see attached
SLDPRT). Then I did a save as dxf and selected the face. During the
export, you get the option to delete entities. This is where I deleted
the lines I used to close my two splines into a single closed contour. I
tried nearly every option and spoke to others and found no way to go directly
from a sketched spline to a dxf spline, without first creating a feature in
SolidWorks. The DXF file is attached also.
MLC8 Part1.SLDPRT.zip 50.7 KB