SpaceClaim can be slow during a pull if there is a lot of data. If the curves are composed of thousands of tiny curve segments, it may cause a a long operation or a freezing. Or it may be a graphics display problem. Or there my be some problem with the data from DXF. Are you able to use Fill tool first to make a surface? Turn on the path fill option.
You don't need to use assembly mode. You can open both DXF files into the same DesignModeler file or SpaceClaim file. For DM, just use "File > Attach to Active CAD Geometry" twice, or "File > Import External Geometry File" depending on whether you have the associative interface or direct reader installed for AutoCAD:
For SpaceClaim, just use "Assembly > File" to import into the same file.
Do the gear teeth have complex curvature? If not, it would be very easy to create the gears new. You can sketch one crest-and-trough and copy around to make the full gear.
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