I have seen two ways how to cut curved piece of ivory:
When the tusk piece is known as solid and clean of any cracks, then using a piece of plywood and gluing the tusk to a position that yields maximum gain-using wooden washers, wedges etc. and using another strap of plywood to clamp it down just for added chatter resistance. No lost material because of sanding.
The other method is not that cheap, but is usefull for even cracked pieces or pieces where there is suspicion for high internal stress because of drying or so.
Dissolve beewax in clean acetone and apply it several times over the tusk and buff it with a cloth realy well. You may choose not to apply on obviously solid parts, which is at times the better choice. Use wooden dovels and wedges to position the piece in a makeshift cardboard or so box, which have about 20-25% of the dia of the tusk clearance to all sides.
The tusk is then suffused by the cheapest resin used in dental replacement mfg. as a supportive and test model material. The beewax is to seal the craks and porosities, to prevent the resin from impregnating the ivory, which at times can happen. The buffing will clean the hard, even surfaces, where the resin will get enough grip.
Spray the blade with water and cut the piece in span of several days or maybe even weeks, to prevent scatter of cut pieces due to suddenly relieved stress.
If there is known crack, take the first cut along it as to get an idea how the crack is propagated and if there are inner, secondary cracks.
This is not my hands-on experience, this comes from old, damn renowned gunmaker.