.The planks have to be glued down flat on a hardboard sled. I use hot hide glue over a melamime sled.When thicknessed to the correct dimension, remove the stock by heating with a heat gun or handheld hair dryer. I slide a putty knife under the veneers or planks while moving the heat gun.
This a bit messier than double face tape,but almost always works for me.
The knives must be sharp,the cut as light as possible,straight grain stock almost always works well. I have done some thicknessing with curly grain stock, more hit and miss.Most of the time I use a card scraper on any stock that is not straight.
mike