Purpleheart contains silica which is very hard on steel. I once carved a mandolin back & neck from it and spent a lot of time sharpening. You might have better luck if you cut as much as you can on a table saw from both edges and just use the band saw on the bit left in the middle. If you only have 6" of capacity on the band saw a table saw might do the whole job. Start with about an inch of blade exposed rip from both edges (flip end over end) raise the blade and repeat.