DaninVan: The wood is in this case Sapele but I would be using other hardwoods such as beech, oak and mohogany.
The bit is new, use only for, perhaps, a total running time of 20 mins. However, some people say that tool bits are not as sharp as they could be out of the box.
Do you think doing that cut in more than one pass might help?
Alternatively, do you think strengthening the face where the tearout occurs with a tape of some sort stuck on? Perhaps the brute force of the cutterwould still tear out fibres.
If I can't solve this problem I will have to confine any decoration (by inserted contrasting hardwoods) to cuts along the grain. Quite a design limitation.
A totally different aproach would be to make the top and bottom pieces slightly smaller all round, effectively providing when fitted the same slot for the hardwood strips. It would be easy to get the eges of the top and bottom to the correct size without tearout, perhaps by finish planing the long edges of the top/bottom by hand and sanding the short (cross grain) ends (sanding does not cause tearout). When gluing on the top/bottom, it would have to be very accurately centred, otherwise the hardwood strip, when finish sanded to size in place, would vary in width. Hum!
Or perhaps I'll stick to plain boxes.