R. Peterson wrote:
Your wedge is the likely culprit for the slipping. It looks like the grain is running across the wedge. This would allow the wedge to flex upwards at the ends and reduce the surface friction area. On a low angle plane you need all the surface area you can get to hold the iron in place.
Try a new wedge with the grain running longitudinally. And maybe scuff the face of the chisel a bit if it still slips.
The grain is running lengthwise, but face grain up. I think I'll make another wedge with edge grain up, maybe a wood that isn't quite as slick as walnut. Also the scuffing sounds like a good idea. Thanks.