If you have a damaged chisel and want to get the original 25 degree angle back before sharpening, this is a jig you’ll want to build. I use it with my homemade belt / disk sander and just clamp it to the disk table:

Homemade all round, I’m using my hybrid bar clamp to hold the guide in place.

The guide is easy to make from scraps and you can change the size to suit your needs and the machine you’ll use it with:

I show how it works in this video:

The results are a perfectly flat face that you can further refine on your sharpening stone:

The basic dimensions: