A vertical is what is needed in knife making. A horizontal is something not needed, but nice to have. I've been making knives part time since 1995. I saw on another forum were a poster had done a WIP and built one to use 2x48" belt. After looking I had most of what I needed to make one. The reason a horizontal is helpful is to make sure everything is square, will be helpful in making folders, and all the grind marks will be on the long part of the blade.
If I had to buy one of these grinders, I would do without to buy other need equipment first.
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poppaclutch what are you going to do with your grinder?