As described in the instructions, you should first do a "practice grind" on an old ski to get a feel for how to handle the device and for the correct feed along the edge.
I did this on a 12-year-old ski. The first grind was a bit rough because I pushed it forward too quickly. I was skeptical as to whether the grinder would really be any good given the low drive power.
The second grind was better and finer than what you get on new skis in the shop.
In the driving test, the old ski held up incredibly well on a hard artificial snow slope with icy sections. I'm impressed.
(I used a belt grinder for 20 years. The results from the new grinder are much more precise. No comparison. A quantum leap).
The 3rd cut is really smooth and almost racing-like.
I can recommend the device with a clear conscience.

Peter (28/12/2018), Austria
Orders: 5
Reviews: 2