Wiper is a part of the roller stone assembly. One can buy it separately also. Wiper is designed to scrape the side of the drum and guide the ingredients towards the stones for effective grinding. Wiper is needed for uniform grinding. It should be close enough to scrape the chocolate from the drum but far enough to avoid scraping the metal part of the drum. Stainless steel wipers are also available.
Checking the gap between the wiper and the drum:
Once fixing the wiper to the roller stone assembly, place the complete assembly inside the drum. Place the drum on the melanger and lock the lock arm. Now take a piece of paper, fold it into two and slide it up and down between the wiper and the drum. Paper should move freely. That is all the gap you need.