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Could bringing my knife in for sharpening cause a kashrus problem?

Facilities that offer knife sharpening services will likely use the same sharpener for kosher and treif knives (or for milchigs and fleichigs), but that itself does not pose a kashrus problem as long as the knives are clean. This is because the sharpening process does not involve heat, and whatever warmth is caused by the friction is not a problem halachically. Similarly, one may use the same sharpener for milchig and fleishig knives in their own home as long as the knives are clean.

However, when bringing knives to a sharpening facility there’s the concern that one may inadvertently be given a non-kosher knife instead of the kosher one they had given in. This issue can be avoided by having a tvius ayin (being very familiar with the knife and its identifying marks), or by some other method of ensuring that one gets back the same knife.

Another concern is that many of these facilities wash the knives together in hot water prior to sharpening them, and the taste of non-kosher food may be absorbed by the kosher utensil at this point. Therefore, care must be taken that one’s own knife is clean when giving it in, so it isn’t washed at the sharpening facility. If one can't be sure that it wasn't washed by them, it should be kashered afterwards, prior to use. #538^⁠1