When I bought my checkering file a couple of years back, I had to buy a 30 lip because the 20 lip ones were just not in stock anywhere. 30 works, but 20 would be better and I plan to get one soon. Another "cut down" a 20 lip file to 2-3 rows on an EDM machine and says that you can do "inside curves" of fingers groves down to at least 1/2 inch across with the "new" two line file. It just takes longer than doing 20 at a time, but the full sized file only really works on flats or "outside" curves. My thought was to cut the sides parallelogram shape so you could get 2 kins on one said and maybe 3 or 4 on the other to speed things up on bigger arcs. The files cut on both sides and are "safed" on the edges.