Pinch turn results

The diameter to length ratio limitation I discovered was 30 to 1. Some rules I established in random order: Tools have to be on center, above center or below center would squirt the part towards the center increasing vibration. Equal depth of cuts caused problems. Rather than divide the cuts in half i.e. 1.25 stock size .625 finish size .156 per side per tool. I divided the area in half so each tool removes the same volume. By splitting the difference in half you remove about twice the volume with the large diameter as the finish diameter. The pinch occurs by splitting equal volumes of material. The length of cut was 30 diameters; 18.75. I found the best results by using the bottom turret for roughing and the upper turret for finishing. I staggered the finish tool one half the tool nose radius in Z. I used Iscar DNMG 432 inserts. The customer still needs to experiment with grades. The outer tool wears before the inner tool because of the higher surface footage at a larger diameter. Like an insert drill you use one grade for the inner diameter and another grade for the outer diameter to balance the wear. I was able to repeatably take a 1.25 diameter down to .625 x 18.75 in about a minute. The pinch seems to be limited to roughing only. The finish tool has to have a roller follower support it throughout the length for mirror finishes. The sfm is 800 and the individual chip load was .01. The first depth was .131 per side (.988) and the second depth was .182 per side (.625). The limitation on a non pinch situation is about 20 diameters. The pinch buys you about 10 more diameters. These rules are subject to change as discovered.