The bigger nose rads may be giving you a smoother cut because of the increased cutting area. Some times a fine point will be giving you a grainy ring appearance. The bottom of the three also has rake on so may be you are not knowingly cutting behind the point and it is giving you a rough edge (A problem i get at work if i do not put 18 degrees on my inserts and sharpen the top edge in the process). As long as the rad is sharp edged the bigger rads actually cut smooth and do not over heat as there is more material to defuse heat to. Black bar scale may be hard work for the finer point and blunt it also.Playing about with some crappy 40mm black bar on the lathe to see what sort of surface finish I can get. Usually I just accept what I get, but best learn this stuff now before I get a job it matters on, eh?
Same tool, same setup. 1000rpm, 2mm DOC, 5 TPR, and the tool rotated so swarf can't catch and mar the finish. Centre height set for each.
The best one is the top one. Chips break properly, and gives a mirror finish. No chatter marks. Works great on just about everything as long as you feed it in hard enough.
The middle one works OK. Chatters a bit towards the free end of the bar, but still gives a mirror finish. Fine, it is a roughing insert after all. But I can't get chips to break, it just gives long hot strings that are strong enough to choke a man. Maybe it's the soft material.
The bottom one is crap. It should be the sharpest of the bunch and work with shallow cuts, but no matter what I do, it chatters and gives a crap finish all over. Faster, slower, deeper, shallower, hot, cold, whatever.
Now I understand the designations and nose radius etc, but going on the limited knowledge i have, the bottom insert should be better for shallow cuts and give a better finish... But it doesn't.
So... What am I missing? Good people of the forum, please tell me what my random ebay inserts are doing
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