Annealed 4140

[f350ca]

The inserts Im using produce tight curls like a spring in this case about 1/4 inch dia but with 4140 its ductile enough that they don't break so the cuttings will keep going till they catch something and twist off. Some of the cuttings were 6 or 8 feet long.

[PixMan]

(01-21-2013, 07:55 AM)Mayhem Wrote:

(01-20-2013, 12:13 PM)PixMan Wrote: ...the finish you see is usually a result of short chipping in the turning operation. It's a necessary evil with a long cut, otherwise the long stringy chips can get wrapped around the part, the machine and/or the operator!

Ken - can you elaborate on what you mean by "short chipping" - is this the desirable little "9" shaped chips or something different. I get a similar finish in 1040 (feeding by hand). The only difference is I get some full circumference lines from the inevitable pause in hand cranking.

Yes Darren, I do mean that short chipping is to get those short tight 6's & 9's. I can't tell for sure which number they are!

In negative rake insert such as those Greg said he was using, there's a new chipbreaker design from Walter that is just fantastic in the gummy steels that don't like to break chips, such as 1018 and 1026 DOM tubing. It's their "MP3". I'm hoping they develop something like it in the positive rake screw-down inserts because it just kicks a**.



Those bumps just behind the cutting edge are taken from their inserts developed for machining titanium, another material that's really tough when it comes to breaking chips. The material coming off the cutting edge hits the bumps and gets curled, split and broken. Very cool.

[Mayhem]

Now that looks more like an expensive cuff-link than an insert! Lets hope Walter does apply this to positive rake inserts.

Now here is a question I hadn't considered previously - given that industrial machines will have the necessary grunt to use negative rake inserts (which also offers twice as many cutting edges), why do insert manufacturers continue to produce positive rake inserts?

[PixMan]

Positive rake inserts will always be around because they're most-often used in small diameter boring bars where you'd need the additional side clearance angle in smaller bores. They also come in small sizes, down to at least a tiny CPGT1.51.2-PM2 that I have for a 1/4" boring bar. It's a 3/16" I.C. size. No way would any negative rake insert I know of work in the minimum bore size that bar can do, which is .310".

Here's another reason for them, in larger I.C. sizes. A negative rake profiling tool such as the MVJNR-163D holder carrying a VNMG331 insert shown in the photo below can (technically speaking) only turn in the direction that it's tipped to a negative angle, as the back edge is then just rubbing. By contrast, a similar shape positive rake insert such as a VBGT331 would sit flat on the holder and have 5º side clearance angle all the way around. Now you can easily use that tool to "back turn", where a VNMG couldn't do well.



There *is* only one type of negative rake insert I've ever seen for really small work, and it wors well. That's a solid carbide internal insert-type threading bar. It's smallest recommended bore is also .310".