Positive vs negative in a hobby shop

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Batmanacw

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I started out with CCMT inserts because my first new lathe (10x30) was pretty small and low hp.

My old Cincinnati 12x36 lathe ran Trigons pretty well. I gave that tool to my buddy who bought that lathe.

The trigons had more edges to use at a much higher price. I don't remember chip control being much better. I can get Chinese CCMT inserts that run fine very cheap.

Now I have my old Carroll Jamieson 16x40 lathe and I bought a CCMT holder for it and it really cuts nicely. The top speed is only 510 rpm.

What advantage does a negative rake insert offer me, not just on my big slow machine, buy also if I replace my whimpy 10x30 with a more substantial lathe?
 
For me, it comes down to material and deflection. Alloy steel on large diameter rigid work is a shoo in for negative rakes. Soft material or thin/flexy work means the advantage goes to positive angled tools. The rabbithole always goes deeper in these discussions, but I'm pragmatic enough to keep the decision matrix simple. So as usual, it's different horses for different courses. I guess that's why we have so much shop storage dedicated to the machines...
 
Negative rake tools hold up better, especially in interrupted cuts. The can also offer more cutting edges per insert. I have some tools that use the old CNMG-432 inserts for my Heavy 10. They don't get used much, but sometimes.

 
I'm pretty much with @pontiac428 - My lathe is a 14x40, 7HP. I use negative rake tools mostly for hard materials and stick to positive rake for most everything else. Hard materials don't have to be large diameter though - it's relative. A 1/2" (hard) dowel pin is definitely something I'd attack with negative rake.

GsT
 
I don't see enough need for the negative inserts to invest the money right now. I don't do more than 0.1" per side on my old CJ lathe in mild steel. My maximum speeds are pretty slow.
 
I would say all of this is a generalization until you start looking at specific insert chip breakers. I have some wngg inserts that some might assume are a negative rake, (Iscar wngg330-f3n-p ic20) but essentially are a positive rake tool by virtue of having a deep angled chip breaker. They are scary pricey at about $20/ea, I believe they were a bit cheaper when I got them a few years ago. But they work a treat in cutting softer metals and have lasted quite a while in my shop, where I'm not the most skilled at lathe work. With the sharp edge, they are not suitable for steel and will chip out pretty quickly (at least with my clumsy work).

Not trying to sell anything, just pointing out an example where there are occasional exceptions to what I would consider useful rule of thumb comments, in case someone comes along and assumes all WNxx (or whatever) have a negative effective cutting rake.

Most of my lathe work is on a 14x30 w/ 3HP and 1000 RPM max.
 
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rabler said:
I would say all of this is a generalization until you start looking at specific insert chip breakers. I have some wngg inserts that some might assume are a negative rake, (Iscar wngg330-f3n-p ic20) but essentially are a positive rake tool by virtue of having a deep angled chip breaker. They are scary pricey at about $20/ea, I believe they were a bit cheaper when I got them a few years ago. But they work a treat in cutting softer metals and have lasted quite a while in my shop, where I'm not the most skilled at lathe work. With the sharp edge, they are not suitable for steel and will chip out pretty quickly (at least with my clumsy work).

Not trying to sell anything, just pointing out an example where there are occasional exceptions to what I would consider useful rule of thumb comments, in case someone comes along and assumes all WNxx (or whatever) have a negative effective cutting rake.

Most of my lathe work is on a 14x30 w/ 3HP and 1000 RPM max.
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Like rabler says I think any negative insert with a molded in chip breaker is really a positive insert. If the chip starts cutting and the chip starts down to the floor in my mind it is a positive insert.
 
I have opted for WNMG 0604 inserts on all my general turning tools and boring bars. Lots of options for various conditions but very rarely use anything else than a steel grade with an 04 radius.
 
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