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The benefit of true right-handed and left-handed turning lathe tools is obvious: You can get right up against the shoulder or face with them:

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The benefit of neutral-handed turning lathe tools is also obvious: You can cut in both directions or plunge cut:

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So then what is the advantage of LH and RH turning tools that don't let you get up to the shoulder? enter image description here enter image description here

It would seem these would be niche tools for when you need a to be able to plunge cut and get closer to the shoulder with a single tool, but my understanding is that these tools are actually more commonly used than the true LH, RH, and neutral tools. They can't face or get as close to the shoulder as a true LH or RH tool but also can't be as strong as a neutral tool (which can get more material behind the cutting tip).

Are facing and 90 degree shoulders just a lot less important than I think they are?

Images from: https://www.mcmaster.com/lathe-tools/carbide-tipped-turning-lathe-tools/

DKNguyen
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1 Answers1

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So it turns out the tools that can cut a 90 degree shoulder have what is called "negative lead-angle" which results in just the tip cutting and also results in clearance. This allows the tool to face and also turn a 90 degree shoulder.

Conversely, the tools I was asking about in this question have "positive lead-angle". The reason these tools exist is to take advantage of this positive lead angle which cuts along a greater portion of the edge. This results in increased tool life since more of the edge is being utilized and distributes the cutting force along more area which helps reduce certain kinds of failures.

The disadvantage is increased cutting forces for the same depth of cut, increased radial forces, and greater potential for chatter. The increased radial forces result in more part deflection and less cut precision.

What this means is that tools with positive lead-angle tend to best for roughing where tool life and speed matter more than finish and precision.

The reason they are more common than neutral tools is that unless you have extremely large positive lead angles, the neutral tool ends up being too narrow and fragile due to symmetry. The LH and RH tools with positive lead angles don't have this problem since they only cut in one direction so the opposite side can be bulked up with lots of material to support and strengthen the tool.

DKNguyen
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