Okay, Bamban and I discussed his work before deciding on what tool geometry to use. He is turning 416R stainless steel rifle barrels. (I have not turned 416R before.) It comes prehardened to somewhere between Rc 24 to 36 so maybe semi-hard. Like all stainless steels, 416R will work harden so we needed a tool that would cut with lower forces to keep the tool from bogging down in the cut and producing heat. We also needed to clear the chips out fast to keep cutting temps low. Most of the work is general turning and facing but rather than just make a turning tool, we chose to make a general purpose tool and a knife tool.
HSS can cut 416 but a tungsten cobalt tool would handle the heat and abrasion better. We chose a Rex AAA bit because it has 5% cobalt but 18% Tungsten, the highest tungsten content of any bit made by Crucible. This would allow the tool to handle high heat and abrasion.
Here are the specs we chose:
- Side and end relief = 15 degrees
- Side rake = 25 degrees
- Back rake = 10 degrees
The relief angle of 15 degrees is about 50% more than called for in this material. I chose this to reduce cutting forces but also to improve the finishes. By lowering forces, the tool should cut more freely and this should reduce heat.
The side rake is 7 degrees more than the angle table calls for. The goal is to greatly improve chip clearance while also reducing cutting forces. Given that the relief angles are increased and side rake is also boosted, the risk of reducing edge life is a real thing so I encouraged Bamban not to hog with this tool.
Back rake is increased by only a few degrees. This is intended to keep the cutting forces focused juuust to the side of the nose radius. It would have been nice to increase back rake more but then we would need a larger nose radius to finish well and this would lead to deflection, less cutting action and more heat production so I opted for more side rake and kept back rake reasonable. Focusing the forces here will enable the tool to rough well and face very accurately.
We chose a general purpose shape and a typical knife tool shape but used the angles above for both tools. The nose radius on the turning tool was held at about 1/64", while the knife tool radius is smaller than that. The reasons for these radii are to reduce radial forces to improve accuracy first; finish is a secondary consideration. If a better finish is required, the nose radius can be increased.
This is an example of how we can take a general purpose turning tool shape and alter its geometry to accommodate the peculiarities of the material. I will admit that since I haven't turned this material myself, the angles chosen were a guess ... but it was an educated guess. We chose a tool blank with the desired properties so it should hold up under frequent use; I think we chose well. Here are the tools right off the belt sander:
My thanks to Bamban for allowing me to share the specs on his tools. I wanted to show you how a tool blank was selected and how the angles were chosen. Bamban already told you how he altered lead angle to both rough and finish, which is helpful.
Now we have a tool that is proven to work with 416R. If you have a tool geometry you would like to share, please feel free.