Mikey, I appreciate your explanations and guidance, thank you. I have a couple questions:
Can you point me to theory regarding the relief angles and how they play into cutting forces? Although they do not go into any degree of detail, the slides
here and
here do not describe the cutting forces as dependent on the relief angle. I know you said you did not want to get into some of this; perhaps the answer is that Merchant's assumption does not hold (or depends on small relief angles)?
In
post 652, you describe work hardening as a result of temperature. I've always understood the phenomenon as strain-induced hardening (as reflected on
Wikipedia or
here). If possible, I'd like to better understand the underlying principles and conditions that induce work hardening when cutting. Sharp tools seem like they would help by minimizing deformation in the workpiece. As a practical matter, I understand that taking sufficiently large cuts will avoid the issue.
Now we're getting into an area that gets complicated. First of all, the Merchant Equation essentially addresses the relationship between the rake angles, shear plane friction and shear plane angle. It does not address the relief angles. Second, it applies to the Orthogonal cutting model where cutting forces are applied perpendicular to the work surface, whereas our tools are actually Oblique cutters where the cutting forces and rake forces are applied at an angle. While the impact of the rake angles on the shear plane applies, the Merchant Equation does not. Well, it does but only when it is modified.
To my knowledge, the relief angles are essentially viewed as clearance angles in the literature. Unfortunate because they matter. I looked for some reliable model regarding the relief angles and found little so I based my understanding on what I could prove at the bench; I had no other option. I took a conventional tool and measured the power required to move that tool through a cut, then varied only the relief angle and measured power with each cut. I no longer have the numbers available but essentially what I found is that as the relief angle increases the power required to move the tool through the cut decreases. The amounts are small but they are there. What is interesting is that on a Sherline lathe, where the carriage is moved by turning a hand wheel at the end of the lead screw, you can feel a difference in the force required to move that tool through a cut when you compare a conventional tool with a high relief angle tool.
I'm sure there is some cutting model out there that addresses this but I am just not aware of it, and I looked hard. What I can say is that there is a lot more to the relief angles than just clearance. What's more, if you look at the finish produced by a conventional relief angle and compare it to that produced by a modified relief angle, there is a definite improvement in the finish produced by the modified tool. If relief is only for clearance then you would not expect an improvement. At this point, I've decided that I'm okay with what I know to be true, although I would be even happier with a model that explained it.
You have to understand that the vast majority of the information on the net applies to carbide tooling, not HSS. There is a lot more available now than when I looked at this stuff initially. In fact, the net was rudimentary back then so the only option available to me was to look at books and to sort it out at the bench. What actually happened was that I derived most of my conclusions by what I could measure and by the results I saw at the lathe. Then I tried to find models that explained what I saw and discovered the Merchant Equation, which explained what I was seeing with the rake angle mods I did. I couldn't find anything about the relief angles. There is a LOT of stuff on the cutting forces; look it up.
With regard to the cutting temperatures, Kennametal describes it thus:
Work hardening ... is caused when heat generated by the cutting tool transfers to the workpiece material and causes plastic deformation. The process is similar to a heat treatment of the workpiece but on a lower scale.
Essentially, what is happening is that the lathe tool is generating enough heat to harden the part at the point of cut. The thing to focus on is that while the tool is generating the heat, how that tool is used is just as, if not more important than, the tool itself. Hence, the tool must be sharp, it must efficiently remove the chips and it must be kept moving continuously. Anytime the tool stops cutting while in contact with the work elevates temps and this includes taking light cuts with a nose radius that deflects instead of cutting.
You will find that working with stainless or other potentially work hardening material is something you have to do from time to time. I work with a lot of stainless and have no issues with HSS tools; I do have problems with carbide inserts at times, though. To save my fingers, here are some tips I wrote in another thread:
- Mount the work piece as rigidly as the lathe allows. Keep work extension as short as possible and use a live center when needed.
- Mount the tool as rigidly as possible, keeping extension of the tool to a minimum.
- Use tools with smaller nose radii to reduce deflection. If the tool deflects, it does not cut and if it does not cut it will build heat.
- Use tools with high positive rake to enhance chip clearance so that the heat goes out with the chip. 303 has high ductility so chips do not break easily; they string. Even so, getting the chips out fast removes heat and high positive rake tooling does this.
- Use sharp tools. For me, HSS with larger side and back rake angles works well. I don't use carbide for this material, although that is certainly not an industrial practice. I opt for cobalt tools that handle higher heat better and I keep the tool sharp.
- Use lubricants. I prefer sulfur-bearing cutting oils but have found AnchorLube to work well. This is not so much to cool the cut; it is more to lube the cut without boiling away.
- Attend to cutting conditions. The smaller the lathe, the more important this becomes. The speed, depth of cut and feed have to be adjusted to allow the tool to cut continuously; if you dwell in the cut then heat builds and the work hardens. Stainless, especially Austenitic steels like the 300 series, is not particularly hard so cutting speeds in the 100-120 SFM range is recommended but I have found that on smaller lathes it is better to go slower so that we can keep up with the feeds. I normally use 60 SFM. Depths of cut need to be realistic and applicable to the lathe in use; smaller lathes require smaller depths of cut in order to keep feed consistent. Feeds in industry may be measured in IPM but that is meaningless to a hobby guy who is cranking manually; here, our speeds and depths of cut have to be adjusted to enable us to turn the wheel at a consistent and achievable rate and that comes with experience.
- The final thing I can think of is to use a steel that is easier to cut. 303 is the sulferized version of 304 and cuts better. 416 does not contain nickel and is much easier to machine than the 300 series.
I hope this answers you. Your tools are important, yes, but you are
more important.
