Material type and material condition (hardness) determine machineability, which dictates sfm. Machinery's Handbook actually does a very good job of explaining it, but it goes into great length to do so.
The shirt answer is from the blue.
Just a few points:
First of all, this book recognizes only two types of tool materials: HSS and carbide forgetting that there are a number of entirely different HSS and carbides. Such “small” things as coating, structure (coarse or micrograin), tool geometry, surface integrity required are just ignored.
Second, the listed cutting regimes are strange. For example if you try to select the cutting speed for 1045 steel having HB 225-275 then you get 300 fpm. If you select the cutting speed for austenitic stainless steel 303 having the same hardness, you get 325 fpm. Is it strange or what? I can provide you with a number of such examples which show useless of these data
Viktor
I agree with you that references like Machinery's Handbook have some limitations in that they do not necessarily properly account for the potentially significant effects of tool materials, tool coatings, etc. However, what does that have to do with 1045 steel and 303 stainless steel having different machinability ratings for a given hardness level? Machinability depends on material microstructure, and the microstructure of a plain carbon steel like 1045 (hardness of 225-275 HB) is different from the microstructure of an austentic stainless steel like Type 303. One can obtain a plain carbon steel like SAE 1045, an austenitic stainless steel like Type 303, a copper alloy like C17000 (2% beryllium-copper), and a titanium alloy like Ti-6Al-4V all with a hardness range of HB 225-275, but they will exhibit substantial differences in machinability because of differences in elasticity and plasticity, strain hardening, fracture behavior, etc. Perhaps I misunderstood what you wrote, but machinability cannot be directly correlated with hardness, most simply because hardness is not an intrinsic material, but the result of elasticity and plasticity effects.
I have never argued that hardness determines machinability. I just pointed out an obvious nonsense from Machinery’s Handbook according to which the cutting seed in machining of stainless 303 steel is higher than that in machining of 1045 steel providing that they have the same hardness. Metal cutting is the purposeful fracture of the layer to be removed so the energy needed to fracture a unit volume of the work materials can be regarded as the product of the strain at fracture and stress. Because the stress at fracture is directly proportional to the hardness, the mentioned work materials have approximately the same stress needed for fracture. When it comes to the strain at facture, it is 3 folds higher for 303 steel compare to 1045 steel. Therefore, the cutting force and cutting temperature in machining 303 steel is much higher than those in machining of 1045 steel. Moreover, 303 steel has much lower thermal conductivity so a greater part of the thermal energy generated in cutting goes into the tool raising its temperature. As a result, the cutting speed in machining of 303 steel should be much lower providing that the cutting temperature is limiting factor for tool life. I have experimental data to support my story including the measurement of the cutting force and temperatures distributions for the mentioned work materials.
Viktor
