why cutting speed of soft material like aluminium is high ?
Simply because it can be. Most aluminum alloys won't create enough stress on the tool that the machinist can either take deep cuts or have a high surface speed. I also know our machinists like a high speed and very small depth of cut to provide a nice finish cut.
There are a lot of factors that can go into what the optimal cutting speed and depth are. Material (obviously), feed rate and type of tool being used are other factors. Usually, companys will have developed their own guidelines based on their experiences with acceptable tool wear and resulting produced parts. If you are in a production environment, especially a very high rate production, you start looking at things like the Taylor Tool Life Equation.
All good stuff Fred.
My favourite reason is that aluminium has a high thermal conductivity. Since heat is an important factor in machining (easily damages cutting tools and workpiece), aluminium is nice to machine since it will practically suck heat away from the cutting tool like a big sucky thing, allowing you to spin the tool right up.
High cutting speeds are preferred for 'softer' materials, i.e. low shear stress and in the case of Al, low elastic modulus and low yield strength, because the softer materials develop a built-up edge which provides for a rougher surface.
As for high thermal conductivity, I once tried to 'cut' an Al plate with an oxy-acetylene torch. Due to a heavy buildup of oxide and crud, I didn't not know the plate was Al (it was a cover to a sump). It was taking a long time, and then I noticed that the metal was puddling very easily. I finally figure I was trying to cut Al, and IIRC, I just a hack saw and cut it.
I think you've all hit on some good explanations for cutting speed, but the one issue that isn't resolved regards metals that are purposely made "free machining" such as 303, or 416 stainless steel, leadloy, B16 (sometimes called CDA360) brass, etc...
These metals are much easier to machine, generally because of alloying elements such as sulfer. I've heard that sulfer for example, reduces the shear strength between crystals, but that still doesn't strike me as a complete answer because one might also expect that those materials would be much lower in shear and even in tension than the same materials without sulfer added, and that simply isn't true.
So why does the addition of sulfer or other alloying elements help increase machining speeds but have little or no affect on strength? For example, with the addition of sulfer to 304, we essentially get 303 with the same mechanical properties except for machinability which is increased by a factor of 2 or 3 (going from memory).
Good question Q.
I was under the impression that sulfur was not the only thing added for an alloy to be labelled 'free machining.' I believe the one I have heard the most in addition to sulfur is selenium. Anyways, I dug up this article. As you will see here, it appears that the main area of focus is on replacing the lead constituent with other elements. Lead acts as a cutting tool lubricator since it has such a lower melting point.
I'm sure Astronuc will have some more guidance on this.
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