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  1. #1
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    Default Cuttability of different metals ?

    Was watching this video and wondered ..?

    "Peeling feeling" net trend, in tools - videos - HomemadeTools.net

    What is the name of the parameter that defines how easily a sharp tool can cut a piece of metal ?

    Did some googling on metal strength parameters and got horribly confused. Shear strength, youngs modulus, ductilty, this, that, the other, etc etc etc

    Is there any one single parameter that one of you experienced chaps can look at and then say "Yes I need some of that stuff to easily be able to cut this thing on my lathe/mill"

    I get the impression that experienced machinists know which materials to use for which jobs without needing to know too much about youngs modulus, shear strength, ductility, point load strength etc.

    I understand that material that is easy to cut may not be the best material to use for a given replacement part but am curious regarding what the various parameters might be that define machinability and whether or not something is "free cutting"

    Any good apprentice texts I should read ?

  2. #2
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    Check out this link, it may fill in a few blanks for you. https://www.productionmachining.com/...-carbon-steels

  3. #3
    BobL is online now Member: Blue and white apron brigade
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    SB - this topic is a can of worms. What you have effectively asked for is a course on metallurgy. You re correct in that most machinists don't worry about much or any of this metallurgical stuff and just remember that some brands and/or alloy numbers machine well and others don't, and experiment heavily with cutter profiles and speeds to optimise their machining.

    From Karl's link,
    The machinability of carbon steels is determined by a number of factors. Microstructure, cold work and content of carbon, sulfur, manganese, phosphorus, nitrogen and lead all play a role in improving machining productivity.
    To expand on this a bit further in a "back of the envelope" kind of way.
    Metals are typically made of two basic structures, "crystals" which consist of many many atoms, and the "atoms" themselves.
    Some metals have organised or disordered crystallised structures meaning the crystals are similarly sized and content while other have stuff all over the place.
    Pure metals are mostly one or a couple of different types of atoms (elements), while alloys like steels can have many different types of atoms.

    Cutting effectively involves "penetrating" and "tearing" (shearing) apart crystal structures and their insides so it depends how strongly they are held together . The roles of crystal size, arrangement and content is complex, some crystals are quite large and and play little role in cutting while some smaller crystals play a significant role. To demonstrated the importance of crystals can be seen in hardening and tempering processes which rearranges the crystal size/content to dramatically alter machinability.

    Since nearly all "cutting" involves an initial "penetration" of a metal surface then the "hardness" parameter is paramount. If a metal cannot be penetrated it cannot be cut.
    Some metals may be quite soft initially but work harden rapidly while being cut - this comes back to hardness again and this also affects the finish.

    Once the surface is penetrated the metal is usually cut by a shearing process so the shear modulus of a metal becomes significant. Although they are correlated, shear is slightly different to hardness and is usually what cause cutting surfaces to look ragged and cutting edges to wear quicker. This explains why although some metals may be relatively easily penetrated they can be "tough" so chips won't form and tear quite as easily as their hardness suggests.

    Despite all this, just looking up the hardness and shear modulus won't give you clear sailing to easy machining.
    For example, an important factor that enables a blade to keep cutting once it has achieved penetration relates to how "sticky" the metal is, so something like copper or aluminium and some steels are relatively soft, but they can stick to and clog/blunt cutting edges which in turn dramatically affects cut speed and the finish. This is also affected by cutter speeds, cutter profile etc. Perhaps surprising the additions of small amounts of what we might call sticky metals, like lead, often improves overall machinability.

    Clear as mud?

  4. #4
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    Bill the "parameters that define how a sharp tool cuts steel" are 1/ condition of the machine 2/ mass of the machine 3/rigidity of the machine.
    If you have a look at the cutting edge on most carbide inserts they have a small radius, ie not sharp like hss.

  5. #5
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    Quote Originally Posted by steamingbill View Post
    What is the name of the parameter that defines how easily a sharp tool can cut a piece of metal ?
    To answer this part of your question: The term, as you've used it, is "machinability", and metals are often rated for their machinability by suppliers.

    I always approach a design from the point of view of which material is best for the job, what is readily available, cost, then I solve the machinability questions.

    What's the worst material I've ever had to machine? Stainless steel! But I suspect that's only because I've never attempted to machine tool steels.

    Graham.

  6. #6
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    Quote Originally Posted by BobL View Post
    What you have effectively asked for is a course on metallurgy.

    Clear as mud?
    That may not do a lot of good.
    Made and poured a few tonne of ferrous and nonferrous metals in the years I pretended to be one, even did a bit of gold extraction when I moved to WA.
    None of this prepared me for driving the tool into a piece of spinning metal on the lathe or a spinning tool to fixed metal of a mill.

    I don`t have much in the way of advice other than to practice, experiment and carbide tips.


    Now Young`s modulus, is that a semi vertical line with a squiggle in it or ………

    Tony

  7. #7
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    Nothing in life or metalwork is free and often you find that one property is traded in order to gain another. Quite often machinability is gained, while weldability is sacrificed, strength gained and machinability lost.
    Find the right material properties for the application and then solve the machining afterwards as Oldbikerider posted. Thankfully most of us only dabble in one offs, so we can fiddle around without worrying about production efficiencies and tool wear which add yet more factors to the machinability equation.

  8. #8
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    Thanks for all the tips

    Summary :

    Machineability is affected by a number of metal properties.

    A bigger heavier massive stronger faster lathe with the right bits makes more things become free cutting.

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