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Angle of HSS turning tool to work

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by Jmanb13, May 3, 2017.

  1. mikey

    mikey Active User H-M Supporter-Premium

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    Ken, I'll try to explain back rake but you must keep in mind that while it is, in fact, an angle on a turning tool it is one of two angles that determine a plane. This plane is actually a combination of the side rake and back rake angles and together they form the top horizontal surface of a turning tool. This plane can be angled in a positive, negative or zero direction, depending on how these two rake angles are ground. Here you can see the back rake angle from the side; it is that angle under the bit sitting on top of the ground tool:

    CRW_4592.jpg

    Note that it slopes from the tip of the tool so that the back of the angle is lower than the tip; this is called positive back rake. Keep in mind that the entire top horizontal surface (the top plane) of the tool is also sloping back in a positive direction. Side rake is the other component of this top plane and it is sloping away from the tip as well so it is also positive.

    IMG_4595.jpg

    So, as you can see, the top surface or plane of the tool is actually a flat plane, the angle of which is determined by the side and back rake angles.

    Both rake angles can be positive, negative or zero. Just look at the tool from the tip. If the angles slopes away from the tip then the rake is positive. If it slopes toward the tip then its negative. If there is no slope to either rake angle then it is a zero-rake tool and is simply flat on top.

    I know your question pertains to back rake but no tool angle works in isolation and the rake angles are no exception. The rake angles are the most important angles on a turning tool because they have a tremendous impact on cutting forces and cutting temperatures. In fact, they are the key set of angles that change when a tool is ground for different materials. If you look at a turning tool angle table, you will see that the relief angles change a little for each material class but the rake angles are the main variables that allow the tool to work with different materials. As the tool cuts, it separates the chip along a plane called a shear plane and each material we turn has an optimal shear plane angle that clears the chip from the cut. This is why the rake angles change for each material. I will stop with the shear plane garbage now but just know that back rake shortens the shear plane and thereby reduces cutting forces significantly. You can grind a tool without back rake but your tool will cut with higher cutting forces than it should/could.

    So, what does back rake actually do (aside from its impact on the chip)? Basically, it alters the direction and speed of chip flow and also determines where the cutting forces are directed.
    • Chip flow is largely determined by side rake. In general, cutting forces will run perpendicular to the side cutting edge. When you add in back rake, it changes the direction of the chip flow and the more back rake you have, the faster the chips exit the cut and this reduces cutting temperatures.
    • If your tool has no back rake, all the cutting forces are focused on the side cutting edge. If you add back rake, you shift the cutting forces toward the tip of the tool and the more back rake you have, the more tip-ward the shift will be. This is why finishing tools, which cut with the nose radius, generally have larger back rake angles. On the other hand, roughing tools that cut mainly with the side cutting edge will have lower degrees of back rake. Facing tools cut mainly with the side cutting edge but up near the tip so these tools generally have more back rake than roughing tools but less than finishing tools. If you wish to see a video of how back rake shifts cutting forces, look up a video on tangential tools.
      Tangential tools have what amounts to significant back rake and as it cuts you will see the chip curling off the very tip of the tool. As the depth of cut increases you can see that it peels off a chip at the very tip.
    In general, back rake is an important angle. It plays a significant role in reducing both cutting forces (by shortening the shear plane) and cutting temperatures (by boosting chip flow out of the cut) and allows you to focus the cutting forces on your tool so it does what you need it to do. If you need more strength at the side cutting edge, reduce back rake. If you need better finishing potential, increase back rake. If you are cutting materials that work harden easily then getting the chips out of the cut faster helps to reduce temperatures at the point of cut; increasing side and back rake will help do that.

    Between the two rake angles, side rake is more important than back rake. Side rake greatly impacts on cutting forces because increases in side rake narrow the included angle at the cutting edge, thereby allowing for better penetration into the material. Back rake adds to this effect by narrowing the included angle at the end cutting edge and this also improves penetration (on top of the other things back rake does).

    There are as many ways to grind a tool as there are tool grinders. Some of us think back rake is not important; some, like me, think it is very important and I use it to help me make the tool do what I want it to do. Other than my threading tools and brass tools, all my turning tools have side and back rake angles ground into them and I tailor the amounts of each rake angle to suit the material I'm working with. Its okay not to grind back rake into your tool but it will cut better for you if you do.

    Hope this helps, Ken.
     
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  2. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    Thank you for an explanation that makes sense.
     
  3. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    If you insist on HSS tooling grind it however you want until it works .
    All of the grinding how to guides and youtube videos are general guidelines, this does not mean that grinding a tool this way will achieve your particular goals. As much as one may expect excellent results going by the book this is often not achieved.
    Trial and error is how it is done, select a tool or grind that works for the particular job at hand.
     
  4. benmychree

    benmychree United States John York H-M Supporter-Premium

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    Very articulate Mike; I fly more by the seat of the pants, but the way we grind tools is to get the results we want, I just go to the grinder and create a tool that I think will work best for me base on experience rather than theory; mostly as I have said, I avoid using much back rake because of its tendency to weaken the tool, but it is pretty much moot because most metal is removed by carbide inserts by hobby folk and professionals alike.
     
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  5. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    Mike, I know I promised not to bother with the "why's for now and only stick with the "how"s until I get comfortable with operating my mini lathe but the more I find out about the impact of cutting forces and cutting temperatures the more important it is to know why especially with such small lathe so thanks for making it all clear enough to understand and so interesting .

    All these youtube videos we watch could be a great source to educate ourselves but it could also be a source of confusion, I'm glad you are around to clear up, define, and interpret it all .
     
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  6. mikey

    mikey Active User H-M Supporter-Premium

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    I totally get what you're saying. You know how to grind a tool to do what you want on your lathe based on your experience so what works for you, works for you. Back rake, no back rake - if it works, that's what counts.

    For what it's worth, I simply wanted to help Ken understand one aspect of tip geometry to aid in his understanding of this complex subject. What I do encourage folks to do is grind tools with and without back rake (or any other tool angle) and come to their own conclusions. I have ground hundreds of tools, focusing on figuring out how each tool angle works and what it does. For me, this is not theory but I don't expect anyone to take my word for it; grind a tool and see for yourself.
     
  7. Desolus

    Desolus United States H-M Supporter - Premium Content H-M Supporter-Premium

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    I grind one for the sole purpose of chip controll.
     

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