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- Dec 20, 2012
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- 9,422
Sorry, Bob. I meant to respond to this earlier but I got busy doing stuff for my wife; she doesn't understand that machining stuff is more important than getting the house ready for our annual Christmas party. Not done yet but I wanted to get to this before it cools.
You are correct, in part. One of the functions of back rake is to move the chip away from the edge and increasing back rake will accelerate chip flow but that doesn't affect how the chip breaks; it only affects flow. The same is true for side rake; the more side rake we have, the faster the chip ejects from the cut. The value of this is that it gets the chip out of the cut fast and that removes heat fast. This reduces work hardening and the occurrence of a built up edge so it is a big deal. However, the rake angles don't make a ductile chip break faster by themselves ... it requires a deep enough cut to do that.
A big cut produces a thick chip. As that chip hits the back of the ground part of the tool, it curls. Due to the thickness of the chip it tends to form cracks and with enough flow velocity, the chip breaks. Enough flow velocity is achieved with a high enough feed. Therefore, when you take a big cut at a high enough feed rate with an aluminum turning tool you will see chips instead of strings.
The raised chip breakers on carbide inserts work the same way. If you take a light cut with a carbide tool in aluminum, it will string. Take a big cut fast enough and the big chip hits the elevated chip breaker and the chip breaks. It isn't the elevated chip breaker that does it all; you need enough depth of cut and feed to make it work.
Now, consider what a grooved chip breaker does. This groove is sunken below the top plane of the tool. A ductile chip tends to flow right over the top of it and it has little effect. Well, at least in my experience it does. I have ground many kinds of chip breakers and did not find them of value but that's just me. I found that in order for a chip breaker to work I had to take a big enough cut with a fast enough feed rate ... but was it the chip breaker or the cutting conditions that caused the chip to break? I suspect it is the latter. In my view, a chip breaker has to alter the direction of flow in order for it to influence the chip so it has to be raised, not sunken below the plane of the tool.
So, this whole chip flow stuff gets a bit complicated, and the tool's geometry is only one part of it. The user has to know how to use the tool to best effect.
Oh, as for softer materials like brass and copper, a raked tool will work but it can dig in if the tool is not sharp. I know for sure that a Square tool will cut brass just fine but I will use a zero rake tool for brass in preference because of the finish it produces. A flat topped tool for brass will take a big cut and leave a beautiful finish, and I believe this has to do with how the chips break with that tool. Try a square tool and a flat top tool in brass and look at the chips. Compare the finish and the accuracy of the cut and you'll see the differences.
Anyway, this is all just my opinion on this stuff. I hope it hasn't confused you.
You are correct, in part. One of the functions of back rake is to move the chip away from the edge and increasing back rake will accelerate chip flow but that doesn't affect how the chip breaks; it only affects flow. The same is true for side rake; the more side rake we have, the faster the chip ejects from the cut. The value of this is that it gets the chip out of the cut fast and that removes heat fast. This reduces work hardening and the occurrence of a built up edge so it is a big deal. However, the rake angles don't make a ductile chip break faster by themselves ... it requires a deep enough cut to do that.
A big cut produces a thick chip. As that chip hits the back of the ground part of the tool, it curls. Due to the thickness of the chip it tends to form cracks and with enough flow velocity, the chip breaks. Enough flow velocity is achieved with a high enough feed. Therefore, when you take a big cut at a high enough feed rate with an aluminum turning tool you will see chips instead of strings.
The raised chip breakers on carbide inserts work the same way. If you take a light cut with a carbide tool in aluminum, it will string. Take a big cut fast enough and the big chip hits the elevated chip breaker and the chip breaks. It isn't the elevated chip breaker that does it all; you need enough depth of cut and feed to make it work.
Now, consider what a grooved chip breaker does. This groove is sunken below the top plane of the tool. A ductile chip tends to flow right over the top of it and it has little effect. Well, at least in my experience it does. I have ground many kinds of chip breakers and did not find them of value but that's just me. I found that in order for a chip breaker to work I had to take a big enough cut with a fast enough feed rate ... but was it the chip breaker or the cutting conditions that caused the chip to break? I suspect it is the latter. In my view, a chip breaker has to alter the direction of flow in order for it to influence the chip so it has to be raised, not sunken below the plane of the tool.
So, this whole chip flow stuff gets a bit complicated, and the tool's geometry is only one part of it. The user has to know how to use the tool to best effect.
Oh, as for softer materials like brass and copper, a raked tool will work but it can dig in if the tool is not sharp. I know for sure that a Square tool will cut brass just fine but I will use a zero rake tool for brass in preference because of the finish it produces. A flat topped tool for brass will take a big cut and leave a beautiful finish, and I believe this has to do with how the chips break with that tool. Try a square tool and a flat top tool in brass and look at the chips. Compare the finish and the accuracy of the cut and you'll see the differences.
Anyway, this is all just my opinion on this stuff. I hope it hasn't confused you.
