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Models for grinding HSS Lathe Tools
- Thread starter mikey
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Just a thought, but working through the development of a tool would be very helpful. You've mentioned being unsatisfied with Sherline's right hand tool. Understand that you've probably lost the actual memories of improving it in the sands of time, but maybe you've got one laying around still?
I guess where I'm mainly unclear is what the specific angles do. I understand the relief angles are mainly to clear the the work surface, the rake angles are to help clear the chips and the radius is a compromise between durability and precision. What I don't understand is what the effect of a small angle say 5 degrees of relief does compared to a larger relief of say 20 degrees assuming both keep the work piece from rubbing against the tool.
I understand that even for experienced machinists some, maybe much of this is trial and error, but it would be nice to at least have a feeling for "if this then try that".
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- Dec 20, 2012
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I'll keep these things in mind as I try to figure out an approach. I am trying to decide how much to include because there is a LOT to this discussion. I will include the function of each angle, what a change to each does and how to decide how much to change.
The difficulty here is trying to say something in a clear and simple manner when the subject matter is anything but clear and simple.
If anyone can do it, it'll be Professor mikey.sounds like a good plan to start with.
What he said! We have time, think it out and when your ready to get started I'm sure we will be here. This should be very interesting!
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- May 10, 2017
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I’m excited about this going to the next level. Mikey has generated great interest in tool grinding and has done an excellent job of explaining the process of grinding the different angles that make a cutting tool. I’m looking forward to learning more of the mechanics behind the tools next.
- Joined
- Dec 20, 2012
- Messages
- 9,422
Okay, we’re going to discuss how a turning tool works. I decided that I am NOT going to discuss cutting theory (Oblique vs Othogonal cutting models or the Merchant Equation) because it would make this discussion hopelessly complex. What I will attempt to do is give you practical information that you can actually apply in your shop.
So, what happens during an actual cut?
Have you actually watched your tool cut? If you haven’t paid attention to it, you should. What you will see is that the initial point of contact is the side cutting edge, followed by the nose radius. As the tool cuts, the chip peels off and exits at a downward angle that is about midway between the side and end cutting edges (assuming you have both side and back rake). The side cutting edge and nose radius produces the chip but the nose radius and end cutting edge produces the FINISH when the tool is turned toward the chuck; the side cutting edge and nose radius produces the finish on a finishing cut when the lead angle is turned more toward the tailstock. Rarely, if ever, is the end cutting edge on anyone’s radar but it should be because it’s important.
In point of fact, every single angle on the tool is important and each one has a function. You will often see guys state that you only need to get the tool angles close and its good enough. I swallowed that line for a number of years and was frustrated because of poor tool performance. Granted, this was on a Sherline lathe but that was actually a good test bed because if a tool cuts well on that little lathe then it is a good tool. After literally thousands of test cuts I know that a change of even a degree or two of a particular angle can make a difference in how the tool cuts.
Okay, let’s discuss the different tool angles on a turning tool. Here is a graphic for reference:
The Side and End Cutting Edge Angles
These angles define the shape of the tool as seen from the top of the tool. Of all the angles on a turning tool, these two are the least critical. They determine how much mass is in the tip and whether or not you can access a shoulder but beyond that you have a lot of leeway in how the tool is shaped. When we ground these angles for our models we drew them out as inked lines but in time you will not need to do that.
If you have a QCTP, my suggestion is to stick with a general purpose shape; this gives you enough mass in the tip to tolerate changes to your relief and rake angles without worrying about the tip snapping off in use and it allows you to cut into a shoulder without rubbing.
The Relief Angles
Of course, we have two: the side relief angle and the end relief angle.
Most people believe that the only function of the relief angle is clearance and that once you have it, then you have it. Accordingly, it matters little if you use 5° or 15°, right? However, don’t forget that the relief angles are also one half of the cutting edge and, together with the rake angles, they form the cutting edges of the tool. Moreover, they partly determine the included angle of those cutting edges; the greater the relief angle, the narrower the included angle of the cutting edge.
On the left we have the typical turning tool as seen from the end of the tool. The side relief angle combines with the side rake angle to form an included angle. On the right is an increased side relief angle and combined with the same rake angle, the included angle is narrower. What this narrower angle does is improve penetration as the tool moves through the material and as a result, it takes less power to make the cut. On a Sherline lathe, just this single change (from 7 degrees to 15 degrees) will allow a 1/3 deeper depth of cut. AND it improves the finish.
The angle tables we all use to grind tools calls for relief angles of 8-10 degrees and we are led to believe that this is all we need for “clearance”. Now we know that maybe there is a bit more to it than just clearance.
So, how does this affect the end relief angle? Recall that I said that the end cutting edge is what produces the finish when the tool is roughly perpendicular to the work. If you look at the tip of your tool you will see that the end cutting edge is angled downward, right? In this position, the end cutting edge does not cut a chip; it shears. The bulk of the material is taken off by the side cutting edge and nose radius and the end cutting edge shears a fine finish. When combined with the back rake, the included angle at the end cutting edge is like a razor held to the work at an angle that shears cleanly, and the greater the included angle of this edge is, the sharper your shear becomes. Does that make sense?
Now, take a look at your turning tool angle table. Most tables call out for about 2° less end relief vs the side relief. Presumably, this is to add more mass to back up the tip of the tool for strength. In all the years I’ve used a turning tool I have never seen a tool tip break off because I didn’t follow this recommendation. Therefore, I recommend you use the same amount of relief for both the side and end.
As in all things, there is no such thing as a free lunch. Increasing the relief angles weakens the cutting edge so tool life may decrease. It isn’t that you will break the edge off but the tool may dull more easily with a lot of heavy cutting. For most of us in a non-production setting, this is no big deal. If you lightly hone your tools after use they will hold up for many years without issues. However, if you plan to use a tool for heavy cuts in hard materials it is wise not to increase the relief angles too much. Be conservative with your relief angle changes and use the rake angles to alter the included angle at the cutting edge instead.
The Rake Angles
Again, there are two rake angles: the side and back rake angles.
Of all the angles on a turning tool, the rake angles are the most important because they have a significant impact on penetration, finish, cutting forces (and where they are focused) and cutting temperatures. Just as importantly, they do this with almost no downside or penalty.
Personally, when I think of the rake angles I actually think of them as planes because that is what they really are. On two edges of each plane is a cutting edge formed by the relief angle and the top rake surface of the tool. The area behind the cutting edges provides a pathway for chip egress. The angle of this pathway is a combination of both rake angles.
If you grind a tool with only side rake on top of the tool you will see the chip peel off directly perpendicular to the side cutting edge. This tells you that the cutting forces flow perpendicular to the side cutting edge. Now, if you add back rake you will see two things: the origin of the chip will shift toward the tip and the angle of the chip flow will angle back and exit roughly in a line between the side and end cutting edges.
Now you know that you can shift the origin of the chip, the area where the cutting forces are focused, by altering back rake; the more back rake you use, the closer to the tip the forces are focused. As this occurs, the chip thins and accelerates, the result of which is a reduction of cutting forces, cutting temperatures and an improved finish.
Okay, so how do the rake angles reduce cutting forces? I will try to simplify this as much as possible but have a look at this graphic that I blatantly stole off the net:
This graphic shows chip formation during an Orthogonal cut. Our tool does not cut this way; it is an Oblique cutting tool. However, the effect of the rake angles on the shear plane is the same. Note that the depth of cut in both cuts in the illustration are the same. Also note that the line, Ø, is actually a PLANE that is as thick and wide as the chip itself.
What you are seeing here is that as the rake angle increases, the shear plane angle increases and the length of the plane decreases and this leads to chip thinning, a reduction in the force required to produce the chip and an increase in chip velocity as it exits the cut.
EDIT: the above paragraph is true; it is also as clear as mud. The picture above, while accurate, has the wrong perspective; turn it 90 degrees clockwise to see what happens during a cut on the lathe.
What I am saying is that as the rake angles increase (applies to either side or back rake), the included angle of the cutting edge increases - it gets sharper. This causes the chip to thin out and break easier so that it takes less force to move the cutting edge through the cut; the result is a reduction in cutting forces. What we see at the lathe is that it takes less power to make the cut so that a smaller, less rigid lathe can take a cut that it normally could not take when a conventional tool is used. The chip also exits the cut faster so that cutting temperatures are reduced because the heat goes out of the cut with the chip. The effect of rake on the shear plane is WHAT happens; a reduction of cutting forces and cutting temperatures is the RESULT, and trust me, this is a big deal insofar as the results you see at the lathe.
Okay, okay, this is getting more into cutting theory than I intended. Suffice to say, the rake angles are important. What you should realize is that BOTH rake angles do this to the chip and their effects are additive. I subjected you to this in order to make sense of the following benefits of the rake angles:
Finally, the nose radius
I have seen all sorts of recommendation for the nose radius, from the “go big for better finishes” to benign neglect. Here is the reality – the nose radius is, for lack of a better term, a shearing device.
Take a good look at your nose radius and you will see that it is actually a really complicated feature. It angles up due to the back rake and also angles to the side because of the side rake. It also has a huge undercut because of both relief angles.
Recall I said that the nose radius and end cutting edge produces the finish on your part and that they do this in a shearing action. The nose radius actually serves as both a transition to the end cutting edge and a participant in the shearing cut the end of the tool produces. As such, the radius doesn’t need to be huge to do this job; it just has to be there.
What’s more, the nose radius is the key cause of radial deflection. Since the tool is more or less fixed, radial cutting forces will move or deflect the work away from the tip of the tool. The bigger the nose radius, the greater the deflection. If the work piece is big and rigid enough you may not see an effect but on a smaller diameter work piece it can deflect enough to produce a taper or chatter.
So, if you want an accurate turning tool that cuts with minimal deflection but still finishes well, keep the nose radius small and rely on the end cutting edge and back rake to produce the finish you need.
How small is small? For general purpose tools used for most materials, I suggest a 1/64” nose radius. For aluminum, brass and plastic tools, 1/32” is enough.
Earlier in this thread I said that I never felt the need for a Shear Tool because my regular tools finished well enough for me. Now you know why I said that; my nose radius IS a shear tool but unlike a regular shear tool, this one can take any cut I can dial in.
Okay, now you know what the angles do and how the tool cuts. Let me hear your questions or comments and we’ll move on to discuss how you can use their properties to modify your tools to work better for you.
So, what happens during an actual cut?
Have you actually watched your tool cut? If you haven’t paid attention to it, you should. What you will see is that the initial point of contact is the side cutting edge, followed by the nose radius. As the tool cuts, the chip peels off and exits at a downward angle that is about midway between the side and end cutting edges (assuming you have both side and back rake). The side cutting edge and nose radius produces the chip but the nose radius and end cutting edge produces the FINISH when the tool is turned toward the chuck; the side cutting edge and nose radius produces the finish on a finishing cut when the lead angle is turned more toward the tailstock. Rarely, if ever, is the end cutting edge on anyone’s radar but it should be because it’s important.
In point of fact, every single angle on the tool is important and each one has a function. You will often see guys state that you only need to get the tool angles close and its good enough. I swallowed that line for a number of years and was frustrated because of poor tool performance. Granted, this was on a Sherline lathe but that was actually a good test bed because if a tool cuts well on that little lathe then it is a good tool. After literally thousands of test cuts I know that a change of even a degree or two of a particular angle can make a difference in how the tool cuts.
Okay, let’s discuss the different tool angles on a turning tool. Here is a graphic for reference:
The Side and End Cutting Edge Angles
These angles define the shape of the tool as seen from the top of the tool. Of all the angles on a turning tool, these two are the least critical. They determine how much mass is in the tip and whether or not you can access a shoulder but beyond that you have a lot of leeway in how the tool is shaped. When we ground these angles for our models we drew them out as inked lines but in time you will not need to do that.
If you have a QCTP, my suggestion is to stick with a general purpose shape; this gives you enough mass in the tip to tolerate changes to your relief and rake angles without worrying about the tip snapping off in use and it allows you to cut into a shoulder without rubbing.
The Relief Angles
Of course, we have two: the side relief angle and the end relief angle.
Most people believe that the only function of the relief angle is clearance and that once you have it, then you have it. Accordingly, it matters little if you use 5° or 15°, right? However, don’t forget that the relief angles are also one half of the cutting edge and, together with the rake angles, they form the cutting edges of the tool. Moreover, they partly determine the included angle of those cutting edges; the greater the relief angle, the narrower the included angle of the cutting edge.
On the left we have the typical turning tool as seen from the end of the tool. The side relief angle combines with the side rake angle to form an included angle. On the right is an increased side relief angle and combined with the same rake angle, the included angle is narrower. What this narrower angle does is improve penetration as the tool moves through the material and as a result, it takes less power to make the cut. On a Sherline lathe, just this single change (from 7 degrees to 15 degrees) will allow a 1/3 deeper depth of cut. AND it improves the finish.
The angle tables we all use to grind tools calls for relief angles of 8-10 degrees and we are led to believe that this is all we need for “clearance”. Now we know that maybe there is a bit more to it than just clearance.
So, how does this affect the end relief angle? Recall that I said that the end cutting edge is what produces the finish when the tool is roughly perpendicular to the work. If you look at the tip of your tool you will see that the end cutting edge is angled downward, right? In this position, the end cutting edge does not cut a chip; it shears. The bulk of the material is taken off by the side cutting edge and nose radius and the end cutting edge shears a fine finish. When combined with the back rake, the included angle at the end cutting edge is like a razor held to the work at an angle that shears cleanly, and the greater the included angle of this edge is, the sharper your shear becomes. Does that make sense?
Now, take a look at your turning tool angle table. Most tables call out for about 2° less end relief vs the side relief. Presumably, this is to add more mass to back up the tip of the tool for strength. In all the years I’ve used a turning tool I have never seen a tool tip break off because I didn’t follow this recommendation. Therefore, I recommend you use the same amount of relief for both the side and end.
As in all things, there is no such thing as a free lunch. Increasing the relief angles weakens the cutting edge so tool life may decrease. It isn’t that you will break the edge off but the tool may dull more easily with a lot of heavy cutting. For most of us in a non-production setting, this is no big deal. If you lightly hone your tools after use they will hold up for many years without issues. However, if you plan to use a tool for heavy cuts in hard materials it is wise not to increase the relief angles too much. Be conservative with your relief angle changes and use the rake angles to alter the included angle at the cutting edge instead.
The Rake Angles
Again, there are two rake angles: the side and back rake angles.
Of all the angles on a turning tool, the rake angles are the most important because they have a significant impact on penetration, finish, cutting forces (and where they are focused) and cutting temperatures. Just as importantly, they do this with almost no downside or penalty.
Personally, when I think of the rake angles I actually think of them as planes because that is what they really are. On two edges of each plane is a cutting edge formed by the relief angle and the top rake surface of the tool. The area behind the cutting edges provides a pathway for chip egress. The angle of this pathway is a combination of both rake angles.
If you grind a tool with only side rake on top of the tool you will see the chip peel off directly perpendicular to the side cutting edge. This tells you that the cutting forces flow perpendicular to the side cutting edge. Now, if you add back rake you will see two things: the origin of the chip will shift toward the tip and the angle of the chip flow will angle back and exit roughly in a line between the side and end cutting edges.
Now you know that you can shift the origin of the chip, the area where the cutting forces are focused, by altering back rake; the more back rake you use, the closer to the tip the forces are focused. As this occurs, the chip thins and accelerates, the result of which is a reduction of cutting forces, cutting temperatures and an improved finish.
Okay, so how do the rake angles reduce cutting forces? I will try to simplify this as much as possible but have a look at this graphic that I blatantly stole off the net:
This graphic shows chip formation during an Orthogonal cut. Our tool does not cut this way; it is an Oblique cutting tool. However, the effect of the rake angles on the shear plane is the same. Note that the depth of cut in both cuts in the illustration are the same. Also note that the line, Ø, is actually a PLANE that is as thick and wide as the chip itself.
What you are seeing here is that as the rake angle increases, the shear plane angle increases and the length of the plane decreases and this leads to chip thinning, a reduction in the force required to produce the chip and an increase in chip velocity as it exits the cut.
EDIT: the above paragraph is true; it is also as clear as mud. The picture above, while accurate, has the wrong perspective; turn it 90 degrees clockwise to see what happens during a cut on the lathe.
What I am saying is that as the rake angles increase (applies to either side or back rake), the included angle of the cutting edge increases - it gets sharper. This causes the chip to thin out and break easier so that it takes less force to move the cutting edge through the cut; the result is a reduction in cutting forces. What we see at the lathe is that it takes less power to make the cut so that a smaller, less rigid lathe can take a cut that it normally could not take when a conventional tool is used. The chip also exits the cut faster so that cutting temperatures are reduced because the heat goes out of the cut with the chip. The effect of rake on the shear plane is WHAT happens; a reduction of cutting forces and cutting temperatures is the RESULT, and trust me, this is a big deal insofar as the results you see at the lathe.
Okay, okay, this is getting more into cutting theory than I intended. Suffice to say, the rake angles are important. What you should realize is that BOTH rake angles do this to the chip and their effects are additive. I subjected you to this in order to make sense of the following benefits of the rake angles:
- Improves penetration by reducing the included angle at the cutting edges, independent of the relief angles. This reduces cutting forces without weakening the cutting edges, a very big deal.
- Thins the chip and accelerates chip flow, thereby reducing both cutting forces and temperatures. Most of the heat in a cutting operation goes out with the chip so this matters. A good case in point is an aluminum cutting tool; a positive rake aluminum cutting tool will rarely, if ever, develop enough heat to have a built up edge. The chip is moving too fast and cool to allow chip welding – pretty cool, eh? (sorry, couldn’t resist the pun!)
- Allows us to accommodate different materials. Different materials require different amounts of side and back rake to cut efficiently. This has to do with how the chip forms and breaks … and I am NOT getting into that.
- Back rake allows us to tailor where our cutting forces are focused. By increasing back rake we can shift the forces toward the tip to enhance finishes or improve the facing capability of a tool. By reducing back rake we can shift the forces to the side cutting edge to allow for heavier cuts while relying on side rake to reduce cutting forces and temperatures. Back rake isn’t even on most people’s radar; too bad because it is one of the most important angles on a turning tool.
Finally, the nose radius
I have seen all sorts of recommendation for the nose radius, from the “go big for better finishes” to benign neglect. Here is the reality – the nose radius is, for lack of a better term, a shearing device.
Take a good look at your nose radius and you will see that it is actually a really complicated feature. It angles up due to the back rake and also angles to the side because of the side rake. It also has a huge undercut because of both relief angles.
Recall I said that the nose radius and end cutting edge produces the finish on your part and that they do this in a shearing action. The nose radius actually serves as both a transition to the end cutting edge and a participant in the shearing cut the end of the tool produces. As such, the radius doesn’t need to be huge to do this job; it just has to be there.
What’s more, the nose radius is the key cause of radial deflection. Since the tool is more or less fixed, radial cutting forces will move or deflect the work away from the tip of the tool. The bigger the nose radius, the greater the deflection. If the work piece is big and rigid enough you may not see an effect but on a smaller diameter work piece it can deflect enough to produce a taper or chatter.
So, if you want an accurate turning tool that cuts with minimal deflection but still finishes well, keep the nose radius small and rely on the end cutting edge and back rake to produce the finish you need.
How small is small? For general purpose tools used for most materials, I suggest a 1/64” nose radius. For aluminum, brass and plastic tools, 1/32” is enough.
Earlier in this thread I said that I never felt the need for a Shear Tool because my regular tools finished well enough for me. Now you know why I said that; my nose radius IS a shear tool but unlike a regular shear tool, this one can take any cut I can dial in.
Okay, now you know what the angles do and how the tool cuts. Let me hear your questions or comments and we’ll move on to discuss how you can use their properties to modify your tools to work better for you.
Last edited:
- Joined
- May 10, 2017
- Messages
- 1,199
Mike, you referenced the tool at different angles to the work, angled toward the chuck, perpendicular to the work and finish cuts angled toward the tail stock. This is the same tool on the same work at different phases of the job? Roughing to finish cuts? Do you normally shift the angle of the tool to utilize the different features of the tool throughout the job?
I have plenty blank tool bits, I’ll get out on the lathe and pay closer attention to things mentioned here.
Great write up, thanks
I have plenty blank tool bits, I’ll get out on the lathe and pay closer attention to things mentioned here.
Great write up, thanks
- Joined
- Dec 20, 2012
- Messages
- 9,422
Yeah, when we rough the tool is roughly perpendicular to the work or maybe slightly canted towards the chuck and in that position the end cutting edge is in play. When you turn into a shoulder the nose radius and end cutting edge is definitely in play. When doing sizing or finishing cuts the finish is produced by the nose radius and side cutting edge. The important point is that both the side and end cutting edges and the nose radius are important and I brought it to your attention because nobody ever pays any attention to the end cutting edge. They seem to think that the finish on the work piece magically appears or that the side cutting edge produced the finish all by itself - not true.