Hey Guys. This post is in response to a request for more information about how the various tool angles work and how that information can be used to modify your tools if you should choose to do so.
Now, this is going to look very much like a tutorial, which implies that I have some sort of expertise that qualifies me to do a tutorial. I will tell you outright that I don’t. I’m just another hobby guy like you who happens to have done some experimenting over the years and has formed some opinions on how tool geometry works. I am not a tool engineer so the information here is worth exactly what you’re paying for it.
I want to clarify something up front. I am going to simplify this discussion and take some liberties that allow me to transmit concepts more easily. I’m not lying; I’m avoiding a complex discussion that would otherwise require a long article instead of an easily communicated set of ideas. For example, if I say that increasing a relief angle helps penetration (even Machinery’s Handbook uses this term) that may not be entirely correct. To be correct, I would have to launch into how increasing the angle causes the chip to shear off before the flank of the relief angle contacted the work and then go into the physics of how that affects the three cutting forces and the various types of chip deformation and … well, you get the idea. This is a forum thread and like I did with that article that Joe linked to I would prefer to just keep it intuitive and simple so I can transmit concepts. The important thing is that these concepts, when applied to a turning tool, actually work in real life. If I can get a budding tool grinder to understand these concepts and help him grind a good working tool then the liberties I am taking are acceptable to me. I hope it is acceptable to you.
Okay, let’s begin with the bottom line:
What really matters is that you understand what every angle on your tool does; only then can you develop control over it to make your tool do what you need it to do.
Edge Angles and Lead Angles
The side cutting and end cutting edge angles determine the shape of the tool but do not directly alter cutting forces. Nowadays, the older tool shapes are not used as much by hobby guys because our QCTP allow us to rapidly change our tool position. A general purpose tool is more common and provides strength, access to shoulders and more mass that is more forgiving of angle modifications. However, this general shape does require more attention to our lead angles that the side cutting edge angles used to provide automatically because lead angles do directly alter cutting forces.
Remember that when the side cutting edge angle is not at the standard angle provided by a traditional tool then the lead angle will be determined by the side cutting edge angle as you have positioned it and an imaginary line perpendicular to the direction of feed.
As lead angle increases, so do cutting forces because more of the side cutting edge contacts the work. On rigid work pieces this results in better finishes but on thin work this can cause deflection and chatter. I won’t go much more into this other than to say that we want to use as much lead angle as possible (to enhance finishes) as long as there is no chatter. If chatter develops, reduce your lead angle before altering speed and feed.
Relief Angles
There are two relief angles on a turning tool – the side relief and end relief angles. They are critical angles because they form half of the cutting edges of the tool.
The primary role of a relief angle is penetration, especially side relief. I’m not talking about penetration in the sense of how easily the tool enters the work; I’m talking about the ease with which the tool moves through the work as it cuts. If you visualize that a greater relief angle will form a more acute included angle at the side cutting edge you can imagine how that angle will cut through the material with greater ease; this is essentially how increasing relief angles reduces cutting forces. Nothing comes for free, however, and we do lose some support under the cutting edge so increasing relief angles may reduce useful edge life. On a hobby lathe in non-production situations this is usually not an issue provided we don’t go crazy with our angle changes.
End Relief angles are usually viewed as support angles (when we think of them at all). If you look at a typical grinding table you will see that the end relief angle is usually about 2-3 degrees less than the side relief angle. This added mass backs the tip of the tool for strength. Unfortunately, it also adds resistance to the cut and retains heat. This is fine if you’re doing long production runs with a heavy lathe on hard materials but that’s not what most hobby guys do, or at least this one. Therefore, it is my practice to equalize the side and end relief angles with the intent of improving shearing action at the tip and to further reduce cutting temperatures. I suggest you consider doing the same. A tool ground this way will cut with greater ease that you can feel and I have yet to break a tip off, even with heavy cuts.
Rake Angles
Of all the angles on a turning tool, the rake angles are the most important. They are usually thought of as chip clearance angles, mostly because it’s true. However, they are also the critical upper half of the side and end cutting edges and play a major role in tool penetration, tool life, and cutting force and cutting temperature reduction.
Side rake is the most important variable in your tool’s geometry. Like side relief, increases in side rake will increase the included angle of the side cutting edge so penetration is enhanced and cutting forces decrease as a result. Unlike side relief changes, this advantage comes without affecting support under the cutting edge so tool life is actually improved. Moreover, since increases in side rake promote better chip evacuation we also see reduced cutting temperatures. No other angle change provides so many benefits with so little risk. When lower cutting forces and temperatures are your priority, think about side rake first, relief angles second and back rake third.
Back rake interfaces with the end cutting edge so increases in back rake increases the included angle at the end edge. While you might think this will have little effect, remember that as back rake increases the cutting load shifts from the side cutting edge toward the tip and this can significantly improve finishes. If you want to see the effect of increased back rake, watch a video on how a tangential tool cuts. These tools have what amounts to substantial back rake and as it cuts you will see the chip spiral off the tip. That’s what increasing back rake does – it focuses the cutting forces at the tip. Conversely, as back rake decreases it transfers the cutting load to the side cutting edge. Knowing this allows you to shift where the cutting load is focused; this is a rather useful thing to know, I think. Finally, increasing back rake creates a more positive rake at the cutting tip; this enhances chip flow so cutting temperatures are further reduced.
Changes to side rake and back rake are the closest we will come to a free lunch in tool grinding. Far and away, they are the most useful angles when altering turning tool geometry. We can now see why Machinery’s Handbook says studies show that “… cutting forces and cutting temperatures decrease and tool life increases as side rake and back rake become more positive …” We have mostly upside with very little downside with rake angle changes provided we keep our changes within reason.
The Thinking Part
I hope this gives you a clearer picture of what the various tool angles do, which should make it easier to choose which angle to change to enhance the performance aspect you wish to improve. It is important to understand that while we can change a single angle to enhance our geometry, we can also alter multiple angles on the same tool and the effect will be additive. An example of how you might modify a tool with multiple angle changes may help illustrate this.
Say we intend to cut a material that is fairly hard and is known to work harden. 1144 Stressproof steel is a good example of such a material. On a heavy lathe, this steel cuts and finishes very nicely but on a smaller, less rigid lathe it can be a challenge. 1144 roughs well at low speeds but likes very high speeds to finish well. If the surface work hardens due to high temperatures in the cut it can be difficult to take a lighter sizing cut and still hold tolerances or produce the fine finish this steel is capable of. In order for us to work effectively with this material we need to think about how alterations to our tool geometry can enhance our ability to machine it before running into the rigidity and power limits of our lathe.
Clearly, we need to reduce cutting forces so we can rough with a decent cut at a feed rate that won’t stall the lathe or slow it much; this reduces the potential for work hardening. For the same reason we also want to reduce cutting temperatures as much as we can. From the discussion above we know that our first choice will be to increase side rake; this will greatly reduce cutting forces and temperatures with a single change. To further aid in cutting force reduction we could also increase side and end relief by a few degrees without endangering the cutting edge. We can also increase back rake to reduce cutting forces and temperatures even more while improving our finishing potential. Smaller nose radii cut with lower cutting forces because it is easier to bury it in the cut for added support so we will keep our nose radius at about 1/64” and depend on the increased back rake to aid in finishing potential.
Will such a tool really work? Yup, it does. I have one exactly like it, shaped with the above reasoning. On my personal tool, the side and back rake angles were increased by 4 degrees each, while the relief angles were increased by 2 degrees. It will take a much heavier cut than a standard tool and doesn’t seem to work harden the material in the process. As a result, sizing cuts are very accurate and the finish is very good.
Now that we have an idea of how to think of which angle(s) to change the question then becomes: how much do we change each angle for a given tool? The answer is … it depends. Each lathe will differ in degrees of wear, rigidity, power and how the user likes to use that lathe. It also varies with the material being cut. Therefore, there are no pat answers for how much to change an angle.
There are, however, general guidelines we can use. Keep in mind that these are guidelines, not rules. Break them and see what happens – I did.
· The smaller the lathe the more latitude you have with the number of angles you change and the degree to which you can change them. This is because you will take lighter cuts due to the limited power and rigidity you have so the chances of damaging even a heavily modified tool is small.
· The more mass in the tool shape, the greater your latitude for angle changes. For this reason, roughing and general use tools are the most forgiving of shapes for us tool modifiers.
· The harder the material and/or the greater the potential for work hardening, the more emphasis (bigger angle changes) you should put on modifying side rake. I also suggest keeping back rake near baseline to shift the cutting load to the side edge for most hard stuff. You can also boost the relief angles a small amount but be conservative to maximize support under the side cutting edge. Stainless is an exception here; it likes larger relief angles and on a small lathe I add more side rake, too.
· When finishing potential is your main priority think back rake and relief angles first. Do not forget that lead angle can be very useful when using the tool.
· To be very clear, when we speak of modifying angles we are speaking of increases to the baseline values already listed in the typical angle table. For example, if side relief is listed as 10 degrees and we wish to increase penetration we may increase side relief by 2-3 degrees for a final angle of 12-13 degrees.
Note also that each angle will vary with the material being cut. That being the case there is no one tool that works for every material and my best advice is to grind at least one good general purpose tool for every material group you commonly work with. Clearly, you must also be familiar with the cutting characteristics of your chosen material (hard, soft, work hardens, etc.)
If you choose to stay with the traditional tool shapes then remember that a rougher cuts with the side cutting edge and tip, a facer cuts primarily with the side cutting edge near the tip and a finisher cuts primarily with the nose. For roughers I change mainly side and back rake; for facers I alter side and end relief and side rake but back rake is left close to standard angles; for finishers I increase side and end relief, side and back rake and increase the nose radius. I leave it to you to figure out why these changes work for the tools involved.
In general, increases to the baseline angles in a grinding table of between 1-5 degrees will produce significant improvements when changing any one angle but when changing multiple angles remember that the improvements will be additive. A few degrees added to both the relief and rake angles may allow you to nearly double your usual depth of cut on a really light lathe so you do not need to be overly aggressive with your changes, especially when changing multiple angles. In fact, it is better to be conservative and selective at first and add more angle in the areas that need it once you see how the tool cuts.
It should now be clear why a tool rest that is settable to precise angles is very useful to a tool grinder. Quite often we will change an angle by only a degree or two so I suggest you leave free hand grinding to those with big lathes and focus on controlling your geometry changes with a good rest. Remember that half the angles at the tool tip are also determined by the angle YOU hold the bit at when grinding them so your skill matters … practice!
The choice of whether or not to alter your tool is up to you. If you choose to change it then I hope this information helps you think through what you’re changing and why you’re changing it. Once you know that and have chosen conservative angle changes to try then grinding the tool is a simple matter of setting your grinding table to the angle you need and grind away. Once you see how the tool cuts you can fine-tune your angle changes until the tool cuts exactly the way you want.
Good Luck!
Mikey
