General Purpose Turning Tool
I’ve already discussed what this tool does and pretty much how to grind it. With all we’ve discussed and my other articles, grinding a tool should be pretty old hat by now. In deference to Tweinke, I will just show how a left hand turning tool would be laid out. The grinding is the same – just grind to the lines. The rake angles would be the same but ground on the other side of the belt or wheel vs a RH tool. So, a mirror image of a RH tool.
Before I can explain why the Square Tool is ground the way it is, I have to briefly discuss cutting forces and how an angle table is used so that it makes sense.
Cutting Forces
There are actually three cutting forces in play whenever a tool comes in contact with a work piece. They are Tangential forces, Radial forces and Axial or Feed forces. I blatantly stole an image off the net because I’m too lazy to draw my own:
Tangential forces (Fc in the pic) push down on your lathe tool when the tool is buried in the cut. They comprise about 75% of the sum of all the forces your tool experiences so anything that reduces or mitigates this force is a good thing.
Radial forces (Fr in the pic) push the tool out of the cut at a 90° angle. They are about 1/3 – ¼ the magnitude of Tangential forces but they matter because this is what causes
deflection. Since the turning tool is somewhat rigidly mounted and is mostly non-compressible,
Radial forces will cause the work piece to flex away from the tool instead. Our goal is to reduce this force to the extent we can to improve accuracy.
Axial forces are feed forces (Ft in the pic). In general, axial forces do not directly affect the cut. As the tool moves toward the chuck, the forces it encounters are these feed forces.
Each cutting force can actually be measured with strain gauges attached to the tool but I don’t own any. When I experimented with this stuff over 20 years ago, all I had was an ammeter hooked up to the motor of my lathe. My theory was that as I altered each angle of a turning tool, it would alter the cumulative cutting forces and I would be able to see a change in the motor load. I admit this is a really crude and insensitive set up but it was all I had at the time. Sadly, I lost the numerical data but I remember the results and here is what I think I know:
- Cutting conditions matter. Assuming that motor load implies a change in cutting forces, as depth of cut and feed rate increase, cutting forces go up. As cutting speed increases, forces actually go down.
- Increasing the relief angles reduces cutting forces. The more relief angles increase, the lower the forces. However, this also weakens the cutting edge so changes here must be conservative.
- Increasing side rake reduces cutting forces more than increasing relief angles does. [It does this by shortening the Shear Plane length. The Shear Plane is an actual plane that extends at a 90° angle from the cutting tip through the thickness of the chip. As the Shear Plane length gets shorter, cutting forces go down, and vice versa.]
- Increasing back rake reduces cutting forces (same Shear Plane nonsense) but less than increasing side rake does.
o Back rake has more of an impact than you might think. If you look at the tool from the front you can see that the included angle at the side cutting edge is comprised of the side relief and the side rake angles. If you look at the tool from side you can see that the end relief and the back rake form another cutting edge, the end cutting edge. Alterations to either rake angle alters Shear Plane length so both impact on cutting forces.
o Back rake also shifts the focus of cutting force concentration. In general, cutting forces will run perpendicular to the side cutting edge. This is reflected by the direction of chip flow. If you look at a tool ground with relief and side rake angles only, the chips will flow perpendicular to the side cutting edge. Now, when you add back rake you are adding that second included angle at the front end of the tool and that changes the focus of the forces. Now the chip flow direction is no longer perpendicular to the side cutting edge; it angles down and away from the tip of the tool. As back rake increases, the point where the chip leaves the work piece shifts closer and closer to the tip of the tool and when that happens the finish improves. Moreover, if you look carefully, you’ll see that chip flow rate actually increases and the chip thins out.
So, let’s summarize this into something useful:
1.
Increasing the relief angles reduces cutting forces. It also improves finishes because it reduces rubbing under the side cutting edge. Increasing relief angles removes support under the cutting edge; use restraint when the tool will see high cutting loads.
2.
Increasing side rake reduces cutting forces more than increasing relief angles. As side rake increases, chip flow accelerates. Since much of the heat in a cutting operation is retained in the chip, increasing side rake also reduces cutting temperatures.
3.
Increasing back rake reduces cutting forces but less than changes in side rake does. It accelerates chip flow so it also contributes to cutting temperature reduction. It also changes the focus of cutting force concentration and improves finishes.
4.
As cutting speed increases, cutting force magnitude decreases. This is useful, especially in harder materials. When you’re trying to come in on size, take a lighter depth of cut and increase speed and this will reduce Radial forces so there is less deflection. The tool cuts with greater ease and more accuracy.
You can use this information to modify any turning tool should you decide to do so. I’ll show you how to apply it shortly.
The Angle Table
In general, what defines the function of a lathe tool is its shape. How well it works and the material it works with is dependent on the angles the tool is ground with. If you look at a lathe tool angle table, you will see that the angles are material-specific.
The table angles define the tip geometry of a conventional lathe tool. You would choose a shape for your tool and then set your tool rest to grind the side and end (front) relief, then re-set the table for your side rake and angle the tool at your back rake angle and then grind both features at the same time. Each angle would change as you grind tools for each material type. The table is simple and easy to use.
When these tools are used on a larger lathe as they were intended, they work well. Unfortunately, when the same tool is used on a smaller, lighter, less rigid and less powerful lathe, they can produce cutting forces that may be excessive. If this is an issue for you then you might consider altering the geometry of your tools.
There is nothing sacred about these tables and there are no rules or laws that say you must grind your tools to these parameters. I don’t, and I don’t think you have to, either. However, the table is still useful because it gives us baseline values that we can modify to create a tip geometry that is more useful to us. We’ll get to this shortly but let me explain why the Square Tool is designed the way it is.
The design of the Square Tool is a compromise that allows it to work with multiple materials and with the lowest cutting force production I could manage. If you look at the angle table above, you will see that, on average, side relief is around 10-12°, end relief is 8-10° but the rake angles are all over the place. In order to reduce cutting forces, I increased the side and end relief angles to 15°; this is enough to lower forces but not enough to significantly reduce edge life. I settled on 15° of side rake because my tests showed that it resulted in a significant reduction in cutting forces but going beyond 20° seemed to increase edge wear more than I wanted, especially in medium carbon steels. I settled on 15° of back rake because this also reduced forces but more importantly, it shifted those forces just to the left of the tool tip. If you take a big cut with this tool, you’ll see the chip curling just to the left of the nose radius. This matters because this tool must be able to rough well but the tool also finishes pretty nicely. Placing the cutting force focus near the tip also allows the tool to face well because this is where a facing tool cuts.
Fooled you, eh? I bet you thought the 15° angle thing was just so I could give it a snazzy name like Square Tool – nope, not the case. It does work well with most common shop materials, so much so that I use it for most things except harder materials and stainless steel. This tool reduces cutting forces enough to enable my little Sherline lathe to at least double its depth of cut when compared to a conventionally ground tool so it does what it is designed to do. If you choose to reproduce it, I hope it works as well for you.
Modifying Your Tools
I’ve already written most of this stuff elsewhere but I’ll try to solidify it with a few examples. Say we want a general purpose turning tool for stainless steel. Whenever you need to cut something, you have to know the general machining properties of that something because this influences your tool angles. There are many formulations of stainless steel and I’ve only worked with 303, 304, 316L and 416. None of these is especially hard as supplied but they do work harden readily; dwell in a cut for too long and it gets hard enough to make you wish you were using a carbide tool.
Let’s decide that we’ll use our general purpose shape and that we want to accomplish two major things with our angles. First, we want to reduce cutting forces so that the tool cuts more freely, and secondly we want to prioritize cutting temperature reduction. Both goals are aimed at minimizing work hardening so we can come in on size. Having a nice finish would be nice as well.
The standard relief angle for SS is 10° but 12-13° would reduce cutting forces without affecting edge life too much. Standard side rake is 15-20° but I would take it up to 25° to reduce cutting forces, improve chip evacuation and keep cutting temps low. Standard back rake is 8° and here we need to make a choice. If we leave back rake at baseline then the tool will focus the cut at the side cutting edge. If we increase it, the tool will cut more freely and may finish better but may not be able to rough as well. In cases where I need to push the tool fairly hard, I try not to boost back rake too much so I would opt to increase back rake only a few degrees – maybe 10° max. You might think this won’t do anything but remember that the effect of these angle changes is cumulative; it will make a difference.
I would also keep the nose radius on the smaller side – 1/64” or so. The reason is to minimize radial forces. Why is it important for this tool? Because high radial forces will increase deflection and when the tool deflects it doesn’t cut and when it doesn’t cut, it builds heat.
You should know that the nose radius on any cutting tool has a big influence on radial forces; the larger the radius, the larger the radial forces. In general, radial forces will increase until the depth of cut exceeds the nose radius. Once the nose is buried in the cut where it is fully supported then radial forces tend to stabilize. It follows that the smaller the nose radius, the sooner it gets buried and supported so try to keep your nose radii on the smaller side when cutting hard stuff or stuff that work hardens.
This is getting to be a long post but let’s quickly look at a general purpose tool for aluminum. The Square Tool will work but not as well as a tool optimized for the material.
We can see that standard relief angles are 12 and 8 for the side and end. Since aluminum is fairly soft, we can easily increase both to 15° without endangering the edges. Side rake is already at 18°, which is a pretty healthy amount so I would leave it as is and boost back rake. Standard back rake is already huge at 35° but by increasing it to 40° we do several things: we reduce cutting forces, we focus all the cutting at the very tip of the tool to enhance finishes and we greatly accelerate chip flow to reduce cutting temperatures (because aluminum gets gummy when it heats up). Nose radius can be a bit larger, no more than 1/32”, and it will finish very well.
I have a tool ground exactly like this, with the same reasoning, and it will easily take a 0.25” deep cut on my 11” lathe. I know for sure it will go a lot deeper but I haven’t needed to waste material or prove the tool; it works for me. It has never developed a built up edge if I use WD-40 to lube the cut and the chips flow right off the tip as you would expect. It produces a near mirror finish when roughing and a mirror finish when finishing. It cuts with very little radial deflection so what you dial in is usually what you get. If you must take a lot off the diameter of a work piece, dial in a heavy cut and increase feed. This tool will create chips, not stringers, under those conditions and the tool will cut with minimal effort. You might give this one a try.
Okay, so that is the “how” of tool mods. Learn about your material then choose which angles to change to accommodate it. How much to change an angle is a guess but I try to keep them conservative, about 25-40% more than the baseline angle. In most cases, this only amounts to between 2-5° per angle but the changes are cumulative and a little bit can go a long way. You need to try this process to get a feel for it.
How do you tell if your geometry changes actually do something? What I do is grind a tool to the standard angles and see how it works. Then I grind a second tool to standard angles and then I modify one angle and assess. Then I change another angle and reassess. I keep this up until I’ve modified all the angles I want and then I focus on optimizing the amount of angle change. Maybe a degree more side rake will help, or a bit more back rake may improve the finish. The only way to know is to try it and see. When I have the tool cutting exactly the way I want it, I grind a keeper from a blank I trust.
This is already incredibly long and I’m repeating what I’ve said elsewhere but of all the tools you will grind, your general purpose tools will be the most useful. Now you know how to grind it, how to modify it, how much to change it and how to assess it.
You can take it from here.
