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It's been some time since @Aukai asked this question and I finally found some time to respond to it.
"How do I properly position a lathe tool" is a really common question posed by new guys. I tried to cover this in post #107 of this thread but I guess it wasn’t clear so I’ll give it another go but with more detail. I will address turning tools only.
Here is a common illustration on positioning various lathe tools:
The old style tools used in this illustration were purpose-ground and each tool shape was meant to be used for a specific operation with the shank of the tool held perpendicular to the work piece. The value of the illustration is to show which part of the tool cuts for a given operation. For example, look at the picture for the RH facing tool.
See how the area just behind the nose radius is in contact but there is a gap further back along the cutting edge of the tool? The area of actual contact is usually only about 1/16” to 3/32” long, starting where the nose radius transitions to the side cutting edge. If you want to maximize the finish, use more edge but the amount shown is usually quite good.
Despite the fact that a modern general purpose tool is shaped a bit differently compared to the old style tools, the area of contact remains much the same as the old tools for a given operation. We simply turn the tool with our QCTP so that the contact area of our tool assumes a similar angle to the older style tools and we’re good to go. This is the short answer for how we position our tools and yes, it is that simple. The precise angle is not critical when using tools this way; your tool gives you a lot of leeway with regard to tool angles.
This is how you’re supposed to use your tools because it has proven to work for over a hundred years. Why fix it if it ain’t broke?
Of course, I don’t actually use my tools quite this way. Instead, I use my lead angle, edges, speed and depth of cut to minimize cutting forces to the extent I can. I do it this way because I prefer to use the geometry of the tool to make it do what it is supposed to do. I will tell you what I do and why, but this is for information only and does not constitute a recommendation.
First, let me tell you about edges. Every turning tool has three of them – the side cutting edge, the nose radius and the end cutting edge. The nose radius is a transition zone between the other two edges but it usually participates in the cut whenever one of those edges is in play. At any given time, you may be cutting with one, two or even three of these edges.
Second, any time an edge is in contact with the work you will generate cutting forces. For any given depth of cut, the more edge contact you have the greater the cutting forces will be so our goal is to minimize edge contact whenever we can.
Cutting force reduction is a big deal, and this is especially true on our smaller hobby class lathes. You may recall that I said before that the lead angle of the tool has an impact on cutting forces. The lead angle is the angle formed between the side cutting edge and a perpendicular line from the work piece.
When the shank of your tool is perpendicular to the work the lead angle is equal to the side cutting edge angle. For the sake of discussion, let’s call this perpendicular shank thing the Standard Angle.
Look at the point of contact in the illustration above. Using our standard angle as a baseline, turning the tool CW increases the amount of side cutting edge in contact with the work so for any given depth of cut, those forces will be higher relative to the standard angle. Conversely, turning the tool CCW, as in the middle of the picture, reduces edge contact and cutting forces decrease.
The benefit to turning our tool CW is that it improves the finish. The benefit of turning it CCW is that it reduces cutting forces, and a little bit goes a long way. So, how do we use this information?
Say we are using our Square Tool in the standard position to take a big cut and we encounter chatter. The lathe operator’s mantra for chatter is to “reduce our depth of cut or increase our feed” OR we can turn our tool CCW to reduce the lead angle. This reduces the amount of edge contact at the side cutting edge, which reduces cutting forces, and the chatter will usually resolve. Typically, very small changes in tool angle will be sufficient to resolve the chatter.
So, you must be thinking that if a little bit is good, wouldn’t more be even better? Yes, more can be better but the tradeoff is that the finish may suffer a bit. Quite often, when I need to take a big roughing cut, I will turn the side edge in a negative direction towards the chuck in almost a facing position to minimize cutting forces. How much can you turn? I turn it so most of the nose radius is engaged but none of the end cutting edge touches the work. In this position, the side cutting edge contact is as small as it can be for the depth of cut you’re taking. I try to start off with stock close to my finished size so big roughing cuts like this are usually not necessary but when you have a lot to take off, remember this one.
In addition to reducing the amount of edge contact by turning the tool CCW, you are engaging more and more of the side rake angle on top of your tool and you will see the path of chip egress change so that it comes off almost perpendicular to the side cutting edge. Given that side rake is the most influential tool angle insofar as cutting force reduction is concerned, you are essentially cutting with the least amount of force the tool is capable of in this roughing position.
So, unless I have to rough heavy, I usually rough with the tool in the standard position to maximize the finish when roughing. If the lathe strains or I encounter chatter I simply turn the tool CCW enough to eliminate it. If I have to rough really heavy then I go negative. If altering tool angle is not enough, I reduce my speed a little and increase feed a little and that tool will cut really heavy. Simple, right? This just uses everything you already know to enable your lathe to perform maximally.
This angle change thing works for both heavy roughing as well as turning into a shoulder, where your tool and side edge is essentially in a facing position. This puts the point of contact just aft of the nose radius and allows you to cut pretty heavy but also allows you to face out at the shoulder. Again, very simple and it works. Hope this makes sense.
So, how does this edge contact thing work for finishing cuts? If you recall, when you do not have a shoulder to contend with I recommend you turn the tool CW toward the tailstock to improve your finish. The contact patch is small, along the side cutting edge and part of the nose radius. Depths of cut are usually very small and I increase speed a bit to further reduce cutting forces (you do recall that increasing speed reduces cutting forces, right?). In this position, you are shearing with the side edge. More accurately, you are shearing with side rake. This thins and widens the chip and you will see the chip come off perpendicular to the side edge, indicating that side rake is the key angle in use. As you feed the tool towards the chuck you are essentially dragging the tool backwards and the chip direction can actually flow towards the tip of the tool. When this happens, watch the finish; it will be really good.
When we have to take sizing or finishing cuts into a shoulder, we can do the same thing except we will use the end edge instead of the side edge. To size, we turn the tool so the area on the end edge just aft of the nose radius contacts the work and we move into the shoulder and then face out. When finishing, we engage a tiny bit more of the end edge, increase speed, reduce our depth of cut and shear the work to size. Facing out in this position is done with the side cutting edge as usual. Ignore the finish in the picture; it is prop just to show tool position. In actual use, the tool is turned a bit more CCW than in this shot but you get the idea.
That’s basically how I turn on the lathe; sorry, nothing very mysterious. I use my edges and lead angle to manage cutting forces when roughing and I shear with the tool to size and finish. It does not take skill to do any of this, merely awareness of what you’re doing and why. I suspect that as you go along you will develop techniques of your own, which is how it should be. Hope this helps.
