Models for grinding HSS Lathe Tools

I thought I would share this bit I finished up last night. I usually use inserts these days, but I broke out this old bit ground for aluminum as I need to turn some nylon this evening. it has about 10° of front and side clearance, 15° of side rake, and about 35° of back rake. I touched up the old grind on the bench grinder, and then honed it on some cheap diamond hones.

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A knife tool for aluminum - nice job, Dan. Let us know how it works.
 
Ken from ontario said:
I affirm that statement to be true, I do have one of Mike's square tool that he personally ground and honed . I know he calls it a "general purpose"tool but IMO it is a perfect tool for dimensioning, when I need to take off the last few thou, and also to get the best possible finish, this square tool hasn't let me down yet and based on the way I use it , it never will.
It will be a good challenge for you Mike to grind a shear tool that can outperform your square tool but I am looking forward to seeing what you'll come up with.
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Thanks, Ken. You hit on why I don't have a shear tool; I don't feel the need for it. However, until we do a comparison then it is just plain hubris to think that way. First, I need to visualize how the shear tool cuts and see if I can make the best shear tool I can make. Then I'll grind a finishing tool that is purpose ground to compete with it so its at least a fair comparison.

So, guys, why don't we do this. Lets put our heads together and figure out what the ultimate finishing tool would look like - shape, relief and rake angles, nose radius - and see if we can sort out what the tool would look like. Then I'll grind a shear tool and our finishing tool and we'll see which one comes out on top. You guys know enough to figure the geometry out - lets see what you come up with.
 
Dan_S said:
Here you can see the finish in 6061-T6 aluminum and MDS filled nylon. You get long stringy chips because there's no chip breaker however you get the benefit of a really nice finish directly off the tool and very very low tool pressure.View attachment 245842
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Looks good, Dan!
 
Okay, I think I understand how the shear tool works. Basically, you have a cutting edge on an angle that contacts at a tiny point tangent to the outer surface of the work piece. The cutting action is very much like that when cutting with a knife tool that has a small (1/64 - 1/32" across) flat ground at the tip instead of a nose radius; that tool also finishes really nicely. Basically, it slices off a very small shaving. However, because the edge of a shear tool is so long only a small contact point can be allowed if the cutting forces are to be controlled. If your depth of cut is too deep, tangential and radial forces climb rapidly and the tool will chatter and/or the finish will suffer.

I think the angle of the cutting edge would be most efficient when the side face of the tool is kept near vertical. This would minimize the amount of edge in contact with the work so it should reduce tangential forces. I'm thinking of grinding that face at a negative 5 degrees to start with and will increase it (go more negative) and see how that affects depth of cut capability and finish potential. At some point, I should be able to find the optimal angle for the side face.

Once I find that side angle, I'll focus on the end cutting edge angle and see what is needed to cut clean. I am less concerned about clearance up front. What I want is to lower the radial forces and I think minimizing the amount of horizontal contact the tool has with the work will do it. I know from experience that I need at least 12 degrees of end clearance to cut clean so I'll start there and go up. When edge life falls off, I'll have reached the limit.

Eventually, we'll have a shear tool that is optimized for both depth of cut and finishing potential. Then we'll grind a finishing tool to see how well it compares to this shear tool. I already know how I would grind it but will wait for input from the group to see what you want to do with it. At the end of this experiment, we should have two tools that should finish well. In terms of utility (facing, turning into shoulders, sizing), the shear tool has already lost that competition.

Okay, I'm comfortable with my plan. I await your input, guys.
 
Okay, 24 hours and no input so let me start this off and ask questions to stimulate you.
  1. Shape - I will use a finishing tool shape. Some of you may not have seen this shape before but it has the most lead angle of all turning tools. This is so that when the tool shank is perpendicular to the work, the lead angle is canted significantly toward the tailstock. As we know, shifting the lead angle toward the tailstock increases cutting forces but it also greatly enhances finishes as long as you keep your cuts light. It will also take a fairly healthy cut if the lead angle is reduced by turning the tool and it can cut into a shoulder and size a work piece very well, indeed.
  2. Relief angles - should we stay with a standard relief, increase it or decrease it? What do the relief angles have to do with finish potential? If we were to grind this tool to suit mild steel, what would be an acceptable relief angle?
  3. Side rake - side rake doesn't influence finishes much but it does reduce cutting forces, temperatures and chip flow rates. Given the changes to the relief angles, would you be conservative or would you increase side rake? If you increase it, why are you doing it?
  4. Back rake - back rake has a significant impact on finishing potential. Would you increase it? If so, why?
  5. Nose radius - if you ask most machinists about nose radius they will tell you that the bigger the radius, the better the finish. This is true to a degree but a finishing tool actually cuts more with the side cutting edge so the nose radius really doesn't play as big a role as you might think. What a big nose radius does do is deflect and when the tool deflects it doesn't cut. Consequently, cutting forces and cutting temps go up and holding dimensions becomes an issue. Therefore, my suggestion to you as you design tools is to start off with a small nose radius and change it only if it doesn't produce the finish you need. In my experience,it is always a good idea to know where the tool is cutting and focus the cutting forces there. So, here are the questions - if you want to focus the cutting forces in a specific location, what angle do use to do that? How big a radius to we start with?
As you learn to grind tools, keep in mind that tool geometry is the key to making the tool work the way you want it to. To do that, you need to know what each angle does and how you can change them to suit your needs. Also remember that in order for a tool to be accurate it must cut, and it must cut where you want it to cut and with the least amount of cutting force you can manage.

Jump in and stick your neck out. Its okay to be wrong; that's how we learn. I promise that no one will laugh at you. Well, at least I won't.
 
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In my oinion part of the issue comes down to how you have your lathe set up, and how you prefer to use it. I use a QCTP and it is aligned as close to perfectly square to the spindle as I can get it. This is so that threading tools don't require any fiddling to get aligned. In general I just drop the holder on and go, because all my angles are preset by how the tool is mounted in the holder.

In my opinion tool B above is a roughing tool and tool E is a finishing tool.

I call tool B a roughing tool for two reasons. It can't turn to a shoulder and face a shoulder with out being reoriented, and to me that's a no go. Also because of how the cutting face is oriented to the direction of travel the force vectors want to push the work away from the tool, making it hard to accurately dimension a part.

Tool E is a finishing tool as it can turn to a shoulder and face the shoulder. To be fair the angle of the cutting face means the vector forces want to pulll the work into the tool. However the small positive lead angle, of usually only a degree or two aren't as strong as the negative lead angles on roughing tool that usually set in the 10 to 20 degree range.

I know guys who either don't use A QCTP or have no qualms about reorienting and use nothing more than a tool shaped like A.


In my eyes side rake and back rake depend on the material being cut, so I have different tools for different materials. I've personally found the envelope of acceptable angles to pretty wide so as long as you are in the ballpark of what's quoted in a lot of old texts you are good to go.

With regards to tool bit geometry, tip radius is the most important thing to get correct. The main thing to remember is that you are cutting a fine pitch screw, so the finish is directly related to the tip radius and feed rate. A higher feed means you need a bigger radius to get the same finish as a smaller radius and a slower feed. Calculators like this one are good tool to help think about it. http://www.custompartnet.com/calculator/turning-surface-roughness

the other thing to consider is that back and side rake effects the effective tip radius. The more back or side rake you have the bigger the radius needs to be. This can best be seen by looking at the cross section of a plane and a cylinder at different angles. When cut though at a perpendicular angle the cross section is a circle. At any angle other than a 90 you get an ellipse, and the further than angle is from 90 the more pointy the small end of the ellipse becomes. This image kind of shows it, but its easier to visualize buy just cutting though some round stock at an angle other than perpendicular.
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In my opinion honing had the greatest effect on surface finish and cutting performance. It was a night and day difference when I started honing tools many years ago.
 
Dan_s

This makes so much sense to a newbie like me:

"The main thing to remember is that you are cutting a fine pitch screw, so the finish is directly related to the tip radius and feed rate. A higher feed means you need a bigger radius to get the same finish as a smaller radius and a slower feed."
 
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