There are two principle angles that govern the ability of a lathe tool to cut: top rake and front/side clearance.
First the clearance angle: Imagine a cross section of the cut as viewed horizontally, from the side. The drawing below was to illustrate the clearance angle of a drill, but the same idea applies to a lathe tool if you just turn the drawing 90 degrees clockwise and imagine the work moving instead of the tool.
The top image shows what happens when there is no clearance. The flat face of the tool just slides over the work, like a bearing. It would make very little penetration and would not produce any chip.
The second image shows that when a clearance angle is formed on that face of the tool, it can now penetrate as it can slide down the ramp that that angle cuts in the work piece. I did not draw a chip but it would be there.
The clearance angle is measured from the original flat surface to the actual angle as shown. Clearance angles are commonly between 5 and 15 degrees. The larger the clearance angle, the faster the tool can penetrate into the work. But it also becomes weaker as the clearance angle is increased because there is less metal under the edge.
The clearance angle of a lathe tool is on the front and/or side faces.
The rake angle is the next to consider. This is the angle of the top of a lathe tool or the left side of the tool shown in the drawing. It is the surface that the chip slides over as it is cut. The tool in the drawing has a 0 degree rake angle: that is, it's cutting surface is perpendicular to the work. As the top of the tool is ground to have a smaller angle as seen in the above section view, it is said to have a POSITIVE rake angle. If the top were ground to product a larger angle in cross section, then it would have a NEGATIVE rake. As I said, the tool in the drawing has a 0 degree rake. A positive rake angle would cause the tool to try to dig into the work, while a negative rake would try to force it out of the work. Thus negative rake tools require more force or pressure to keep them cutting.
Clearance is necessary for almost all tools. That includes drills, lathe tools, milling cutters, wood saws, etc.
The rake angle is a more complex subject and often must be determined by experiment. I always start cutting aluminum and steel with a positive rake tool. But this is only a starting poing and I am not the most experience one here on this question. Brass likes a 0 or negative rake to prevent the tool from digging in too fast which can cause tool breakage and vibrations that leave a wavy surface. I use a Dremel to put a 0 degree rake angle on drills for brass.
If you need a starting poing, then the various handbooks and other references will show suggested tool shapes and angles for various materials.
As both the clearance and rake angles are made more positive, the included angle of the tool at the cutting edge becomes smaller and the tool will become dull faster.
Those are the most important angles. The tool will also have an angle as viewed from the top. Most tools will have a front face and a side and the angle between them will vary for different purposes. My most used tools have about an 80 to 85 degree horizontal angle between these faces and I grind relief on both of these faces to allow cutting on either or both of them. This is handy when turning a diameter up to a shoulder. On the final cut, I cut right to left up to the shoulder and then back the tool out to finish that shoulder. Other angles are used. A threading tool would have a 60 degree horizontal angle to match the Vee of the threads. And other shapes are possible. Cutoff tools have a square shape. Whenever possible, the sharp point formed by this horizontal angle should be rounded to a small radius to prevent it from becoming dull too quickly. Roughing tools should definitely have a radius here. A finishing tool can be left sharp is needed.
Surface finish is another complex subject. There are some suggestions but again you may have to experiment. I have read that a large radius between the front and side faces helps to produce a better finish. This may be true for some materials but not for others. The first three things I would insure for a better surface would be rigidity. Any vibration in the tool or work will show up in the finish. The second thing is sharp tools - SHARP, SHARP, SHARP. I use a quick change toolpost that allows the tool to be removed and restored to the same position. This allows it to be touched up while the work is in progress. The third thing is a good cutting fluid. It is amazing how much this can help.
Only after these three things are absolutely insured, would I look at changing the tool geometry.
I know this was a bit long, but I hope it helps.
Last edited by Paul Alciatore; 07-07-2006 at 01:27 AM.
Make it fit.
You can't win and there is a penalty for trying!