a question about a positive rake cutter

[tim mathis]

i am going to make some wood lathe turning tools.i am looking for a round cutter with a positive rake. i don't know what a positive rake is and the round cutters that i found online don't say positive or negative rake. any help would be great. thanks and have a great christmas.
tim in centerville , ohio

[HuFlungDung]

The top face of a tool is the face against which the chip impinges and is redirected away from the workpiece. The angle of the top face relative to a tangent touch point on the work is its rake angle.

In lathe work, the tool is typically set with its top face horizontal and with its set height at the same height as the lathe spindle centerline. When the cross slide is cranked in, the top face of the tool remains parallel to the slideway, and touches the work at the tangent point. The top face of the tool is perpendicular to the tangent, so this tool would have neutral rake, or zero degrees.

If the tool face is ground so that the angle between the tangent and the face is greater than 90 degrees, then it is positive rake. To conceptualize this, the tip of the tool still remains on the centerline of the spindle (work) and the top face slopes down below the centerline the further you move back from the tip. This eases the chip flow so that the chip has a moderate amount of redirection.

A negative rake tool also has the tip touching the work at the centerline but the rake face moves up above the touch point. This causes chip flow to reverse in the sense that it hits to tool face and flows upwards as it moves back along the tool. This is why negative rake tools take more power and apply greater force to the workpiece.

For a round, positive rake tool, you would need something like an insert with a countersunk top, only the countersink would continue from the screw hole right up to the edge of the insert. Such a tool would have poor chip flow characteristics because the chip would collide and pile up in the bottom of the hole. A quarter round tool would overcome this chip flow problem and allow the chip to flow out one side.

[Jarwalcot]

I have a Training Manual Kennametal put out years ago that you might find useful. I would upload the file but it is to large for our current file size limits (72MB). If someone has an FTP site that I could upload it to, let me know.

Cheers,