Metal Lathe Tools: Cemented Carbide

* Temperature at which the tool material begins to soften appreciably. For HSS, it depends on the level of cobalt in the HSS.

Side Clearance Angle should be 4° to 6° greater than Side Relief Angle.

In selecting the Side Relief Angle, the sharpener should:

• carefully chose the correct angle, as a deviation of even 2° (plus or minus) will lower tool life appreciably.
• chose the lower angle from the table below for heavy cutting
• chose the higher angle from the table below for finish cutting.
• decrease the angle by 2° when making high impact cuts (e.g., work with significant gaps or hard inclusions).

Special Notes :

1. When grinding the Side Clearance Angle, the sharpener should leave the top 1.5 mm (¹/₁₆ inch) unground. This is because the rough grinding of the carbide introduces cracks and makes it more brittle. By stopping short of the top, this is minimized. Then, when grinding the Side Relief Angle, a high grit wheel should be used to make the edge smooth with minimal cracks.
2. When grinding the Side Rake Angle, the sharpener should be sure the top surface is finished with a very high grit. The roughness of this surface affects the chip damage to the tool, and rough surfaces are more susceptible to damage than smother ones.

Front Clearance Angle should be 4° to 6° greater than the Front Relief Angle.

The Back Rake Angle, as shown in the picture to the right, is positive.

• 5° - 8° to simply control chip direction
• -5° to protect the finishing point of the tool

If a combined Negative Back Rake Angle and a Back Rake Angle is used (as shown in the picture), the width of the land for the Negative Back Rake Angle is 1.5 - 2 x the feed per revolution.

Another reason to introduce a Negative Back Rake Land is to reduce the ability for introducing breaks or cracks on the intersection between the Back Rake and the Front Relief. This is sometimes called "stoning off" or "dubbing" the edge.

Special Notes :

1. When grinding the Front Clearance Angle, the sharpener should leave the top 1.5 mm (¹/₁₆ inch) unground. This is because the rough grinding of the carbide introduces cracks and makes it more brittle. By stopping short of the top, this is minimized. Then, when grinding the Front Relief Angle, a high grit wheel should be used to make the edge smooth with minimal cracks.
2. When grinding the Side Rake Angle, the sharpener should be sure the top surface is finished with a very high grit. The roughness of this surface affects the chip damage to the tool, and rough surfaces are more susceptible to damage than smother ones.

(can be more than 90°)

Top View - Point Angle

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Top View - Radius

• Point Angle
  • For carbide, the width of the Point Angle should be:
    • Soft materials : 2 x the feed rate
    • Hard materials : 3 x the feed rate
  • The Point Angle and the Front Cutting Edge Angle should be the same.
• Nose Radius
  • If the sharpener wishes to use a Nose Radius, the Point Angle values should be ignored.
  • For carbide, the radius of the Nose Radius should be the greater of the following two values:
    • 2 x the feed rate
    • depth of cut / 8

Increases in the Side Cutting Edge Angle will :

• decrease chip thickness and increase chip width (retaining the same area of the chip)
• decrease power usage