The insert corner radius determines the strength of the cutting point, the maximum admissible feed and the surface finish of the workpiece.
Ground rule: Select the largest possible corner radius.
Roughing
The following minimum radii, depending on feed, are recommended for rough turning :
Radius r 1.6 x feed f for insert shapes C and S
Radius r 2.5 x feed f for insert shapes D and T
Recommended maximum feeds, based on approach angles 750 - 1050
| max. feed f (mm/rev) | |||||
| 0.4 | 0.8 | 1.2 | 1.6 | 2.4 | |
| 0.25 | 0.5 | 0.8 | 1.0 | 1.6 | |
| 0.16 | 0.32 | 0.5 | 0.63 | 1.0 | |
Note : Higher feed values are possible under favourable conditions. Under non-rigid conditions lower values should be applied of smaller radii selected, especially if vibrations occur. Larger radii are generally recommended for cast iron,cast steel and interrupted cuts.
In finish
turning exacting demands are placed on surface
finish and part accuracy.
To determine approximately the surface finish to be
expected in turning with feeds > 0.1 mm, the following formula for theoretical
roughness height Rth can be used:
| Rth = 125 x f2 (µm) r |
|
| Theoretical roughness height Rth | ||||||
| 0.10 | 0.12 | 0.16 | 0.20 | 0.25 | 0.32 | |
| 0.4 | 3.20 | 4.50 | 8.00 | 12.5 | 20.0 | - |
| 0.8 | 1.60 | 2.20 | 4.00 | 6.3 | 10.0 | 16.00 |
| 1.2 | - | 1.50 | 2.80 | 4.2 | 6.3 | 10.70 |
| 1.6 | - | - | 2.00 | 3.2 | 5.0 | 8.00 |
If the theoretical roughness height Rth is assumed to be
roughly equal to Rz, the ten-point height (ISO), the roughness average Ra can be inferred,
which however does not show a fixed relationship to Rz. A conversion ratio of Rz:Ra 4:1 is
generally appropriate.
| Fine Finishing | Finishing | Roughing | Violent roughing | |||||||
| P01 P05 P10 P15 P20 P25 P30 P35 P40 | ||||||||||
| P45 M 10 / K 10 M 15 M 25 M 35 | ||||||||||
| 350 | Cutting speed Vc
(m/min.) 280 200 150 100 |
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TN 1000 TN 2000 TN 4000
|
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An appropriate carbide grade is equally important while selecting an insert for machining operation on a particular material. Keep it in mind the type of operation, cutting speed selected before opting the Carbide grade. The combination of right chipbreaker profile with right Carbide grade would be helpful in acheiving the maximum productivity
Good surfaces are acheived with
- higher cutting speeds
- inserts with sharp cutting edges
- positive rake angles and positive chipbreaker geometries
- use of cermets
- rigid machining set-ups
- use of easily machinable work materials
- use of cutting fluid
