In special cases for instance steels with low strength (e.g. elastic limit 37) stainless
steel, copper or aluminium it might be better not to grind the chip surface as a plain
area but to grind a radius. This radius the so-called undercut enables the discharge
of chips here.
The choice of the grinding angle
The grinding angle is in a certain ratio to the thread profile and equals the support
angle, the infeed chamfer and the cutting angle insofar that the cutter is running pa-
rallel to the axis of the workpiece. Thus a perfect thread profile can be cut.
The grinding angle consists of the support angle and an additional angle.
Grinding angle = holder angle + supplementary angle
The chasers with short and extra short infeed are delivered with a standard
grinding angle of 7°. The chasers with medium and long infeed are delivered
with a grinding angle of 5°.
The following table provides you with information on the additional angle for sharp
threads with normal infeeds. It applies for the most common cutting angles from 20°
to 25°:
Cutting angle
Supplementary angle for
long throat
medium throat
short throat
20°
3,0°
3,5°
5,0°
25°
3,8°
4,5°
6,5°
In order to calculate the additional angle for other cutting angles and special first cuts
the following alignment chart applies: (Figure 8)
All you have to do is to read off the first cut or the correspondent first cut angle on
the right scale. Then draw a line covering the corresponding cutting angle from the
middle scale to the left scale. There you can read off the additional angle.
The choice of the grinding angle:
In special cases where cylindrical threads are cut without a guiding presser foot there
is no additional angle.
Therefore the
grinding angle = holder angle
When cutting conical threads to the width of chasers which is done without a guiding
presser foot an additional angle is needed, too. It is necessary to equal the tapering
of the chaser in order to bring the cutting edge in a parallel position to the axis of the
workpiece. This additional angle amounts to 1° on a thread with a cone 1:16.
Checking of the grinding angle with the setting device
Table 2:
Supplementary angle
Figure 8:
Alignment chart for deter-
mining the supplementary
angle
12
Operating Manual ZR 26-I
Chasers