Undercut
-
or a
recess
- is a special area of the finished part that
serves as a clearance area. Undercut is often machined between a thread end and
a shoulder, for example, to provide a suitable fit during parts assembly. The
most common undercut at the end of a thread is a 45
_
chamfer,
usually the same size as at the thread start. Its depth is typically a little
greater than the thread depth. Roughing multiple repetitive cycles allow a
direction change, if
Type
II
is programmed. Other
undercuts may have virtually any angle. In CNC lathe programming, the single
most important consideration is the back angle clearance of the cutting tool.
The
illustration shows general calculations for the three most common turning or
boring inserts. The lead angle is normally 5
°
for the 80
°
insert,
and 3
°
for the 55
°
and 35
°
inserts.
Back angles can be easily calculated - they are show here as 5
°
, 32
°
, and
52
°
for 80
°
, 55
°
, and
35
°
inserts respectively.
Large
(deep) undercuts may have to be programmed with one or more roughing passes,
depending on the material and the tool nose radius.
As
the word 'hard' in the title indicates,
hard turning
is
a method of turning and boring parts that are made of material with a high
degree of hardness, typically over 45 Rc (Rockwell hardness). Although the
range of parts that can be hard-turned is large, typical applications include
roller bearings, special gears, forging dies, axle shafts, injection valves and
nozzles, and other parts that have been hardened
before
the final machining. The main reason many CNC
machine shops adapt hard turning technology is to eliminate grinding
operations. If that is not possible or practical, the least that is expected from
hard turning is to
minimize
the grinding operations. In
either case, it is the question of economics and productivity (as well as
competition).
Hard
turning does not require any special programming format or structure - in fact,
the program looks just like for any other turning (or boring). What is
different are the tools and the method of applying them to the cut. The most
affected are the depth or width of cut, and the selection of spindle speeds and
cutting feedrates.
Copyright © 2006
Industrial Press Inc.