7. Cylindrical turning and facing of long cylindrical parts
Long cylindrical parts can be machined with the part held in a
chuck, between centres, but also sometimes using collets. The use of
counter-supports is always necessary.
The cutting is done on the circumferential surface or end face
of the part.
- You should always select the safest way of chucking/clamping
for the relevant part The more chucking/clamping face you have, the more safe is
the machining process.
- The tailstock with live centre and the stationary or follow
steady rest are used as counter-support. Longer parts necessitate the combined
use of tailstock and steady rest or of several steady rests.
- The use of the steady rest prevents deflection of long shafts
and the production of chatter marks as well.
- For parts mounted overhung, the steady rest (without using the
tailstock as counter-support) permits facing and boring operations to be
performed, if it is used to support and guide the freely rotating end of the
part (see Fig. 12).
- The contact faces of the steady jaws must be round and even.
Sufficient lubrication during the machining process is necessary.
- For non-round or uneven contact faces the use of a scroll
chuck (Fig. 24) is recommended.
- Areas of the part succeptible to deflection should be
sup-ported by stationary steady rests mounted on the lathe bed. The length to be
turned is then subdivided.
Figure 24 Scroll chuck
1 tailstock jaws,
2 scroll chuck,
3
tailstock,
4 work gripping in driver,
5 bearing surface
- Follow rests are mounted on the tool slide and perform the
same feed movement as the turning tool. In this connection attention is to be
paid to the facts that
· the steady rest
top jaws must always be softer than the material of the part to be machined,
· the steady rest must be
precisely aligned,
· the pressure of the top jaws
on the surface of the part must not be too high (heat expansion),
· the top jaw contact faces must
suit the diameter of the part,
- If any part shows heavy deflection, it must be straightened
before machining.
- When machining long parts, the turning tool should be mounted
slightly above centre to avoid hooking.
- The cylindricity is to be checked and, if necessary,
corrected.
- A negative rake angle (tool face and side relief of the
cutting tool) is useful for machining parts with a high length-diameter ratio.
What is the basic difference in the use of a stationary and
follow
rest?
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What basic rules are to be observed when using a steady
rest?
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