ComputerAided_Design_Engineering_amp_Manufactur.pdf

The first three possibilities are considered the most common work-holding methods used for one-off
or small-batch components. The decision about the holding method is based on a set of rules using
length-to-diameter (L/D) ratio, weight, maximum diameter, and minimum diameter of the component.
The following rules are employed to classify a component as short or shaft for determining the workholding
method:
(a) If L�D � 2 then the part is a short component
(b) If (L�D � 4) and (maximum dia. � 100) then it is a shaft
(c) If (L�D � 4) and (maximum dia. � 100) then it is short
(d) If (2 � L/D � 4) and (minimum dia. � 15) then it is short
(e) If (2 � L/D � 4) and (minimum dia. � 15) then stiffness is to be compared. If its stiffness when
held between centers is greater than that when held in a chuck, then the part is considered a shaft;
otherwise, it is a short component.
After classifying the part, the following guidelines are applied to determine the work-holding method:
• Short components are usually held in chuck (CO method).
• A component classified as a shaft is preferably machined between centers using a dog driver (BC
method).
• If the shaft is a heavy component, e.g., more than 350 kg, a chuck is used to drive the shaft (CC
method).
• If internal features are present on the shaft, then use of steady rest is necessary.
• The shafts with L�D ratios greater than 12 are considered as non rigid (Kovan, 1959) and two
steady rests are employed.
• Steady rest is not used on the side clamped in a chuck.
To calculate L�D ratio, the maximum and minimum diameter of the part can be obtained from the
geometrical model of PDIR. Weight of the component can be estimated from these dimensions. Some
short-cut methods of finding the holding method are also employed. For example, the part name and
the drawing code (which is usually based on the company standards) are available in the global data of
PDIR. These attributes can be used as reference for the shop floor practices to determine the holding
method. For example, if the name of the part is “rotor shaft,” then it is obvious that it has to be held
between centers. In such cases, the above calculations need not be carried out.
Deciding the Number of Settings
A setting has been defined as being that part of the machining operation accomplished during a single
clamping of the workpiece being processed (Kovan, 1959). In this context, reversing the part on the same
machine and shifting the part from one machine to another can also be treated as different settings.
The majority of rotational parts can be machined within three setups. A sample flow chart for
determining the number of setups for a “chuck only” component is shown in Figure 5.28. Refer to Hinduja
and Huang (1989b) for more details on finding the number of settings.
Attaching the Pockets to Setups
Only in the case of a single setup condition can all of the pockets be machined from the blank to get the
final part. If the part needs to be machined in multiple setups, it is necessary to establish the accessibility
limits of the pockets in each setup. To relate the pockets to different setups, the concept of demarcation
lines used by Hinduja and Huang (1989b) is found to be a suitable approach.
The accessibility of the pockets is determined by placing the demarcation line (DL) at either end of
the maximum diameter of the external profile. The pockets lying on either side of DL are placed in
different groups. DL thus establishes a limit in such a way that pockets belonging to different groups
cannot be accessed in the same setup; however, pockets of the same group can be machined in more
than one setup. In the case of internal features, minimum diameter is the criteria to set DL (Figure 5.29).