02 Mechanical Controls Flashcards
Basics of mechanical controls
- Oldest kind of controls for machine tools
- Used predominantly in automatic lathes
- The workflow is saved in mechanical elements (e.g. Cams, Curves, Arms etc.)
- The mechanical elements must be replaced for each workpiece change because of their fixed shape
- Machine set-up time-intensive
A distinction is made between:
o Special control curves, developed through barrel cam
o Disc curves, developed through cam plates
o Geneva mechanism
Functionality of mechanical controls (cam controls)
- The control is generated purely mechanically
- Cam gears create time-dependent speed and movement characteristics
- Movement path and speed are stored within the geometry of the curve
- Curves can be two-dimensional (disc curves) or three-dimensional (cam drums)
-> Disc curves store movement factors via radius and angular relationships
-> Cam drums save factors via angular and axial length relationships radius is constant - The required movement path and velocity are transferred from the measuring pin of the transfer element to the movable component
- The transmission mechanism is composed of additional mechanical elements
Advantages of mechanical controls
- Simplicity
- Less error-prone
- High setting speeds
- Exact adjustment paths
- Suitable for line and mass production
- High productivity
Disadvantages of mechanical controls
- Transmission of large forces over long distances is unfavorable (backlash and deformations within the elements)
- Additional moving masses of the control systems adversely affect the dynamic performance
- Workpiece shape changes require the exchange of mechanical components (Low flexibility, Long setup times)
Machine elements for mechanical controls: Barrel cam
Rotational movement of barrel is converted into a translational movement by tracing the barrel cam groove with a mechanical scanner
Benefits of the barrel cam: tight fit between curve and scanner
Disadvantage: backlash between scanner and curve
Aspects to consider when realizing mechanical controls via barrel cams
Working stroke and rapid return stroke must be connected through radii to the non-load strokes (to assure a uniform translational speed of the pushrod)
Lead angle of the control cam must be large enough to avoid self-locking
Machine elements for mechanical controls - Cam Disk
o Machine elements for controlling non-uniform motion sequences mechanically
o Designed with control curves either on the side of the surface or on the diametrical surface
o The forces on the piston (e.g. spring, inertia, process) acting on the disc have to exceed those exerted on the piston by the disc movement and geometry to avoid lift-off
o The transfer of the curve movement is backlash-free
Machine elements for mechanical controls - Geneva Mechanism
o Geneva mechanisms are a form of mechanical control that transform a uniform rotating movement of the drive shaft into a periodically interrupted rotating movement of the output shaft.
o For transport rolls, rotary indexing tables and tool turrets, intermittent movements are required
o It is composed of a disc with a role and a Geneva cross; the disc is moving at constant speed
o It can have a different number of slots
o The role on the driving disc slides in a slot of the Geneva cross during each rotation, turning the Geneva cross by a specific rotation angle
o The output rotation angle depends on the number of slots of the Geneva cross
o Other mechanism for the generation of intermittent movements are gear wheels and gears with crossing shafts
Automatic Lathe
- The mode of action of mechanical controls is explained on automatic lathes
- Short cycle times required due to the large amount of workpieces
- Medium to long setup times are acceptable
Advantages and Disadvantages of Automatic Lathes
Advantages
- High productivity
- High operational safety
Disadvantages
- Long setup times
- Complex implementation of manufacturing processes
- Low flexibility
Single Spindle Lathe
- Different tools are aligned in a row and mounted in the turret
- After a brief turret tool entry into the work piece, a longer inactivity follows -> Tool spends more time in storage than working
- Additional tools are distributed over cross slides and are used in parallel
- Mechanically controlled single-spindle turning lathes are only produced by a few companies
High Speed Clutches
Provide switching and support functions
Functions:
Bar feed
Workpiece clamping
Speed control
Geneva motion switching
Switching function of the attachment cam shaft
Characteristics of multi spindle lathes
- High productivity
- Low Flexibility
- Long Setup times
- High complexity for process design and planning of the cut distribution
Structure of a multi-spindle lathe
o Single drive motor (all movements are realized by gear drives and cam mechanisms)
o Central main spindle drive Same rotation speed on all spindles
o Feed for cross slide and longitudinal slide
o Rotating tools
o Workpiece feed and workpiece clamping
o Drum indexing and drum locking
Multi-Spindle Lathe
- One workpiece can be produced simultaneously on each of the four to eight spindles
- Each spindle position has its own tool kit which accomplishes the infeed or rather the cutting motion
- After processing the workpiece in one spindle position, the spindle is moved into the next position by rotating the spindle drum -> In the next position the workpiece is processed by the next tool kit in that particular position
- Simultaneous machining of several pieces is achieved -> The number of simultaneously machinable workpieces equals the number of spindles
- Part production times can be just a few seconds -> Much higher productivity than single-spindle lathe
Benefits of the cam control
- Cost-efficient and robust
- Very short workpiece production times (security checks can be omitted)
