Rotary Axes

5. Mechanical Design and Modelling

5.5 MRM Motion Modules (Modular Axes)

5.5.5 Rotary Axes

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5.5 MRM Motion Modules

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5.5 MRM Motion Modules

The DC motor that has been implemented in all axes contains an internal worm gearbox that amplifies the output torque from the armature shaft. The second benefit derived from the internal gearbox is that it behaves as a braking mechanism. The gear component of the box cannot be rotated by an applied load and can only be rotated by the worm mechanism attached to the armature shaft. The direct coupled solution provided a basic, cost effective solution, however for industrial implementations it is recommended that the interface be coupled to the motor via a secondary gearbox. Table 5.8 summarizes the specifications of this module. Calculations for the loading specifications may be found in Appendix C.3 and C.4.

Table 5.8: Specifications of the Cutting Head Rotary Module

Module Data Cutting Head Rotary Module

Module Code (CHR = Cutting Head Rotary) MRM-CHR01-360-T2A-T2B

Module Control Resolution 0.703 ^o

Module Range ± 360^o

Moving Interfaces Type Two A

Static Interface Type Two B

Max Speed 70 rev/min

Stall Torque 20 N.m

Max Allowable Torque On Moving Interface 12 N.m

Max Normal Load on Moving Interface 250 N

This module has been designated as the A-axis, as it provides rotary motion about the X-axis in the drilling configuration. The HTM for this module is given by equation 5.21; the limits are with reference to local frame „i+1‟. Refer to Appendix B.5 for calculations and illustrative diagrams.

^𝑖+1_𝑖𝑀_{𝐶𝐻 𝑅𝑜𝑡𝑎𝑟𝑦} =

1 0 0

0 𝑐𝛾 −𝑠𝛾 0

0 𝑠𝛾

0 𝑐𝛾

0

−138 00 1

−360° ≤ 𝛾 ≤ 360° (5.21)

Tilt Table Module (B-Axis)

Figure 5.21: The Tilt Table Module (B-axis) Tilting

Interface

Guide/

Support Arch Static Interface

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5.5 MRM Motion Modules

The tilt table module illustrated in Figure 5.21 was designed for use in the drilling configuration.

The module is capable of having a work table attached to its upper interface and provides an optional rotary axis. The table tilts the work part relative to the cutting tool to provide different planes of machining. The module was designed entirely out of aluminium, to create a strong light weight mechanism that is easy to reconfigure and easily moved by the drive motors of other modules in the kinematic chain.

Figure 5.22: Drive Mechanism of the Tilt Table Module

The drive mechanism of the module consisted of the upper interface plate being moved by a leaver arm as illustrated in Figure 5.22. The lever arm is coupled to a drive shaft, which is in turn coupled to the motor. The drive shaft fits into the lever arm and the connection is secured by a shear pin. The shaft is also supported at both ends by two deep groove ball bearings. The rotary motion of the upper interface is guided by two stepped arches at either end of the plate; these arches also support the load placed on this interface, protecting the drive shaft from bending. The DC motor that has been implemented in this axis contained an internal worm gearbox (as in all the other axes). The internal gearbox behaves as a braking mechanism, permitting motion to occur only by the actuation of motor. It should be noted that this mechanism is a direct drive mechanism as there is no torque conversion between the motor and the interface tilting mechanism. Table 5.9 summarizes the specifications of this module. Calculations for the loading specifications may be found in Appendix C.3 and C.4.

Table 5.9: Specifications of the Tilt Table Module

Module Data Tilt Table Module

Module Code (TT = Tilt Table) MRM-TT01-90-T3-T3

Module Control Resolution 0.703 ^o

Module Range ± 90^o to the horizontal

Moving Interface Type Three

Static Interface Type Three

Maximum Speed 70 rev/min

Stall Torque 20 N.m

Maximum Allowable Torque On Moving Interface 12 N.m

Max Normal Load on Moving Interface 240 N

Guide Arch Pairs

Drive Shaft Lever Arm

Motor Coupling

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5.5 MRM Motion Modules

The module is designed for attachment to the work table slide module and provides rotary motion about the Y-axis in the drilling configuration; it has therefore been designated as the B-axis of that system. The HTM for this module is given by equation 5.22; the limits are with reference to local frame „i+1‟. Refer to Appendix B.6 for calculations and illustrative diagrams.

𝑖+1𝑖𝑀𝑇𝑖𝑙𝑡 𝑇𝑎𝑏𝑙𝑒 =

𝑐𝛽 0 𝑠𝛽

0 1 0

−𝑠𝛽 0

0

0 𝑐𝛽

0

−50𝑠𝛽

−50𝑐𝛽 − 950 1

−45° ≤ 𝛽 ≤ 45° (5.22)

Rotary Table Module (C-Axis)

Figure 5.23: Rotary Table Module (C-axis)

The rotary table module illustrated in Figure 5.23 was designed for use in the drilling configuration. The module is capable of having a work table or other modules attached to its upper interface. This module was also designed entirely out of aluminium, to create a strong, light weight mechanism that is easy to reconfigure and easily moved by the drive mechanisms of other modules.

Figure 5.24: Rotary Table Drive Mechanism Worm Gear

Box

Rotating Interface Thrust Bearing

Housing Coupling to Gearbox

Motor Coupling Rotating

Interface

Aluminium Frame

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5.5 MRM Motion Modules

The drive mechanism consisted of the upper interface being rotated via a reduction worm gearbox. This interface is supported by a thrust bearing at its base. The worm gearbox is coupled to the drive motor by a 12 mm steel shaft. The worm gearbox in the module and the internal worm gearbox belonging to the motor both prevent the mechanism from being moved by the load, permitting motion to occur only under the control of the motor. Table 5.10 summarizes the specifications of this module. Calculations for the loading specifications may be found in Appendix C.3 and C.4.

Table 5.10: Specifications of the Rotary Table Module

Module Data Rotary Table Module

Module Code (RT = Rotary Table) MRM-RT01-360-T3-T3

Module Control Resolution 0.0125 ^o

Module Range ± 360^o

Moving Interfaces Type Three

Static Interface Type Three

Maximum Speed 1.25 rev/min

Stall Torque N/A – Prior failure of gearbox

Maximum Allowable Torque on Moving Interface 20 N.m

Max Normal Load on Moving Interface 250 N

The module provides rotary motion about the Z-axis in the drilling configuration; it has therefore been designated as the C-axis of that system. The HTM for this module is given by equation 5.23; the limits are with reference to local frame „i+1‟. Refer to Appendix B.7 for calculations and illustrative diagrams

^𝑖+1_𝑖𝑀_{𝑅𝑜𝑡 𝑇𝑎𝑏𝑙𝑒} =

𝑐𝛼 −𝑠𝛼 0

𝑠𝛼 𝑐𝛼 0

0 0

0 0

1 0

0

−1520 1

−180° ≤ 𝛼 ≤ 180 (5.23)

Idealized Performance of Rotary Axes

The cutting head rotary module and the tilt table module both possessed direct drive mechanisms with no torque conversion between the motor and the driven interface. Ideally (no friction) these modules should display the torque and speed characteristics of the drive motor due to the direct coupling. On the contrary the rotary table module possessed a mechanism where the rotating interface was driven via a secondary worm gearbox. Equations 5.24 and 5.25 relate the speed and torque of the input shaft to the output shaft of a single stage reduction gearbox.

𝜔_{𝑜𝑢𝑡𝑝𝑢𝑡} = 𝜔_{𝑖𝑛𝑝𝑢𝑡} 𝑁_{𝑤𝑜𝑟𝑚}

𝑁_{𝑔𝑒𝑎𝑟} (5.24) 𝑇_{𝑜𝑢𝑡𝑝𝑢𝑡} = 𝑇_{𝑖𝑛𝑝𝑢𝑡} 𝜔_{𝑖𝑛𝑝𝑢𝑡}

𝜔_{𝑜𝑢𝑡𝑝𝑢𝑡} (5.25) These equations have been used in conjunction with the motor performance data to generate Table 5.11. This table displays the performance characteristic of actuation torque versus speed for the MRM rotary modules.

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