Series 3: Manufacturing Automation Article 18 Scope for Automation in large size work-holding fixture

Work-holding fixturing solutions which are readily available focus more on mass production, precision-repeatability and ease of location and clamping BUT there are many much bigger practical problems when it comes to large size job machining:

1.?????? Large size jobs are unlikely to be mass produced and large machines are costlier and few hence variety of jobs and machining operations are required to be done at the same machine … you do not usually have the luxury of making an automated dedicated SPM line.

2.?????? Handling and loading / unloading is risk prone and time taking especially if the job has no lifting provision and those have lopsided centre of gravity.

3.?????? Due to larger overhang some part of the job could sag / bow due to its own weight if it is not fully resting. The wall thickness where the job is held / suspended may or may not have adequate strength and rigidity to bear the weight of job and withstand deflections from machining forces.

4.?????? The input material may often be raw casting or forging or fabrication so there may be no pre-machined reference surface / bores or features which could be used for locating or even directly resting the job on a table.

5.?????? Usually, all six faces + angular faces may require machining with perpendicular cutter, with an absolute accuracy level that may look daunting. Typically, boring accuracies of 10-20 microns is expected even for a job size of 4m x 3m x 2m. Refer picture and figure out how many setups would be required for a job like that.

6.?????? Levelling and alignment take up maximum manual effort and time for all setup changes.

7.?????? Fixture / Clamping could foul with cutter or spindle body while running the CNC program; if clamping is done by jugaad method manually it may not be possible to do prior collision test in software.

Some ideas which are not so common but could add lot of productivity advantage:

·???????? Support the irregular non datum bottom surface of job on four lockable hydraulic jacks located under four corners of the job. A portable central multi-jack control unit does levelling in coordination with CNC using a laser distance measuring device mounted as a cutter in CNC spindle using the top surface of the job or with a straight edge mounted on the top surface.

·???????? At four corners there are four adjustable columns with short arm that carries Nitrogen springs. After levelling all jacks are lifted by same amount to build clamping pressure.

·???????? After clamping the jacks are locked by operating conical locking device on stem of jack so that hoses and the controller could be moved elsewhere.

·???????? Similar arrangement may be provided across left and right side of the job for automated aligning.

·???????? For the case depicted in picture, a motorized indexing may be provided with locking on index plate according to the setup through PLC

·???????? There could be redundancy in hydraulically operated swinging clamping arms and in event of fouling, the particular arm which will foul could be selectively moved out temporarily in coordination with CNC – PLC control when the cutter path reaches there.

·???????? Presently CNC is CNC and Fixture is a Fixture and generally they do not talk to each other much during machining; going further there appears to be lot of scope of integrating them together.

·???????? It is an attractive proposition to embed vibration and force sensing in fixtures for mass produced items where signature signals for OK jobs may be used to detect any abnormal machining condition for subsequent jobs automatically and flag alarm.

Conclusion : There is a lot which is yet to be done in respect of Work Holding Automation

AVINASH KHARE?????????????????