QC T continued

Harold Hall


As commented on on the first page the design was established with a wish to limit the need for precision. Whilst this has been largely achieved there is a need for repeatability between parts making the choice of the manufacturing method used an important one. This is mainly to ensure that the clamping method is capable of securely holding each holder to the base. The cam action of the locking spindle does of course provide some room to cope with minor differences.


The semicircular grove (B) in the holder was produced by clamping two holders back to back and drilling down through the join. I did this with just a drilled hole but later felt that reaming this prior to separating the parts would have been worthwhile and would suggest this is dome. Even with this done it is inevitable that there will be minor differences in the depth of the grove between the two parts and other holders and  subsequent operations have been chosen to cope with these differences.


Having made the Body (1) and fitted to it one Locating Key  (6) and one Spacer (7) this can then be used as a jig for positioning each holder in turn to mark out the position of  hole (C)


Mount the body onto the milling machine table using the central hole and a fixing stud, then mounting the first holder onto this, clamping it with a toolmakers clamp of similar. With a centre drill in the machine spindle use the X and Y table movements to establish the position for the hole to be drilled ensuring the table clamps are set. With that done, mark out the position of the hole in the first holder using the centre drill. Then fit and mark out each additional holder in turn ensuring that the tables are not moved during this process. Adopting this method will compensate for any variation between holders in the depth of the grove. A 10mm drill can now be placed in the machine spindle and each holder returned for final drilling, again without moving the tables.


The Body can also be used as a drilling jig for drilling the 8mm hole in the Operating Piece (31). Packing, having a suitable thickness, is placed between each flat on the Clamp Piece (32) and the Body and the whole assembly clamped to permit drilling to take place. Do ensure that the Clamp Piece is upright before clamping for drilling.  


Note the comments in Sk. 2 regarding the position of the hole when choosing the thickness of the packing pieces.     

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