One way of holding round and some other shapes – square and hexagonal is by using collet blocks.
Collet blocks used as rotating collets are covered elsewhere. Collet blocks are covered here because they are often used in vices where they are only rotated in a very simple way.
Collet blocks are blocks of steel designed to take a specific style of collet. One of the most common is the Hardinge 5C collet. Usually there are two blocks – one is square and the other is hexagonal. Since both have parallel sides either can easily be held in a vice though the hexagonal one might not fit well in some smaller vices. The collet block should be held in the center of the vice’s jaws.
Fig. Square collet block 112
Fig. Hexagonal collet block 113
Fig. One way to hold a hexagonal collet block 546
The beauty of the collet block is that it turns a round shape, the collet, which can be hard to hold into a square or hexagonal shape, which is easy to hold. Of course, only the end of the workpiece can be machined but if the part in the collet is not needed it can easily be turned off in a lathe.
But the collet block can be used to do more than this. This is because the block can be rotated in the vice to give 90° turns with the square block or 60° turns with the hexagonal block. This allows for a very simple and cost effective form of indexing.
If a large number of bits are being machined by holding them in a collet then it can be useful to have a collet stop. In the case of the 5C collet this screws into an internal thread in the end of the collet. This ensures that each part in pushed into the collet by the same amount.
The only problem with this is that the position of the collet block laterally in the vice is not well defined. This can be done using a stop fitted to the collet is the case of the 5C type collet, or a stop fixed to the vice or a separate one bolted to the table of the milling machine.
Collet blocks for ER type collets
These work the same as the 5C collet blocks.
Fig ER collet blocks
One difference is that the closing nut is a larger diameter than the collet block. This can be got round by using short parallels to hold the collet block.
Fig holding ER collet blocks