Drilling precise holes in granite.

[tonyfoale]

I am making a new CNC machine which will use a granite table and slides. More details in forthcoming posts.
The trucks for two linear rails will be bolted to the granite plate, holes are needed to glue in aluminium bushes for bolts to hold the trucks.
Here is short video (< 1 min) which gives a quick overview of drilling through a granite slab with diamond faced core drills, in particular the use of an aluminium ring sealed with plasticine to contain water and the abrasive debris from the granite. I did not want to wash the abrasive granite grains over the mill and I wanted the water to prevent granite dust floating in the air if done dry.

The granite slab was mounted on a milling machine and the DRO was used to precisely position the holes.
Here are a few stills but at less than 1 min I suggest the video.

The water and debris containment
Click for full size

The drilling, note the cooling water discolouration with granite debris.


The debris from two holes, captured in preference to being washed over the mill.


Note the plywood support under the granite plate to protect the mill table from the drill and the water after the drill has broken through.


The core drills


The aluminium bushes


After the bushes were glued in place with filled epoxy (JB Weld) the slab was put back in the mill and the bolt holes were drilled through. If the bolt holes had been drilled first then the positional vagaries of drilling granite and of the concentricities of glued inserts would be unlikely to result in the accuracy that I require.