I have made a back plate for mounting a 150mm 3 jaw chuck. It is a SanOu brand which has 4 8M threaded holes for mounting. I have done the shoulder to fix into the jaw's register. The next step is to drill 4 holes for mounting the chuck. I painted the holes on the jaw and pressed against the back plate. It left an impression on the back plate. I assumed that it should be pretty close.

I have made centre holes first and measured again the distance between 4 holes. However, I have done the centre hole in one go and then 8mm drill bit for holes. I have difficulties on getting the 8 mm drill bit on the centre of the centre hole. I have tried my best to centre the drill. When holes are done, I have no luck of matching them with the threaded holes on the chuck. It needs a lot of precision.

My question is:
1. Should I use 8.5mm drill bit to build in some tolerance.
2. Should I do the centre drill, followed by 8mm drill bit. In that case, centre drill and 8mm drill should be in line
3. What is the best practice of doing this. I am sure that it really requires accuracy and tolerance consideration.
4. What is my option now, perhaps re-drill?


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