the best method to get out the Centre bit

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andyf:
When chain drilling, it might help to spot for each hole, then go round the first time drilling every other hole, then go round again filling in the gaps. Otherwise, the drill can tend to veer off into the hole you have just drilled. Having a hole on each side as you go round the second time evens things up, so the drill can't make up its mind which way to veer off.

Incidentally, there's a nice little graphical calculator to work out the number and location of the holes which can be downloaded from here: http://iwr.ru.ac.za/~iwdf/lathe/pcd.html . It can pay to fiddle around with the number of holes, or reduce the diameter of the "big hole", until it comes up with a drill size which you have available.

Andy

lordedmond:
Thanks all for the input

Looks like its lots o drilling :(

Stuart

andyf thanks for the link its an unfortunate thing that I do not have a windows pc , I use a Mac so exe do not work

mike os:
put it in the lathe & holesaw most out... no need for a pilot hole so you get a useable blank, then bore out to size

lordedmond:
thanks I will look into the costs of a hole saw

Stuart

picclock:
Hi Stuart

I thought you just wanted to get it centred :doh:.

FWIW I have cut through 1/2" with 1/8" carbide 3 flute mills - so its possible but takes a while (my machine only goes up to 2000 rpm) and you have to clean out the groove every circle. You would have to turn it over and repeat to get the depth, although with a programmable table it wouldn't be too bad. I've done this with 131mm blanks in 10mm S275 steel OK, and your only doing 3.5"

You may be better off turning it on the lathe. If you turn a groove from the end about 1/2" wide and deepen it with repeated passes allowing it to narrow as you proceed.

But the fastest way (and the most boring ::) ) is probably to drill but you have to add the 'tidy up time'.

Best Regards

picclock

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