I drilled one hole with a 10 mm drill rather than 8mm
. I can use the blank for a sierra but I will need to drill out to 10.72mm.
Anyone have any ideas how I can successfully do this?
OK - so the real question here is that you drilled a 10mm hole, and now you need to accurately enlarge that to 10.72mm (27/64"). That's relatively easy. The key is in how you set up the blank so that the 10.72mm hole will be exactly concentric with the 10mm hole.
Mount the 10mm bit in a Jacobs chuck in the tailstock, and slip the drilled blank over the bit. Then, bring the tailstock with the Jacobs chuck up to the headstock, and grip the blank in a scroll chuck. What this is basically doing is using the 10mm bit as a mandrel to align the blank on the axis of the Jacobs chuck when you grip it in the headstock scroll chuck - so that when you later switch to a larger bit, the bit will remain exactly aligned with the center of the 10mm hole.
Then, back away the tailstock, and replace the 10mm bit with the larger bit.
If possible, start drilling with a standard spiral bit rather than a brad point or bullet point bit. The conical tip of a spiral bit will center fairly easily in the 10mm hole, unlike a brad point or bullet point bit that relies on the tip to start the bit, and then uses the walls of the hole to maintain alignment. But once you get a hole started, you can exchange for a brad point or bullet point bit, and it will remain centered.
If you don't have a 10.72mm (0r 27/64") spiral bit, you can still drill the hole with a brad point or bullet point bit. Align the blank as described above, but when you start drilling with the larger bit, be very careful as you advance the bit toward and into the blank; if you start the hole
very slowly, it will likely remain centered on the 10mm hole. And of course, once you get the bit started, it will be aligned by the walls of the hole and not rely on the tip of the bit.
This technique can also be done with a drill press (eg, pillar drill), but it will be a little more 'fiddly'. You will need to hold the blank in a vise and use a clamp to lock the vise to the table so that the 10mm hole remains aligned with the axis of the drill after you change to the larger bit.
