accurately enlarging a hole.....

[Michael Schuch]

I'm interested in hearing how people would reliably concentrically enlarge an existing 1/2" through hole in something like the face of BB plywood or MDF to 3/4" for about half its depth , preferably with only the types of tools the average serious amateur woodworker would own. Tolerance of about .005 -.01"?

Another possible approach is to drill a 3/4" hole in a piece of scrap wood with the fostner bit. Then clamp the scrap piece on top of the work piece lined up properly then use the hole to guide the fostner bit into the work piece.

If concentricity is a major issue (assuming the work piece is round) mount the work piece onto a lathe then spin the work piece instead of the drill. A dial indicator would allow you to line up the 1/2" hole with axis of the lathe very acurately making the 3/4" perfectly centered over the 1/2" hole.

[Richard Coers]

I bet you don't get .005" concentricity with a step drill. A step drill won't guide in a smaller hole since there is a cutting flute there as well. Just the slightest side feed pressure or the bit following the grain a little, and you'll enlarge the small hole to get an oval. .005" is like a sneeze drilling in wood.

[mreza Salav]

I have done this a few times and works quite well even on pieces you can't take to a drill press (like side of a cabinet). The trick is to make a stepped hole first in a scrap piece to work as a guide: drill half way (Say 3/8") using a 3/4 bit into a pice of MDF on drill press. Then while the piece is locked use a 1/2 bit to make a through hole into the half hole. Now put the piece and align over the hole you want to enlarge, e.g. use a 1/2" dowel to line up the 1/2" holes, clamp the piece guide and use a 3/4 bit and the half way 3/4" hole as guide to drill your large hole.

[Andre Packwood]

Hi Al

Maybe you could use a 3/4 inch flush trim bit such as this one:
https://www.toolstoday.com/v-7980-19...0aAo1JEALw_wcB
and replace the ball bearing with one of 1/2 inch diameter. Then you would simply plunge the bit to the required depth if the 1/2 hole is not too tight.

Andre

[Rob Luter]

A few great tips here:

[Rich Engelhardt]

Is it possible to drill the 3/4" part of the hole first?
That would make things a lot easier.

[Prashun Patel]

Combine the dowel plug with the clamping as your instinct says. That will work fine. In the future drill the wider counterbore first