accurately enlarging a hole.....

[al ladd]

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"?

[Carl Beckett]

I might put a snug fitting forstner bit in the existing hole. Then tighten on the drill quill. Then clamp the piece in place snug to the table. Then remove the bit and replace with the larger bit.

[Bill Dufour]

Counterbore with pilot that can be removed and exchanged. Common for metal work.
Two days ago I needed a 1/4" pilot so I took a 1/4" bolt and drilled it axially for a 3/16 hole. Cut off the head and cut the short length I needed. Cut and soft soldered a bit of 3/16 rod for a shank and used it. Worked fine. Should really have silver solder or brazed it.
Bill D

Gives you an idea what I am talking about. Stupid Amazon does not mention size.
https://www.amazon.com/HHIP-2007-0021-Piece-Interchangeable-Counterbore/dp/B00N40FDD2/ref=sr_1_4?crid=6X0WC7OLVEMX&keywords=interchangeable+pilot+counterbore&qid=1641844427&s=industrial&sprefix=interchangeable+pilot+counterbor%2Cindustr ial%2C138&sr=1-4

Ebay now for $25

[Steve Jenkins]

Probably with a rabbiting bit on a router.oops edited because the bearing needed would probably not fit in a1/2” hole

[glenn bradley]

I might put a snug fitting forstner bit in the existing hole. Then tighten on the drill quill. Then clamp the piece in place snug to the table. Then remove the bit and replace with the larger bit.

This is what I commonly do for alignment in preparation for a secondary operation.

[Jerry Wright]

HF step bit. Self centers in 1/2 hole, them bore to desired depth.

[Steve Eure]

Another way that I was taught is to cover your existing hole with masking tape (painters tape), then take your 1/2" bit,align with your hole, and using the point, make a dimple in the tape. Remove the bit and insert your 3/4" bit. You now have a dimple or small hole to align your larger bit.

[Bruce Page]

I might put a snug fitting forstner bit in the existing hole. Then tighten on the drill quill. Then clamp the piece in place snug to the table. Then remove the bit and replace with the larger bit.

+1, that’s how I would do it and I’m a retired machinist with a milling machine.
The only thing I would do differently would be to chuck the alignment tool first and then bump the plywood into position, then clamp it down. Either way will work If the plywood piece is small and lightweight.

[Lee Schierer]

Cut a short piece of 1/2" dowel that fits in the hole. Mark the center of the dowel on one end. Place the dowel in the hole so the marked end is up. Drill the hole larger to the desired depth with the larger forstner bit.

[Bruce Wrenn]

Tolerance of about .005 -.01"?Your wood can change that much overnight due to humidity changes. It's woodworking, not machining.

[Brian Holcombe]

There is no chance a 1/2” hole is changing by .005-.010” overnight unless it has been cut in green wood. OP is cutting ply and MDF, both materials are reasonably stable.

[Richard Coers]

Tolerance of about .005 -.01"? Tolerance for what? Diameter? Depth? Concentricity? I prefer a router with a guide bushing and template. If that works with your extreme tolerance of .005" is another question. Curious of the application now with that kind of metal working tolerances.

[al ladd]

Thanks for all these replies.

I was curious about step drills-- I have one coming, never used one before.

Sounds like the consensus is for a forstner bit in existing hole aligned on drill press and clamped in place -then switch bits. I guess I was unsure about the reliable tracking of a forstner bit guided by its rim like that. The dowel (or plug)suggestion is interesting too, and maybe used in conjunction with the forstner bit trick would be better yet . The other possibility I've considered is router with template guide, either changing bit, or template guide size with template left clamped in place. The job will utilize router with template guides already. But I know unaligned template guides can create problems.

The need for accuracy is in concentricity, but that will require comparable control of diameter -depth not so much . The application is for a threaded fastener, guided through fairly tight tolerance bushing, to align with a fixed nut below, securing a fence to a table. Maybe I'm exaggerating the need for precision. I know how annoying it is trying to thread a knob into a misaligned pair of parts. I suppose I can give more room in ID of bushing for the fastener to wander in, but then it might be harder for the fastener to find its threads. It's something that will be assembled and disassembled fairly frequently in use.

The item is a high precision jig that I hope to offer in a kit form, built with a router bushing guided template and unusual hardware I'll supply. So I'm trying to figure what will be doable for average woodworkers with average tools. It will be important to keep the fabrication by the builder not-too-fussy, and hopefully fun, and the end product needs to be accurate and easy to use.

[Michael Schuch]

Probably with a rabbiting bit on a router.oops edited because the bearing needed would probably not fit in a1/2” hole

This is exactly what I was thinking. I have a flush cut bit that would easily fit in a half inch hole and a set of all difference size bearings I can use with it.

[Michael Schuch]

HF step bit. Self centers in 1/2 hole, them bore to desired depth.

Irwin makes a single flute step bit that does an excellent job keeping itself centered. Better than the two flute step bits. I have enlagred holes in saw blades with one without throwing the blade off center.

[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-7980-190-50.html?ne_ppc_id=15153909030&gclid=Cj0KCQiA8vSOBhCkARIsAGdp6RTgX6NyfReb0qNOL2rN iETwqguXuFK03h2-EGwt2kA2_jkhWHWEgk0aAo1JEALw_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:


https://www.youtube.com/watch?v=hlnu1qXGzbI

[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