Best way for expand 3/4” dog holes to 20mm

I would just use a regular twist drill. Don’t try to use an auger , spade, or forstner etc. Even a cheap junky one like this would probably work since its mostly drilled out already. The taper will center itself in the 3/4 hole.

https://www.amazon.com/uxcell-Diameter-Twist-Straight-Drilling/dp/B00G9TX0YE

I’d try a 20 mm forstner bit. To prevent side travel, first bore though a piece of scrap, then clamp this scrap over your existing hole, and drill. If there are many holes in a line, say 4” from the front of the bench then make the scrap to have the correct distance for a repeatable jig. Just remember to stay vertical. Good Luck.
ps- verify that the clamps are 20 mm and test fit on the scrap piece before drilling the bench.

I think SMP’s idea would work better and quicker. but still test after one hole.

Gotta love Festool being unique for everything.

As 20mm bits may be expensive, you could route a large diameter jig for the outside of your router and use a 1/4 spiral bit. Not as easy, but you may have everything.

Ugly way: rasp. Could do the taper.

I’m assuming that you can find a 20mm bit of some sort. I would plug the 3/4” holes with dowel, mark the centre of the dowels and drill them out using the 20mm bit.

-- Grant Wilkinson, Ottawa ON

I would begin the hole with a step drill that goes to 20 mm. Then you can continue with the 20mm bit of your choice. It would center the hole

-- Just trying to build something beautiful

Why not just use a file or rasp? It’s only 1mm that you need to remove. .............. Jerry (in Tucson)

-- Jerry (in Tucson) www.woodturnerstools.com

You can use one of these chucking reamers3!472545890348!(make a handle for it) and get good results….but so would a 20MM twist drill. Those reamers can be had in 20MM size.

-- Our village hasn't lost it's idiot, he was elected to congress.

SMP has it right. Regular old twist drill will work quick and easy.

Forstner bits need a center point or they’ll chatter and chew up the edges.

Filling the holes and redrilling is a lot of work. Why not just drill new holes a little away from the originals and save all the plugging & gluing?

Routing 1/2 mm off the sides of the hole is scary if you have to do one, dozens would be a chore esp if 2” deep. You’ld have to build a precise jig to make it feasible. Clamping is an issue unless you use a dog hole jig and even the the last few holes won’t have the right dia.

Hand reaming will give you a forearm like a fiddler crab.

Follow SMP’s advice. Mcmaster.com has the 20mm bit here

Alternately just drill separate 20mm holes so you can use either 20mm or 3/4” dogs.

-- The hump with the stump and the pump!

Another vote for a 20 mm twist drill. It’ll ream your hole fast and clean.

-- I collect hobbies. There is no sense in limiting yourself (Don W) - - - - - - - - Kenny in SW VA

+1 for the 20mm twist bit. Self centering and easy, just make sure it is sharp.

Festool isn’t really unique in utilizing 20mm holes. It’s simply the standard in Europe – where they are headquartered. If there’s anything unique , it’s that we here in the USA are the only place that doesn’t use the metric system.

and I’m not looking to start a debate on whether on not we should adopt it !

I absolutely agree with SMP on the way to enlarge. 20mm twist bit is the only way to fly.

If only you were making a bigger jump, a simple tricks is use a hole saw that just fits the hole, then put it inside the larger one and mount both on the mandrel.

Since it’s such a small jump, I’m in favor of the guide, since even a spur or Forstner will want to walk and a twist will want to tear out at the top.

One more possibility is to use a step bit to just start the 20 mm indentation for the Forstner or spur or even the twist bit.

+1 for the 20mm twist bit. Self centering and easy, just make sure it is sharp.

- controlfreak

-- Andy - Seattle USA

if you’re lucky enough to have a plunge router with two identical motors, put a 3/4” bit in one to use as a centering guide, then clamp the base… swap the motors, with the second one having a 20mm bit, then plunge

been doing it that way for a while, but i have all the right gear; see if maybe you can borrow, if need be