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

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Forum topic by Italo78 posted 04-23-2020 04:11 AM 3367 views 0 times favorited 14 replies Add to Favorites Watch
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18 posts in 2354 days

04-23-2020 04:11 AM

Topic tags/keywords: mft 20mm dog holes 34” festool

Does anyone have a suggestion on the best way to expand the 3/4” holes on my workbench to 20mm Festool style. I received a few clamps from a relative that are used with an MFT table and will not fit on my workbench. I was debating between a 20 mm drill bit or a 20mm boring router bit. Any thoughts on getting the best results. Thanks ahead of time

14 replies so far

View SMP's profile


5301 posts in 1361 days

#1 posted 04-23-2020 05:36 AM

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.

View Jeff's profile


321 posts in 841 days

#2 posted 04-23-2020 05:37 AM

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.

View tvrgeek's profile


2378 posts in 3105 days

#3 posted 04-23-2020 09:06 AM

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.

View gwilki's profile


375 posts in 2929 days

#4 posted 04-23-2020 12:04 PM

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

View Kaleb the Swede's profile

Kaleb the Swede

1993 posts in 3425 days

#5 posted 04-23-2020 01:04 PM

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

View Nubsnstubs's profile


1819 posts in 3185 days

#6 posted 04-23-2020 01:09 PM

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

-- Jerry (in Tucson)

View Fred Hargis's profile

Fred Hargis

7480 posts in 3949 days

#7 posted 04-23-2020 01:30 PM

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.

View Madmark2's profile


3433 posts in 2044 days

#8 posted 04-23-2020 03:45 PM

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. 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!

View HokieKen's profile


21690 posts in 2594 days

#9 posted 04-23-2020 04:00 PM

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

View controlfreak's profile


3730 posts in 1057 days

#10 posted 04-23-2020 04:10 PM

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

View xedos's profile


537 posts in 756 days

#11 posted 04-25-2020 08:42 PM

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.

View Kelly's profile


3899 posts in 4400 days

#12 posted 04-25-2020 09:51 PM

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.

View Andybb's profile


3485 posts in 2059 days

#13 posted 04-26-2020 12:04 AM

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

- controlfreak

or a Set of 3 step drills on Amazon Prime

-- Andy - Seattle USA

View davezedlee's profile


66 posts in 2280 days

#14 posted 04-26-2020 01:13 AM

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

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