20mm router bit

[domscan]

Hi trying to work out best bit for the job so any advice much appreciated

need to cut 20mm diameter recessed holes approx 16mm depth into a 20mm deep work piece
these holes are to accommodate 20mm diameter disc magnets...for a knife magnet rack

holes need to be perfectly flat bottomed and with ideally no central pivot point hole as per that created by a forstner bit..I guess I could use a forstner bit on a drill press and grind of pivot point and a template to keep everything steady but would prefer to accomplish with router if possible.

saw a thread on the 491072 hinge boring bit - anyone know if does this leaves a flat bottom or is there a pivot hole (and if so any idea on depth of said hole?)
this could be the perfect bit as on same thread i hear that it is more like 20.2mm which gives a bit more wiggle room for the delicate magnets

a retailer here in UK also said that either the 490971 edge trimming cutter would do the job??

confused...

 

[AstroKeith]

The 491072 has a centre pivot. Threw mine away after it got worn out so cant measure it I'm afraid. About 2mm from memory.

My experience with cutters like the 490971 is that although they will plunge, they aren't realy designed for that so expect a slightly ragged start to the hole.

Intrigued why you want a perfectly flat bottomed hole when its going to have a magnet inserted?

 

[afish]

https://www.freudtools.com/products/04-552

 

[Roseland]

The Festool Zobo Forstner bits have a removable centre, with a choice of short pin, long pin (for angled holes) and drill bit.  If you start with the short pin in, and then remove it you will end up with an essentially flat bottom.

Andrew

 

[Jim_in_PA]

I have the Freud bit previously mentioned and it works well. The singular downside is that it's only a .25" shank cutter, although that means it can be used in the smaller OF1010 when that's desired.

 

[Cheese]

holes need to be perfectly flat bottomed and with ideally no central pivot point hole as per that created by a forstner bit..

saw a thread on the 491072 hinge boring bit - anyone know if does this leaves a flat bottom or is there a pivot hole (and if so any idea on depth of said hole?)

The 491072 will leave a very small diameter pocket that's about 2.5 mm deep.

I'm unsure as to why you don't want the pocket as it will be hidden by the magnet plus it also provides a place to put in a dab of silicone or construction adhesive to help secure the magnet.

Don't use a cyanoacrylate glue as it is brittle and it doesn't like to adhere to the nickel plating on the magnet. Eventually the magnet just falls out unless you're press fitting the magnets into a recessed hole.  [mad]