Flat bottomed 20mm dia holes.


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24 Jul 2020
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Hello, I am looking for a solution for drilling holes for Hafele Onefix connectors, 20mm diameter. I would normally use forstner bits for this kind of job, if not the fact that I will be drilling in 15mm plywood and the hole itself is 12.5mm, so I'm a bit worried about the bad point which will be very close to drill through. I will have a solid jig, made of steel (something like concealed hinges jig) that will hold the drill vertically so I wonder if I can simply use widely available routing bit to achieve flat bottomed holes? Or is there anything against doing this?
Btw. I know about the existence of forstner bits with removable center pins but I would like to hear your opinion on the use of router bits first. Any advice will be much appreciated.
You could use a router with a guide bush and a straight cutter. Make a template with edge location. Depending upon the router bit and guide bush you will have to adjust the hole diameter in the template
I was going to suggest the same as Hornbeam, or a slight alternative make up a jig for a bearing guided cutter like this

Not sure if that'd be feasible though as I know those connectors can be pretty small.
Or get 2 forstner bits and grind the brad point off one. Start your hole, do maybe 8 to 10mm and finish with the other.

If you have a router, i would do hornbeams suggestion
Drill a hole through a piece of ply with your forster bit to use the ply as a guide, then grind the point off the bit as others have said
My 35mm bit isn't the same 35mm as the blum hinges 35mm, naturally. It is also a bit pointy for 18mm boards despite filing some off the tip. In the end I used it on a scrap, used my spindle sander to round the hole to a size that actually fit the blum hinges and then used a router using that hole as a template. Worked fine. As always there are many methods of skinning a dog.