flat bottomed 25mm hole

[Essex Barn Workshop]

Hi,
I need to make 4x 25mm round holes to take an extending table spare leaf mechanism which folds the leaf away under the table. Easy enough with a forstner bit, except ...

I dont have any depth below the hole, it will be really close to the overall thickness of the piece of wood and the brad point on all my bits would puncture the surface. I even bought online a 25mm bit that said it was flat bottomed, but no that also has a brad point.
I do then now have a spare 25mm forstner bit, so am considering starting the hole off with a brad pointed one, stopping less than half way then grinding the brad point off of the spare and finishing with that.
Any better ideas? I have the usual array of tools (routers, drill press etc)
Thanks

 

[Phil Pascoe]

Use a 25mm router cutter with a bottom cut? Experiment with some scrap to get the depth right You would probably need a jig of some description to ensure the correct position of the holes.

 

[spb]

Alternatively you could drill most of the depth with the forstner bit, then use a much smaller straight router cutter to flatten the bottom. Which is easiest probably depends on how the holes are set out and what references you have to guide the router - if you've just got a mark for the centre of each hole and no other convenient reference point, then drilling it part way lets you use the hole itself to keep the router in place.

 

[Essex Barn Workshop]

Thank you, i should have mentioned that the holes aren't circular, they are 3/4 circles as they overlap the edge of the wood. I feel this might make a router problomatic? There wouldn't be a guide surface to edge against.
Thinking as i type, I suppose i could drill a 25mm hole straight through a piece of scrap, clamp that over the hole in the workpiece and use the scrap as a guide?|

 

[Doug B]

You could use a Forstner drill bit FB D 25 CE-Zobo with removable centre, drill part way with the centre inserted then remove the centre for a flat bottom hole.

Edited, wrong link

 

[Essex Barn Workshop]

I didn't know they existed! Thanks, bit nervous about £27.60 plus delivery to cut 4 holes though.

 

[marcros]

Thank you, i should have mentioned that the holes aren't circular, they are 3/4 circles as they overlap the edge of the wood. I feel this might make a router problomatic? There wouldn't be a guide surface to edge against.
Thinking as i type, I suppose i could drill a 25mm hole straight through a piece of scrap, clamp that over the hole in the workpiece and use the scrap as a guide?|

for 4 holes, the cheapest and quickest solution may be to grind the point off the second bit. a no brand one would be alright for 4 holes, I wouldn't waste a decent one on it.

 

[Terrytpot]

A hole/circle cutting jig. Here’s a link to the one I copied Even though it’s shown here doing quite large holes you can move the fulcrum point almost all the way to the centre so holes smaller than 20mm can be cut and overlapping them isn’t an issue using this method.

 

[Steve Maskery]

I'd just use an ordinary router bit with bottom cut, a guide bush and a template. I'd have to do a bit of maths for the right diameters, but it's a bit early in the day for that
S

 

[Myfordman]

I'd just use an ordinary router bit with bottom cut, a guide bush and a template. I'd have to do a bit of maths for the right diameters, but it's a bit early in the day for that
S

+1 Use a piece of scrap clamped to the edge of the workpiece to avoid breakout spoiling the job.

 

[Yorkieguy]

You could finish the bottom of the hole with an 'end mill' ordinarily used for metalworking, but not a cheap solution for a one-off job:

https://www.ebay.co.uk/itm/4-Flute-...655432?hash=item4404769148:g:wQUAAOSwCmZZRyR2

 

[Essex Barn Workshop]

thank you all, great replies and ideas. As this is likely to be a one-off attempt at just 4 holes, I feel that the simplest solution is probably best. I'll grind the brad point off of an expendable bit and use that to finish off the four holes.

 

[sammy.se]

Start the hole with the forstner bit, then a simple template trim bit like this to achieve the desired, perfectly flat bottomed, hole.
(I'm assuming you have a 1/4" trim router, or smallish router)

I used this but with a hinge jig yesterday, no issue of gouging out parts you want to keep, even if the hole overlaps the edge of the wood, since it's guided.

 

[Ttrees]

Might it be worth a shot to drill close to depth with the forstner bit and to finish off to depth, grinding off the tip from a spade bit?
They work well enough in reverse with some honing.

Be easy to try out on some test holes first.

 

[Richard_C]

Thinking how I might do it.

Can you go as deep as you can with the forstner, which will give you a nice clean edge on the bit you will see once the mechanism is in, and finish bottom by hand with chisel and mallet because you will never see the bottom of the hole. Might take a while but sounds like it is a one off job.

Or start as above and carve out bottom with hand held dremel or similar if you have one.

 

[Essex Barn Workshop]

again, thank you all. Some great ideas.
I think I'll spend an hour or two this afternoon with a scrap piece of scaffold board trying to achieve a perfect, flat bottomed 25mm hole!

 

[galleywood]

What if you drill the 25mm hole straight through.
Then from the other side, route a shallow shape to let in a thin piece of wood, to give you the correct depth to the 25mm hole.
It could be just glued or glued and screwed.

 

[TheUnicorn]

I haven't got any forstner bits to hand, but the brad can't be more than a few mm? If you're leaving that little material in the tabletop I'd be concerned about dimpling the top. Is there an option to drill out a little less, leaving more wood at the bottom (top?) of the hole, and pack out the top of the wood. obviously you would need to pack out elsewhere too for alignment of the mechanism.

For what it is worth I'd cut the hole with a good bit, finish it with a blunted bit, and clamp the end to protect from tearout

 

[Essex Barn Workshop]

Had a fun hour grinding down one bit, starting this offset hole ( so I could see the depth) with another before finishing with the ground off one.
many thanks to all who responded.
this was, obviously, just a piece of scrap wood.

 

[TheUnicorn]

still can't quite see the mechanism that is meant to work there, will you post a picture when its finished please