advice needed

[inthegutter]

Hey all,

I'm very much a newbie so pardon the ignorance.

What would anyone advise is the best way of making a 75mm diameter ~2mm deep circular recess* in oak? I'm also keen to avoid buying a new expensive drill bit that I'm only going to use a few times (a large 75mm Forstner bit would work I guess but I'm loathe to pay ~£30).

Thanks for any advice!


* This is for a coffee table project where I want to inlay an aluminium disk which will act as both a "design element" and also a "built-in" coaster.

 

[marcros]

router, guidebush and a template would be my suggestion ahead of the forstner, but you would need to cut the hole in the centre, using a forstner or similar.

You may be better taking it to somebody who already has the kit to cut the holes

 

[Glynne]

A router with a circle cutting jig would work. Something like a false MDF base with a pin to rotate your router. Peter Lloyd uses this for some of his boxes.

 

[AndyT]

What tools do you have? A router would work if you have one, but there's a chicken and egg problem in making the template.

A low tech cheap way would be to use a hole saw to define the circle, then remove the wood inside the circle with a chisel. You can make a 'poor man's router' by wedging a chisel through a hole in a scrap of wood. (Search Paul Sellers' blog for pictures of this.)

To avoid making a hole through the job with the hole saw pilot bit, temporarily glue a thicker scrap over the centre of the circle.

 

[inthegutter]

Hey all,

Thanks for the advice.

- I don't have a router though I can easily lend one so I might experiment with that (though it does again seem a bit of overkill for my problem).
- I'd be worried about the low tech way (using a chisel to remove inside the circle) as I only want to remove ~2mm and want a nice smooth recess for the aluminium inlay to sit on.
- I do also have an expansive flat bit that just about reaches the right size. Though this will leave holes they'll be covered by the aluminium inlay so this may be easiest.

 

[lanemaux]

The method Andy points you at is fairly accurate amigo, The chisel is used with bit of wood to create a jigged cutting plane for stock removal. Once set up it can be very accurate. It is then very similar to a Hags Tooth , a hand tool used for bottoming before we began torturing wire to get magnetic fields to leap through hoops.

 

[Cheshirechappie]

I do also have an expansive flat bit that just about reaches the right size. Though this will leave holes they'll be covered by the aluminium inlay so this may be easiest.

It might be a good plan to try this on a piece of scrap first; if my experience with expansive bits is anything to go by, it might prove very difficult to achieve an even depth, especially at that diameter.

I think AndyT's method is a sound one. I might add a couple of details, though; the first thing I'd do is place the disk in position, and scribe round it with a scalpel or similar. It might pay to clamp the disk if you can, and take care to keep the scalpel blade tight to the disk diameter, especially on the long-grain bits (the knife might be very keen to follow the grain of the wood). Scribe lightly at first to establish a line, then follow up two or three times more heavily to make sure the surface fibres are neatly severed. Then excavate out the bulk with a chisel, keeping away from the knifed line. Trim to even depth with a hand router set carefully to the thickness of the disk, and then using a narrowish chisel vertically, creep up to the line. Finish with a paring gouge if you have one, but if not, use a carving gouge or even a narrow chisel to nibble out the last millimetre or so (if the 'wings' of the carving gouge dig in, don't worry about it - you won't see the marks once the disk in in place). Clean up to depth with a chisel used bevel down.

That might sound frightening, but with care, patience and a methodical approach (and sharp chisels), it'll be quicker than you think; try one in scrap first to get the feel of it.

 

[custard]

A 75mm coaster isn't much use unless you'll only be drinking espresso from those dinky little cups. You really need 100mm, maybe even 120mm if your aim's a bit off!

 

[Owl]

What does it matter if you go deeper then 2mm, or even if the bottom is rough and not smooth. If you use epoxy to set the aluminium disc in then you can get it at whatever level you want. I agree with Custard about 75mm not being big enough.

 

[Baldhead]

Hey all,

Thanks for the advice.

- I don't have a router though I can easily lend one so I might experiment with that (though it does again seem a bit of overkill for my problem).

Borrow a router and buy on of these
http://www.fiskars.co.uk/Crafting-Sewin ... te-Circles
Place a spacer between the template and the table top (use carpet double sided tape) one of the holes on this template is 3" diameter, use a template cutter in the router, job done.

Baldhead