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I need to mount an undermount copper sink to our new kitchen surface. The surface is 4cm oak, which will be varnished with 12 coats of marine lacquer to protect the oak from water.

What I want to achieve is to cut out a very precise hole for the sink (I should add the sink is rectangular with curved corners), such that the cut out wood can be reinserted as a lid over the sink for additional workspace.

Elsewhere I cut out some rougher holes for domino hobs with a jigsaw but found it nigh on impossible to maintain straight and vertical edges.

I guess a cut width of about 4 mm will be necessary.

What tool or technique can I use to achieve my goal?

  • Good to have you here at the DIY stack exchange. Since you would like to use the cutout as a lid it will be important to understand what width of cut will be acceptable. – Michael Karas Oct 11 '19 at 8:11
  • @MichaelKaras Thanks Michael. Good point. I'd guess 1mm ideal, with 3mm starting to get on the side of it being an unsightly gap? The lip of the undermount sink is about 20mm all round, so having a resting surface is not really a constraint. – Sentinel Oct 11 '19 at 8:33
  • @MichaelKaras Actually, no, correction... the gap the tool would have to cut out would have to be significantly wider. 10 layers of marine varnish on the hole and cutout is going to be how thick? Probably an extra 2mm? – Sentinel Oct 11 '19 at 8:42
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    I wonder if you would get different answers on woodworking.stackexchange.com – UuDdLrLrSs Oct 11 '19 at 15:02
  • You may wish to include very specific details on the sink in question, as that can impact correct answers. [Neighbour recently installed a live-edge slab counter top around a porcelain lined sink, but failed to account for seasonal expansion of the wood... They are currently sourcing a new sink to replace the damaged one for want of a small expansion space between it and the wood...] – TheLuckless Oct 11 '19 at 22:47
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Use a router with a centering bar. A really fine finish can be achieved.

If you need a square or rectangular or different hole then bits of batten secured down can give you that shape...

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    Thanks, Mike, can you point me to a link please to what you mean by centering bar? A quick search seems to point me to some circular hole cutting devices, though that be me using incorrect search terms. – Sentinel Oct 11 '19 at 8:34
  • Think of a compass, then make a bar from wood to control the position of the router... just need a locating point on the work... – Solar Mike Oct 11 '19 at 10:39
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    Great answer+. Making a jig you can clamp to the surface might b better. You'd have to fasten the centering bar to the piece the OP wants to use. – JACK Oct 11 '19 at 12:14
  • Gotcha. I updated the question to emphasise that the hole is actually rectangular. – Sentinel Oct 11 '19 at 12:44
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    Also, don't try to cut the whole 4cm at a time. Start with a very shallow depth cut and then increase the depth incrementally each circuit around. – Arluin Oct 11 '19 at 15:16
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If the sink is a drop in sink, you have about 1/2" to 3/4" (1-2 cm) rim that fits over the edge of the counter. This makes the problem fairly easy, as 1/8" (3 mm) wanderings with your jig saw aren't a big deal.

If the sink is an undermount one, then you need a really precise hole, and if you are going to reuse the cutout as a lid you need a narrow kerf.

To cut a rectangular hole, use a plunge router and a 1/8" bit. Make battens that are the router's baseplate radius away from the edges. E,g, If your router as a 6" diameter base plate, the battens are placed 3" outside the marks for the cut out. Don't forget to account for the diameter of the drill bit. If you use a 1/8" bit this may mean the battens are 3 1/16" outsdie or 2 15/16 depending on whether your line is for the opening for the cutout.

You can hold the battens in place with double sided carpet tape.

You can't take the entire 4 cm in one pass. I would suggest taking out half a cm per pass. The inside should be supported too so it doesn't tilt and tear as you do the last edge.

If the shape is other than a rectangle, then make a template. You can can router bits with a bearing on them. The bearing rolls along the edge of the template (so the template is bearing radius too small or too large for a centered line.

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Continuing on the laser idea, consider water jets. These are commonly used for cutting thick stacks of fabric without distortion to the material. The trick here is finding a suitable machine. Best of luck.

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  • I considered that, too, but it would do some damage to the wood to get it soaking. You'd raise the grain and cause swelling and possibly checking. If you want to try, your local iron supplier may have one for cutting sheet steel. – isherwood Oct 11 '19 at 18:18
  • Waterjets can actually cut thick materials much better than laser cutters - I would ask a local shop on their capabilities, but wood should be able to be cut without abrasive, and if you put it on top of a sacrificial piece you should be able to protect it from blowback. One concern would be potential splitting or wandering during piercing, which you might be able to avoid with a pilot hole. 4mm should be easily achievable as a kerf. – IronEagle Oct 11 '19 at 18:33

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