How to slightly enlarge a worktop hole for replacement kitchen sink

Question

The task

I am replacing my kitchen sink. The existing sink sits in a rectangular hole (with rounded corners) in the kitchen worktop. The worktop is about 1½" (40mm) thick and is made of some kind of particleboard (chipboard) with a laminate top surface.

The new sink is about ⅜" (10mm) wider and the same depth front to back. However it also has a smaller overhang.

Consequently I need to make the hole about ¾" (20mm) wider and about ⅜"(10mm) deeper. If I keep the sink centered that means cutting back ⅜"(10mm) and 3/16" (5mm) from each side.

This will have to be done with the worktop in place and from the top.

Tools?

I don't have a suitable power tool (I have drills, sander, angle grinder, Dremel) so I'm wondering what to buy or hire.

From what I've read, the usual tool for this job is a jigsaw but I'm concerned that it will be difficult to cut such a thin slice around the hole using a jigsaw.

Question

Should I buy a jigsaw or a router and what issues should I look out for?

Answer 1

I would scribe a line and then use a coarse wood rasp (like a "cheese-grater" type planer) by hand. You will have more control than with a power tool, since it is such a small amount to remove. Then you could smooth it out with coarse sandpaper.

Answer 2

If there is adequate room at the rear of the sink, or if you worry less about centering and just do the sides and the front, a router will do the job nicely. A laminate trimmer may be a better choice on the "fit's the back of the sink" part as they have a smaller base, but they also have more limited bit size/power - still you could do it in a few passes, and they cost less to buy.

The secret to nice edges with a router is to build a fence or template - a board supported the same distance away from the desired cut as the edge of the base is from the bit. Freehanding it will tend to wander quite a bit more than following a fence. Firm control of the machine is also needed, especially when working on a narrow strip of material as you will be in this case, at least at the front.

Answer 3

In the end I bought the cheapest jigsaw I could find in the nearest store. I tidied up with a rasp of the type suggested in Jimmy's answer

The problems I encountered with the jigsaw were ...

• The blade wandered from vertical by a really remarkable amount. This didn't show up in my initial testing with scrap wood. I found this impossible to avoid but maybe it gets better with practice.
• Because the blade wandered, I tended to cut about six inches then sever the hanging piece and restart the cut more vertically. On one side this left a slightly stepped edge (though the top edge was still close to OK.
• I bought a downward cutting (reverse) blade to try to avoid chipping the laminate surface as I had to work from the top. However the only size I could get was too short and kept jumping out. This was alarming enough for me to switch to a longer normal (upward-cutting) blade I'd bought at the same time just in case. The chipping of the laminate was pretty noticeable but not severe enough to extend beyond the lip of the sink.
• I really should have rigged up some kind of dust extraction, even with a laser-line, it was very difficult to see where I was cutting, wearing a dust mask makes it impossible to blow the dust away.
• Using masking tape to draw my line on didn't work too well as the jigsaw tore off strips of tape that ended up stuck to the sole-plate and made work more difficult.

Cutting very thin slices with the jigsaw was surprisingly easy. My worries about this were unfounded.

Using the hand rasps was hard work, that's why I didn't use them for the whole job. They are very satisfying to use though and did a good job of smoothing out the wandering jigsaw cuts.

In the end the top-edge of the cutout was OK but the section looked pretty awful. This doesn't matter as it isn't visible. I had been pretty conservative with my marking up so I needed to use a hand-rasp and files to take off the last millimeter in most places , to smooth the edges and bring the sides of the aperture to vertical. I used a short version of the surform tool and a conventional wood-rasp. I used a half-round file for the corners.

This answer is just to record what I actually did in case any future reader finds it useful. That's why I made it community wiki to not detract from the other answers. I've upvoted the other answers and accepted one that I found useful.

Answer 4

There are a few tools you can use.

It all really depends on what you feel most comfortable using.

When I cut out my sinks I use a power saw. Mark your cut lines. Then start the saw an drop it in. Then you use a jigsaw to finish the corners.

The reason I use a power saw is because you get straight cuts an is quick.

If you chose to do it all with a jigsaw the blades can bend as cutting an is not as neat. Also does not always work if there is stuff under that you don't want to cut into.

A router / laminate trimmer could be the easiest for you. If you run it of a guide, problem with this is that there will be more dust. But may be easier for you with the 5mm cuts.

But like I said it all depends on what you feel comfortable using.

Sinks have a lip of about 10mm that will cover up your cut.