Cutting circles with a jigsaw

Oversize unless your rasp magically adds material...

He said cm.

Given: plywood, jigsaw wanted: circular bit of plywood, diameters 60-80 cm, small hole in the middle is OK.

Yeah: "make a jig", i.e. an arm with the wanted radius, attached to the jigsaw base, screw middle down and cut around.

But: I seem to recall there's a catch there somewhere, or a best way of doing this, in positioning the radius arm with respect to the blade. Should the radius arm attach to the jigsaw a) ahead of the blade, b) exactly off to the side of the blade, or c) further back from the jigsaw blade?

Thomas Prufer

You can buy a jig that has a drill down the centre and three jigsaw blades that adjust out on a spiral to about 90mm diamtere. It works quite well and stays balanced in use.

You definitely want it balanced and on three point cutting support or it will snatch the moment the blade tips touch the surface.

If it is not in line with the blade any wiggle will change the radius.

Personally I would cut undersize and use a rasp...

A holesaw will leave you a circular bit of ply:

Don't suppose you have a router, hence asking?

My feeling is a jigsaw isn't going to make an accurate job of this - so perhaps easier for a one off to simply cut as near as possible and plane or file down round?

He wants 600-800 mm.

In answer to the OP, the blade needs to be at 90 degrees to the radius. As others have said, you might want two radius arms, to hold the jigsaw steady. But it's easier just to use a rectangular piece of ply with a hole at one end for the pivot?

How about this approach? A jigsaw table with a blade stabiliser mechanism. Could be worth it if you are going to do this repeatedly.

The table is also equipped with a T slot for circular cuts?80~?450??) and a foot pedal for maximum efficiency and control.

A jigsaw is not what I'd recommend, they often wander all over the place & require endless steering to get back on track. There are various other ways depending on exactly what you have. One possible is to saw it a little outside the line, mount the disc in a drill & hold coarse sandpaper upto it to sand it as it spins. Not too fast :)

A more suitable power tool would be better, but this approach would get you ther.e

NT

Use a spare piece if ply as a base. The saw end needs to be as long as the saw base for stability. The hole end can be narrower. Start of by cutting a big hole with a spade bit in the base accommodate the saw blade. As has already been said the blade should be at a dead 90 degree angle to the hole.

To start the cut in the wood, before mounting the saw and base drill a hole on the outside of the intended radius using a spur point (for positioning) wood drill to accommodate the blade.

Use a thick blade for maximum rigidity. At that radius the size of the blade relative to the curve isn't a big issue IMO.

It goes without saying that the work will need to be firmly clamped ideally with a minimum of two clamps Maybe most important of all don't hesitate to stop the operation and reposition and re-clamp the work as many times as is necessary; making sure at all times that you won't be cutting into the support, bench or whatever.

michael adams

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Oh, yeah, ta. I thought mm.

TW

Router will make a neater job and most routers are easy to make an arm using one of the guide clamps hole.

Yup.

Ages ago, I got a Lidl mini router kit with jigs for cutting either holes or disks. Has proved invaluable for making speaker baffles. A normal router often won't cut small enough holes - and a holesaw maybe not available in exactly the right size. It is odd in that it uses US spec (imperial) Dremel cutters, which aren't so easy to find in the UK.

You want the blade on the radius. I would make a ply guide say 5" wide and as long as needed. Stick a hole through the jigsaw end with a spade bit to allow some clearance round the blade, and then pin and glue blocking to the ply to trap the base plate of the saw, with the blade poking through. Then add a screw etc for the centre pin.

A quick search turns up this which looks about right:

That's a faff, although just a small one.

I've had to do similar with my router recently. Fortunately there were some holes already in the base plate, so I just screwed it to the supports.

As my jigsaw is an ancient B&D, I'd have no qualms about drilling a couple of holes in the baseplate if needed.

Then add a screw etc for the centre pin.

Yes ever tried cutting holes for speakers in cabinets, I mean takes ages. Brian

Got it. And I have both hollow-ground sawblades and blades with a set, plus pendulum action, so I can adjust the kerf to suit.

Yup, though I may be using sheet metal -- depends on what's kicking around in the odds & ends corner. And there's two holes in the base, so It'll probably be just two screws to connect tot he base.

Aaaargh, and a big one, too. Though in my case, I can make a big divot, 'cause I'm notching the edge of most circles 22x44mm wide, and can put the divot in the waste:-)

(Why ever do people show a ten-minute video to show something that would be self-evident in a drawing that can be grasped in seconds?)

Thomas Prufer