Thread: Best way to cut accurate circles

If you have a router you could make a circle jig like this blokes: How To Make A Circle Cutting Jig For The Router

When I need accurate circles I've had good success with rough-cutting the blanks to within a few mm and then using a pivot on a large disk linisher to sand it down to final size.

Dunno about the .5mm accuracy though.

Best way = CNC
If you don't have one, make yourself an adjustable jig for a router/trimmer.
If you don't have a router, buy one, then make the jig

I need to do this occasionally for some record player plinths that I make. I just rough cut a circle on the bandsaw and use my lathe to turn a perfect circle, I also get to sand the edges easy mode this way too. I press my rough cut blanks against a faceplate and then bring the tailstock up to apply pressure. It leaves a mark in the underside but that isn't seen so I don't care and it eventually gets drilled away to accept the record player pickup arm. But if I wanted both faces to be good, I would do the same thing but use a block of timber in front of the tailstock so I don't bruise my workpiece.

Kuffy, I need an accurate hole in the timber, not an accurate disc. Or have I mis-interpreted your post

oh you want holes...I was talking about discs. But I make the hole, in which my discs fit into, via a router trammel. Cut most of the way through with the trammel and then flush trim the rest afterwards. Can't cut all the way through because the center point of the trammel is part of the offcut which becomes loose as you finish the cut.

No way! What timing, I was about to do a review of this tool:

...the Carbitool Hole Cutter Adjustable Circle Hole Cutter | Benelongs - Woodworking Accessories

I bought mine from Benelong's. I email Dan (he is an accommodating guy) and buy most of my router bits from there. He posts on the same day and allows pickups from his showroom too.

For $20 and $3 postage, this is an absolutely first class bit of gear. I was honestly expecting a two-use-wonder and an absolutely piece of junk. Boy was I surprised. This kit comes with both carbide and HSS drill bits, plus the same for replicable blades. it comes with a good quality Allan key and it fits ALL the sockets (not one of those #%^* tools that use three different keys...)

i make speakers for commission occasionally. I've done it 6 times this year alone (and another for a forum member only this week). The speaker openings need to be exact.

so I use the Incra ruler to very accurately measure the upper/lower/left/right and centre, use a nail to punch a divot on the exact cross, then drill a 5mm hole. The Carbitool hole cutter in the drill press, running at the slowest RPM makes a very clean, very accurate and predictable circle.

the setup of the edges is dead easy. This is why I use the Incra to scribe where the outside should be. The very tip of the blade hits this line. The "reverse" blade is set a smidgin inside that. Simply cut about half way, flip over and finish it off.

100% recommend this tool.

hope this help. Say hi to Dan if you buy off him.

Evan

One of these in a drill press https://www.bunnings.com.au/sutton-t...lesaw_p6350213
Is 121mm near enough, or does the 0.5 matter?

I used to make a lot of nest boxes from 18mm Hoop Pine, quite a number of different size holes required. I used the same Carbitool cutter that woodpixel showed. Great tool.

A bit more. Here are the holes (87mm and 37mm) and again with roundovers added. These are not sanded... this is how they come off the tool.

I see from the description that the tool goes to 120mm. You need 120.5mm... The end-cap nuts come off and you can get another 5mm on each side out of it.

I also see absolutely no problem setting the tool to 120.5mm (or any x.5mm) for if you accurately lay out the lines (using an Incra rule) you can exactly set the very tip of the cutter right on that line.

This is exactly what I did with the speaker holes and while I don't have a photo with calipers, I can attest they were exactly correct.

Ev

DSC03154 (1024x685).jpg DSC03168 (1024x697).jpg

Thanks for this information, woodPixel, looks promising. A couple of questions:
Q1. Could you not draw a circle of the correct diameter with a compass and set the tip of the cutter to that?
Q2. Looking at the brochure on your link, what is the point of the left hand cutter? The bevel is facing the wrong way, so do you really need that cutter? With this setup as shown, only the right hand cutter needs to be set to the correct diameter of the hole.
Q3. That tool looks pretty dangerous, whirling around on the end of a drill press, being difficult to judge where the end of the bar is. Looks like an accident waiting to happen. Do you paint the extreme ends with a fluorescent paint to make a danger zone that is quite visible when the tool is rotation at high speed?

Q1 - if I had a compass, then absolutely thats the way I'd do it, but alas, I'm just a poor boy (I dont even have work shoes and my socks have holes and its cooollddd).... But, the Incra rule is so easy it isnt funny. Its on me when Im in the shop, plus a micropencil. Just stick the end in the hole and a magic line appears that is exact. In this case, since I needed to pinpoint the centre (Audio people are absolute nutters for exactitude) then it was 50% done anyway....

Q2 - The cutter can be flipped, but I assumed it was to cut the "inside" of the circle. Thats how Im using it. The blades are shaped like a #10A scalpel or kiridashi. I set the outside one right on the line and in "inside" on about 2mm to the inside of that mark. This scores and cleans the track out (this is only my observation).

I was thinking about it when I bought it and it seems that the nature of a curved line will require a groove wider than the one cutter makes, as it is straight and its a curving cut. On smaller circles the problem is exacerbated. By having two cutters offset slightly it makes a wide grove.

This is only my logic, but it does make a clean cut.

Q3 - Speed. On my TotalTools ElCheapo it is set to the very lowest speed. I believe this is 180rpm. I simply selected the slowest it had.

Why? Cos I watched this imbecile doing a "review": https://youtu.be/kBzu0GOjHTo. Its hypnotic. I was seriously hoping to see a monster catch, his sleeve being caught or it taking a finger off. It was, I admit, a bit disappointing as I was hoping at least to see a fingernail removed.

Looking at the Carbitool, its sharp. Really sharp. Why does it need to be going like a jet engine? A chisel doesn't, nor does a profile plane. The first hole I drill being a nicely pre-punched 5mm, then the centre point of the cutter does a great job of aligning everything. No wobbles, no frightening noises or motion-blurred physics busting rotation.

Its smooth and balanced and I don't feel threatened by it at all. Its predictable and I haven't had a catch yet.

If I were to lay one criticism in my overly rosy one-sided review is that the centre drill bit is quite long. One has to pile up a fair scrap underneath to give it room. I do clamp this down, but the work is simply lightly restrained against the torque being applied to it. i.e. the work isnt clamped. The downward pressure doesn't need to be Conan for it to be effective. The 5mm predrilled offers good guidance for the main bit and it doesn't want to go wandering off.

One thought I did have when first using it, is that I do set my table at 90° to the press. I use a digital tilt gauge for this, but if you just spread the arms wide and drill down until the cutters jjuuuuusssttt touch a scrap, this would let you register the tabletop flush and flat according to the chuck (after a few love taps of the table), which seems a better way of doing things.....

A router with a bearing guided rabbeting bit and a couple of 5mm pieces of melamine (Baltic Birch is also acceptable) about 305mm square is all that is needed to make a circle template.

The concept is using the bearing to guide the rabbeting router bit to make larger and larger holes in a controlled series of steps.

The process is to use double side tape to hold the two pieces of melamine together. Drill a hole in the center of the two pieces of melamine. Then with the router bit set to the depth of a smidgen more than the thickness of one piece of melamine, cut a rabbet in one of the pieces of melamine. Flip the two pieces of melamine over and repeat the cutting of the rabbet. This will give you a template.

The secret to getting size hole that you want is the size of the initial size of the drilled hole. The math is simple. Start with 120.5 and subtract twice the size of the rabbet. Keep subtracting until you get to the reasonable size of a drill bit for the initial hole. Note that you'll want a drill bit larger than 25mm due to the diameter of the rabbeting bit.

To use the template, first cut an approximate hole using a jig saw. Then use a bearing guided flush trim router bit riding on the template to make the hole.

Thanks for this detailed reply, woodPixel, I will give it a go. I just use a cheap compass from my kids drawing set that they had for school years and years ago. You can get them at Officeworks, cheap as. ( I still have a variable protractor I used for Engineering Drawing back in the 1960's - used it recently too )

But I will be painting the extremities of the bar, so as to see the danger zone when it is rotating