cutting perfect circles

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firstascent2002

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Hi guys, I am still making my self settering gear..The latest challenge is to cut perfect of ply wood to make the vane mount out of. Are there any tricks out there to this. The circles need to be 2 x 30 cm diameter, 1 x 28 cm diameter and one 31 cm diameter. Once cut they all need a further circle cut from exactly their centre of about 8cm. Is there a magic tool I can buy to do this or is it just bitter trial and error (more error that I would like thus far) with a jig saw?

Thanks
Jamie
 
Assuming you don't have any of the tools suggested. But assuming you do have a jig saw. Firstly decide where your centre will be on the piece of ply. Partly screw in a screw and then measure out from your centre the required max dia. put a loop in a piece of cord and place it over the screw, adjust the other end with a pencil attached to the required diameter. Draw a circle with the pencil going around the centred screw, keeping the cord tight. There you have a circle.

You can now either cut the circle with the jig saw, finishing off to the exact size or you could always rig up a length or cord on the jig saw as you did with the pencil, using the cord to control the arch of the circle.

the smaller centre hole can be ct when you have finished, using a hole saw.
 
Why bother asking,you seem to have it sussed, i used the same method to paint a heli landing circle on a ship i was working on,the simple ways are the best.
 
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Obviously this will cut circles as well as ellipses - but you get the idea.
 
Obviously you mark and cut the outer circle first. if you make the mistake of cutting out the inner one, you have lost your centre.

I'd use a strip of metal rather than a cord. Imagine a length of meccanno, or perhaps a strip of wood with two holes in it. Cord will stretch a bit, also the pencil point wobbles about, so you find the start and finish don't quite meet.
 
roto tool, or whatever its called. One of those odd tools that you have little use for, but when you need it, its brilliant.
For general use, a router might be more useful overall.
 
If I was doing it, I would drill a 1/16th dia hole where I needed the centre. I am fortunate in having a large pair of dividers .I would then place one point in the 1/16th hole and draw a circle, gradual for a start then gradually deepen,this stops the ply from delaminating later,I would then turn the ply over and do the other side the same. Then use a jigsaw nearly up to line then finish with a 1/2 round file.As you have marked both sides it wont be a ragged edge.
 
See Rutlands or better still the router template at Router Compass

I have the Trend version of the Axminster one and used it to cut several dozen fire resistant MDF circles and shapes that adorn the wall of a local childrens ward.

Best tool for this sort of work is a router - if anyone gets stuck just drop me a PM with the sizes & materials needed and I'll sort you out.
 
You don't say what tools and equipment you have available, but assuming you have a router (the sort which has 2 6mm bars which run through the base to hold the fence) it is fairly easy to make a little trammel centre from a small block of hardwood, 6mm hole + self-tapper to hold it onto one of the bars, with a panel pin sticking down to locate in the centre of your ply disc. Set it at 150mm from the cutter and you've got a 300mm dia circle. Or of you're rich buy one of these.
If you have a disc sander it's very easy to cut a disc out slightly oversize, panel-pin it thro' the centre to a base board which you clamp to the table - just rotate the disc on the pin as you sand the edge it you have a perfect circle - we do this all the time at work as it's so much quicker than using the wood-turning lathe.
 
[ QUOTE ]
Obviously you mark and cut the outer circle first. if you make the mistake of cutting out the inner one, you have lost your centre.

I'd use a strip of metal rather than a cord. Imagine a length of meccanno, or perhaps a strip of wood with two holes in it. Cord will stretch a bit, also the pencil point wobbles about, so you find the start and finish don't quite meet.

[/ QUOTE ]

You could use a length of Paton band, which plumbers use for supporting length of pipes under floor joists, its a metal strapping with holes all along its length, any plumber should be able to gibe you a couple foot of it.
Incidently I cant see why you cant mark your outer circle and cut it with a jig saw, then dress it up like I did when I made a porthole window in my top washboard, and used a 7" hole saw for the inner hole.
 
Its two things actually: a router mounted on an ellipse cutter. They are both available from any good woodworking power tool shop.

I have a fairly cheap Bosch router which SWMBO gave me for Christmas a few years ago. If I had a choice now, I would have a Makita or Dewalt 1/2 chuck (with 1/4 collet adapter) professional machine, but what I have works ok ish. If I had the room I'd have a proper router table as well. I make do with an old kitchen unit shelf mounted on a Workmate. ith very little effort and the right bits you can make handles, mouldings etc etc.

I even made gratings with the router once, but it struggled with routing inch wide slots in the stack of 1"by 1" teak I had clamped together. It took several cuts and a lot of patience.
 
Have a look at the Triton router. Excellent product as are their other offerings. Loads of safety features and very powerful, but not the lightest. Great company to deal with too.
 
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