advice needed on cutting an arc or circle

[alwyn]

I would like to make an arc of timber mm X 25 mm with an outside diameter of 300mm. I have thought of making a circle 300 mm diam and 25 mm thick then cutting the outside ring off - but what is the best way to do this. I do not have room for a band saw.
I made a circular table top for the same project by roughly cutting the shape with a power jig saw then sanding to perfection with a belt sander. But that only solves the outside. I have a coping saw but cannot make it cut the arc I would like on this thick timber. I have a router fixed to a table top but find it dangerous to use the router bit free hand so to speak.

can anyone help please?

[Wood Borer]

alwyn,

A router whether fixed to a router table or used freehand can be used to cut an internal curve or in your case a circle.

Make a template and use a flush cutter or make a circle cutter jig for your router.

There are plenty of threads using these techniques on this Bulletin Board.

[Robomanic]

A trammel might be what you need. This was referenced in another thread:

http://www.woodworkingtips.com/etips/etip030201wb.html

You could use it to make your template then trim up your workpiece, or cut the piece directly.

[joe greiner]

A router compass might be suitable: http://www.woodworkforums.ubeaut.com.au/showthread.php?t=50372

This is similar to Shannon's (Robomanic's) "trammel." For either one, use hot-melt or other adhesive to attach a small piece of wood for the centre point to engage, so as to prevent marking the workpiece.

Cheers,
Joe

[alwyn]

[quote=Wood Borer;903110]alwyn,

A router whether fixed to a router table or used freehand can be used to cut an internal curve or in your case a circle.

It was using the router freehand that caused me to ask the question in the first place. I am not quite sure what mistake I made - probably rotated backwards for a moment - but now I have a hole in my palm and the work piece is some distance off. I wont be wood working again until I can hold things firmly.

[Wood Borer]

alwyn,

What I meant by freehand was "not using a router table".

For over 20 years I didn't have a router table and I was able to use my router to perform most tasks by using templates and fences including cutting shapes far more complex than circles.

You have to firmly secure the workpiece and the guides with lots of clamps and then you can use your router freehand but it is being guided by the templates and fences.

PS I hope your injuries aren't too severe.

[joe greiner]

alwyn,

Your distress probably arose from taking a too-aggressive cut with free-hand routing. You should have better results by either (or BOTH) controlling the router position with a template or other guiding means, AND progressively increasing the depth of cut in small increments.

There's no hard and fast rule about traversing the pattern (clockwise or anti-clockwise), but keep in mind that most routers spin clockwise when viewed from above. As you progress across various grain alignments, the bit will tend to "climb" cut, which pushes and pulls the router away from the intended line.

I suggest you practise utterly free-hand on some scrap timber, with very little depth of cut, to become familiar with its response to a pencil line. Your own experience is your best teacher in this case.

Don't give up the ship, mate.

Cheers,
Joe

[soundman]

for a one off circle or arc a router is definitely the go....I've choped out my fair share of speaker holes this way and it is no problem

you should be able to manage this with nothing more than came with your router or a piece of scrap thin sheet material.

I have modified my hitchi router guide by drilling a hole and filing to a slot in the casting....I remove all the other parts of the guide.
some router guides come with some sort of provision for a pin to be used

Dont think you can hold a centre reference that isn't driven in.... you won't

I sink a screw or nail into the centre of the desired circle. then hook the giude onto the screw...adjust the circle dameter by sighting up on the line.and away you go...in 25mm you would want to take a couple of passes....probably 4 or 5 unless you have a big machine and a lot of confidence.
I will happily punch straight thru 18 mm ply or MDF... but I have a big machine and am specificaly set up for chopping circles....for most people 5 or 6 mm per pass is reasonable... I would advise a 1/2" bit.

If you dont have a suitable guide
simply screw a suitable piece of flat sheet goods... masonaite thin MDF.. whatever measure from the correct side of the router bit and drill a hole..stick a nail thru that.

realy this is a basic router task....I probaly could have had it done in the time it took to type this post.

make sure you secure both the keeper piece and the waste....I often work on a vacume box... but you wont have one of those.... fine brads or double sided tape will be the go onto a sacrificial backing.

it most certainly does matter which way you go arround a template or even traverse a straight edge.
you want the rotation of the bit to draw the machine against the guide fence or reference......

If you traverse the wrong way the router will be driven away from the reference and the result will be a disaster.....unless you are doing very light cuts you will not hold against the force

so if cutting to a centre pin you need to travel clockwise, because the router will be drawn outward.
if you are cutting to the inside of a template curve likewise clockwise.
cutting the outside of a template curve anticlockwise.

cutting large curves in thick material with a hand held jigsaw will always be disapointing....keeping to the line is hard and no matter what you do the blade will not stay purpendicular.

for doing this work nothing competes with a router.

Oh dont even think about this without a guide it will come out a dogs breakfast

cheers

[alwyn]

I did eventually build a router jig to do the inlay work but all the rest was done with a jig saw and a belt sander. the big problem I had was that I was using the off cuts from the table top to make curved brackets

[Wood Borer]

That's a nice piece of work.:2tsup:

Well worth the effort of making a jig?