Thread: Drawing an Arc based on a known Arc radius

You must be drawing a lot of arcs.

I have about three router templates for various arcs, and that's usually enough.

However, you could get a batten (long straight edge ruler, I use some wooden blind slats) and attach one to your arc template. With a screw in the center and some wedges or scrap on the ends, it should move to an appropriate shape.

One of the magazines I looked at recently (maybe the forumites will remember which one had this) had an adjustable arc jig with the ends held in place and the center adjusted by a threaded knob.

Either of these will give you different size arcs which you would measure by looking at to see if it looks right. To get an exact 2.24 meter arc (or whatever) might take some measuring up, and depending on what you're doing may be overkill.

Tex

Domenic, are you asking us to do your homework?
Regardless, welcome, hang around


I don't fully understand your question.

By "arc template" do you mean a piece of ply or MDF 1500 x 1200 with a 1500 arc across one end?
If this is the case, you can't modify it to cut any radius other than 1500 without recutting the arc on the end.

OR do you mean you have a batten 1500 or so long with a pin one end and the router the other that will cut an arc of 1500?
In this case, making it infinitely variable is relatively straight forward.

For the range of arc sizes you mention, I suggest two templates connected by rigid posts of various length
• one template will be a false base for the router,
• the other the mount for the centre pin.
The neatest way to make the template infinitely adjustable is to connect them using a dovetail slide. However, a few band clamps and a piece of 2in galvanised iron water pipe will also do. It has to be iron because copper and PVC are too flexible.

I'll describe the water pipe version:
Both templates are 500 long and have three saddles 150 to 200mm apart on which the water pipe sits. The pipe is held in place by a band clamps fastened to a captured nut on the underside of each saddle.
To adjust for length, loosen each clamp, adjust to the required radius, tighten the clamps. For a larger radius, substitute a longer section of water pipe.


ian

Attach a router to a pipe clamp that will slide along a piece of pipe.
Have different pipes for the different lengths (i.e. under 1 m, 1 - 2 m, 2 - 3 m...) other end of the pipe is a pin.
Measure from the pin to the router cutter.
Turn on router, push it in an arc.

Is this what you mean?

If I understand the question, you don't want to be mucking around with 5m long extension arms. So, if you've already got a template with a 4 m radius groove in it, I can't see why your proposal of riding trammel runners in the groove with an extension arm wouldn't work. Not sure about your question of whether to ride the runners straight. My interpretation of the scheme is in the pic. You'd probably have to incorporate a plate of some kind to eliminate flex in the extension arm, but apart from that, it would give you an r+x radius curve, where r=radius of your template groove, and x = the distance between the groove and the router bit.

The only drawback here is that the greater the radius of the extension compared to your template, you'll get less distance around the circle with the bigger radius.

wow

thank you TEX B, IAN, CLINTON1 and ZENWOOD

This was my first post to this forum thank you for taking the time to help me out.

Zenwood what you described is spot on.. thank you for the diagram ( what program did you use ?)

My arcs are never more than a meter in distance.
"whether to ride the trammel runners straight in the groove " you answered that also in your diagram. (I thought that they had to be offset)

I better tell you why I asked the question in the first place.

I have to make wooden molds for making cast stone pavers for pool surrounds.

Some orders are very specfic.. and having to draw the full arc you need a lot of space that is flat... and i dont really have the luxury of space.

Thank you for confirming what i was thinking.

thanks again

regards
Domenic

The runners in the pic are conceptual: a real system made like that would flex all over the place, and give you a rough edge. A solid plate with trammel runners fixed to it would be easier to use. Also, the wider apart the runners, the easier it would be to use because there would be less twisting moment on the extension. An option would be to fix wheels onto a couple of extensions (which could be made adjustable with a couple of wingnuts), which ride on the outside of a template. You'd have less tendency with this to bind in the groove. The pic shows an implementation of these two ideas.

BTW: The drawing program is called Sketchup, available as a free download from Google.