Duplicating arcs

Hello all,

New to the site and shapeoko, I love it so far!

I tried searching for a solution to what i’m running into but haven’t quite found a similar scenario in any threads yet. If I did miss it my apologies! Any help is much appreciated…

I am trying to carve out a snug fitting piece of acrylic for this device to rest into and am having trouble trying to accurately duplicate the slight curve on the two sides of this rectange… I’m a little scrambled trying to measure the curve and putting into cad with confidence that it’s not just good enough but rather a duplicate of the curve. Any thoughts on to how I can accurately measure and transfer into cad? Thanks in advance for any advice.

Why not just use the two widest measurements and make rectangle. If you ascetics dictate the slightly rounded sides then trace the image, scan it and save it as an SVG. You need to create and/or save the SVG in 96 DPI otherwise it may change size on you when imported. There are several tricks to get an oversized and/or undersized SVG by drawing a rectangle the correct size and make the SVG larger/shrink until it fits the rectangle. If you image program cannot output an svg then download the free application Inkscape. Save in Inkscape as a “Plain SVG”.

How to create SVG in Inscape

Open your image in Inkscape – select it
Click Path – Trace Bitmap – this will open a Trace Bitmap window
Click ‘Update’ to be sure image looks OK – then click OK and close window
You will now have 2 images - (I only get 1) the top one is the traced image, move that one aside, select the bottom one and delete
Be sure your new image is selected – click File – Save As - Choose type as Plain SVG – hit Save

You will now have an SVG file that “should” open just fine in Carbide Create

If i am hearing you correctly, i should trace the curve onto paper and then scan it for an svg file? thanks for your input

1 Like

You could place the photo, scale it so that the ruler is to scale, then re-draw, see:



Three easy options:

If the curve is an arc of a circle (likely, as this is the most common form, since it is easy), use the sagitta formula to get the radius (see wikipedia). Then you can form the arc exactly

It is fairly likely that if not a circular arc, it is that it is a spline arc, as formed by bending a spline (stick of uniform section) using three pins. You can match this and use the spline tool from any drawing package

As suggested, take a picture (or scan on a flatbed scanner- much more accurate, but you need to be able to put it on a scanner) and trace or match using splines in your favourite package.

If your goal is just to have a snug fit, other options are match three points of contact and insure the rest is not in contact, intentionally machine with maybe 1mm clear and fill the gap with castable resin (you can prevent it from bonding to the base with clear packing tape), or any number of other options.

For the corners, a radius gauge is very handy, but you can probably match it with a drafting circle template closely enough.

As a general guide, you can NEVER have enough measuring tools (physical tools, as well as conceptual models)


this is great. thank you

thanks for your help!

It isn’t really difficult to mark the three points (two ends and a middle) and trace a curve between them. Sometimes, over-engineering is rampant! :smiley:

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.