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Hello to all -

First of all, I am relatively new to Qcad and I'm running version 3.27 on a Windows 10 platform.

I'm trying to use the Freehand drawing tool to trace over a line art image to convert it to a file that can be cut using a CNC plasma cutter.
Inevitably when I trace a curved line the curve is not as smooth as I'd like.

Is there a tool or technique I can use in Qcad to make smoother curves of an arbitrary shape?

Hello Iowa_Guy - welcome to the QCAD forum.

Iowa_Guy wrote: ↑

Fri Mar 10, 2023 11:12 pm

I'm trying to use the Freehand drawing tool to trace over a line art image to convert it to a file that can be cut using a CNC plasma cutter.
Inevitably when I trace a curved line the curve is not as smooth as I'd like.

Is there a tool or technique I can use in Qcad to make smoother curves of an arbitrary shape?

It's difficult to answer the question without seeing the art image. However - I assume you are looking for a Spline tool instead of the Freehand tool. There are two of them - one with Control Points and one with Fit Points. Most likly Fit Points is the right choice.

Additional there is also a Trace Tool in QCAD. The quality of the result depends on the source image. If you post the art image to your post then we can tell you more about your options ...

I am replying to this thread because this seems like the question that most closely describes my issue. I am struggling to draw a fair curve to define the perimeter of a tear drop, or symmetrical airfoil sort of shape and have attached my pathetic file. I've tried various methods using the spline tools and by using fit points. I've achieved something that looks more like a lumpy potato. See the attached DXF file.

I modified an existing drawing which was too short for the purpose. I was trying to offset the "left" end of the shape by about .125 in, and then fair the resulting offset back into the outside shape.

Is there some way to select the elements of my lumpy curve and tell q-cad to "take this lumpy looking line and fair it smooth"?

In the alternative, what would be the best way to draw this shape from scratch?

@ctdahlework,
Hi, and welcome to the QCAD forum.
Starting a new topic and referring to this post with a link would have been a better option.

Automatically, not really, QCAD does not know how to generate a correct 'airfoil'.

ctdahlework wrote: ↑

Thu Feb 20, 2025 12:32 am

I modified an existing drawing which was too short for the purpose.

With the original data I would try to get a nice fit with a fit-point spline.
Starting with some fit-points.
With the spline selected, dragging fit-points by their markers, adding/inserting additional fit-points if that is required.
Until the fit is acceptable.

To enlarge that, one could scale that up.
Scaling non-uniform to enlarge it more in one direction.
Not that this would be aerodynamically correct.

At the right there are 5 Arc shapes.
When selected and combined to a polyline ('Polyline from Selection' (OC)) these are almost tangentially connected.
Slightly better at the right extreme (179.9997881 and 180.00021583 degrees or only 0.00021190 and 0.00021583 degrees off)
Less good for the highest and lowest point (179.85837827 and 180.06026577 degrees)
Only based on these differences -> Not 100% symmetrical.
Still, almost smooth.

At the left there are 2 fit-point splines that form somewhat of a sharp corner.
Fit-point splines end normal, 'relaxed'.
With only 3 points each, chances are that this results in a corner or in not tangentially connected.
It would be better as one symmetrical open ended fit-point spline with 5 points.
With the central point a little to the right.

As one complete spline it will also not be that easy because of the small radius on the right side.
Typically the fit-points must be more dense at high curvature and less dense at low curvature.

Store copies of intermediate results, of fit-point splines.
For example ... Except when undoing some of the last changes:
- An offset will typically be a polyline, a way back is almost impossible.
- Splitting/trimming a fit-point spline will result in a control-point spline, there is no reverse method.

Trash things that are no longer required in the long run.

Regards,
CVH

Thank you for your detailed reply and I apologize for not getting back to this sooner. I wear lots of hats and my CAD hat is up on a hook for a few weeks.

I skimmed your reply this morning and realized that I need to read it much more carefully when I have some time to experiment. Hopefully mid week.

Looks like what I really need to do is take a couple of hours and just play with all the spline tools using the original drawing as a template instead of trying to modify it's lines.

I came up with a kludgey, but workable solution for my immediate problem. I still need to spend some quality time learning my way around the spline tools, but your advice above kick started my brain and I am grateful.

I used the "ellipse inscribed in a quadrilateral" tool to clean up the "leading edge" of the patch panel posted above I drew a rectangle with three sides tangent to the left-most apex of the curve and to the top and bottom of the curve. I drew a construction line through the two latter tangent points, measured the distance from that point to the leading edge, and then drew the final side of the rectangle at twice that distance from the leading edge.

The resulting ellipse was a perfect match for the footprint of the original part.