[solved] Drawing multi radius curves

[Paul Scott]

I am inexperienced with QCAD and have got stuck with making the attached drawing composed of 3 curves of 2 radii.
Could anyone show me how to get them even and connecting smoothly.
I am using QCAD version 3.25 on Windows 11.
Regards, Paul.

[petevick]

I would think using Ellipse with radii or Ellipse with diameters would work for you - find them in the Draw>Ellipse menu

[Husky]

Hi Paul Scott - welcome to the QCAD community.

Could anyone show me how to get them even and connecting smoothly.

There are different ways to solve this puzzle - what do you think about this approach ...

Shape 3 radii.gif (2.36 MiB) Viewed 2861 times

... or this one?

Shape 3 radii 2.gif (2.1 MiB) Viewed 2857 times

[CVH]

I would think using Ellipse with radii or Ellipse with diameters would work for you

Sorry Pete but the request was "3 curves of 2 radii".
An ellipse curve continuously changes in curvature.

Regards,
CVH

[CVH]

how to get them even and connecting smoothly.

Paul, "connecting smoothly" with arcs is called tangentially connected.
As you can see in Husky's GIF's that can be accomplished by trimmed roundings or with the tangent arc/circle methods and trimming.
There are more and better suited tangent circle methods but these require more and correct trimming.

What I miss in your sketch is the overall height but I think that you can manage to add a horizontal line at the proper place.

Regards,
CVH

[petevick]

I would think using Ellipse with radii or Ellipse with diameters would work for you

Sorry Pete but the request was "3 curves of 2 radii".
An ellipse curve continuously changes in curvature.

Regards,
CVH

An ellipse can be constructed by using the concentric circle method, which is what I'm assuming the two commands I pointed use. The request was indeed asking for two radii, which is effectively what the concentric circle method uses.

[CVH]

An ellipse can be constructed by using the concentric circle method.

Not to my knowledge.
A concentric circle (CC/CG) is again a circle with the same center but with a different radius.
I can not create an ellipse using CC or CG.
There is nothing like concentric ellipses ... CC or CG based on an ellipse is a shape approximated by a spline.

The request was indeed asking for two radii, which is effectively what the concentric circle method uses.

Any circle has but one unique radius, there is nothing as a concentric circle with two radii.

Any of the ellipse methods creates an ellipse or ellipse arc.
The term radii is short to use as tool-tip but ambiguous, they are correctly called 'semi-major axis' and 'semi-minor axis'.
If the major axis is equal to the minor axis we get a special case what is circle or an arc.
The chances that an ellipse arc explode to 3 tangentially connected arcs with the required radii is second to none.


Regards,
CVH

[DerekGove]

This may be going over what others have said, but here's the basic construction for what you're needing. It's not a pretty QCAD diagram, but it was simple enough to do with pencil and paper to help show what's going on with the fundamental geometry.

P1120307-cropped.jpg (79.24 KiB) Viewed 2647 times

First off, draw your verticals 22mm apart, and decide where the centre for your top arc will be.

Next you can draw an arc with a radius of 18mm touching for the top curve.

Because the radius of the other smaller arcs are 4mm, and these have to touch both your top arc AND the sides, the centre of these circles must fulfil both these conditions:
1) be on a line parallel to your vertical lines. So draw a vertical line 4mm inside your main shape.
2) be 4mm away from your R18 curve, so 18mm - 4mm is 14mm. Draw an arc with the same centre as your R18 curve with an R18.

Where these intersect is the centres of your R4mm circles, and there will be no sharp change in curve between any of your lines. In fact, the point the curves join can be determined by drawing a line from the centre of your R18 and the centre of your R4 circles. Where this crosses the circles is the point of tangent.

If this hasn't been helpful, I hope it has been interesting.

Regards,
Derek

[CVH]

This may be going over what others have said, but here's the basic construction for what you're needing.

Thank you Derek for the exact ruler-and-compass construction method.
We can replicate your sketch using construction lines in QCAD and deleting them afterwards

BTW, this is not how QCAD solves these puzzles using the tangent methods nor using the round method.

Meanwhile Husky has entered a feature request for a 2 tangent and radius arc tool ... With trimming if possible.

Regards,
CVH

[Paul Scott]

Thank you to everyone who replied to my enquiry. I have tried the various methods suggested and successfully progressed by drawing.
Your ideas have been very instructive.
Regards,
Paul.

[DerekGove]

An ellipse can be constructed by using the concentric circle method, which is what I'm assuming the two commands I pointed use. The request was indeed asking for two radii, which is effectively what the concentric circle method uses.

Just a quick follow up to something I picked up on early on in the post...
While Paul wasn't wanting an ellipse, I can understand how Pete thought he did, and there is a construction for an ellipse called the concentric circle method, which uses concentric circles which have diameters equal to the major and minor axes of the desired ellipse. It's also known as the auxiliary circle method, and is one of the quickest and simplest ways to draw an ellipse.

P1120308.JPG (69.02 KiB) Viewed 2516 times

Cheers
Derek