Techdraw: Dimensioning an Ellipse?

Discussions about the development of the TechDraw workbench
chrisb
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Re: Techdraw: Dimensioning an Ellipse?

Post by chrisb »

Moved to TechDraw forum.
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domad
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Re: Techdraw: Dimensioning an Ellipse?

Post by domad »

AzAv8r wrote: Thu Jun 09, 2022 11:30 pm .......
Hi AzAv8r, greetings to the Community!
This is how your elliptical profile cam could be sized, for the proifile I used the polar quorature (usable for any curvature), obviously it must be enriched with dimensional tolerances, shape and surface finish.
I am attaching your revised file in the 3D modeling flow and with the dimensioned technical drawing inserted.
Attachments
TCamv4a-2.FCStd
(192.52 KiB) Downloaded 13 times
example polar dimension.png
example polar dimension.png (291.37 KiB) Viewed 520 times
Workshop_Notes
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Re: Techdraw: Dimensioning an Ellipse?

Post by Workshop_Notes »

domad wrote: Sun Jun 12, 2022 1:57 pm This is how your elliptical profile cam could be sized
That is a very nice drawing. However, due to the way an ellipse is represented in FC, the dimensions are not correct.

I thought you had done something very clever, and in some ways you have, but it loses its cool factor when you just dimension the FC representation of an ellipse (a B-spline) rather than produce dimensions from the mathematically correct ellipse.

You can use the parametric form of an ellipse to work out coordinates of the ellipse at each point it intersects the radial lines. Then work out the length of those lines and feed them into the dimension.

See attached pdf for the calculated dimensions and the ones pulled from the drawing. It is interesting to see that the FC approximate ellipse is always smaller than the true ellipse. Apart from the axes intersections, which are defining dimensions, all the calculated ones are bigger, by up to 2.5% in this case.

At the very least, reporting everything on the drawing to multiple decimal places is highly misleading as it implies a level of accuracy that is not there.
Attachments
ellipse generator.py
(1.34 KiB) Downloaded 9 times
ellipse ss.pdf
(11.14 KiB) Downloaded 16 times
domad
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Re: Techdraw: Dimensioning an Ellipse?

Post by domad »

Workshop_Notes wrote: Tue Jun 14, 2022 5:48 am ........
Hello Workshop_Notes, greetings to the Community!
Thank you for your intervention, I assure you that I know perfectly the "conics" and its equations (for those who do not know it is analytical geometry), however my intervention was not aimed at verifying the accuracy of the elliptical calculation used by the algorithm of FreeCad, but rather on how (mormatively) it was possible to represent it in a technical drawing elaborate, better defined as "table setting" or "workshop elaboration".
So I suggested that this representation can be implemented through "polar dimensioning", where the letter "M" represents the "Modulo" (length of the rays defined below), while the Greek letter "φ" (phi) represents an angle, specifically defined "Anomaly", ie that angle formed by the reference axis with the ray conducted from the origin to the point.
As you certainly know, the "Sketcher" workbench allows you to draw ellipses, in two different ways, and consequently to constrain them, therefore to draw half-lines, having ends coinciding with the point of intersection of the axes (origin) and the points lying on the ellipse, allows you to set an alley of "Distance" which will force FreeCad to report us a "redundant constraint error, however, define it as" Reference ".
These "References" have been recalled, using the appropriate function, in the spreadsheet to be displayed in the technical drawing in table format.
So according to your calculation, "Sketcher" would draw ellipses of dubious precision?
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Re: Techdraw: Dimensioning an Ellipse?

Post by Workshop_Notes »

domad wrote: Tue Jun 14, 2022 12:18 pm So according to your calculation, "Sketcher" would draw ellipses of dubious precision?
In a very early post in this thread, papyblaise tells us that ellipses are represented inside FC as B-splines.

That is how it is. Right or wrong, we cannot do a lot about it.

But the thread is not about the internals of FC. The OP wants a technical drawing that contains dimensions to allow another person to reproduce the item she has created. The technical drawing is a means of communication from one human being to another. The human being who created it knows she has created an ellipse. The human being who reads it wants to see dimensions that allows him to make it (or another one).

The danger is that someone will follow your method and believe in his heart that he is dimensioning an ellipse (i.e. that your post answers the original question) when he is not doing so. Having no dimensions at all (and thus being forced to produce them in a different manner) is far better than dimensions that are incorrect. If we know that the object in FC is not an ellipse, we cannot naively use dimensions from that object.

If we want to achieve the task of dimensioning an ellipse, answer the OP's question and advance the FC project, another method is needed. In effect, for situations where FC does not exactly represent the object, we have to override its dimensioning tools, use the correct mathematical representation of the object and force it to lie.

I do not know FC internals, but if you draw up a list of 'objects' and 'mathematically correct representation', the ones where 'no' is written down will be few in number. A method for solving the current problem will stand a good chance of working with other objects in the same category.
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Re: Techdraw: Dimensioning an Ellipse?

Post by domad »

Workshop_Notes wrote: Wed Jun 15, 2022 8:10 am
In a very early post in this thread, papyblaise tells us that ellipses are represented inside FC as B-splines.

That is how it is. Right or wrong, we cannot do a lot about it.

But the thread is not about the internals of FC. The OP wants a technical drawing that contains dimensions to allow another person to reproduce the item she has created. The technical drawing is a means of communication from one human being to another. The human being who created it knows she has created an ellipse. The human being who reads it wants to see dimensions that allows him to make it (or another one).

The danger is that someone will follow your method and believe in his heart that he is dimensioning an ellipse (i.e. that your post answers the original question) when he is not doing so. Having no dimensions at all (and thus being forced to produce them in a different manner) is far better than dimensions that are incorrect. If we know that the object in FC is not an ellipse, we cannot naively use dimensions from that object.

If we want to achieve the task of dimensioning an ellipse, answer the OP's question and advance the FC project, another method is needed. In effect, for situations where FC does not exactly represent the object, we have to override its dimensioning tools, use the correct mathematical representation of the object and force it to lie.

I do not know FC internals, but if you draw up a list of 'objects' and 'mathematically correct representation', the ones where 'no' is written down will be few in number. A method for solving the current problem will stand a good chance of working with other objects in the same category.
Hello Workshop_Notes, greetings to the Community!
Unfortunately I have read but I have not been able to answer before as busy.
Given that I do not want to open controversy in any way, but to put the modalities of dialogue back on the "right path".
I think you should be more humble and less presumptuous, above all respectful towards others, even if they express, in your opinion, things that you consider wrong.
I asked you a question (with the hope that it would put you in "alarm") but unfortunately you answered by putting in a bad light, not me, but those who work on the calculation algorithms of FreeCad and therefore "undermining" the reliability and validity of FreeCad .
The following is the ellipse equation that FreeCad algorithm refers to to generate precise ellipses:
[X ^ 2 / a ^ 2 + Y ^ 2 / b ^ 2 = 1]
where "a" and "b" are the semi-axes of the ellipse.
My technical drawing shows that the ellipse, centered at the origin, has:
a = 13.096875;
b = 10.4775.
also detectable by Sketch007- "K" PROFILE DETAIL.
Now knowing the angle (called "anomaly") of the first point on the ellipse, also detectable from the drawing,
φ = 21.1330365284794 °
and knowing the size of the module (marked by the initials M) which can also be seen from the drawing,
M = 10.73159918443412
through a trivial trigonometric calculation we obtain the values ​​of "X" and "Y".
So it is enough to insert these values ​​in the equation of the ellipse to determine its equality, which is always verified.
To check the other values ​​of "M" just add the 5 ° progression to the first "anomaly" (φ = 21.1330365284794 °), you will see that all the modules listed in my drawing are correct.
Conversely, if you insert the values ​​of your modules that you have published in your "ellipse ss.pdf" file you will notice that in no case they verify the equality (... = 1) of the ellipse equation defined above.
I am attaching a spreadsheet file with which the first four points of your data have been verified in comparison with those derived from my technical drawing.
Attachments
ellipse data control.ods
(15.97 KiB) Downloaded 7 times
ellipse data control.png
ellipse data control.png (100.29 KiB) Viewed 361 times
chrisb
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Re: Techdraw: Dimensioning an Ellipse?

Post by chrisb »

I don't really understand where the idea comes from that ellipses in FreeCAD are always B-splines and that they are not accurate. An example may illustrate this. I use numbers which allow using integral number for the radii and the point coordinates.

It is known that for any point on an ellipse the sum of distances to the focus points is constant. In case you doubt it: it can also be derived from the formula given by domad.

Let's take this ellipse. It is drawn in Sketcher as ellipse, not B-spline:
SnipScreenshot-fafd82.png
SnipScreenshot-fafd82.png (13.09 KiB) Viewed 310 times
and place an arbitrary point on it, then the sum of distances to the focal points is always 26:
SnipScreenshot-d82bc2.png
SnipScreenshot-d82bc2.png (25.65 KiB) Viewed 310 times
SnipScreenshot-a10f31.png
SnipScreenshot-a10f31.png (27.2 KiB) Viewed 310 times
and this is how it looks in TechDraw:
SnipScreenshot-d8f225.png
SnipScreenshot-d8f225.png (27.68 KiB) Viewed 310 times
SnipScreenshot-f86daf.png
SnipScreenshot-f86daf.png (29.29 KiB) Viewed 310 times
Looks pretty accurate to me.
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domad
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Re: Techdraw: Dimensioning an Ellipse?

Post by domad »

chrisb wrote: Fri Jun 17, 2022 10:59 pm .......
Hi chrisb, hello to the Community!
Many thanks (!) For your testimony and accurate demonstration, it is the demonstration that FreeCad is a very valid alternative, to the commercial, of the highest quality and reliability! :D :D
AzAv8r
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Re: Techdraw: Dimensioning an Ellipse?

Post by AzAv8r »

Shalmeneser wrote: Fri Jun 10, 2022 10:34 am (Center Marks don't work ?!)
I have studied this drawing and all the properties/controls related to both the view and the sketch, and cannot determine how to get the ellipse sketch shape to display in the TechDraw view. I do recall seeing something about displaying construction geometry from sketches when I first started trying TechDraw, but when I search now I'm not finding anything useful.

The ellipse does display in the geometry model views, just not in the TechDraw views. I used the projection group you showed me to create the views, as you did in your example.

Any pointers?

BTW, I did upgrade to 0.020.
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Shalmeneser
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Re: Techdraw: Dimensioning an Ellipse?

Post by Shalmeneser »

1) Select the step of the object in the construction tree AND the Sketch001 (+Ctrl).
2) Add your view
3) Modify the ellipse line : select then TechDraw_DecorateLine : style=dot
3) Modify the ellipse line : select then TechDraw_Quadrants
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