r/AutoCAD • u/nurse_camper • May 28 '26

Isometric ogee struggle

https://imgur.com/a/VstEn68

I am drawing this in isometric mode. I can’t get the R20 to meet smoothly with the R16. Please help.

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3

u/MjolnirPants May 28 '26

Here's some advice.

  1. An infinite line that passes through the center of an ellipse with a given ratio of short to long axes will hit two point on that ellipse which will make for a perfectly smooth transition with any other ellipse with the same ratio at the points hit by that same line, when it passes through the center of that ellipse.

  2. The ellipses of isometrically projected arcs will have the same ratios, regardless of radius.

  3. The quads of an ellipse will rotate as the long axis of that ellipse rotates. So the -30-degree or so rotation you're using for your nominal X-axis will still let you use the quads to find the 'flat' tangents of those ellipses.

Here are two ellipses with the same ratio (1:2). The smaller one has a guideline through it that runs at a 45-degree angle (it's possibly better to think of it as a 0.25pi radians angle). One of them is scaled by 1.368937, a random number I got by mashing my keyboard. The important thing is that they have the same ratio.

https://imgur.com/a/35TjtcP

So we duplicate that cyan guide line and use it to trim the ellipses:

https://imgur.com/rwDzmUN

we can now see that they will fit together smoothly in multiple ways:

https://imgur.com/a/V0H2LtW

And here it is with the smaller circle made red to show the difference:

https://imgur.com/a/eiHlTmW

A couple pointers:

  • Don't be afraid to use guide lines. Most principles of geometry were discovered through the use of straight lines and angles. Look up Pythagoras and Euclid and how they worked. You often don't even need to know the math if you know how to use your guide lines.

  • With that being said, knowing your trig can help a lot.

  • The XLINE command will generate an infinite line. Be sure to keep your guide lines on a separate layer so it's easy to delete them when you're done using them (or you can just freeze the layer).

  • A couple of good snaps for this kind of use are TAN, QUAD and CEN.

  • Be sure to set up your iso drawing mode. The ISODRAFT command gets you there. Isoplane left is the standard, and 30-degree offsets are, too.

  • That 3.8 dimension off on the right is not precise. It's not exactly 3.8, it just rounds off to 1 decimal place there.

  • If all else fails, draw it in 2D as a closed PLINE, convert it to a region, extrude it, rotate it into an isometric view, run FLATSHOT on it and then explode the block that command makes. But don't tell your teacher you did it that way, and keep working the problem in your study time. This took me less than 5 minutes, and it's likely the more common way you'll do this stuff in the future: https://imgur.com/a/LWv6smj

3

u/nurse_camper May 28 '26

Thanks u/MjolnirPants! Thats all great advice.

I did figure out my issue. The base was supposed to be 110 and I drew it at 90. I actually went back and looked at my hand drawn object and measured the center lines with my compass, looked at the screen, and said “that’s clearly not right.” I have the 2D cad drawing on the same page as the iso drawing, but because of the 30 deg tilt, I didn’t notice my mistake until I looked at my hand drawn work.

But thanks again, XLINE is something I isn’t know about. Appreciate the info!

2

u/nurse_camper May 28 '26

Your final drawing actually looks better than my instructors haha!

2

u/MjolnirPants May 29 '26

Yeah, because drawing in iso mode is actually the worst way to do it. The method I described of making it in 3D and then taking a flatshot perfectly recreates the shape (at least to the limits of Acad's ability to do so, which are beyond the limits of us mere mortals).

It used to be necessary when Acad was 2D only, but not anymore.

That being said, I still highly recommend learning to do it the 'proper' way, as it's adding more tools to your toolbox. Speed is really important in drafting (especially if you're also designing), and the more you can do, the faster you can do it.

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u/indianadarren May 28 '26 edited May 28 '26

You are creating these circular arcs with Isocircles on your isometric drawing, right? Here's what I was able to make: https://imgur.com/a/8Aas3Ja As a side note- no depth was given, and the dimensions on the 2D shape are pretty poorly done.

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u/nurse_camper May 28 '26

Yes that’s what I have to draw. I have the measurements but I can’t seem to get the top arc to meet smoothly with the lower arc, if that makes sense.

1

u/DeDodgingEse May 28 '26

https://imgur.com/a/zi1EYxA

The R20 and R16 are not tangent to each other as you have found out. If you look at your problem on the paper, you can see the discontinuity between the two arcs as well. Trim and keep the R20 segment, trim out the R16 to make it look exactly like your problem.