topic Re: Arc tangent to line and arc in Civil 3D Forum

Arc tangent to line and arc

AllenJessup — Tue, 28 Oct 2014 18:02:44 GMT

I thought this would be simple but I can't get it to work.

I'm trying to draw an arc that's tangent to a line at a specific point and tangent to an arc. I drew the line, started the arc command, hit enter to force the arc tangent to the line at the endpont and then tried to use a Tangent OSnap to hit tangent to the arc. This is what I got. Note the Tangent snap at the midpoint of the arc.

If I accept the Tangent snap it does draw the new arc to the midpoint of the existing arc. Definitely not tangent. Does this mean there is no solution? I think I'm missing something. I'm usually aiming for a set radius and have no trouble. This is kind of a Best Fit situation. The only criteria are that the new arc be tangent to the red line at the intersection with the yellow line and tangent to the red arc.

Allen

Re: Arc tangent to line and arc

_Hathaway — Tue, 28 Oct 2014 19:33:45 GMT

I was able to replicate this behavior as well. If you know where your going to tie into the red curve at you may need trim the arc at that point, establish tangent bearings and recreate the curve manually through dual circles at the tangent points and then a circle from where the two circles intersect (the new curve radius point) and then trim accordingly.

Or just fillet until you get something close to what you want. I'm not sure how close you need to get.

Re: Arc tangent to line and arc

AllenJessup — Tue, 28 Oct 2014 19:41:55 GMT

That's sort of the problem. The only thing controlling where I'll hit the red arc is wherever a tangent curve will hit it. I've got a "close enough for government work" solution. But with $6 grand of computer and software I thought I might be able to do a little better.

Thanks.

Allen

Re: Arc tangent to line and arc

_Hathaway — Tue, 28 Oct 2014 20:01:17 GMT

I have tried to no avail to get this to work using arc, curve and even alignment 'curve' functions. In this instance I would recreate it the old fashioned way with trimming, cutting, etc....until you get the curve data you want.

I never use the 'tan' snap so the process you've described is new to me.

Re: Arc tangent to line and arc

AllenJessup — Tue, 28 Oct 2014 20:07:01 GMT

By using Circle by TTR (Tangent, Tangent, Radius) and taking a couple of guesses on the radius. I've gotten it to withing 0.000091m of where I want to hit. No problem with construction. Just an annoyance!

Allen

Re: Arc tangent to line and arc

Pointdump — Tue, 28 Oct 2014 20:21:13 GMT

Allen,

Yup, definitely time for an Iterative Government Solution.

Dave

Re: Arc tangent to line and arc

Neilw_05 — Tue, 28 Oct 2014 21:34:08 GMT

Assuming you are using plain cad elements you should be able to solve it by applying parametric constraints. Will that work for you?

Re: Arc tangent to line and arc

Neilw_05 — Tue, 28 Oct 2014 21:44:28 GMT

BTW it might be simpler to create some construction elements to establish the tangent points.

Re: Arc tangent to line and arc

Joe-Bouza — Tue, 28 Oct 2014 21:47:08 GMT

What happens if you temporarily extend the larger arc beyond the extension of the tangent? does the osnap give better results?

Re: Arc tangent to line and arc

Neilw_05 — Tue, 28 Oct 2014 22:07:42 GMT

http://screencast.com/t/McpGijus

Re: Arc tangent to line and arc

AllenJessup — Wed, 29 Oct 2014 16:09:54 GMT

@Anonymous wrote:

Assuming you are using plain cad elements you should be able to solve it by applying parametric constraints. Will that work for you?

Doesn't seem to. If I constrain the proposed arc to the existing arc it move the other end. If I then constrain the other end to the tangent line the line is offset to be tangent to the arc. So that doesn't seem to help.

@Anonymous wrote:

BTW it might be simpler to create some construction elements to establish the tangent points

Yes. I usually would. But without knowing the tangent point on the arc I haven't figured out how to do that.

Allen

Wait a Minute! I think I may have gotten the constraints to work by applying them in a different order. I'll do some testing and post back.

Re: Arc tangent to line and arc

AllenJessup — Wed, 29 Oct 2014 16:05:45 GMT

@Joe-Bouza wrote:
What happens if you temporarily extend the larger arc beyond the extension of the tangent? does the osnap give better results?

No. I tried that. No luck.

Allen

Re: Arc tangent to line and arc

AllenJessup — Wed, 29 Oct 2014 16:51:51 GMT

No. It changes the tangent point along the line by. 0.055 m. I'm sort of disappointed.

Allen

Re: Arc tangent to line and arc

Joe-Bouza — Wed, 29 Oct 2014 17:20:28 GMT

All these things consider leads me to believe the starting PC creates a non-solution to the reverse curve.

filleting the tangent to the reverse curve should yield the correct PC and tangent length, no?

Re: Arc tangent to line and arc

Neilw_05 — Wed, 29 Oct 2014 18:22:02 GMT

I was able to make the constraints work as seen in the screen recording. It may be you haven't applied them correctly. You'll need to apply constraints to fix the line and target arc in place before applying the tangent constraints to the new arc.

Using parametric constraints is overkill if all you need is a static one shot solution. I'll have a look at the geometry to see if I can find a solution using construction elements.

FYI that other "horrible" CAD platform does this nicely.

Re: Arc tangent to line and arc

mathewkol — Wed, 29 Oct 2014 19:42:36 GMT

This is actually a pretty easy thing to accomplish.

create the line and second arc as fixed objects.
add a "free curve fillet between two entities" and specify a bogus radius.
grab the curve end point nearest to the end of the line you want to match and manually move the curve end point to it. This changes the radius to the exact number.

See my screencast recording

Matt.

Re: Arc tangent to line and arc

AllenJessup — Wed, 29 Oct 2014 20:09:01 GMT

Looks good Matt. I just got out of a meeting and have an hour to clean up a few things. Then off until Monday or Tuesday. I'll give this a try then. I had tried with an alignment but didn't think to pull the snap back to the point line you show. I think that will do it!

I'm attaching a drawing with a copy of my entities in case anyone wants to try in the meantime.

Allen

Re: Arc tangent to line and arc

jmayo-EE — Wed, 29 Oct 2014 20:36:45 GMT

As Joe said fillets should work to unless the dims don't meet the deed...This is what I would have tried. I like Matt's niffty trick as well.

Re: Arc tangent to line and arc

Joe-Bouza — Wed, 29 Oct 2014 20:42:30 GMT

Very good Matt

Looks like it worked for me

Re: Arc tangent to line and arc

mathewkol — Wed, 29 Oct 2014 20:44:28 GMT

Thank you Mr. Bouza 🙂 and Mr Mayo.

Matt Kolberg, Applications Specialist

c: 604-803-5963
4040 Graveley Street
Burnaby, BC
V5C 3T6
Improving field to finish productivity

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