Arc tangent to line and arc

[AllenJessup]

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

[_Hathaway]

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.

[AllenJessup]

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

[_Hathaway]

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. 

[AllenJessup]

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

[Pointdump]

Allen,

Yup, definitely time for an Iterative Government Solution.

Dave

[Neilw_05]

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

[Neilw_05]

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

[Joe-Bouza]

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

[Neilw_05]

http://screencast.com/t/McpGijus

[AllenJessup]

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@Neilw wrote:

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

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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.

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@Neilw wrote:

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

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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.

[AllenJessup]

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

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No. I tried that. No luck.

Allen

[AllenJessup]

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

Allen

[Joe-Bouza]

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?

[Neilw_05]

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.

[mathewkol · Accepted solution]

This is actually a pretty easy thing to accomplish.

1. create the line and second arc as fixed objects.
2. add a "free curve fillet between two entities" and specify a bogus radius.
3. 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.

[AllenJessup]

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

[jmayo-EE]

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.

[Joe-Bouza]

Very good Matt

Looks like it worked for me

[mathewkol]

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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