Joining Arcs to Lines using constraints

David Waight

Dear All,

I am having great difficulty creating a constrained model where I need to join Arcs to lines but only one end is tangential.

I have attached two drawings (Snaplok12 bottom one of the two) that has all four Arcs joined tangentially at both ends and it seems to work perfectly.

When I try and do the same as the top drawing in Snaplok12 everything becomes a complete mess (see Snaplok9).

I am probably doing something completely wrong but I cannot figure a way to keep the Arcs connected, as soon as I change one of the other dimensions (even ones that aren't referenced or constrained by the Arcs, they become either too large or too small, disconnect themselves completely from the Lines or bulge out the wrong way as in Snaplok9.

I have managed to make this work by only having one panel of the 4 part design, with just one of the Arcs but as soon as I introduce more elements everything stops working

Hopefully somebody can shed some light on what I am doing wrong as I have spent most of the trial period trying to make this work.

Many Thanks in advance,

David Waight

Roy Klein Gebbinck

You are obviously going to need additional constraints to control the non-tangential connections. Because without them 'anything goes' as you have seen. I think angular constraints for the arcs can work here.

For example:
If I look at the total angle of the arcs marked rad1 and rad2 in "Snaplok12 Standard.dwg" I find:
Bottom (default): rad1: 90 dgr; rad2: 110 dgr
Top: rad1: 61 dgr; rad2: 78 dgr
In both cases the "subtractionAngle" is ca. 30 dgr.
I would add this "subtractionAngle" as a parameter and set it to the default value of 0.
And then add these angular constraints to the rad1 and rad2 arcs:
rad1: min(90-subtractionAngle,90)*
rad2: min(110-subtractionAngle,110)*

* According to the Help you have to 'Use the list separator character as defined on your system: , (comma) or ; (semicolon).'

Anthony Apostolaros

David, I don't understand your question, but just from looking at the drawings you posted I think this might possibly be the answer:

1. Delete the old arc
2. Use the FILLET command to create a new one

If the assembly is a polyline rather than individual lines and arcs, you can skip the first step. The old arc will be deleted automatically.

Roy Klein Gebbinck

... Actually you don't have to use the 'min' expression as in my previous post. Since you are the designer you are unlikely to use a wrong value for the subtractionAngle.
Angular constraints:
rad1: 90-subtractionAngle
rad2: 110-subtractionAngle

David Waight

@Roy,

Thanks for the replies, I think you have taken me passed my level of knowledge with your answers!

Could you possibly provide me with an example .dwg indicating what exactly you mean?

Sorry to not understand what it is you are trying to tell me.

I appreciate the help none the less.

Regards,

David Waight

Roy Klein Gebbinck

Attached "Snaplok12 Standard modified.dwg".

I have removed 4 _gctangent constrains and added 4 _dcangular constraints.
Use the -PARAMETERS command to change the "subtractionAngle" value.

My initial thought was that 2 _dcangular constraints would be enough since the arcs have _gcequal constraints. But it seems that _gcequal only works on the radius.

Snaplok12 Standard modified.dwg

Roy Klein Gebbinck

Improved formula for ang5: 180-ang1-subtractionAngle

David Waight

Roy,

Many thanks for your help, I have played around with your example and it seems to work quite nicely.

I added the amended ang5 to your drawing and then experimented again.

I finally settled on 70º for ang4 and 160º for ang5.

These now shouldn't need to change when resizing the overall design.

I must admit I hadn't thought to add additional user variables to control other variables so I have learnt something new which I can include in my next attempts.

Thanks very much for all your help.

David Waight 

Greg Skelhorn

Indeed Roy, your contribution to this thread has showed me some new tricks also... Thanks.

Roy Klein Gebbinck

@ Greg:
If you are interested in 2D constraints, and have not yet done so, you should definitely check out:
The power of 2D dimensional constraints in BricsCAD, or how to transform a smartphone into a tablet in couple of clicks