how do I change the radius of a 3 point arc after I've drawn it?
Just to clarify @Washingtonn 's suggestion: Changing the radius alone, via the Properties palette or by grip-editing and the Radius option [there are two ways to get to that], will leave the center point in place, and change the position of the Arc's curve, meaning the endpoints will move. If you want to do something different, such as to change the radius but leave the endpoints where they are, that will change the location of the center point. You can do that by grip-editing, dragging the midpoint grip, but I don't know of a way to specify a particular radius in the process. An AutoLisp routine could certainly be written to do such a thing.
You could use : _GcFix command on the 2 ends
Then _DcRadius and dble-clic to change the value...
Try the attached ArcChangeRadiusKeepEndpoints.lsp with its ACRKE command. Lightly tested. It will do multiple Arcs, and it remembers your specified Radius to offer as default on subsequent use. But [so far] it has a limitation, in that because it uses the ARC command in start/end/radius option sequence, it always draws the smaller of the two possible Arcs for a given pair of endpoints and radius. These two Arcs have the same endpoints and radius:
The command will always draw the red one, even if you are starting with something close to the yellow one. If you need to do this with Arcs of greater than 180° included angle, that would involve some calculation, but it could be done.
It does not yet work in other-than-World Coordinate Systems, but could be made to.
It also reports the quantity of Arcs [if any] that it did not change because their endpoints are too far apart for the specified radius. An enhancement could be for it to also identify those in some way [change their Layer or color or linetype, or leave them selected/gripped/highlighted, or something]. And of course it could use the usual stuff [Undo begin/end wrapping, *error* handling].
@tramber wrote:
You could use : _GcFix command on the 2 ends Then _DcRadius and dble-clic to change the value...
I wasn't aware of those, not having worked enough with constraints -- pretty cool. That approach has the benefits that it works as expected on greater-than-180° Arcs, and requires no custom command definition. It has the drawbacks [if considered drawbacks], compared to my ACRKE command, that it must be done individually on each Arc, and doesn't remember a default value.
