topic Re: how do I change the radius of a 3 point arc after I've drawn it? in AutoCAD Forum

how do I change the radius of a 3 point arc after I've drawn it?

emmanuel_boakyePJCYR — Mon, 09 Oct 2023 11:36:59 GMT

is there a way to change the radius of a 3 point arc after I've drawn it? I want to be able to type in a specific radius number

Re: how do I change the radius of a 3 point arc after I've drawn it?

Washingtonn — Mon, 09 Oct 2023 12:01:07 GMT

Select the arc's midpoint arrow grip and enter the desired radius OR with the properties palette opened, select the arc and enter the desired radius value in the Geometry section.

Re: how do I change the radius of a 3 point arc after I've drawn it?

Kent1Cooper — Mon, 09 Oct 2023 12:40:42 GMT

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.

Re: how do I change the radius of a 3 point arc after I've drawn it?

emmanuel_boakyePJCYR — Mon, 09 Oct 2023 12:56:33 GMT

I want the end points to remain where they, I have to meticulously drag the mid point until it gets to the radius number that i want and that is not easy to do cause I have to be extremely precise when dragging the mid point and I cant even get it to land on the exact number that i want.

Re: how do I change the radius of a 3 point arc after I've drawn it?

tramber — Mon, 09 Oct 2023 13:41:34 GMT

You could use : _GcFix command on the 2 ends

Then _DcRadius and dble-clic to change the value...

Re: how do I change the radius of a 3 point arc after I've drawn it?

Kent1Cooper — Mon, 09 Oct 2023 14:02:09 GMT

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].

Re: how do I change the radius of a 3 point arc after I've drawn it?

Kent1Cooper — Mon, 09 Oct 2023 14:14:05 GMT

@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.