Arcs change to Bezier curves and segments

[Kevin Jenness]

I am setting up to run a curved moulding in two concentric pieces. I need to cut out the pieces with concentric arcs prior to profiling. I drew the inside arc, offset the second, then offset the fourth (outside) from the first one ( and moved it), and offset the third curve from the fourth. Using the measure tool the first shows as an arc, the second and fourth as Bezier curves and the third as a group of line segments. If I use edit/curve fit vectors on the second through fourth "curves" to make them arcs they become a set of disjointed arcs with slightly differing radii within each curve. Why is this happening and how can I go about drawing concentric curves efficiently in VCarve Pro?

[FixitMike]

Simply draw each one using the Draw Circle tool. Just enter the new diameter in the box.

[IslaWW]

Kevin...
You can also use the arc tool. Either click or define the start and end points and drag or edit in the form to use an exact radius.

[adze_cnc]

You could still use the "edit/curve fit vectors" tool if you select replace with Beziers rather than arcs.

[Kevin Jenness]

Thanks for the suggestions. I may draw these in Designcad as that allows for drawing parallel arcs without changing the geometry.In that program I can draw an arc with given endpoints and radius, offset it, and the resultant curve and any derivatives remain circular arcs with identifiable radii.

Playing around with VCarve I see that if I draw a circle , select it and measure its span/contour properties it is defined as four quadrants, as are any offsets from the original. If a circle is trimmed against a line and measured, the result is a Bezier curve with no defined radius, as is an offset from an arc. I assume the offset is parallel to the original arc at all points, but am not sure. Offsets from the Bezier curves are defined as groups of line segments.

Clearly I am expecting VCarve to operate in a way it was not designed to do, and have to use a work-around to get the result I would expect in a "conventional" line-intensive cad program.

[IslaWW]

Kevin...
Do you have a reason other than knowing there are arc properties to a particular vector for doing this? Are the vectors being cut improperly?

The reason I ask is that all curves and arcs are executed in the control as numerous segments. CNC machines make point to point straight line moves, period. The segments are generated by the CAM application for all but G2 and G3, which are generated by the controller. Assuming all things are set properly, there should be no difference in the cut.

[stanv77]

I am not sure on what you are trying to do. I think it is to move an identical arcs
a certain distance apart. If this is what you are trying do then copy the arc and paste
it and then move it the where you want it. I hope this will help.

[Kevin Jenness]

Gary,

You are right that the proof is in the cutting. I have not had any problems in the past cutting curves that nest together properly. I understand that cnc operations are point to point,and as long as the points are close enough together and the paths are smooth I will get a "close enough" (very close) approximation to circular arcs.

This issue came up when I cut a template for a cabinet rail for this project using imported dxf vectors and to my surprise got a series of straight line segments instead of the expected concentric circular arcs. At this point I don't know if I did any work in VCarve on those vectors. I can say that when I import that file again into VCarve the arcs show up as true arcs. At the time, I redrew the arcs in VCarve and the machining went properly.

My next task was to cut out blanks from one larger blank for a cornice moulding to be profiled as two nesting pieces. I started with the tightest curve and offset it to define the opposite edge of one moulding section. I then copied the second curve and moved it over, then offset it to get the outside shape of the second piece. At that point I was curious enough about my first mishap with the curved rail template to measure the arc/span properties of those curves, and found the results described above. I did not want to proceed with the cutting until I understood the program function better. I felt that the "curve" that showed as a series of line segments would surely not be acceptable, and I was wondering whether the Bezier curves resulting from offsetting the first arc were in fact truly parallel to it. That was the reason for posting my query.

I decided to cut some shorter curves using the same drawing sequence today in order to get practical test results. Imagine my surprise when all the offsets, whether from the first arc or from subsequent offsets all are defined as arcs. I have to wonder what is different from today to yesterday,aside from closing and reopening Vcarve.

I assume(?) that a Bezier curve resulting from offsetting an arc is a true circular arc. Any thoughts? If so, the only practical difference for drawing is being able to identify the radius of a particular curve with the measure tool.

As always, thanks for the help.

[IslaWW]

Kevin...
I was trying to find out if you had any real cutting issues with the segmented arcs. If so, I would help tune your controller.

It is a common misnomer that gcode arcs (G2, G3 commands) are actually cut as a true arc rather than a series of straight line segments, which of course, is not true. CNC machines cut straight lines. Even when cutting some of the nicest circles.

[TReischl]

A bezier curve is NEVER a true circular arc. That is by definition.

A true circular arc is a point in space and a radius. It can be drawn with a compass.

The same cannot be said for a bezier curve. It is a parametric curve. As such all that can be done is to calculate points that lie on the curve and then connect them.

Interestingly, even though the software shows nodes and handles and it appears to be a bezier curve it is not. It is actually 4 true arcs. You can prove this to yourself by grabbing one of the nodes and moving it, then posting the work and take a look at the g code. It will be 4 either g2 or g3 moves. Unless you are using a pp that does not output arcs.

Edit: In the last paragraph I was writing about the Circle tool.

You could think of the difference between Circle and Bezier as the difference between analog and digital. The circle being analog.

[Adrian]

In VCarve beziers have blue nodes and outlined handles. Arcs have black nodes and outlined handles. Line segment nodes are black with no handles. Circles made with the circle tool are four arcs. Offsetting should still yield 4 arcs on a VCarve created circle that hasn't been messed with. Offsetting other shapes the arcs can be converted into line nodes if it's not possible to "respect" the shape while maintaining arcs.

[TReischl]

I never noticed the color difference Adrian!!!!

Amazing, been looking at this software 10 years and never noticed that, thanks for pointing it out.

[Kevin Jenness]

Thanks for the responses.

Adrian, your observation about the node colors is helpful. Interestingly, in the file I posted, the curves that are identified as Beziers by the measure tool have black nodes.

TReischl, do I understand you correctly to say if a bezier curve resulted from offsetting an arc it would not be precisely parallel to the original arc? To put it another way, is it impossible for a Bezier to be congruent with a circular arc?

I am still confused by this episode. A colleague pointed out that Autocad will "dumb down" the display of arcs, portraying them as line segments until you hit "regenerate" at which point they show up as true circular arcs. He wondered if there was a setting in VCarve that acted similarly. As I said in my last post, the program now produces arcs when offsetting arcs, but at one point it acted differently.