QCAD.org Forum

I am working a project that I would like to offset several lines (arcs). I'm trying to make them into a polyline so I can offset them all at once. I thought I was done until a circle appeared in the offset. Screen shot attached. Any thoughts

Please attach the drawing file and please note the forum rules (in red above), thanks.

KHE wrote: ↑

Wed Jun 14, 2023 4:39 pm

I thought I was done until a circle appeared in the offset. Screen shot attached.

Sometimes QCAD can not make an offset with very small arc-segments.
It is usually a mathematical question. EDIT: Especially with tangentially connected arcs.
QCAD doesn't trim arc shapes pair-wise before merging them to a polyline, your base polyline.
If the connection point matches within a certain tolerance only one endpoint is retained but the bulge factor of both arcs are used.
Changing an endpoint, how minor the change is, with the same radius and center then that should be reflected in the bulge factor.
An offset of such a shape can be problematic.

One can clean up or simplify the base shape so that this doesn't occur.
Trimming arcs pair-wise before creating a polyline is the best solution in such cases.
EDIT: You will need the original arc shapes for that ...

This is a small (almost full) arc, probably to be tangent, but it may occur as larger errors too.
See Eagle offset faults.dxf here: https://www.qcad.org/rsforum/viewtopic. ... 497#p24417

Regards,
CVH

Thanks, lots of extending, clipping, editing and I got my job done.

KHE wrote: ↑

Thu Jun 15, 2023 2:30 am

Thanks, lots of extending, clipping, editing and I got my job done.

It mostly does ... And 'lots of' tells me that you didn't had the original arcs or their source to start over with.
Once merged to a polyline there is no way back ...
... The polyline arc-segments their radius, center, begin and end angle are altered.
Exploding such a 'misformed' polyline back to arcs can worsen things ...

In a way the same occurs with line-segments:
https://www.qcad.org/rsforum/viewtopic. ... 977#p33158
That only affects endpoints and/or the angles of the lines.

Regards,
CVH

Windows 11 Home
Version: 3.28.1.0 (3.28.1)
Internet: QCAD.org
Build Date: Mar 31 2023
Revision: ca7ec14
Qt Version: 5.13.2
Architecture: x86_64
Compiler: MSVC++ 14.0 (2015)

G-Code (Offset) [mm]

I hope you don't mind me asking this question about toolpaths. It is a continuation of my current project. All of the offsets are related to toolpaths that I am creating. I created a test file and have attached it. I have labeled the lines with numbers. In my actual drawing things are more complex but the exact same question arises.

Profile #1
I window selected all, filtered out the number and created a simple toolpath of the red lines. Qcad choose to follow 5, 1, 2, 3, 4, 6. Logical here. But what if I want the order different?
Profile #2
I selected, in order, 6, 4, 2, 1, 3, 5. Qcad calculates the same toolpath. I want it to follow 6, 4, 2, 1, 3, 5

I know I can relocate the start point on each line (and always use start of polylines) but I can't figure out how to tell Qcad a specific order to follow. I want to optimize my tool flow as I have many toolpaths (over 100).

KHE wrote: ↑

Sat Jun 17, 2023 8:38 pm

I want it to follow 6, 4, 2, 1, 3, 5

Not sure about the start points but I assume this is the order you are looking for, right?

Thanks for your reply. Yes you are correct. In actual fact I just want to be able to reorder them to any order.

KHE wrote: ↑

Sat Jun 17, 2023 9:47 pm

In actual fact I just want to be able to reorder them to any order.

You have to create for each entity a single Profile Tool path in that particular order. A bit laborious but I'm not aware of any other option ...

In this case - add first a Profile tool path for entity 6, then a new one for entity 4, then a new one for entity 2 ... you get the picture.

Thanks, I was hoping for something better. This method will add 40 toolpaths. I'm already at 100 +. Thanks for the confirmation of what I was already thinking.

KHE wrote: ↑

Sat Jun 17, 2023 8:38 pm

I hope you don't mind me asking this question about toolpaths.

No we don't but have a new look at the 'Forum rules' (in red above).
One question per topic makes the forum searchable.
The title of your toolpath question is now 'Re: Offset mystery circle' ...


May I ask why those 100 or more cuts must be executed in a specific order?
If it is for holding down your pieces you might consider using TAB's.
For thin materials a vacuum table or sticky supporting pads may be a solution too.
In a way we can also exploit the fact that after a cut QCAD/CAM looks for the nearest startpoint to group certain cuts in one toolpath.

KHE wrote: ↑

Sat Jun 17, 2023 8:38 pm

I want to optimize my tool flow

Traversing in sequence (5, 1, 2, 3, 4, 6) is 311.71" in total ... For (6, 4, 2, 1, 3, 5) that is 423.20".
And in fact this won't be a big difference as traversing is done at the highest FEED's in X & Y available.
There is more gain to make in plunging an retracting.
As engraver I tend to hover at less than Z1 over the workpiece ... For each letter it needs to plunge and retract.
See: https://www.qcad.org/rsforum/viewtopic.php?f=74&t=10063

The issue is that there is no fixed order when querying the selected items.
Nor the handle, nor the drawing order is relevant.
The handle is a fixed thing on creation, the drawing order is re-ordered on save/reload.

Regards,
CVH

Good afternoon CVH

The sample I made was just for an example. Qcads path for the that example would be fine and faster. I am making a guitar body with many shapes. Traversing does waste time when the next cut should be right be side. I have be honest there are some time its faster just to let the machine go where it wants as the it doesn't really take that long. I can only send a screen shot but creating a preferred flow by creating more toolpaths is tedious. I would like to select a bunch of line and have the ability to arrange in my order. I'll just have to more aware of how I create the order or just let the machine do it's thing. It would also be nice to add a toolpath at a specific point in the tree. It always creates a new toolpath at the bottom and I have to move it up. On my computer that is very slow (I forgot a couple of toolpaths).