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Author Topic: How can I trim the remainder of a circle when using two tangents?  (Read 572 times)

May 13, 2019, 02:31:53 AM
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noxou

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Hi,

New to Inkscape with some experience of other programmes.

I'm trying to draw a gasket for laser cutting. I created the using basic shapes and tangent lines. I would like to remove parts of a circle between two tangent lines (trimming in autocad). I have deleted segments between nodes as an option, but that leaves a small line segment that I am having trouble removing. What would be the best technique for this? I have included some pictures of what I'm trying to do.

Any help would be much appreciated.

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May 13, 2019, 03:41:11 PM
Reply #1

brynn

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Welcome to the forum!

Referencing the small screenshot inside the red rectangle, have you tried deleting the end node?  Does that delete part of what you want to keep?  Or not enough of what you want removed?

If deleting that end node is not a perfect fix, then you'll need to add a new node at the point where what you want to keep is on one side and what you want to remove is on the other.  Then just delete the nodes on the side you want to remove.

You're welcome to share the SVG file, if it seems like this isn't correct.  But from what I can tell from your screenshots, I think what you would have wanted to do, is add new nodes on the circle, at the points where the tangent lines touch it.  Or possibly add the new nodes first, and snap the tangent lines to those nodes.  I guess it depends on your approach to drawing the design which way you go.

But in any case, with the new nodes in place on the circle, it gives you a place to break the circle and delete the unwanted parts.

Or another way.....  And again, it depends on your construction method.  If there would be a way to use some other shape, like another circle or a rectangle, or even just a straight path, which intersects with the circle at the appropriate places, then you could use a path operation, such as Difference or Intersection, to delete the unwanted part of the circle.

Like for example, if I "read" your design correctly, the places on the small circles where the tangent lines touch represents a line that is tangent to the large circle, at the point where the small circles touch it.  So possibly that little bit of geometry could be used.  To use a straight line to cut the circle, you would use Path menu > Cut Path.  Then delete the unwanted part of the circle.

Edit
Oohh, I just thought of another, possibly better way, than adding new nodes.  Select the tangent lines (one at a time) using the Selection tool. Duplicate.  Make sure they are on top of what's left of the circle (this button on the control bar  :mtt: or Object menu > Raise to top).  Grab a corner arrow, and while holding Ctrl and Shift, drag slightly.  This makes the duplicate tangent line longer, so that it intersects with what's left of the circle.

Now select what's left of the circle and the duplicate tangent line (at the same time), then Path menu > Cut Path.  It won't look like anything happened, but it did cut whatever is left of the circle.  Now you just need to delete the unwanted part.

Let us know how it goes :)
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May 13, 2019, 11:31:31 PM
Reply #2

noxou

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Hi Brynn,

Thank you so much for the detailed response! I've gone through your suggestions and I think the crossing line method works for me. It requires a bit of work, but it does work, so problem solved for which a big thank you!

The first method (adding node) works too, but is not as precise; i.e. I can add a node at approximately the right spot and then cut, but it's 'close, not geometrical' if you know what I mean. The intersection with a line method (option 2) works more precise I think. I drew a line as you suggested, from within the circle to the tangent point. Then extended it by dragging. So the cut is then at the precise point.

The last method didn't work for me, but I may have misunderstood your instructions though? I've copied the tangent line, pasted it back in place, put it on top and then tried to extend it along its path. One problem is that it won't let me extend along it's path (not sure why, maybe because it's a tangent line?) and the second is that it's a tangent, so if I would extend it, it wouldn't cut the circle (as a tangent touches at one point by definition), but I'm probably missing the point. Perhaps you can clarify? I've attached the VSG should that help.

Sorry for all the questions, just trying to get my head around Inkscape and its ways of working. The outcome will be that a classic motorcycle will be back on the road by using this design to get a laser cut gasket  :-D

In any case, many thanks for helping me!

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May 13, 2019, 11:58:16 PM
Reply #3

brynn

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In the last method, you need to draw a different tangent line.  Well, it's just a line.  But it's tangent to the larger circles, and tangent at the point where a smaller circle touches the larger circle.  Maybe I can add some notes to your file.....  I'll attach it shortly....

Edit
Well, I didn't have to do much to show you.  Since you have everything already set up so precisely, I just had to draw a 45° line.  And since you have snapping set up as well, it snapped perfectly tangent to one of the larger circles.  I colored it red for clarity.

If you move that line out to the smaller circle, the place where it intersects with the smaller circle is the same place where you want the other lines tangent to the smaller circles to start.  Since I'm not familiar with this gasket, I'm not sure what decides where those lines need to start.  So I'm not sure exactly how you would place the red line across the smaller circle.  But you must have had some idea about that, unless you just kind of "eyeballed it".  But anyway, with the red line on top of the circle, Path menu > Cut Path will break the circle in the proper places.  Then you just delete the unwanted part.

Or, since you're interested to learn, there's another way, which gives the same result, with one less step.  So much with Inkscape is a matter of preference, and learning what works best for you.  Anyway, notice the blue rectangle I made.  With the rectangle on top, select both the rectangle and circle, and do Path menu > Intersection.  You'll have the same result, and it saves the step of deleting the unwanted part.

One more note.  This might be unnecessary.  But it seems, from what I can tell (I don't have any kind of cutter) that some cutter machines raise up the cutting head or laser at the end of a path.  You probably would get the same result either way.  But to make sure it cuts all in one go, without starting and stopping, you probably want to join the end nodes of all the paths, so that you end up with a single, closed path.

To do that, switch to the Node tool.  Select 2 adjacent paths, like one tangent line and the remnant of the circle.  Presumably, the end nodes will be right on top of each other, which means it's really hard to select whichever node is on the bottom.  So to select them both, you have to drag a selection box around them, just around the 2 end nodes which are on top of each other.  Then click Join Selected Nodes button on the control bar (or use the key shortcut, whatever it is)(I tend not to use key shortcuts, if there's a button for it, but that's just me).  Then continue around the whole thing, until you have a single path.

Actually, it might be possible to join them together in one step, instead of joining one segment at a time.  Or at least join half at a time.  Select all the path segments.  Then while holding the Shift key, drag a selection box around each set of overlapping nodes.  (It looks like there must be at least 16 segments, maybe 24 segments? So there would be one less number of sets of overlapping end nodes.)  Then Join Selected Nodes button.  It can be a little tricky sometimes.  Once in a while, I find what I thought was an end node, actually there are 2 nodes there, and that keeps it from joining properly.  It might join the wrong segments together, or it just doesn't join them.  But once you get some practice, you'll learn how to sort out the problems, and be able to take little shortcuts like that.

Sorry for all the text.  I just enjoy helping new users learn and sometimes I get carried away  :)
« Last Edit: May 14, 2019, 12:45:49 AM by brynn »
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May 14, 2019, 01:20:26 AM
Reply #4

noxou

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Hi Brynn,

Thanks again for such a speedy reply - I understand what you mean now. I've tried out those two approaches and that works nice and quick. I guess it's about being creative with what support structures you use and getting some experience with it. Using these approaches it's readily doable - I think I'm getting the hang of it as well as the node concepts.

Thanks for the tip on joining the nodes! I didn't realise the impact it would have on how a cutter operates, but now you mention it you're of course right with that. The machine would just follow instructions and like a 3D printer the head would withdraw from the piece to move to the next segment to process, potentially leaving a messy result or tiny pieces uncut. The gaskets are metal reinforced, so they might not get loose from the material, so that's a great pro tip.

I really appreciate you taking the time to point me in the right direction - it really helped me along. I'll post back whether mission gasket was successful. It's a great use case for software like this - these gaskets are no longer made (or only at extortionate one-off prices) and given the reinforced metal core cutting them by hand is also not really an option. This way anyone who needs one can take the svg and get them cut at the local fabrication lab without having to have Autocad or other software that can be disproportionately costly for what you need it for incidentally.

Many thanks!
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May 28, 2019, 03:33:18 AM
Reply #5

noxou

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Hi Brynn,
Just wanted to show you the successful outcome of the little project - gaskets came out of the laser cutter perfectly and my classic scooter is back on the road due to Inkscape!

Many thanks!
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May 28, 2019, 08:05:47 AM
Reply #6

brynn

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Oh that's great news!  We love to see the results of projects where Inkscape was used.  And you're welcome!

I guess I never really thought about it before.  But I see really old cars on the road sometimes.  This gives me a whole new appreciation for what the owners have to deal with, to keep them working.
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"Be ashamed to die until you have won some victory for humanity" - Horace Mann