Polylines with real arcs for CNC Router?

Hello, my problem is as follows.

I would like to send 2D shapes that have arcs for rounded corners to a CNC router for cutting. For simplicity imagine im using a single object whose shape is a square with 4 arcs making up the rounded corners.

If i draw the rounded square in SU and make it into a group or component and then send it to a CAD/CAM software called Torchmate, then after applying a male toolpath i wind up with 8 different toolpaths - 4 for the edges and 4 for the corners, rather than a single and and correct toolpath.

If i go back to SU and use the weld extension then i get a single polyline (welded lines and arcs) and a single toolpath in Torchmate but i lose the real arcs as they become these array of edges that are strangely still registered as arcs in SU.

If i use the bezier spline extension i wind up unable to create polylines with the specific shapes i am seeking. It seems that none of the arcs in bezier spline match the basic 2 point arc in SU.

So my question is this: is it possible to create polylines in sketchup while retaining real arcs, or do i have to delve into Autocad or some other program?

Thank you.

Arcs and circles are always represented as poly lines in SketchUp. However, the defining parameters of the circle are remembered and if you export as a 3D dwg or dxf file, true arcs and circles will be created in the export.

Exporting a SKP as a 2d DXF always makes circles and arcs out of segmented lines.
However you can export the SKP as a 3d DXF - even if it is effectively 2d geometry - and then circles and arcs export as real CAD equivalents - with no segmenting etc.

I am aware that exporting with 3D DXF will retain true arcs and circles. That is what i am doing. The problem is that the rounded square produces 8 toolpaths in Torchmate which means that sketchup is exporting them as detached lines and arcs rather than a single polyline as is the case with Autocad.

And if i import the shape into Autocad and try to join the 4 edges and 4 corners into a single polyline then nothing happens. It is as though the arcs in sketchup are not recognized the same way as in Autocad. This in turn means i would have to redraw everything in Autocad and that is undesirable.

So i will ask again, is it possible to somehow weld lines and arcs into a single polyline in Sketchup while retaining real arcs?

I’m not familiar with torchmate, I use dxf exports from SU for cnc that are only composed of lines and arcs. I import them into an “importing environment” of the machine software which you set cut direction, tools etc. At this point all the geometry is single toolpaths just as yours, but there is a “join all connected” command which (obviously) joins all connected lines within a certain tolerance into one tool path. Which in my case solves the problem. I don’t know whether torch mate would have such a feature?
In my case, if I remember rightly all the geometry has to be pure lines and arcs for it to be read correctly by the machine in the first place.

In answer to the weld question, I’m not too sure but I might have a go at it later and get back…

Any ‘welded’ arcs and edges will retain their original segmentation in the exported ‘curve’.
You can of course increase the arc’s segment count - either when you first make it or later using Entity Info - provided that you have not incorporated it into 3d geometry.

There are several ‘weld’ tools to make a single path from selected lines and arcs/curves - as a single ‘curve’ - aka a polyline.

My own ‘welder’ is: https://sketchucation.com/pluginstore?pln=TIG-weld

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I thought that might be the case using weld tools.

While you can increase the segment count for nicer arcs, bear in mind that that each segment will now produce a line of machine code.
So an improved arc of say 96 segments will produce 96 lines of program. Instead of a single line arc command. The resulting program file gets long very quickly.

So accepting the 3d DXF’s 4 lines + 4 arcs for a rounded-rectangle is probably better than the potential 4 lines + 96x4 arc segments in a welded polyline !

So no free lunch :unamused:

Thanks guys.

Ok, so IanT, my machine software (Torchmate) also allows me to join all attached lines into a single polyline to produce a single toolpath. However i have two problems with this approach. The first is that i would have to shell out 1000$ for the machine software (i am currently using a free educational version), and secondly the drawings made in Sketchup even when grouped or made into components get dislodged in the machine software. Apparently the ordering of drawing the 2D shapes in Sketchup affects how they are read by the machine software if they are not polylines (welded).

Nonetheless, my point is that i face two difficult obstacles with using the machine software myself.

However the fact that you mention that CNC routers like to use real curves is important. I read that somewhere else. So for me that means i should not use segmented arcs but instead real ones.

And that leads me to TIGs point, which is that in SU i cannot weld lines and arcs while retaining true arcs, which of course would mean SU is a dead end for me.

This means i have two things to say.

One, it sure would be nice if the SU developers added a polyline feature that would retain real arcs that can be used for CNC router since at present its basically impossible to use SU alone for drawings arcs for the CNC routers.

Secondly, i have to decide on whether to go with the machine software or Autocad, two difficult and costly choices. That is a real bummer since i was hoping SU would handle everything.

Don’t get me wrong, segmented will work but has the drawback of both program length and quality. I supervise a woodworking machine shop. I walk round and run my hands on finished pieces to check cutter sharpness. Unless you had an arc of an incredible amount of segments, I think I would feel it and you would probably be able to see it too.
I’ve found my way around the problem, which sometimes I do have to go back and redraw certain arcs that just don’t want to play. It can be frustrating sometimes, but they’re always true arcs I use never segmented.
I can only give you my limited opinion. What’s best for you at a level of quality you can accept, you’ll have to decide.
I agree with you on the whole SketchUp arc thing, it is a shame. Even with the 3d export it’s not always plain sailing and doesn’t always go as planned.

Ok, so i did misunderstand you, it is possible to use segmented arcs but they are not as pure and smooth as a real arcs. This is common sense that i should have understood since some designs need segmented arcs.

So that means i can go with SU but only if i use segmented arcs. However, here is where the next dead end occurs for me. With my SU 2017 the amount of segments added to arcs with short lengths is very limited. This means if i wanted to use a high amount of segments then i have to go back to SU 2016 or draw very large scales in SU 2017 and then scale them down, two solutions that are highly undesirable.

So in the end SU is just not designed for CNC machining. It simply doesn’t support polylines or a high amount of segmentation for arcs. Indeed this is a shame since SU is such a beautiful program.

I suggest that you try the “Dave method” for dealing with SketchUp’s inability to directly create very short edges. It avoids the need to scale back down (which might introduce mistakes in one’s workflow):

The “Dave Method” is a good idea for overcoming the segmentation issue. However my other problem with the positioning of some components being shifted to a new place when exporting to DXF just makes designing parts with SU for CAM not worthwhile. I would have to redraw every component in the right order in SU since that seems to be the factor in producing the placement of the components in the machine software. And that to me is absurdity. Perhaps the machine software i am using is flawed and is causing the shifts in positions or perhaps its indeed SU. Whatever the case, i basically have no choice but to go through Autocad, where i get true arcs and no dislodging of the components.

I’m confused by this statement. Do you mean that the entities are located at the wrong coordinates (what I would think “placement” means), or that they are not exported in the same order as your CNC software wants them (e.g. touching edges are not always exported sequentially one after the other, the way a tool path would want them)? Other than a bug, I can think of no reason why an exporter would output a SketchUp entity at incorrect coordinates.

So far as I know, a SketchUp model’s database does not capture any explicit memory of the order in which entities are drawn, and the initial ordering can be altered during editing anyway as it often cause an existing entity to be replaced by a new one. So, a simple traversal of the database is not likely to yield entities in the order you drew them. But an exporter targeted to CNC ought to be smarter than that. It ought to know to output based on connectivity not simply on database order - more work, but certainly sounds feasible. Most likely the developers of the dwg/dxf exporter did not adequately anticipate CNC.

Its a coordinate issue.

This is what i draw in SU:

This is how it shows up in Torchmate (CAD/CAM Software):

This is what i draw in Autocad:

This is how it shows up in Torchmate:

I draw the exact same object (neither of them polylines) both saved as DXF version 2013, and then import with the exact same settings in Torchmate. If it works with Autocad then that means that its not Torchmate but indeed SU that is the problem.

But if i export from SU and open up the same component in Autocad it comes out normal. However if i try to join all the edges and corners into a single polyline in Autocad then i am denied. It is as though the arcs and or edges in SU are not the same as the native ones in Autocad.

For this reason i am left with no choice but to either create everything in Autocad or create the designs in SU and then export them to Autocad for tracing.

This brings back some memories :grinning: The arcs seem to get mirrored about the Y axis in some way, they don’t always all do it, though maybe just one sometimes.

What happens if you open the dxf export from SketchUp, in AutoCAD, the same?

Can you share the SketchUp file of the rectangle?

As stated above the DXF export from SU when imported into Autocad comes out normal without the corners being dislodged. However i cant join the 4 edges and 4 arc corners into a polyline in Autocad for some reason. If i draw the same detached edges and arc corners natively in Autocad then i can join them into a polyline.

Rounded Square for SU Forum.skp (21.6 KB)

First thing I noticed was that the rectangle is a component, I always explode the component container before exporting, only loose geometry is exported and I delete the face out of habit. (I’m don’t know if this has a bearing on the arcs flipping in Torchmate). Then I check to make sure the arcs are intact and not segmented, if they are segmented I trace over them at this point. I exported with the setting shown, although all I really want is the edges…

Opened up the file in Draftsight and the arcs are retained in this case as you can see the centrepoints in the corners. So far so good…

Then I attempted to make all the entities into a single polyline, I were able to do it in this case…

I’m just here trying to help, but I think the reason you are not able to make a polyline is because you are not exploding the component before exporting the DXF from Sketchup, maybe you could give that a try?

FYI I always export what I call the “profile” into DraftSight. I then use this to create all manner of geometry for cutting jigs to hold workpieces for the final program in the machine. If I did it all in SketchUp it would just be more to fix if it goes pear shaped.
So it goes SketchUp>Draftsight>Machine.

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Nothing worked.

I created 2 squares one exploded and one a component, and then two rounded squares, once again one exploded and one a component. I exported them as DXF using edges only with dimensions but one with Autocad R14 and the other with Autocad 2013.

I imported both files from Torchmate with varying import settings and the problem of arc dislodging remained.

I imported both files from Autocad and again the same issues happened. There was no dislodging but i was not able to join the exploded edges and corner arcs of the rounded square. However, i was able to join the exploded square. This means the source of the problem seems to be the arcs.

Of course with another file with varying shapes where i had all components welded in SU i still wound up with coordinate dislodging of some components in Torchmate. Only in this case each component as a whole remained in tact but simply had its position changed.

So all together i just cant explain what is going on.

My hopes were for at least being able to import the designs from Autocad where i would simply join them into polylines, but that is not possible. My only options are to go through Autocad and to create from scratch or to trace the exports from SU.

Oh the joys…
I just put the last dxf (lines and arcs not poly) into the machine software and yes all 4 arcs are dislodged. I went back to SU and re traced the arcs and tried again, with the same result. For that example I would have no other option but to redraw the arcs in Draftsight which will fix it.

I did notice your part is very small 4mm across if I remember, so I tried a larger one of 300mm across.
Here’s the weird bit: At first I drew one corner and then double clicked to create the other 3 corners automatically. This ended up in the machine with the first arc ok but with the double click ones dislodged.
I went back and redrew the other 3 corners independently in SU and then put that into the machine. This time there were no dislodging and when I hit “join all connected” a full toolpath were created.

In all cases, all of them look fine in Cad but only get flipped in the machine software. I’ve experienced this before many times and wish I could give you a straight answer to what happens but there doesn’t seem to be an obvious pattern with it.
I just get on with it and fix it. I knew this caveat when I took SketchUp on but I figured the pros outweigh the cons. I’ve even been offered “better” solutions but turned them down as I feel I can deal with it at not too much of a cost. (But I only want SketchUp really😁)

To be fair I could write paths by hand for that part in about 60 seconds and I often do simply manually create programs as it’s faster than messing with any cad/cam software. But if I design a complex part in SU that requires several pieces to be machined at a time with a jig to go with it, I then do the export method as often theres just too much information to easily process.

I do understand the issue, If I find a straight solution or notice anything that improves it I’ll post it.