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10x14 rectangle overlapping a 12mm circle equidistant
me_here910
Member Posts: 23 ✭
in Drawings
Like this:
That takes 6 clicks in my CAD, but a month and 2 "learning paths" later, and I still cannot work out how to do it in onshape?
That takes 6 clicks in my CAD, but a month and 2 "learning paths" later, and I still cannot work out how to do it in onshape?
-1
Best Answer
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owen_sparks
Member, Developers Posts: 2,660 PRO
me_here910 said:
So, why do you think I typed all of that? (Perhaps I'm lazy, stupid, a troll?)owen_sparks said:The time taken to type the posts above should far exceed the total time required to learn how the OS sketch environment and solver works.Hi. No! That's not the intention of my comment at all. I meant no disrespect whatsoever. (I'm happy to PM a phone number if you'd like to say hello and have a brief chat.)The message I apparently failed to get across is that the sketcher is to most users a very natural thing to use that requires next to no explanation at all. It it's failing to make sense to you currently then relax, as soon as it does make sense you'll never give it another thought. The mental effort / stress should go into the design, not to driving the software!I'll admit It took me a little while to grasp how to make the best use of constraints to convey robust design intent, but things like:-blue = un-consrained geometryblack = constrainedenclosed regions automatically turn greydotted lines are construction geometry and don't form enclosed regionsetc seemed to make sense from the offset.If you're seeing things happen other than what's expected then either your browser is glitching (Firefox and chrome should work just fine) or you may have some preconceptions as to how a function should work and they may actually work slightly differently. That said a 3 point circle / center circle / vertex is pretty much that in any software.If I can help then please shout, if not then please know folks on this forum are a decent lot and are only giving up their time trying to help.Cheers,Owen S.Business Systems and Configuration Controller
HWM-Water Ltd7
Answers
HWM-Water Ltd
Select the "center point rectangle" tool. You may need to click on the down arrow next to the "corner rectangle" tool to find this tool.
https://cad.onshape.com/help/Content/sketch-tools-rectangle_center.htm
Click first where you want the rectangle to be centered, then click anywhere to finish placing the rectangle.
Type "10 mm" and press enter, then type "14mm" and press enter. (No need to select the dimension tool, right after you place the rectangle, if you just start typing it will start to fill in the dimensions of the sides of the rectangle).
Select the "center point circle" tool.
https://cad.onshape.com/help/Content/sketch-tools-circle_center_radius.htm
Click first on the center point of the rectangle, then click anywhere to finish placing the circle.
Type "12mm" and press enter. (Again, if you just start typing after placing the circle, it will fill in the dimension).
4 clicks, three types
Not quite.
Click once to pop up circle menu
Again to select the tool.
Once more for the centre of the circle. (What if I want it offset from the origin?)
(Try typing to set the size;no go.)
Once more to set a random size of circle.
NOW I can type '1' '2', 'enter'.
Damn. I caught the '3' key and got a 123 circle. How to correct it?
Try clicking the dimension. Nope that created a new circle.
Hit ^Z, and that circle disappears; but so does the sizing, its back to the random size; but no dimension to click on (not that that worked anyway), so how do I size it?
Try mousing over it (ithighlights) Try typing. Nothing.
Try selecting it. (the 1 appears on the cursor) try typing. Nothing.
Try right-clicking .... hm. maybe properties ... Nope. Maybe Transform ... oh, that did something. A weird thing popped up and a cursor that seems to be inviting me to left-click.
{Go grab a coffee guys...I may be a while ....)
Nope. It'll allow me to move the circle in a variety of interesting ways (some more interesting than others) and I can type numbers to control the distance/angle relative to the start position, but once you tried one, and initiate another to try and get back, as they are all relative and transient, there's no way to know what their accumulated effect is, so no way to put it back where it started.
So, once again: select the circle, right mouse, delete sketch entity. (At least I didn't have to throw the entire sketch away this time -- though as the circle was the only thing on it, the effect is the same.)
Click once to pop up circle menu
Again to select the tool.
Once more for the centre of the circle. (What if I want it offset from the origin?)
(Try typing to set the size;no go.)
Once more to set a random size of circle.
NOW I can type '1' '2', 'enter'.
Phew. Got it right this time.
Now click the rectangle menu,
Click to drop down the rectangle menu,
Click to select centre point rectangle.
Click origin to mark the centre.
Click some random position.
Type '1','0' try 'tab' to get to the other dimension.(Nope, does nothing.) Hit 'enter'.
Hm. The random high value hasn't changed to an edit field?
Try clicking it -- nope that starts a new rectangle.
Click white space. Click scissors drop down, click trim.
Click 7 more times to remove the segments of the unwanted rectangle.
Phew. EXCEPT... its left 5 blue dots behind and the trim tool won't touch them.
And the height of the rectangle I do want is still wrong, and the ghost dimension that was there when I tried to click it and got the rectangle I didn't want, has no disappeared. And nothing I click or mouse over seems to bring it back for me to edit.
%$$$ this. Click the red cross in the sketch dialog, throw it all away and start over (again!).
Click sketch,
click top,
Click circle drop down.
Click centre point circle.
Click origin.
Click rand point in space.
Type '1', '2', 'enter'.
Click rectangle drop down.
Click centre point rectangle.
Click origin.
Click random point in space.
Type '1', '0', 'enter'.
(Tabbing didn't neither did clicking the ghost dimension...so try just typing???)
Type '1', '4', 'enter'.
FINALLY. I got what I wanted. (And a bunch of badly placed and unwanted dimension lines, but hey. Progress.)
So...final tally (ignoring all the dead-ends and blind alleys) for this 'simple' task. 10 clicks. 9 key strokes.
Most of which have to be done exactly in the right ordr because get it wrong and the only option is to throw it all away and start over.
And all to avoid what. Making clicks in the right places to start with.
Back to my preferred CAD to compare.
Click new for a drawing.
Click circle tool.
Click the origin.
Move the cursor until the diameter value read 12 and click.
Click rectangle tool.
Click the grid point at -5,7.
Move the cursor until the dimensions read 10,14 click.
Done. 7Clicks No typing. Everything clear and obvious. No restarts.
----------------------------
Anyone interested in instalment 2?
Where I get rid of the extraneous bits of the square and circle, extrude it to 40mm; and cut an (accurate) M12x1.25 external thread on it?
In my CAD it goes like this.
Click trim tool, and 8 clicks to remove the extraneous.
Click pull tool; click &drag the flatted circle surface a little and then type '4','0','enter'.
Click sketch tool; click the center line of the extruded axle to get a sectional view.
Zoom in top left.
Click construction line tool. Click top left of section, and drag up and right, let go and type "1.25*sin(60)/8" enter.
Click the end of the new cline and drag down and left; let go and type "1.25" enter.
Click the end of that cline, drag up and left; let go and type "1.25" enter.
Click the top left corner of the section again, drag straight down, type "1.25*sin(60)*5/8" enter.
Click the end of that cline, drag left until you crossed both diagonal lines.
Click the 3-point arc tool. Click the crossover points where the horizontal line crosses the two diagonals, drag down til the arc angle reads 120(I type the degree symbol here using alt-248, but that throws away my entire post and dumps me at the forum front gate {grr}.
Click the line tool, overdraw the angled construction lines and connect the ends of the arc to the horizontal of the top edge of the section and close the rounded equilateral triangle. (much easier to do than describe.) 6 clicks.
Click pull tool; Click the face of the triangle. click revolve helix, click the pitch entry field and type 1.25, drag right.
Done. (Some left over construction lines to clean up, 3 clicks in the structure dialog and press the delete key.)
--------------------------------------------------------------------------------------In OnShape? The extrusion, okay. (I ended up with 3 extrusions in the features list, only one of which had any depth, but once I found that this time instead of typing into an invisible entryfield somewhere random on the screen, I have to click in the dialog top left. and overtype the preselected 25mm??
(There's nothing like consistency; and this is nothing like consistent.)
Getting rid of the 8 extraneous line & arc segments what a .... nightmare; but mostly a one time misunderstanding and tired eyes; but I never did find a way to rid myself of those dimensions I did ask for or the point where the corners of the rectangle were.
But, trying to establish a sketch plane along the Y-Z centre line of the extrusion utterly defeated me. The two end faces, and the two flat sides appear to be the only choices.
So, I never reached what I know is going to be the nightmare bit of trying to position anything on a sketch accurately. Constructing an accurate thread profile with finger painting just seems impossible to me.
Sometimes I give up out of shear exhaustion, sometimes I stick it out and learn something new despite my stubbornness. I have no real advice just sayin'....
Been coming here pretty much daily for a month and ... I'm nowhere. (despite and done the "Learning paths".)
Click help and it want to tell me about apps for mobiles.
Search for "Setting a plane axis" and it suggests that click a "Select a linear edge, sketch line, or cylindrical face to get its axis"; but it just doesn't work. I just doesn't.
And the same is true for pretty much everything. Nothing is intuative, and the help doesn't!
(Thanks for the video. I just can't see it. I have 4 different browsers -- FireFox, two versions Opera, and Vivaldi, and none of them will play it.
(I won't use Chrome or MSEdge!)
so, Thanks.
https://www.youtube.com/watch?time_continue=1&v=lbukikTdJ9w
Yes. That video link worked were the original didn't; but now does. (Maybe I was too quick off the mark and it hadn't converted it to a format firefox understands? It said something about html5 when it failed. )
Anyways.
The point about click counting is simply some metric for how easy or hard it is to do something.
Far more important is that (lack of) feedback from the interface. (Ex. usability tester here.)
You mentioned earlier something about "new conventions"; the thing about conventions is to become "conventions" they have to be "a way in which something is usually done."
Usually, in (from memory) every other CAD package I've used, when drawing 2D geometries, you do so on a grid with optional snaps. That grid combined with judicious use of snaps, make positioning stuff absolutely and relatively a 'snap'
The only alternative I come across is a dynamic cursor that displays the current position as you mouse around; but that is usually (optionally) combined with grids and snaps. Those are "conventional".
FreeCAD's Sketcher is the only thing that is vaguely similar, but it still provides a grid and snaps and dynamic cursor position, and a dialog where sizes can be set and adjusted that is always there and always in the same place; and always works the same way.
Basically, no guesswork. No matter what you want to next, you look at the screen and you either immediately know how to do it, or you have a very strong idea of the possibilities. That never seems to be the case here. Ever.
Even trivial things like using this forum are just hard work.
eg1: Any other text editing environment -- whether browser of text editor or entry fields in applications, control+left or right arrow key moves back and forth a word at a time. Here, those keystrokes go to the beginning and end of line.
Trivial you say -- and you are right -- but just another piece of (needless, pointless) unconventional that makes the whole experience so frustrating.
A more relevant example: With your help I've got my 3D solid -- cylinder with flats -- and from watching other videos on youtube, I've managed to wind a helical path around it:
Now I need to sketch a M12x1.25mm thread profile/surface on the X-Z centreline of that extrusion; but despite hours of trying and looking and searching the help; I can find no way to place a sketch plane on that central axis.
(And its not now about being told how to do that -- (you, I and everyone else would rapidly get bored of me asking for every new thing) -- but about why can I not work it out for myself. That is, why does the interface make it so damn hard?)
so how would you do this job in OtherCAD? You have already drawn a symmetric extrusion profile centered to the origin, and why you now want to make additional workplane? - just draw thread profile on one of the existing ones - Right or Top workplanes are good enough.
HWM-Water Ltd
IR for AS/NZS 1100
And its not the browser, because in two other open tabs connected to two other JS apps (SimScale*CAD Exchanger), it works as expected.
Ditto for ctrl-shift-left & right to select words. (but that just and irritation that contributes to frustration; not a show stopper.)
The point was to show the thought processes as I try to discover how to use this interface. How unconventional, inconstant, un-intuative it is.
What am I hoping for when I go to all the effort of typing that blow-by-blow account as I work it through? I dunno; maybe the secret button or code that decrypts the interface and provided the ... for dummies version.
Have you heard of the internet/usenet/forum meme: "Don't ask to ask; just ask?"
Well, if this corollary to that doesn't yet exist; I hearby claim the invention: "Don't tell what you're going to tell me; just tell me!"
17:34 seconds into the video (sorry, "webinar") and he hasn't shown me how to do anything!
Two minutes saying hello.
Five minutes of saying how OnShape is the greatest thing since sliced bread; and how great the collaborative features are for the Enterprise.
Five minutes telling me what he's going to tell me .
Five more minutes showing how to log in; navigate and browse a directory listing.
Finally, there's a part assembly on the screen, this must be the bit where he shows me all the tricks that will make learning to use this thing a breeze.
All the short cuts to *contextual* help.
How to stop tools de-selecting the stuff I preselected before clicking them and then complaining that I haven't selected anything.
And how -- if I really must activate the tool and then make the selection it will affect -- to constrain the things it highlights as I mouse around to only those things that it will accept for this tools. (Rather than everything but.)
But no:
SEVENTEEN MINUTES AND THIRTY FOUR SECONDS IN, AND HE'S NOW GOING TO TELL ME, THAT HE TOLD ME, WHAT HE ALREADY TOLD HE'S GOING TO TELL ME.
Back in the days of those incredibly expensive physical attendance seminars and training courses; the old "Tell them what you're going to tell them; then tell them; and then tell them what you've told them" made some sense:
For the attendees: The three times of telling acted as some level of knowledge reinforcement.
For the trainers: It stretched four hours of material to twelve. Add in a couple of long lunches; an orientation session; and a few 10 minutes Q&A that "We'll get back to you on" And you've a 3 day course that you can charge $1000/per seat for.
But in a video where I have the choice to watch over, step back and forth, pause. IT MAKES NO SENSE AT ALL.
And now: "A project is a container" It holds things. Oh. Now he's gonna tell me about pdfs, videos and text files Wow. Onshape can hold and display them all So clever..
...
HWM-Water Ltd
It took me maybe 3 minutes to work out that I needed to (and how to) define the cylinder mate before I could apply the screw mate to it. (The plethora of mating points displayed on a threaded cylinder with flats is bewildering, but ... I worked it out.)
I love the power and potential of this place/app.
But I had to draw that axle and nut in a different app, and then go through the dance of converting it from that app's native format into a format that this place would upload without throwing away half the information -- which involved segregating the 21 elements in the assembly I constructed in the other app into separate components, converting the format, importing them here, renaming the to sensible (non-autogenerated) names and sticking the whole assembly back together.
Obviously, that is a long-winded and laborious process that I'd much rather avoid. But it was much quicker than trying to construct one component of the assembly here.
[More long-winded ranting ellided.]
I'm obviously not gonna change anything by ranting further; so I'll mark a post above as the answer and go away.
Normal service will be resumed shortly
Then "Sketch mode", then click the plane and the sketch plane sections the shaft at that plane.
I can now draw the thread profile on the central axis of the shaft -- where it needs to be otherwise it would be distorted by the subsequent rotation.
When I exit the sketch mode, the curves become a surface; I select that surface.
I now click the pull tool; alt-click the axis of the shaft to indicate the axis of rotation; check the "revolve helix" checkbox, enter the pitch 1.25, and drag.
Done. The shaft now has a accurate M12x1.25 thread along its length.
Hm. I suspect that there is more in there that I do not understand, than do ... but
If I draw the thread profile on a plane that is orthogonal to the axis I wish to rotate it about ... doesn't that present a problem? Surely the profile has to be in the right plaec in order for the revolve to 'cut' the thread at the right pitch and depth?
The only way I can envisage that working is having drawn the profile in the wrong place/plane, I then have an (probably several) extra step of trying to manoeuvre it into the appropriate position before I can revolve it along the helix to cut the thread.
And beside being an extra step; as the only correct position is in the plane of the shafts axis of rotation, I've just deferred the problem of locating the axis of the shaft.
What it doesn't show me is how you align your sketches to the right place; which is the bit that defeated me.
for the first sketch, make the centre coincident with the origin. It will then work
IR for AS/NZS 1100
By that I assume you to mean. If I set the centre of that axle face about the origin in whichever of the coordinate planes I chose for the sketch, then once the axle is extruded, one of the other two coordinate planes will (coincidentally) happen to coincide with the plane I need.
(But what if that isn't convenient in a more complex drawing?) No matter, I eventually discovered that clicking the arrow next to the offset field in the plane tool dialog, and so I can select of of the axle flats and then offset it back by 5mm. (How dumb are you I here you not say out loud.)
Problem:I did what to me was the obvious thing. As the default setting is 25mm (wny?) and that leave the plane floating in front of the selected face, I typed "-5". Obvious. Not!.
And then, it pops up the ever-so-helpful tip: "Enter a value between 0 and 500000. Current value is -5". (Quite why it feel the need to inform me of the current value when its 5 pixels to the left highlighted in pink I'm not sure; but that's not the problem.)
The problem is that helpful tip completely obscures the 'reverse direction' button -- that it doesn't mention. And it helpfully does so again the next time you cursor over or click in the entry field.
(Fix: Don't bother telling me what I already know "the current value is -5"; tell me "To offset in the other direction, click the button with the arrow on it."; And don't pop the error up to obscure it.)
Moving on. Konstantine's project uses a simple equilateral triangle with one corner made coincident with the circumference of the shaft. Fine for a quick demo, but obscures the next problem.
To define an standard Metric male thread, the root has to be radius'd; and the crown has a flat, which mean the corner of the Eq.Triangle is offset up and to one side of the surface of the shaft.
A picture of the steps of me doing this in OtherCAD: (Note: I'm not promoting that otherCAD, nor even the way I do this in that OtherCAD; it's just the one of the 20 or so I tried that works closest to those I used 20 & 30 years ago, and comes closest to working the way an (ex-ex-ex) profession Draughtsman thinks.)
That very first step of constructing a point in space that must be positioned exactly relative to the corner surface of the shaft defeats me in OS.
I need to draw a line a specific length at a specific angle (from a specific point; but I guess that could be constrained into position later (though why eludes me?))
I've seen in videos where people draw lines of random length and orientation and then constrain them to length and (vertical or horizontal) position (how is that easier or better than drawing it in the right place first time eludes me?).
I looked for a "line angle" constraint, but if it is there, it eludes me?
(Having posted the image I noticed I used sin(60) in the length of that first construction line when I shouldn't have. It should be P/8 not H/8.)
Basically I need to know how to construct the above in OS?
Does dimensioning between two lines at an angle give you what you want?
IR for AS/NZS 1100
It will autofill the options for you.
IR for AS/NZS 1100
HWM-Water Ltd
https://cad.onshape.com/documents/27f4ca0110b2c794c62e67c2/w/5955a1e46aa05b0d87e049e5/e/d4c1f249581c0be123cfde2e
Same public doc as above...
@me_here910, everyone in this forum has bent over backwards to show you that your problems are nowhere near insurmountable, but you seem determined to find every possible (if not necessarily practical) flaw in the OS interface. I have two potential courses of action for you.
Option 1: Stop using OS like a QA. I know it's QA's nature to find fault, but considering that the vast majority of OS users have found its interface to be sublimely intuitive, maybe the problem is you and not OS? Just go with the flow. Is something not on the screen where you think it should be? Well it's there somewhere, so just look around a bit. Stop trying to hammer a nail with a screw driver
Option 2: As I mentioned earlier, you can't please everyone all the time. If OS just isn't for you, maybe you should stick to FreeCAD or AutoCAD or whatever it is that you find most intuitive. I go back and forth between Onshape, SolidWorks, Catia, and Creo depending on the project. They all have their eccentricities, but they all make perfect sense in their own way. Every CAD software out there is just a tool, and OS isn't necessarily the tool for you.
Its "too frustrating" in the
In a nutshell; I see the power and rolling improvements to OnShape; and know that -- if I could only work out how to do the simple stuff -- a community account here would give me everything (and much more) than I need.
For the (much, much) longer version, look inside:
I was immediately at home in it, because whilst is supports direct modelling, it hasn't thrown the baby out with the bathwater. Ie. Has a grid & snaps and tells you where the cursor is in world coords. It allows you to specify the length and angle of lines as you draw them.
As someone who started out as a draughtsman -- big board,pencils parallelogram rulers et.al -- this idea of drawing a a bunch of random lines and the adding dimensions to size and angle them (that I then have to delete because they clutter the drawing and get in the way; or because lines I was forced to attach them to were only construction lines anyway) ; then constraints to tell vertical lines they are vertical and squares that they are squares, just seems ....
It feels akin to a decorator throwing a tin of paint on the floor of the room, and transferring it to the walls using his feet, and then having to run round the whole room again to clean up all the splashes and overspill.
Or writing a letter by throwing a bag of scrabble letters on a table and then manouvering them into the right positions to construct the sentences. OnShape isn't "too difficult"; just very different. And (to me, at this point) arcane. With a user interface that is:
Volume is not substitute for clarity!
(And webinars on "Basic sketching" that get 32 minutes in before a single line is drawn... or demo vids where the producer randomly pulls focus back from the laptop screen to show an interesting shot of the demonstrators right ear; almost invariable just as he moves the mouse and clicks on the icon that performs the magic...)
Buk's corollary: Don't mix marketing with tech. support.
The question you might have asked is: why do I persist in trying to get to grips with something that lays me bare to ridicule and derision?
Because I see the power of the package. If only I can get over the dual frustrations of
a) trying to find simple instructions on how to do the simplest of things:
Draw a line of the required length, at the required angle; in the right position.
b) My exasperation at the design philosophy that, instead of two clicks and two data entries to position a line exactly where I need it,forces me to:
- draw it at some random position
- then add a second unwanted line somewhere in the vicinity;
- then constrain the second line to horizontal or vertical so that I can
- use it as a reference to set the angle of the first line by
- creating a dimension I don't want, that clutters the screen
- and will get discarded anyway when I discard the second line that I didn't want either
I've been hoping someone will explain why doing 5 times the work to achieve the same end result is a good (or "modern") thing to do?I just checked my download history. In the last couple of months I've installed (and usually quickly uninstalled; in no particular order) SketchUp, SketchUp Make, Sculptris, Blender, ModellingPE, EMachineShop, DesignSparkMech, LibreCAD, OpenSCAD, FreeCAD, BRC_CAD, BabaCAD, QCad, GrabCAD, SolveSpace, SpaceClaim, RhinoCAD, Solid Edge Student; AnsysAcademicStudent.
And tried a few other online CADs Tinkercad, Fusion 360.
Some were easy to dismiss as they are inherently 2D, or 2D+.
Some because they are intended for a different audience -- artists and finger painters using touch-screens and mobiles.
Some I couldn't qualify for the registration.
Some failed to install.
Basically, It's been 15 years since I did hands-on CAD and a lot has changed. I'm persisting to use several of the above, including OnShape, in rotation -- moving on from one to the next when the frustration level rises enough -- in the hope that one of them will start to make more sense than the others.
At the moment, that is DSM, but I am very aware of the restrictions -- only meshed output without geometry (ie. STEP/IGES output) -- but I know that if it makes sense to me, and I am forced to shell out for a solution, SpaceClaim or SolidWorks are from the same family and probably won't be too dissimilar.
But if I could get to grips with OS, a community account here would give me pretty much every I want and save several intermediate steps. If only it wasn't so ... um ... different.
I'm very grateful for peoples attempts; but they mostly miss the point.
Take my initial question. How to position a rectangle relative to a circle that it cuts it in the way I need. Answer: use a centre point circle and centre point rectangle and anchor them at the origin.
But that only works if the origin -- the single, solitary snap-point the interface deigns to provide -- is a convenient place to construct this minor part of the bigger picture; and then only if the intersection needed happens (conveniently in this case) to be symmetrical
I'm not unhappy with anyone's responses to me -- they respond, I'm (for the most part) grateful
Whatever frustration I've allowed to show is with the system, the (IMCO) illogical philosophy behind it; and my inability to discover the solutions to even the simplest problems I encounter using it
I've taken the time to lay it all out because I REALLY want to understand it.
And, I learnt a long time ago, unless you describe the problems you encounter, nothing will change because the developers assume everything must be hunky dory.
And unless you describe the difficulties in detail; they get dismissed as "newbie" or "stupid guy" problems.
Of course, the problem with laying them out in detail, is that most readers don't get beyond the first 5 lines.
I understand (the mechanics of if not the philosophy) of the method.
I had looked for a "project to sketch" or similar option, but the "intersection" tool is displayed in that position; and in any case "Use" probably wouldn't have rung the bell.
I followed your driving instructions until I reached this point:
I wanted to draw the blue line above horizontal, but the dialog was in the way (I know it is movable) so I dropped it where it is and then selected it and clicked the "horizontal" constraint. And...the line disappeared!. Completely. No errors or warnings. Just vanished!?
I have no explanation.
But, I thought I would move on and deal with that later, so I selected the tangent arc tool (the radius of the root arc is defined by it being a 120 degree arc tangential to the two lines at 60deg where the lower horizontal line crosses them.
But, trying to place the end or the arc at those crossover points doesn't work -- they aren't seen as snap points. (You had a similar problem in your vid.)
So then I though that it must be because the lines I had drawn were not construction lines. So I multi-selected all the lines and right clicked to switch them to construction ... BUT THE OPTION IS NOT ON THE DIALOG! Instead I need to de-select and then select and convert them to construction one at a time.
Except now it won't let me convert even individual lines to construction; the option simply isn't offered? Bah, throw the sketch away, start from scratch, converting each line to construction as I go. What a faff.
Okay, now I try to position my tangent arc at those crossover points -- BUT STILL IT WILL NOT SNAP to them! Any attempt to position the tangent arc on those points causes the arc to snap to the end of one of the two lines.
Then I looked back at your vid and realised that you weren't snapping the ends of your 3-point arc to the crossover point, just clicking in the rough proximity to them!
The reason I used a tangent arc is doing so ensure means that I don't have to calculate the radius of the arc; there is only one radius that is tangential to both faces at the (carefully positioned) horizontal crossover point. But, if the darn software won't left me select those two positions -- which are the ntire reason for drawing the lines in the first place...
Right now, I'm frustrated, embarrassed and angry and I'm gonna kick OS to touch for another month until the pain of trying to use this bloody interface subsides enough for me to try again.
Thank you for your attempts to help me.
Buk.