Tracing over images.. is Onshape really bad at this or am I?

andy_arbon

I'm hoping it's the latter..

I am a relative beginner at Onshape, but I've had some success at designing models for 3D printing. I am currently trying to model something with complex surfaces that curve in multiple planes, so I took some photos of the object from multiple sides, against graph paper, and I've been trying to extract useful reference points from them so I can approximate the object (I'm not trying to recreate the object; I want to create a dock to hold it).

So, I create a sketch, import an image and draw a single construction line. Once I dimension this the image scales to match - so far so good. After this is where it seems to get insanely complex. Is there any way to just tell onshape to preserve this scale for the image? Because as soon as I do anything else to it, like move it or rotate it to get it aligned with the axis etc, my scaling seems to go out the window and get lost. The best I've come up with is to then dimension the sketch itself using its 'natural' dimensions, locking its size. This seems a bit clumsy but solves that particular problem.

The next issue, which I haven't been able to resolve, is how to trace curves etc from that image and fix them relative to each other without creating a billion undesired construction lines and giving them all dimensions to fix things relative to each other - this always eventually goes wrong when you some how end up overly constrained and can't work out which of the 2000 accumulated lines and constraints might be causing the problem.

I am trying to get my sketch on one plane to correlate with a sketch from another by projecting a fixed point from one to the other and applying a coincidence, but I cannot get this to work without my sketch turning in to a firework display as soon as the final constraint goes in (or possibly more annoyingly, finding that the entirely blue sketch, which Onshape complains is not fully constrained, is also somehow locked in space and can't be moved).

Onshape is excellent and I can't believe that the relatively simple(?) concept can be implemented this badly, so presumably I'm missing something. Can anyone give me any pointers in the right direction, please?

Thanks,

Andy

Matt_Shields

I think you would learn a ton about best practices by taking a couple of Learning Center courses. In this course, they do a couple examples similar to what you are trying to do.

andy_arbon

Thanks. That sounds like it might be useful. I've watched a few youtube videos but they haven't gone in to anything as complex as matching multiple sketches on multiple planes.

I am having an issue with step 6 of that course though. I have prepared the front plane as per instructions:

And the right plane:

However the slides say:

But when I try and create a coincident relationship I get the error 'Some of the entities could not be used'.

The wording of the instruction seems odd.. it 'is coincident', not 'make it coincident' or similar. Am I missing something?

Matt_Shields

That wording is a little awkward. You want the bottom edge of the second sketch to be coincident to the bottom edge of the first sketch. Both circled in red below.

andy_arbon

I get that that's the aim, but am I supposed to be able to apply a relationship to enforce that, or are they meaning to just visually line it up? The diagram shows a coincident constraint on that line, so I'm assuming it's the former, but when I try I get that 'Some of the entities could not be used' message.

andy_arbon

I have tried this every which way I can think of, I've been right back to the beginning and worked forwards, I've tried drawing explicit lines on the first sketch to project in to the second (even though the tutorial doesn't indicate this should be necessary) but no matter what I get 'Some of the sketch entities could not be used'.

I can line it up visually, but that's not what the tutorial shows and would be a bit naff for professional engineering anyway so I can't imagine that's what's intended.

https://cad.onshape.com/documents/591bde193c956df26a74eea8/w/d2698eedc51774ecefe71006/e/7708fbaf5d592805ee8e1d65?renderMode=0&uiState=67d2a4ec386e4037ddb6c9ed

This is where I have gotten to, if you can see how to make the coincident relationship appear please let me know, but as far as I can tell this tutorial must have been valid for some previous version but is now broken.

This probably has helped slightly with my original question because I think I'm getting the message that Onshape wants you to get all the imported images scaled and aligned with respect to each other before drawing anything on top of them, rather than tracing the image and then moving the geometry in to alignment which is what I was doing that wasn't working.

Ste_Wilson

I'm just looking at my version when I did the that, I don't think I have done anything different to you. The dimension constrains the sketch horizontally The constraint to the top plane to position it vertically, not to the other sketch. I think.

andy_arbon

OK, I had the brainwave that I could search one of the thousands of public implementations of that exercise and look at how they did it.

Here's one example. This person created a coincidence between the line of sketch 2 and the corner of sketch 1. Why this is allowed and a coincidence between the two lines isn't, I have no idea, but I've just tried it in my version and it does work. Apologies if that's what you were trying to tell me, Matt, I can see you did circle the corner of sketch 1 in your screenshot.

The wording of the example could be improved though, it's definitely not obvious why one of these should be allowable and the other not.

martin_kopplow

To make things a little easier, it could be a good idea to start this kind of task by creating a centercurve for the grip body and two planes (or mate connectors) for the end faces, which rest on the end points of the center curve. This would provide a flexible yet reliable base for all other curves constructed on top, while at the same time making management of coincidents way easier, mainly by reducing the number of helper entities required and by reducing clutter. This one is a quick and dirty demo without an image to trace over, but it could have been just as well:

https://cad.onshape.com/documents/848fec8c14a567d9d9e7d956/w/54b3dd2b55dd2528906d3299/e/2314715689349302c05e604d?renderMode=0&uiState=67d2e283fef41029b55cd66c