Help with a loft please.

bitingmidge

Hi, I really don't get the concepts of lofts mostly!   As soon as they get a bit complex I spend days trying to figure out what went wrong!

This time, I'm only a day in so thought I would put up my hand.  I am working on a measured drawing of an historical boat model from the original (shipwright's) loft.

I effectively have a wire frame (I apologise for the size of the image!) that's complete except for one or two points which are microscopically misaligned and I'm doing my  best to track them down.  Even so, a loft should work between two or three frames, perhaps with the waterlines as guides, shouldn't it?

I'd really appreciate some advice as to how to go about troubleshooting, and giving me a direction to head in!

Thanks!  

MichaelPascoe

Each sketch guide must pierce the loft profiles exactly. Usually, I use the Pierce constraint for this. If the guide is not touching the profile exactly, it will fail the loft. This is a tedious process to learn, but once you learn how to constrain these sketches, your lofts will not fail in this way.

S1mon

As @MichaelPascoe points out, there are many places where pierce relationships are needed. After fixing the end points of the "Profile Transom 12in" with pierce relationships, I was able to create a fill surface. I could use the various waterline and profile curves as guides, but it ends up being a very lumpy surface.



Unfortunately, the standard spline is a terrible tool for doing this kind of surfacing. I would highly recommend using Béziers.

Here's the curvature plots of the waterlines:


Here are the profile curvatures (with all the sketches hidden):


The basic splines are curvature continuous internally, and with a ton of careful tweaking you might be able to create something that's not so lumpy, and with out the random negative curvatures, but starting with Béziers, will be way easier.

Here's an example of Profile 10inch:



It will be more complicated to make every profile and waterline intersect at every cross curve perfectly with a bunch of independent Béziers, but you might be better off with lofting or filling a surface with fewer internal guides. In my redrawing of Profile 10inch, I only aligned the end points and approximated the internal points.

It may be cleaner to only have one or two internal waterlines and use all the profiles. Or perhaps, use all the waterlines, and only one or two internal profiles. In general you want to use as few constraints as possible to get the cleanest surface possible. The more dense (and imperfect) the guides, the lumpier it will be.

Also, from the size of the model, it looks like this is literally a model boat, not a full size reproduction, so more than likely there will be a lot of hand sanding/finishing. To some degree, you could ignore a bunch of this if you're going to clean things up manually in the real world.

S1mon

If you can make your document public I’m sure we can help.

My first thought is that all of the sections you’re using might not be intersecting everywhere. Perhaps a fill surface with all the interior stuff set as guides with “precise” unchecked would give you a cleaner surface.

MichaelPascoe

Those sketches look very clean. We can get that to loft for you.

@S1mon has the best approach, surfaces will be the way to go on this. Either Loft or Fill one side at a time: left, front, right, back, top, bottom. Then use the Enclose tool to enclose all of the surfaces. If you get stuck, or want more detailed help, share a link and make the document public.

Or, you could do a solid loft, but this will require a careful selection of guides and connections.

dirk_van_der_vaart

You could try this free trial
https://appstore.onshape.com/apps/Design & Documentation/BI4NF3BZAEQXBXOERCYO6NSHJ5Z62TB6ZI472VA=/description
Or this featurescript
https://cad.onshape.com/documents/b8730d44070bc0466ad80444/w/b111d39908fbb971f1521124/e/4d25c05bc32c9ea6f74209d6

bitingmidge

Thank-you so much for this Simon - the document is public and the link is HERE (I apologise for leaving it out in my excitement!)

I feel that I am oh so close to a solution, but every time I try something to get that last few mm it all collapses in a heap again, it's exactly at the point where I give up or find a workaround, but I really want to learn this bit.

I haven't found the "precise" checkbox - am I staring at it without seeing perhaps?

This loft was from frame to frame - with the Deckline (bottom horizontal in the picture) as a guide.   I have no idea if an why it decided to curve itself along the length of the model, but I'll take it. - however it's not actually closed at front of  the keel (top of pic) or at the front at the deckline.   (I'll make the screenshots a lot smaller this time!

bitingmidge

S1mon said:

My first thought is that all of the sections you’re using might not be intersecting everywhere. 

Following my reply I didn't mention this - I think I've checked them all, but there are a lot of intersections and I seem to be chasing my tail, whenever I see an error I can understand I go after it!  (if there's an obvious red node for instance).    Cheers!

S1mon

I’m suggesting using a Fill surface instead of a Loft. Fill will give an option to approximate internal guides if you select guides. This will be a lot more forgiving.

bitingmidge

Hi all, I apologise for the lack of response - the messages have sat as drafts for a couple of days!

S1mon I've still not discovered the "precise" box to uncheck!
 

Thanks MichaelPascoe  I think I've managed to break it in just about every combination of fill - I have learned in the past that surfaces are much les problematic than solids,  so I haven't even tried solids for this one yet.

The link IS HERE -  I have no idea why the delay in posting!

Thanks dirk_van_der_vaart too.   If all else fails I will try Phi - but I can't afford a subscription and would only have occasional use in the long term so would like to resolve this in a way I can use on the next one.   I have installed the Sculpt Surface script thank you, but have had no luck.   

Some time later in the week when I am less dispirited  I will once again check every node and intersection point, "pierce" everything again, and no doubt will find one that has gone astray.

I would appreciate it if any of you have the time to solve my puzzle though!
Regards!

bitingmidge

Hi S1mon - I've tried every flavour of Fill surface, I think -  no errors show other than in the title bar which makes it harder than usual to guess!

 -

MichaelPascoe

Each sketch guide must pierce the loft profiles exactly. Usually, I use the Pierce constraint for this. If the guide is not touching the profile exactly, it will fail the loft. This is a tedious process to learn, but once you learn how to constrain these sketches, your lofts will not fail in this way.

S1mon

As @MichaelPascoe points out, there are many places where pierce relationships are needed. After fixing the end points of the "Profile Transom 12in" with pierce relationships, I was able to create a fill surface. I could use the various waterline and profile curves as guides, but it ends up being a very lumpy surface.



Unfortunately, the standard spline is a terrible tool for doing this kind of surfacing. I would highly recommend using Béziers.

Here's the curvature plots of the waterlines:


Here are the profile curvatures (with all the sketches hidden):


The basic splines are curvature continuous internally, and with a ton of careful tweaking you might be able to create something that's not so lumpy, and with out the random negative curvatures, but starting with Béziers, will be way easier.

Here's an example of Profile 10inch:



It will be more complicated to make every profile and waterline intersect at every cross curve perfectly with a bunch of independent Béziers, but you might be better off with lofting or filling a surface with fewer internal guides. In my redrawing of Profile 10inch, I only aligned the end points and approximated the internal points.

It may be cleaner to only have one or two internal waterlines and use all the profiles. Or perhaps, use all the waterlines, and only one or two internal profiles. In general you want to use as few constraints as possible to get the cleanest surface possible. The more dense (and imperfect) the guides, the lumpier it will be.

Also, from the size of the model, it looks like this is literally a model boat, not a full size reproduction, so more than likely there will be a lot of hand sanding/finishing. To some degree, you could ignore a bunch of this if you're going to clean things up manually in the real world.

bitingmidge

Hi S1mon - I've tried every flavour of Fill surface, I think -  no errors show other than in the title bar which makes it harder than usual to guess!

 -

S1mon

Just pick the edges first. Look very carefully at the two corners where the "Profile Transom 12inch" look like they join the waterline and keel bottom. There will be red dots. That's because you need to have Pierce relationships between the endpoints of the curve in the profile to the curves in the orthogonal sketches. Until you fix that, nothing will work.

bitingmidge

Hi S1mon Thank you once again - I am oh so close, but still no cigar!   Will keep plugging  away every now and then.

I think the nature of the original hand-drawn loft is working against me - as I think you mentioned above - it would be fine if I was just making it out of wood, I could take the minor bumps out of it easily - but the software is not quite so forgiving, and every fix means a break somewhere else.

It's teaching me patience!