How do I create a 3D archtop guitar body part ... multiple issues embedded

mark_casey

I'll try to make this short by stating the end goal and the resources available to me to get there.
I'm trying to create an archtop guitar body part based on a set of purchased plans.  For those that don't know, an archtop has a very complex curvature for the top and back.  I want to create a part that I can ultimately use to drive a cnc mill to carve the top and back and use as a basis for further modified tops/backs.

So, I bought a set of full scale plans in .pdf format.  The intent of these plans is to create full scale templates for tracing onto wood.  There are many stated dimensions, but not for the basic curves of body.  I am able to convert the .pdf into a .dxf.  However, when I try to import it into a sketch, I get a "too many entities" error.  I've stripped out all but the body outline (and editing the .pdf is a pain) and that seems to work to allow import into a sketch, but I am limited with that sketch.  Even then, the sketch shows the body as hundreds of connected points rather than a single smooth continuous curve.  

Taking a different approach, I was able to import the body and contour lines as a single dwg and attempted to use that as a basis to trace the body and contour lines using the spline tool, but I'm left with splines that I can't seem to convert to a single curves.  My intent was to create the body outline and the subsequent contour lines on different planes and then loft between them to create the complex archtop shape.  I can't seem to do that with the spline based tracings.

So, I'm stuck.

If anyone is at all interested in my struggles, I can share the .pdfs or .dxf files for you to play with.

thanks,
Mark
mjcasey@yahoo.com

Jason_S

Can you share you document URL here? Just copy what is in the address bar and paste it here so we can help!

mahir

Try the composite curve feature. It will let you combine multiple adjacent lines/curves into one curve you can use for lofting.

mark_casey

https://cad.onshape.com/documents/c35bf3e197b981efa0814028/w/0fe67074dcf3c0b665d052ee/e/a8ded801d2ac0ac7d0874a8b

Its a bit of a mess as I've been experimenting.

mark_casey

I had tried, but I can't seem to get the composite curve tool to work as it seems it should.  I keep getting an error.

mahir

I took a look at your doc. Looks like there's no easy way to convert your DXF to a spline because everything comes in as a bunch of line segments. Taking a different route, I was able to import a picture of all the curves and size it using the imported outline DXF. This reference image can now be used to create your own splines at different elevations. Looks like you have over 20 iso curves, so it won't be a quick process. Below is a link to my document. It includes the outline and first iso curve. Repeat the same steps (offset plane + sketch w/ traced spline) for each iso curve, and you'll have all the profiles you need for your loft. Don't forget to make the final profile a point at the apex of the "hump".

https://cad.onshape.com/documents/832a7690c58d9a18f29e31f9/w/a651ab765f4a7387e14d4f9d/e/31c39df5fa616b63458c31e8

mark_casey

THANK YOU!

mark_casey

So yes the spline process is tedious, but it does result in curves.

As a test, i created a composite curve of each of the outline and the first contour (I figured out why composite curve wasn't working for me) that you created.  Shouldn't I be able to loft between the two?  When I start the loft command, it won't allow me to select either curve as an a loft profile.

Also, would you mind sharing the steps of importing and scaling to get the appropriate sized part?  That is not clear to me and I can see using it quite a bit in the process.

thanks again.

mark_casey

Here is my current version based off your (mahir) work.

https://cad.onshape.com/documents/32163d7652b7cbaf34e192d2/w/13d059c3cbf186154a1632ca/e/2591166cf15dc3bd4b4dfa42

mahir

Here are my steps to get the image into OS

1. Insert a sketch with some sizing geometry. In this case it was the outline from the DXF.
2. Create the image file you want, preferrably with dark lines on a white background.
3. Import the Image file into OS.
4. Open a new sketch and insert the image file.
5. Dimension and locate the image so that it overlaps your reference geometry (guitar outline).

I don't think you need to create a composit curve. If you're creating a solid loft, you can use sketch regions instead of curves. The hiccup I came across is the corner in the guitar outline. OS is having trouble mapping the very different shape of the outline to that of the contour. After creating all the contours on separate elevation planes, you may need to create guide curves by running a Fit Spline through a couple common points in all the contours. If you had a couple side profile views in your PDF drawing you could have just traced these guide curves. It would have been much simpler than making 2 dozen iso contour curves

mark_casey

I do actually have side profile section views.

See below. 

Of course, I have no idea how I would use them to create the part.

mahir

Check out my doc linked above. The loft is failing due to some strange behavior. @ilya_baran or @kevin_o_toole_1 maybe you know what's going on? I created some dummy contours using simple offsets of the first contour. I tried creating the loft using matched vertices and guide curves. I even created an intermediate contour that doesn't include the corner in order to blend that portion correctly. But the base contour keeps trying to loft in the opposite direction. Can nyone from OS comment on why the normal for this profile seems so wonky?

ilya_baran

Interesting case.  I imagine that the problem is that the profiles are different enough that matching parameterizations is hard -- you may need to help it by splitting the profiles up into pieces and doing the lofts individually.  But @elif is our loft expert -- maybe she has a better suggestion.

mahir

I'm used to vertices not matching well automatically, but I've never seen a loft try to connect profiles in the complete opposite direction.

elif

a couple of things I can see right away - as ilya said the outermost profile has a very different topology and it needs to be split to match others for best results. (or you can create a loft between sketches 1 through 4, i.e. exclude the outermost one to get an idea of what's happening)

Another thing is the usage of guide curves, I'm afraid we might be trying to do something fancy with the propagation of the 3d spline as a guide curve. You might be better off doing vertex matching only. 

So if you remove the outermost profile, and use vertex matching instead of guides you should at least partially get to what you're trying to do.



It would be great if you could file a bug with regards to the guide behavior so that we can look into it more carefully.

mahir

The first profile still misbehaves with only vertex matching. The guide curves were an attempt at fixing that, but they only helped locally. I was just trying to help @mark_casey, but I'll try to get around to logging a bug.

mark_casey

Well that escalated quickly...


Maybe the shape is a little too "organic".  It is meant to be hand carved, but I do know folks have successfully used CNC to carve them.

Thank you @mahir for the link.  It looks very promising.  I'll keep playing with it.

I may be too much of a novice at CAD and certainly at OnShape to tackle this one, but I'm going to get there one way or the other.

Thanks for taking an interest in my project and I welcome any additional feedback.

mahir

I feel like you might be better off importing all the sections as reference and using Section F as a guide curve and the other sections as loft profiles.

homer

u better take this as reference:

https://cad.onshape.com/documents/c1aa5bba6a0139ebb12ce0bd/w/7dbd79899d296fb290aca0f0/e/221fb58cf7a7524ff4d663aa

http://https//formlabs.com/blog/designing-a-3d-printed-acoustic-violin/

mark_casey

Great resources.  Thank you.

brucebartlett

Hi Mark,

You might want to look at @william_chelton 's Guitar. Looks like he tackled the arch top geometry a number of lofts, not just one.  https://cad.onshape.com/documents/2b44aab91ac41d6cd4b6b5d1/w/3a0372585e6b5f19c9345ef8/e/806ee39592023f4df664e91b

brucebartlett

Here's an imported model, (does look like it's done in Onshape) but it also appears to have the top made from surfaces in a similar fashion to the last example.
https://cad.onshape.com/documents/2242b8898166583ba40c4ef6/w/9c6fb428ba24cc6146f9651c/e/5dd2f707f7c383f23a7d8dd5

mark_casey

Thank you.  All helpful.
I'm trying a new approach as well.  I have cross sections.  I'm going to try and use with intersecting extrusion or lofts between them.
We'll see how that works.