Dimensions not correct when exported from OnShape

tim_higham442

After getting the sizes wrong on a panel I created, I made a test object 7.5cm by 8.5cm in OnShape, exported to dxf and viewed it in eDrawings - The dimensions it shows there are 190.5mm by 215.9mm. There's obviously something really basic I'm doing wrong here, but am really confused, if anyone can help please - thanks!

NeilCooke

DXF is unitless and requires the importing system to provide a way to set the units, but it looks like eDrawings defaults to inches with no way to change it (yay). 

You can either set your workspace units to inches before exporting or you can create a drawing with the view of the part (which will import into eDrawings correctly). 

tim_hess427

I just ran into this issue today. I was trying to quote a laser cut part and exported a dxf from the face of the part. The part studio units are mm, but when I uploaded the dxf to the vendor website, it was interpreted as inches. 

I couldn't find a $INSUNITS variable, but I did find $MEASUREMENT based on some googling. Changed a 0 to a 1 in a text editor and re-uploaded. Now, my dimensions are correct. 

https://www.practicalmachinist.com/vb/cad-cam/there-something-dxf-file-header-indicating-mm-inch-units-357982/

EDIT: Also want to note that I uploaded the same original dxf (before I modified it) to two different vendors. One system asked for the units and scaled the file appropriately. The other did not ask and it was wrong. 

NeilCooke

DXF is unitless and requires the importing system to provide a way to set the units, but it looks like eDrawings defaults to inches with no way to change it (yay). 

You can either set your workspace units to inches before exporting or you can create a drawing with the view of the part (which will import into eDrawings correctly). 

alnis

@NeilCooke The DXF format does specify units in the header variable $INSUNITS, but the problem is that Onshape always writes $INSUNITS as 1, representing inches, while the numerical values representing geometry are in the workspace's units, so scaling is only correct when the workspace units are inches. DXFs are only unitless when $INSUNITS is written with value 0.

DXF specification (Autodesk seems to move the documentation around semi-regularly, so I'm also putting in a link to a downloadable PDF):
http://help.autodesk.com/view/OARX/2018/ENU/?guid=GUID-A85E8E67-27CD-4C59-BE61-4DC9FADBE74A
https://images.autodesk.com/adsk/files/autocad_2012_pdf_dxf-reference_enu.pdf

As a workaround, you can switch workspace units to inches, export, then switch workspace units back, and then the DXF should have correctly scaled units.

Edit: looks like I am wrong about the DXF format having units; $INSUNITS does not seem to apply to the whole DXF, but rather seems to be a hint for editors for what scaling to use when inserting additional content into the drawing. Link to the comment in this thread here: https://forum.onshape.com/discussion/comment/70565/#Comment_70565

dirk_van_der_vaart

Never had problem's with exporting from the Part studio, select a face and RMB , select export DXF/DWG.

tim_higham442

Very interesting - thank you for that. Even something as simple as keeping everything as millimetres is not quite that simple! I'll try that workaround - Thanks for your help.

tim_hess427

I just ran into this issue today. I was trying to quote a laser cut part and exported a dxf from the face of the part. The part studio units are mm, but when I uploaded the dxf to the vendor website, it was interpreted as inches. 

I couldn't find a $INSUNITS variable, but I did find $MEASUREMENT based on some googling. Changed a 0 to a 1 in a text editor and re-uploaded. Now, my dimensions are correct. 

https://www.practicalmachinist.com/vb/cad-cam/there-something-dxf-file-header-indicating-mm-inch-units-357982/

EDIT: Also want to note that I uploaded the same original dxf (before I modified it) to two different vendors. One system asked for the units and scaled the file appropriately. The other did not ask and it was wrong. 

alnis

Huh, DXFs from Onshape still show the $INSUNITS variable for me when I export. Maybe it depends on what you are exporting? I also found the $MEASUREMENT variable, but unfortunately changing it didn't make the quoting software I was using correctly interpret the data. I still had to change the $INSUNITS variable from 1 to 4 to get it to get the scaling right (the DXF was exported with workspace units set to mm).

tim_hess427

@alnis_smidchens - That's weird. I did a little more digging and found this post: 

https://forums.autodesk.com/t5/autocad-forum/units-settings-within-dxf-files/td-p/7417774

It looks like $INSUNITS and $MEASURMENT variables apply to different things. Based on the above post, it sounds like $INSUNITS is probably only used when it's needed for some sort of attached object. And it looks like $MEASUREMENT is being incorrectly used by some vendors to guess the original drawing units when it really only applies to lines and hatching properties. 

In the end, it looks like @NeilCooke is right in that DXF is supposed to be unitless, but different implementations of import software use workarounds to try to assume what the original units were - which causes issues when they're wrong. 

alnis

Huh. Very interesting! Looks like I was wrong then! Sorry for not looking into this more before posting!

I guess since it's the "drawing-units value for automatic scaling of blocks, images, or xrefs when inserted or attached to a drawing" it might be reasonable to assume that the scale applies to existing geometry too? I'm not really sure. I haven't used a 2D CAD system before, so I don't know what the standard practices/conventions are. In any case, it does seem to be that a lot of vendors use it for scaling DXFs in their quoting systems.

tim_hess427

It was an interesting rabbit hole for me to dive into on a Wednesday. Learned something new and now know at a trick for fixing DXFs that don't import to things correctly

paweł_kucmus

@NeilCooke I ran into this today and indeed it is confusing. DXF spec https://images.autodesk.com/adsk/files/autocad_2012_pdf_dxf-reference_enu.pdf - page 20 describes `$INSUNITS` (`1` for inch and `4` for mm) but does not explicitly say how to use it.

The end result is that my DXF files (made in a millimeter workspace) have proper values but some reader software read my 500mm as 500inch as all Onshape DXF exports have the $INSUNITS set to `1`. 

If Onshape wants to stick to unitless DXFs that value should probably be set to `0` which the spec defines as `Unitless`.

What do you think?

matthew_stacy

Hmpf!  Lots of discussion, but units for DXF export/import seem baffling and buggy to me.  If my Onshape workspace units are set to mm , at the time of export, then the DXF will scale correctly.  If my workspace units are set to inches the DXF is at 1/25.4 scale.

Goofy.

I would strongly advise importing any DXF files that you export to confirm that the size is correct ... to avoid serious disappointment when FedEx brings your tiny-scale models back from the laser cutter.