JOIN command not working in UCS / OSNAP inaccuracy

[jw-2024]

I am trying to join the two diagonal lines pictured here into one polyline.  I have redrawn these two lines several times (snapping to the endpoints/intersect points of the grid) and find that while the PEDIT command works consistently, the JOIN command has worked on only very rare (albeit some) occasions.  I would like to understand why this inconsistency is occurring.

I notice when I zoom in extremely close that the endpoints of the lines are not actually touching, but I am using OSNAP to the intersection of my grid and the LIST command says the two lines share a common endpoint.

I have found some work-arounds for these particular lines, but they are not sustainable in the long-term if working with many lines across a large file.  These little “cheats” were:

• Draw the lines in the WCS, rather than using the UCS I have designated aligned to the grid.
• Go into the properties of each line (which already display 0 to the eighth decimal place as the Start/End Z value) and manually type in “0” to reset the values.

Things I have tried that have not worked:

• “Replace Z value with current elevation” option enabled
• “Ignore negative Z object snaps for Dynamic UCS” option enabled
• FLATTEN command
• OSNAPZ = 1

I am new to AutoCAD (but have experience in similar programs, like Rhino) and would really like to understand how the program is treating OSNAPs, Z plane, and lines to be able to work with it more efficiently in the long-run, rather than relying on spot-fixes that diminish my trust in my model’s accuracy.

Thank you for any insights you can provide!

[J-Porter]

@jw-2024, I downloaded your drawing and the JOIN command works for me every time.  I'm unsure why your experiencing otherwise.  Has this been an issue in past versions for you?  I'm using 2024, but I also tested it in 2022.  And I tested it numerous times.  Do you have all updates installed for your software?  

As far as zooming in and seeing they're not connected. . .that's just a graphic card glitch.  If you've snapped to the endpoint, it's definitely on the endpoint.   

[jw-2024]

@J-Porter Thank you for your reply. I am a new user so I have been using AutoCAD 2025.  That said, I just downloaded 2024 and still cannot join the lines (I'm using a Windows PC).

I figured the endpoints looking off was a graphics limitation, but it is odd to me that the JOIN operation does respond when I manually reset the z-values to 0.

[pendean]

@jw-2024 If you see a gap, fix it with FILLET command before you start JOIN. Or as you discovered, PEDIT is better at the task. Always know the other ways to get something done. And always look things up in HELP in the program, in this case notice how their example does not look like your objects

Sometimes as a user we have to make an extra effort to help things along in AutoCAD to stay focused and productive, it's not perfect and neither are we using it.

If you wish to alert Autodesk of a flaw in the program or command, open a support case with Autodesk if you are on subscription:
https://www.autodesk.com/support/account/manage/use/support#:~:text=com/plans.-,Contact%20support,-S... 

[tramber]

My way : selecting objects. The blue grips can turn red as you click them again, right ?

A grip in common will show that it moves the 2 objects. If not, the points are not the same.

It is often my first step to analyse a joining issue.

Grips can be useful. They even stick in one direction when you are on ortho... but it is a secret.

Well, if the 2 grips come, following your movement, but it shows an empty gap, then, it can be a graphic card problem.

[AllenJessup]

@jw-2024 I don't have any problems in 2024 either. I tried the unnamed UCS as well as the named ones. Joined every time in both directions.

In coordinate systems near zero the program can go out past 8 decimal places. So it may have something off out there. I rarely use a UCS so I'm not too experienced with them.

A couple of tricks that have worked for me when the Lines won't join.

Try selecting both lines, pick up the end of one with the grips and don't use an OSnap. Let the active grip pick up the inactive grip on the other line.

Filleting the lines with a zero radius almost always works.

[jw-2024]

@pendean I did see in some older tutorials online that JOIN was for coplanar, collinear objects.  So it seems maybe some of that legacy has stuck around even as the JOIN tool has been expanded to allow joining of non-collinear polylines.  Strange it behaves inconsistently across machines and I wonder if there is some setting I have different on mine than other users here, but it is comforting that PEDIT is reliable.

@tramber and @AllenJessup: Thank you for the tips.  I have tried working with the grips and my lines do share a common grip, so they are actually occupying the same endpoint.  Good to have some diagnostic/troubleshooting options for the future if PEDIT ever fails too.

[J-Porter]

Everyone, I think we're missing the whole issue here.  It's obvious that jw-2024 knows the lines need to be co-linear, co-planer, and need to be endpoint to endpoint (and with same start Z-value and end Z-value).  I looked at the example drawing provided.  Why is that jw-2024 can SOMETIMES join the same exact lines that the rest of us can join every time?  

[pendean]

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@J-Porter wrote:

...Why is that jw-2024 can SOMETIMES join the same exact lines that the rest of us can join every time?  

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Ask Autodesk, we are all end users just like you.

@jw-2024 PEDIT (and MPEDIT if you wish to explore) are 100% consistent for what you need as you confirmed and everyone who's ever used AutoCAD for 30+years can confirm work all the time too.

We can also open up the whole world of LISP automation to speed up the core tools and commands (which few if any will write up using JOIN command BTW) to do what you wish even faster 🙂

Best wishes.

[AllenJessup]

@J-Porter Without being able to duplicate the error in jw-2024's drawing. I can't see how to do much more to help.

[J-Porter]

@AllenJessup, I agree. I was just making sure we weren't offering up answers to focused on the "how to" of the JOIN command rather than attempting to help figure out why it's happening.  

@pendean, correct in my long history of using AutoCAD (30+ years), there's no telling why some commands work some but not others.  

[jw-2024]

Thank you all for your timely and thorough replies!  I will chalk this up to a quirk in the software and go with the commands/methods that are working reliably without getting too hung up.