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I am learning how to use the Knife tool and it's not working for me. I try to make a straight line by clicking on two points with Left mouse button, but the second time I click it, the line disappears. I am trying to cut a diagonal line in the small box on right.

Please find attached blend file.

I tried to attach a Blender Screencast but it doesn't allow me to upload avi files, could you please tell me how to do it?

PS While you are at it, could you please check why my vertex snap (or any snap for that matter) is not working when holding down the Ctrl key? I am following a video tutorial and it's supposed to snap at all the corners, but it's only snapping in one or two places. Thanks

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3 Answers 3

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The reason why your knife tool is not working is due to the fact that you have multiple polygons on top the same surface area. What I did was ... I made sure I "removed doubles" by hitting the W key and chose "remove doubles". After which the knife tool works.

As for your screencast, the site blender.stackexchange will only accept .gif files. You can use Screen 2 Gif software to capture the gif but it has to be below 2mb , if you want to upload via stackexchange's internal upload interface. Or else you would have to upload your gif to an external site and add the link to your post.

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  • $\begingroup$ I tried uploading gif via stackexchange but it says "you can't upload files of this type". $\endgroup$
    – A Allen
    Commented Feb 5, 2016 at 13:02
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    $\begingroup$ regarding gifs please read: meta.blender.stackexchange.com/questions/963/… and meta.blender.stackexchange.com/questions/404/… $\endgroup$
    – user1853
    Commented Feb 5, 2016 at 21:10
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    $\begingroup$ Can you elaborate on why multiple polygons prevent the knife tool from working? $\endgroup$ Commented May 29, 2019 at 3:31
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    $\begingroup$ @Kyle Delaney When there are multiple polygon, the slice tool is only able to detect one polygon at a time. You may be slicing one of the many polygon overlayed and have the edges not showing up on screen since it's masked by the other polygon recognised by the graphic cards to be display on top of the one you just sliced!!! Which of cos is a problem. $\endgroup$
    – hawkenfox
    Commented Nov 11, 2019 at 13:28
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For me the answer was to follow this procedure:

  1. K to select knife
  2. C to constrain direction
  3. Z to toggle on cut action
  4. LMB to confirm position
  5. ENTER to make cut
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I was trying to cut out the inner radius of a torus and keep the outer radius, but evidently, in between each square face I had to hold down Shift + LMB to place a cut in between each cross-sectional edge. Then just use LMB to add a cut on the cross-sectional vertices.

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  • $\begingroup$ You might be better off just selecting the inner radius loop, and 'V' .. ripping it? $\endgroup$
    – Robin Betts
    Commented Feb 4, 2023 at 18:20

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