Question about tolerances in plastic extruded parts

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Hi. I have a question regarding plastic extrusion and die making. I am designing a new tool for fabricators that will be plastic extruded. Part of the assembly with be flexible plastic (not yet chosen) but will attach together similar to a dovetail joint, used in woodworking. My problem and question is I want to have the joining plastic move freely together but a tight enough tolerance that they don't bind together. Can anyone help shed light on the matter? I would greatly appreciate any feedback since I am new to all of this. Also, to help understand what I am trying to accomplish, look at the attachment and imagine being able to slide the top section freely back and forth within the bottom parts dovetail recess. Thanks!

Sorry for not being 100% on the lingo. I'm a fabricator who is trying to solve a problem. Amateur inventor :)

Check out the following webpage...

[U][B][URL="http://www.engineersedge.com/general_tolerances.htm"]Mechanical Tolerances and Fits[/URL]
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Probably, a class II or III.

Thanks Kelly. That was exactly what I was looking for. I appreciate the help.