HI, i want to create a support that I can add one to the other by sliding each support to the next one in FreeCAD 19 [ATTACH=JSON]{“data-align”:“none”,“data-size”:“thumb”,“data-tempid”:“temp_1195_1611844137808_557”}[/ATTACH]
I’m a little unclear on whether you want to make 2 parts that you can print & slide together, or you want to know how to slide/move things in FreeCAD.
I would use FreeCADs PartDesign workbench for this. I assume you have the block on X/Y plane, you simply create a sketch representing the nodge you want to add or remove.
It may be a good idea to have predefined measurements e.g. in a spreadsheet, so you can change the size of the connectors and both objects (male/female) change in the same way.
Since you seem to want to create a trapezoid shape, you need the length of the short and the big side and also the angle. I would instead got for the distance between the inner and outer line, as angles can be troublesome. You also need a depth or length for the connector unless it should be as long as the box.
There is a fifth measurement you need, which is not as obvious. Tolerance. You cannot print a 10mm hole and a 10mm hole. This will not work. You need to make the hole bigger or the pin smaller. I would define the overall tolerance as 0.25 or so.
Note: You don´t need to define these value in a spreadsheet. You can define the name within the size definition window. For non existing measurements like tolerance just draw a blue helper line and define size and name. I always use the first sketch as my base sketch (naming the sketch is also a good idea, than dealing with sketch004 or such shit.) Naming stuff may seem to be a lot of work, but it pays of in the end. If you later on see a “12.2mm” you have no idea what that comes from. With naming it says “Base.Constaint.Width+Base.Contraint.WallSize+ 2Ba se.Constraint.Tolerance", so you can not only see it is "Width+WallSize+2Tolerance”, but also see where the measurements where defined.
To create two object that should interlock, I would create a Part and two Bodys, where each of the models will be created. Part is the yellow step stone and body the blue one. They are kind of folders. Part contains the project and body separate pieces like your two modules.
Create a block on X/Y and make sure you center the sketch on the X axis. This allows to use the center point of the coordination system as anker point for the nodge and the pocket. Since you just need to draw a few lines and set the measurements, which, thanks to auto complete, is simple like hell, you are done in a few minutes.
The result is a fully parametric thing. I know this was a lot of information and not so much on your problem, but therefore I created the model as an example. Play with it and learn from it. Try to change the connector shape into a circle on a pin.
I attached the full FreeCAD model below.
It is right below the image. It says 0 views 
Try to reorder the messages. Some modes cut off attachments.
Thank you so much I’ll try this this afternoon and let you know if I made it!