Your problem is that your curves are splines, which cannot be joined to other objects. Even some of the straight lines are splines. If you are new to Autocad, try to avoid splines because they are geometrically very complex and are completely unnecessary for something like what you have in this drawing. You have a couple of choices here:
- Honestly, they should be redrawn as curves and lines and you will be thankful you did in the long run. There are some circles there that are made up of several splines and there is no reason that should be the case. They should simply be circles. Personally, I would redraw the whole thing with the curves as arcs, all the circles as circles, and the lines as lines or straight polylines. You could also use the fillet radius command to make the corner curves.
- If you don't want to redraw everything, you can convert the splines to polylines by using "splinedetit", select the spline, and then choose "convert to polyline". But it only allows you to do one at a time, so I think it would take longer than drawing everything over as curves and circles. And if you take a straight spline and convert it to a polyline and then select it, you will see a ton of grips light up meaning that that polyline is made up of a ton of little segments.
Was this drawing converted from another format or drawn in Autocad from scratch? If it was drawn from scratch, it should have been a very simple line, arc, circle drawing with no splines. There is nothing in that drawing that warrants being a spline from what I can see.
Lastly, many machines (assuming this is an item going into production) will have a hard or impossible time reading spline geometry. Most machining programs like simple joined polyline objects.
