Hi Denman, thanks for posting the models, that really helps!

Yeah for "test2", like Steve and Danny mentioned something happened to "un-solidify" that part, it isn't connected together into just one single solid anymore.

But also notice this area here:




That's where your cutting object just kind of comes very flush and sort of grazes the tire surface. This kind of situation makes things much more difficult for the booleans to calculate.

It's better if you can set things up like this instead:



Here I used Transform / Scale / Scale1D to kind of stretch out your cutting object a little bit so that it has a more definite protrusion through the tire body, that is the kind of thing that will work better for booleans.

I've also solidified the tire body, and set it up to just do half of it and then the results can be mirrored. Also I moved the tire so that it was centered around the origin point, that tends to be a convenient way to position things because it is easy to snap back to that origin point later on.

That base model is attached here as test2_new.zip .


So then the steps to finish it are:

Select the new slightly longer cutter:




Use Transform / Array / Circular to replicate it (looks like you had 45 copies):



Now select the base tire, run Construct / Boolean / Difference, and select all the cutting pieces. With the cutter arranged like this the boolean can be done more easily:




Now to mirror it, select the flat face on the opposite side:



And delete it:



This now has an open edge there, now this shape can be mirrored, and then use Edit/Join to glue the 2 pieces together along that open edge to form the final solid:



- Michael