Defining a function as a truncated taylors series

I am trying to numerically find the error of a truncated series. To do so I want to first define a function that is the truncated sum, I cannot seem to do that. All I am tried is not working.

f[x_]:=Normal[Series[ArcTan[x], {x, 0, 100}]] 

Does not work as f[1] evaluates $ x$ inside $ \arctan$ . I also tried

Normal[Series[ArcTan[x], {x, 0, 100}]] f[x_]:=% 

But that did not work. When I copy and paste the result of

Normal[Series[ArcTan[x], {x, 0, 100}]] 

It works fine. What am I doing wrong?