How to radius the topline of a putter

Something I have been curious about is how to produce the radiused topline that is seen on some putters. I have Redwood Anser with a flat, squared off topline and am wondering how difficult it would be to radius it. I know I can go at it with a file or belt-sander but hand work won't produce a precise radius along the entire length. Do they just mill it with a ball nose mill? Any thoughts from the machinists?

It depends on how large a radius we are talking. If it is a small radius (say 1/2" or less), it can be machined with a corner-rounding milling cutter. Some of the larger radii can be contour milled with a ball nose, like the TaylorMade milled grind wedges. However, don't discount how precise you can be with a file. If you take your time, you can do very clean and accurate work with a file. Sometimes a file is a toolmaker's best friend. You can often create contours on a part that would be difficult or impossible to machine otherwise.

I'm guessing it has to be a fairly big radius across the top - maybe 2 inches or so. I could practice with a file but still wouldn't be comfortable that I could hold a shape and radius along the length of the head.

I've done it when refinishing a putter or two, belt sander and a gentle finesse is all it takes. Also, do the work against the belt only(free moving air portion, that doesn't have the flat backing bar directly behind it, for 90* planar surfaces). If you are gentle and smooth with your motion, once you polish and blast it, you'll never know it was hand made.

Trust yourself with the file, getting it set up to mill the form would probably take longer than the milling itself!! When I've done it in the past I scribble some Sharpie on the top line to give a bit of reference as to how square you are keeping it - hope that makes sense!!

Probably worth getting a small needle file for finishing it off and some fine emery paper too.

I prefer a little radius myself, and use a Harbor Freight belt sander with a 400 or 600 grit belt to get it done. On the sander, there is a small gap between the pulley and flat metal base that I use. I just hit the top line of the putter in that gap and gently rock the putter back-n-forth to get the radius. The rocking motion, combined with the "give" in the sanding belt (because there is no hard surface underneath the belt in that gap), gives a smooth and even radius with no angles. And the best part is it only takes like 10 seconds.

Cool way to do that, thanks for the tip !