Do you believe that grooves or inserts with grooves really improve one’s putting or is it mostly marketing and sound effects?
The face material must be hard and rigid and not impart more or less spin to a ball than a standard steel face. Otherwise, it is in violation of Rule 5-1a in Appendix II of The Rules of Golf. Some exceptions may be made for putters. These exceptions are probably related to the phrase “hard and rigid”.
With this in mind, I believe inserts are used to soften the feel of impact and/or make the impact sound better on those putters with hard, steel faces.
The Frankly Frog putter is made of 6061 Aluminum (aircraft quality) and the effect on the ball and the sound is perfect without an insert.
Bottom line is that there is no evidence that grooves or inserts make golfers putt better. The face must, however, have a rough surface to ensure that there is no slippage during contact and this may be accomplished by three-way milling.
I have been led to believe that the USGA tests putter face treatments to confirm that the spin properties are similar to that of a standard steel face.
Tom, you can’t buy a stroke: you need to earn it through an understanding of the fundamentals and then practice these until they become subconscious moves, like walking.
“The Fundamentals of Putting” will certainly be a good start.