Not to belabor this but, even though the discs are .025" there's enough wobble in the mandrel, chuck, and the operator's hand to dig a trench that's considerably greater than 1/32". Arnie says he built a fixture that allows for a straight line. That probably works just fine. But I don't believe, having tried it myself several times, that a person can etch a groove freehand to the width and depth required by USGA rules.
I had no trouble enlarging already-existing grooves. Kept it under the USGA max pretty easily, freehand. Now, if you had no grooves, or grooves so worn that it wouldn't track in them, then I'd have to agree with TFlan.
Cheers! DaveT
