I've done valve jobs many times on my fathers Black/Decker valve machine. I've faced the valve facing, and valve seat stones many times, but over the last 20 years I've forgotten if the valve facing stone needs to be faced flat, or at an angle? I just got the Snap On machine a few years ago, and finally got around to ordering a new valve facing stone. The new stone, out of the box, has an angle already in the stone, but giving it a try on an old junk valve at 44 deg valve angle, it donesn't look right. Here is some pictures of the valve machine model. Here is the new stone installed where you can plainly see the bevel, inrealtionship to the valve set at 44 degs. Here is the old stone in the foreground where you can see the bevel actually in a reverse angle to the new stone [yes, the new stone is installed correctly I double checked]. And, a shot of the whole valve machine.
Ok, I've found some conflicting info. Some people are dressing the stone flat [ie no angle], and here is a video where the guy is facing the stone at a 45 deg angle.
Well, nevermind. I figured it out, the valve facing stone has to be dressed with a 45 deg angle. All done for the day so tomorrow I will cut the seats, and be done....for a while.
The stone should be flat your chuck that holds the valve is whats adjusted to the angle your looking for. The stone being flat allows the valve to be cut to 30 degrees and any other angle. Most valves will have two angles for improved flow using a stone thats set for one angle will not work well.
Snap On VR300, VR500 and K O Lee (who built the Snap On machines) have the stone dressed at an angle.