attack angle
Art,
Thank you for providing that image. There's something a little curious about it. Specifically, there are a couple of points which make it seem a bit simplistic to me.
First is that when the tooth is rocked back relative to it's direction of travel, the relative depth of cut decreases, until it's eventually nothing. At .025" depth with the "raker" leading the point by .25", the angle is 5.7°. That is, 5.7° from 0 relative depth.
Maintaining that angle (distance-to-depth of 10:1), .038" depth would result in .38" distance to the cutting point, which barely reaches the front of the rear rivet on my 3/8 RS chain. There is fully another 1/8" (1/2" total) of useful distance from the "raker". At .5" distance, the depth gauge would need to be .050" by the formula implied in that attached image.
Another way of looking at it is that at .5" setback and .038" depth, the angle has lessened from 5.7° to 4.3°. So from that standpoint there seems to be some discrepancy, or at least indication of simplicity in the information (since the angle would not be maintained).
When I take a couple of quick measurements on my 3/8 RS chain, I see that the cutting edge of the top plate starts as approx. 5mm and lessens to approx. 4mm at the rear witness mark. That's a 1.25:1 ratio, and applying that factor to the initial depth setting of .025" would result in a setting of .031" to maintain the same cross-sectional area of "chip" being removed (or volume per length of chip).
Now that last bit was not based upon precise measurements, and I would certainly not be surprised to see variation in one direction or the other. Let's assume it's accurate for the moment.
If for some reason the tooth actually does cut with the leading rivet at a greater distance from the bar than the trailing rivet (and I concede that the tendency to attempt to do that would increase as the cutting corner approaches a position above the rear rivet, and even more so as it falls behind the rivet) then that would certainly lessen the <i>effective</i> depth setting. If indeed that occurs, then when the depth gauge is lowered beyond that necessary to compensate for the shorter top plate cutting edge, as further compensation for the altered orientation of the tooth, the vertical angle of the sideplate cutting edge would need to be brought forward to ensure that the point leads by the proper amount on that plane as well. Is any mention of that made in the literature? Have you experimented with <i>that</i>?
This is all armchair engineering on my part. I say that to let you know that I know that. I feel that proportionately lowering the depth gauges is the good and proper thing to do in order to maintain the amount of material the cutter will remove in use over it's lifespan. It's obvious that it actually works, whatever the possible explanation. It's also obvious that the increased distance from the depth gauge to the cutting point of a used chain imparts greater kickback tendencies to it, and that by progressively lowering the depth gauges to maintain the volume of chip produced increases those tendencies even further.
Perhaps my point could be summarized by saying I feel that diagram is a bit more marketing gimmickry for the File-O-Plate than it is scientific. (not to say the File-O-Plate is a gimmick)
By the way, don't you think that the drag of a dozen following cutters, all trying to do that same thing, would tend to pull the lead rivet down to a position directly inline with it's heel rivet and that of the previous cutter? There must be tremendous force on the heel of the cutter link!
