Please educate a newbie on the 4 jaw and interpretation of inconsistent results.
Last indicated part was a CNC machined commercial grinder idler wheel from Beaumont Metals. As such, I am fairly confident that it is at the very least good to 0.002" TIR if not better. Chucked in my 4 jaw there is a facet on the side of the wheel that should be concentric with the diameter and the bore which has a bearing in it. I therefore decided to use this feature as the indicating surface. The outer diameter of the wheel has a severe crown that I want to machine flat or almost flat. That surface is also fairly rough so I am using the previously described smoothly finished facet as the indicating surface. Using a "7 jeweled" HF Chinese test indicator (all I got right now) marked 0.0005", I presume this to be imprecise to that claim. I generously give it 0.002 precision at best. Centering opposite jaws, I can get both sets of jaws within 0.001" on the Chinese test indicator reading. What puzzles me is that one set of opposite jaws (say #1 and #3) will read about 0.003" to 0.005" bigger than the other set (#2 and #4). This is a trend not only on this part but on all previous parts I have attempted to center. Granted that my test indicator is a POS, I should at least get consistently closer readings I would think.
Analyzing this dilemma, first suspect is my indicator. But how can I get very close to center between paired jaws but not so comparing the readings of the two sets. To my best theoretical visualization, if one can center the part between sets of jaws, I should get the same reading at both sets and all the way around when I spin the part. One might say that the part may be out of round. May be true of the other parts I've centered but this wheel is my best "round" part I can refer to since it has been CNC'ed and all. Could my 4 jaw be off center on its baseplate? Worse, could the spindle of my 9" Southebend be bent? I am staining my brain now trying to theorize and guess at what might be causing the inconsistency. Or perhaps and more likely, as a novice, there is something I am not doing correctly in my centering technique. I invite the kind folks here to postulate possibilities that might be causing my dilemma. Thanks for your time.
Last indicated part was a CNC machined commercial grinder idler wheel from Beaumont Metals. As such, I am fairly confident that it is at the very least good to 0.002" TIR if not better. Chucked in my 4 jaw there is a facet on the side of the wheel that should be concentric with the diameter and the bore which has a bearing in it. I therefore decided to use this feature as the indicating surface. The outer diameter of the wheel has a severe crown that I want to machine flat or almost flat. That surface is also fairly rough so I am using the previously described smoothly finished facet as the indicating surface. Using a "7 jeweled" HF Chinese test indicator (all I got right now) marked 0.0005", I presume this to be imprecise to that claim. I generously give it 0.002 precision at best. Centering opposite jaws, I can get both sets of jaws within 0.001" on the Chinese test indicator reading. What puzzles me is that one set of opposite jaws (say #1 and #3) will read about 0.003" to 0.005" bigger than the other set (#2 and #4). This is a trend not only on this part but on all previous parts I have attempted to center. Granted that my test indicator is a POS, I should at least get consistently closer readings I would think.
Analyzing this dilemma, first suspect is my indicator. But how can I get very close to center between paired jaws but not so comparing the readings of the two sets. To my best theoretical visualization, if one can center the part between sets of jaws, I should get the same reading at both sets and all the way around when I spin the part. One might say that the part may be out of round. May be true of the other parts I've centered but this wheel is my best "round" part I can refer to since it has been CNC'ed and all. Could my 4 jaw be off center on its baseplate? Worse, could the spindle of my 9" Southebend be bent? I am staining my brain now trying to theorize and guess at what might be causing the inconsistency. Or perhaps and more likely, as a novice, there is something I am not doing correctly in my centering technique. I invite the kind folks here to postulate possibilities that might be causing my dilemma. Thanks for your time.