I look at it like this...There is no such thing as perfect, only lesser degrees of f'd up. But assume for a minute that you had a truly "perfect action that you wish to verify. Proving it to be perfect is dependent upon your setup and upon the fixture being perfect, too. Both, unlikely at best.
Now lets say we have an action that has .0002" runout at the face. You indicate it in perfectly, to .0000 runout. Where did the .0002 go. Or, lets say you dial it in to show .0002 runout. How are you sure that it's the runout in the face you are measuring or that of the fixture? Hence the difficulty in truly proving either way, particularly when most lathes have this much or more run out when they are brand new and properly setup from the get go. Which are you actually measuring..the machine/fixture or the part? And this is not your typical Chinese lathe I'm referring to. Nor is it checked in a basement shop, with tools that have never been calibrated and at some unknown temperature. How long did you run the lathe before testing to bring the headstock bearings up to temp and what temp is that? Lots of variables worth more than .0002" here. Also, if you really get serious about critical dimensions, you should know that they are typically ground, not cut with HSS or carbide. Tool deflection, part deflection in the lathe, etc, are just a couple of variables that have to be accounted for when machining to these claimed tolerances. Or rather, one can claim them but proving them is another ballgame, entirely.
I've seen a few custom actions that were less than ideal. Some to the point of needing to be corrected by me. But, as a whole, the premium actions are better than they have to be and any time you remove the part from the initial machining setup, you introduce much more room for making it worse, not better. This has little to do with how good you are or how good you think you are, but just plain common sense in the machining world. Of course this is referring to the very good quality actions that are typical. Sure, if it's out very much, i.e., .001" or more, you can very well improve upon that with careful setup, quality equipment and attention to detail.
Remember this, any time you put cutter to steel, the cutter bends. This is called tool deflection. Same goes for the lathe chuck in the spindle bearings.
Just some things to think about when you hear people talk about working in .0002" tolerances in their basement shop.
Now lets say we have an action that has .0002" runout at the face. You indicate it in perfectly, to .0000 runout. Where did the .0002 go. Or, lets say you dial it in to show .0002 runout. How are you sure that it's the runout in the face you are measuring or that of the fixture? Hence the difficulty in truly proving either way, particularly when most lathes have this much or more run out when they are brand new and properly setup from the get go. Which are you actually measuring..the machine/fixture or the part? And this is not your typical Chinese lathe I'm referring to. Nor is it checked in a basement shop, with tools that have never been calibrated and at some unknown temperature. How long did you run the lathe before testing to bring the headstock bearings up to temp and what temp is that? Lots of variables worth more than .0002" here. Also, if you really get serious about critical dimensions, you should know that they are typically ground, not cut with HSS or carbide. Tool deflection, part deflection in the lathe, etc, are just a couple of variables that have to be accounted for when machining to these claimed tolerances. Or rather, one can claim them but proving them is another ballgame, entirely.
I've seen a few custom actions that were less than ideal. Some to the point of needing to be corrected by me. But, as a whole, the premium actions are better than they have to be and any time you remove the part from the initial machining setup, you introduce much more room for making it worse, not better. This has little to do with how good you are or how good you think you are, but just plain common sense in the machining world. Of course this is referring to the very good quality actions that are typical. Sure, if it's out very much, i.e., .001" or more, you can very well improve upon that with careful setup, quality equipment and attention to detail.
Remember this, any time you put cutter to steel, the cutter bends. This is called tool deflection. Same goes for the lathe chuck in the spindle bearings.
Just some things to think about when you hear people talk about working in .0002" tolerances in their basement shop.
