I tend to disagree heartily with Mrbean and here's why:
We make cams from our own designs. In doing so, we see continuous and often predictable profile degradation/deviation trends as you go from design data to the machining master.
There is then more change generated as you go from master to finished cam. We even see differences in the profile when we grind the cam with a "small" (13")versus a big (18") diameter wheel.
Only via empirically derived and applied compensation techniques can we "manage" this in spite of the fact that we've rebuilt and redesigned certain features of our grinder and/or our production processes - the machine simply can't do anything other than what the synergy of its parts will allow - good, bad or indifferent.
There is defininite analog "smoothing" and/or morping that takes place in the manufacturing process which results in profile deviation. Is it enough to matter???
Suffice it to say that it is manageable and/or containable once you learn how to compensate for it. Machine compliance and/or normal tolerances are merely a part of the reason why it is IMPOSSIBLE for even the best analog and some digital equipment to EXACTLY duplicate design intent.
Now, assume you're want to to make an EXACT 10 turns/inch ball screw. If you have ANY lead error in the lead screw of the lathe used to make the ball screw, the BEST you can hope for is to duplicate the lead error in your "master" lead screw (don't forget axial runout potential in the spindle bearings too which gets superimposed as well) - the more probable result will be the amplification/exacerbation of existing lead error - thus, the by-product WON'T/CAN'T be better than the original - ERROR begets error.
Having worked in the bearing industry (machine tools) and now in the cam industry, I know that the reduction of error is MANDATORY as you seek to achieve the production of more precise, error free product. The creation of better machines that do things with less composite error is critical to the creation of lower PPM defects.  |