Usually on a three place decimal for a dimensioned drawing, it calls for no more than +.001" on the OD or -.001" ID, or if they are really anal .0005". How much scrap will they tolerate? Are they providing material or are you? There will be scrap. Inattentiveness, inaccuracies in measuring, product damaged during/after handling, etc.
Asking to hold a tenth is basically asking for one-at-a-time production or piecemeal work, as most machines will constantly be over or under .0002" due to thermal growth. There is almost no way to anticipate the growth or shrinkage of a part at this tight of a tolerance without some really expensive machines (with built-in thermal compensation), even with adjusting offsets after every part. Of course this is also dependent on weather factors, amount of heat put into the part during/before/after the process, climate control inside the building, repeatability of measuring due to differences in operator styles and practices, measuring the part at room temperature with tools that are at the same temperature, etc. We had a part that could only be +.0003"/-.0000" and we could hold it, but we were running that part ONE at a time on a machine that was manually loaded, unloaded and operated, with a hard stop at the end of the program. We had to measure EACH part and adjust the offset up or down after EACH part. This special job was run on a 4 month old machine that ran production 16 hours a day, normally with an insert change every 250 parts and offset adjustments made every 10 parts, measuring every 5th part, using proven programs. Oh, by the way, this machine also cost the better part of $100,000 before barfeeder and tooling. This sounds like a tough job for someone using a hobby-type machine, regardless of brand. Not to spoil the party, just giving some feedback from the industrial side of CNC machining.  |