In the golden olden days of Model Engineering when the main measuring tool was a ruler marked out to 128 Th's and parts where made to fit each other, 5 or 6 thou was considered good but we now have much better tools today.
they were making them perhaps without aid of a micrometer but the does not mean their standards of fit were any less than today. There are many comparative technique to create extremely accurate fits. On the contrary, i think you could make an argument that the more gadgets the newbie is equipped with today the lower the standard of work....why? they never learned the basics, filing, scraping, general bench work, layout etc whereas is less equipped learned many of these skills because there was no other way to do things. I haven't done it, but don't think it would be too difficult to fit a piston to a bore with less than a thou clearance without a micrometer.
Note that in making a one-of, its fit that matters not hitting a nominal dimension. if the the goal is 1 thou clearance between piston and cylinder that's what counts not whether the final bore is 1.000" or 1.005". In fact even with all of our fancy equipment today, its still good practice to turn the piston to the bore.
Invite any one of the pros to your shop to show you how they consistently hit that tenth
of a thou. size.
and
If it's straight and on size within .001 for me, it's Dead Nuts on!
there's, imo, several stops along the accuracy track between these two statements. Within a thou isn't good enough in many situations, yet you better be running a hardinge or monarch in temp controlled environment and have some very nice measuring equipment to consistently claim to hit a tenth. My best digital mic measures in tenths - if tenths is its best resolution, how can someone claim to hit a tenth without a mic that measures to even a smaller increment ...and does so accurately. show me a guy in his garage or even the average job shop running around with mic that accurately measure to a fraction of tenth :
However when it really really matters, like getting the proper press fit on something or housing or shaft for a roller element bearing, you need to machine to better than a thou.... a couple of tenths is a reasonable target imo....when you have to (as per chucks point).... also we tend to work with smaller stuff so clearances and tolerances are smaller.
My German tool and die maker turned high school machine shop teacher would unleash a tirade for filing in the lathe and emery was for finish not fit. Granted he was trying to train us, but the point is valid, once you start what that stuff you've no guarantee the part is round or straight. I do use fine emery to take off the minute burrs and ridges if its a moving part, but I've never lost what that very skilled and disciplined man taught us.....and those abrasive techniques are useless for bores unless you want a broken finger
Chuck point is right on - far more germane to us that how accurate you
can machine to is understands when and what accuracy is required. There are reasons model engineer drawings don't usually have tolerances or even clearance specified. Its up to us, acting as engineers rather than by rote following a tolerance schedule to understand what is required and machine to that - i like the engineers expression: Good enough is good enough! (though good enough is often in the eye of the beholder when we're talking model engines)
