From: jmorton@euler.Berkeley.EDU (John Morton)
Subject: Re: Help me learn from this experience
Date: 5 Dec 1995 20:52:30 GMT
Organization: University of California at Berkeley
In article <email@example.com>,
Vince Miller <firstname.lastname@example.org> wrote:
>I just picked up a cast iron pulley I had bored at a local machine shop. I
>specified a sliding fit on a 2.001 shaft. The dude said ok, and said they'd
>shoot for 2.0015-2.002. I was shocked when I returned to find I was billed
>for 1.5 hours on this job.
>I guess I inadvertantly asked for more precision than I needed? The guy
>said they bored it then had to hone it to get it that close. I just wanted
>a decent fit that didn't need to be pressed.
>Was I taken a little? Is there some book I can read to avoid such gaffs in
The designer faces many pitfalls, and there is no one source of information
on them. Generally speaking +/- .005" should be pretty routine for small
parts. +/- .001 requires more care, and (if mass removal of material is
not involved) approximately twice the time. +/- .0005 is yet more costly,
but doable for external dimensions, and perhaps more troublesome on the
mill than on the lathe. Internal dimesions are not as easily measured,
and for this tolerance may require making a set of plug gauges for the job.
The question here is one of the range of tolerance constituting a
slip fit. A close sliding fit has about .0004" clearance per inch of
diameter; that is the normal clearance for gauges. Therefore you must
be larger than 2.0018" and allow .001" total beyond that to keep the job
in the lathe. The hone is more money because it's another setup, but
the shop without a hone might spend the same money working out a way
to hold +/- .00025. A shop with a good set of Boyce gauges would have
a great advantages (that is the generic name for sprung 3-point gauges
which are set with great accuracy to a Cadillac-type precision height
gauge). Intramikes will do it too, and are yet more expensive.
Therefore the shop charges more to pay for all those nice tools, and
you pay in hourly what you saved in time :-).
John Morton Mechanical Engineering Machine Shop
email@example.com University of California at Berkeley